geometric and kinematic models for detachment folds with fixed and variable detachment depths abstract detachment folds are defined by competent rock units and are cored by incompetent units deformed internally above a detachment horizon we have developed two geometric models to constrain possible geometries and kinematic paths for ideal detachment folds the models each independently relate fold geometry to shortening and to detachment depth model assumptions include plane strain constant competent bedlength constant cross sectional area chevron fold geometry and no bed parallel shear outside the fold detachment depth is constant in one model but may vary in the other thus allowing evaluation of the implications for fold geometry and kinematics of fixed vs variable detachment depth detachment folds formed above a detachment unit of constant thickness constant detachment depth must be initially symmetrical and cannot grow with fixed hinges fixed arc length or a self similar geometry detachment folds formed above a detachment unit of variable thickness variable detachment depth must also be initially symmetrical but any one fold geometry can have a range of possible initial and final detachment depths kinematic paths for folds with fixed hinges fixed arc lengths migrating hinges variable arc lengths and selfsimilar geometries are all possible if detachment depth varies the change in detachment depth during deformation can be determined using the variable detachment depth model if either initial or final detachment depth is known the models demonstrate a wider range of variability in the geometry and kinematics of ideal detachment folds particularly for the variable depth model than is the case for ideal fault bend and fault propagation folds this variability limits the usefulness of simple geometric models for reconstructing the geometry of natural detachment folds balancing cross sections over a sufficient area and evaluating strain may compensate for these limitations

formation of large sulfide mineral deposits along fast spreading ridges example from off axial deposits at 12 43 n on the east pacific rise abstract submersible investigations with nautile near the 13 n hydrothermal field on the epr were made in 1992 we selected diving areas based on deep towed side scan sonar imaging close to a pit crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest 70 m high and 200 m in diameter sulfide mounds formed at a fast spreading ridge 12 cm yr despite an overlap in composition between axial and off axial deposits the general trend reflects real differences in compositions off axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment sulfur isotope variation is minimal for the off axis seamounts and suggests that sulfides precipitated from unmodifed end member hydrothermal solutions axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different pb sources and separate convective systems for the seamount and the axial deposits our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount this configuration is by far more efficient than unstable axial processes to create during several hydrothermal episodes major sulfide deposits near a fast spreading ridge off axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis thus we suggest that off axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast to medium spreading ridges

syn and postkinematic cement textures in fractured carbonate rocks insights from advanced cathodoluminescence imaging abstract in calcite and dolomite deposits in fractures transmitted light and optical cathodoluminescence methods detect crack seal texture in some fractures but scanning electron microscope based cathodoluminescence sem cl combined with secondary electron images and element maps reveals crack seal and cement growth textures where previous sem cl imaging methods found massive or featureless deposits in a range of fractured carbonate rocks patterns and textures of calcite and dolomite cements precipitated during and after fracture growth resemble complex accumulation patterns found in quartz in sandstone fractures suggesting that some apparent differences between carbonate mineral and quartz deposits in fractures reflect the limits of previous imaging methods advances in delineating textures in widespread carbonate mineral deposits in fractures provide evidence for growth and occlusion of fracture porosity

conodont color and textural alteration an index to regional metamorphism contact metamorphism and hydrothermal alteration experimental and field data are used to extend the utility of conodonts as semi quantitative thermal indices into the regimes of regional and contact metamorphism as well as hydrothermal alteration an arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color alteration indices cai above 300 c that is for cai values of 5 through 8 such cai values occur in very low to medium grade regionally metamorphosed contact metamorphosed and hydrothermally altered rocks the uniformity or variability of cai values within a sample together with conodont texture can help to distinguish grades and environments of metamorphism particularly in metacarbonate sequences induced cai by pyrolysis in a water methane mixture at kbar results in retardation of cai and in a disparate mixture of both low and high cai values within each experimental sample in this system color alteration processes above a cai of 2 to 3 seem to change from predominantly carbonization to predominantly loss of organic matter presumably by oxidation and volatilization of oxides these experiments approximate the type of cai mixture characteristically found in conodonts recovered from hydrothermally altered rocks these data indicate that cai values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization

origin and timing of dauphine twins in quartz cement in fractured sandstones from diagenetic environments insight from fluid inclusions abstract electron backscattered diffraction techniques ebsd show that dauphine twins in quartz are widespread in many tectonometamorphic environments our study documents that under diagenetic temperatures

practical approaches to identifying sealed and open fractures for one essential ingredient of permeable fracture networks degree of fracture pore space preservation in large fractures i show how the characterization challenge presented by sparse fracture sampling can be overcome by measuring a surrogate the abundance of rock mass cement that precipitated after fractures ceased opening sampling limitations are overcome because the surrogate is readily measured in small rock samples including sidewall cores and cuttings permitting site specific diagnosis of the capacity of fractures to transmit fluid over a wider range of sample depths than conventional methods allow a diverse core database shows that this surrogate correctly predicts where large fractures are sealed information on timing of fracture opening relative to cement sequence can be obtained in two ways first evidence of fracture movement history and cement sequences in sparse large fractures can be extrapolated to areas having only cement data alternately evidence of fracture timing can be acquired from sealed micrometer scale fractures distribution of porosity reducing cement is commonly heterogeneous from bed to bed and location to location in siliciclastic and carbonate rocks however because patterns of sealed or open fractures cannot be delineated using fracture observations alone surrogates have practical value for production fairway mapping and other applications in which identifying open fractures is essential this study highlights the vital interplay among structural and diagenetic processes for fracture porosity preservation or destruction

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

hydrothermal discharge zones beneath massive sulfide deposits mapped in the oman ophiolite the area in the oman ophiolite containing the volcanic hosted bayda and aarja massive sulfide deposits exposes a cross section of ocean crust and reveals to an unprecedented extent the fossil zones of hydrothermal upwelling that fed these sea floor deposits the fossil discharge zones are elongate areas of alteration and mineralization characterized by numerous small metres to tens of metres in length linear discontinuous gossans the gossans result from oxidation of hydrothermal pyrite replacing primary igneous phases and filling voids and fractures in the altered host rocks the two deposits have separate discharge zones that appear to be sub sea floor extensions of their stockworks the bayda zone extends through the volcanic section into the upper sheeted dike complex and is interpreted as having formed on the ridge crest above an axial magma chamber the aarja zone terminates against a plagiogranite pluton that intrudes the lower volcanic section and is thought to have formed after bayda in an off axis environment structural stratigraphic and compositional characteristics of the bayda and aarja massive sulfide bodies are consistent with this interpretation the geometry of the discharge zones suggests that in both cases upfiow occurred in broad zones at least 400 600 m wide that were elongated along strike i e parallel to the spreading axis

structure and geochemistry of tethyan ophiolites and their petrogenesis in subduction rollback systems suprasubduction zone ssz ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions mantle flow and slab rollback result in one or more episodes of arc splitting and basin opening producing a collage of protoarc and forearc oceanic lithosphere in suprasubduction zone settings the jurassic cretaceous ssz tethyan ophiolites in the eastern mediterranean region i e mirdita pindos troodos kizildag oman generally have penrose type oceanic crust and contain well developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution igneous accretion of these ssz tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins showing a progressive evolution from morb like to iat island arc tholeiite to boninitic extremely refractory protoarc assemblages however there are some distinct differences in the geochemical evolution of these tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamics whereas a major part of the kizildag and troodos lavas shows island arc affinity similar to their counterparts in the pindos and mirdita ophiolites a significant component of the oman lavas indicates morb affinity and the majority of the kizildag and oman data plot within the mantle array between n morb and e morb on the nb yb th yb discriminant diagram furthermore the troodos and oman lavas do not show any particular th enrichment in their multi element patterns suggesting that fluid melt input from subducted sediments was not that significant in generation of their magmas although all ophiolites exhibit geochemical features indicating increased subduction influence during the melt evolution of their younger extrusive sequences and dike intrusions as evidenced by their negative nd values their overall characteristic trace element patterns seem to have been strongly affected by the maturity of the subduction systems in which they developed

clinopyroxene composition of ophiolite basalts as petrogenetic indicator abstract a statistical microprobe study has been carried out on augitic clinopyroxene in high ti to low ti metabasalts from various phanerozoic ophiolites a comparison has been made with clinopyroxenes from mid ocean ridge basalts morb oceanic intraplate basalts island arc tholeiites and boninites leaving aside the effect of cooling rate and of fractional crystallization as major factors influencing pyroxene chemistry their composition appears to be essentially related to the type of host magma both the tio2 and the tio 2 al 2 o 3 in clinopyroxenes are positively correlated with the zr y ratios of the host basalts which is a diagnostic parameter insensitive to alteration increasing regularly from low ti to high ti ophiolitic basalts ti also shows a positive correlation and average d ti cpx rock partition coefficients varying from 0 3 0 6 to 1 from low ti to high ti basalts which compare favorably with those from tongan southwest pacific basaltic andesites 0 3 0 6 and morb 0 75 respectively likewise the average k fe mg calculated for clinopyroxene rock pairs varies from 0 20 in low ti basalts to 0 26 in high ti basalts up to 0 33 in the highest ti rocks suggesting a significant dependence of this parameter on magma composition the compositional correspondence of clinopyroxenes from various ophiolitic basalts with those from oceanic actualistic equivalents is shown in various diagrams concomitantly with increasing tio2 and na2o and decreasing sio2 contents clinopyroxenes from very low ti to high ti ophiolitic basalts are best equated with those from boninites basaltic andesites and tholeiites from island arcs and morb respectively accordingly a parallel increase is shown by the substitutional molecules catial2o6 and natisialo6 within each basalt type fractional crystallization processes are reflected in the natisialo6 and fetial2o6 increase and mgcrsialo6 decrease from early to late crystallized clinopyroxenes

lithologic and structural controls on natural fracture distribution and behavior within the lisburne group northeastern brooks range and north slope subsurface alaska the carboniferous lisburne group of northern alaska has been deformed into a variety of map scale structures in both compressional and extensional structural settings thus providing a series of natural experiments for observing the formation distribution and behavior of fractures in this thick carbonate unit two fracture sets dominate the lisburne group carbonates of the north slope subsurface and the nearby northeastern brooks range fold and thrust belt north northwest striking regional extension fractures probably formed in front of the northeastern brooks range fold and thrust belt in the north slope subsurface this fracture set overprints east northeast striking fractures related to earlier extensional deformation in contrast in the fold and thrust belt the north northwest striking fracture set is overprinted by younger east northeast striking fractures related to subsequent contractional deformation lithology is the primary control on the fracture density of both sets in mildly deformed lisburne copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 20 1996 revised manuscript received november 25 1996 final acceptance june 5 1997 2geophysical institute university of alaska fairbanks fairbanks alaska 99775 3sandia national laboratories ms 0705 albuquerque new mexico 87185 4department of petroleum natural gas engineering new mexico institute of mining technology socorro new mexico 87801 and sandla national laboratories ms 0705 albuquerque new mexico 87185 this study was supported by a department of energy subcontract administered by sandia national laboratories additional support was provided by arco alaska bp alaska chevron exxon mobil and japan national oil company we would like to thank arco alaska and bp alaska for giving us permission to view selected lisburne group cores w wallace for helpful discussions on deformational styles of detachment folds a j mansure for help in interpreting the interference tests and w wallace k biddle w belfield n hurley and j kelley for helpful reviews of the manuscript

the pennsylvanian permian transition in the low latitude carbonate record and the onset of major gondwanan glaciation abstract recent studies suggest a marked expansion of glacial ice across much of gondwana beginning in the earliest permian because expansion of glacial ice results in a lowering of sea level the imprint of ice expansion should be evident worldwide as significant exposure event hiatuses or other evidence for sea level drop at or near the pennsylvanian permian boundary this literature review investigates the signature of an early permian expansion of gondwanan ice through examination of stratigraphic records from eight carbonate dominated palaeotropical regions across pangaea tropical carbonate environments are used because most form in tectonically quiescent regions and are sensitive indicators of eustatic change correlation between stratigraphic sections is achieved using the most current biostratigraphic and absolute time constraints available all studied sections show a sequence boundary or basinward shift in facies at or near the pennsylvanian permian boundary supporting the hypothesis of a significant expansion of glacial ice and global eustatic lowstand beginning in the early permian by contrast a series of mid sakmarian kungurian transgression events in the palaeotropics are interpreted to reflect the asynchronous deglaciation of gondwana the stratigraphic framework developed herein will allow for better correlations among stratigraphic records from gondwana and northern pangaea which will ultimately improve the understanding of how carbonate systems respond to global icehouse conditions such as during the late palaeozoic

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

k ar ages of metamorphic rocks at the base of the samail ophiolite oman hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the samail ophiolite and phyllites farther from the peridotite contact have weighted mean sup 40 ar sup 39 ar total fusion ages of 90 0 3 0 m y and 79 5 3 0 m y respectively the amphibolities represent the first tectonic slice welded to the base of the samail ophiolite after it was detached from the tethyan oceanic crust formation of the amphiobolities occurred no more than 3 to 7 m y after crystallization of plagiogranite in the ophiolite the phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the tethyan spreading axis the k ar ages suggest assuming a half spreading rate of 2 to 5 cm yr that detachment of the samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center using the same spreading rate one can calculate a minimum half width of 300 to 750 km for the tethyan ocean during the late cretaceous

quantitative evaluation of fractures on monkshood anticline a detachment fold in the foothills of western canada monkshood anticline is a well exposed surface anticline located in the foothills in northeastern british columbia extension fractures are well developed in the prophet formation a carbonate shale chert sequence throughout this fold virtually all of these fractures are filled with some combination of calcite and quartz cements the fractures formed and the mineral cements were deposited during the fold development most fractures on monkshood anticline have formed at high angles to bedding and they commonly fall into two to three distinct orientation sets the dominant fracture trend is aligned with the fold axis through the backlimb of the fold but there is considerable variance in the dominant orientation in the forelimb an order of magnitude variation is found in fracture densities across this fold and over two orders of magnitude variation occur in mean fracture aperture these variations in density and aperture do not correlate with particular structural positions fracture trace lengths exhibit power law distribution patterns suggesting fractal character fracture aperture displays a roughly linear correlation to volumetric fracture strain but shows no consistent association with either density or trace length stochastic modeling of the fracture networks on monkshood anticline suggests density is the primary factor affecting fracture network connectivity but fracture size also can play an important supporting role the occurrence of a dominant fracture orientation set can impart a significant directional bias to connectivity measurement stations on monkshood anticline that have both high fracture densities for connectivity and substantial fracture apertures for conductance occur primarily in midlimb positions

syn tectonic magmatism and the development of compositional layering ungava orogen northern quebec canada abstract layer and foliation parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now exhumed middle crustal sections of both archean and paleoproterozoic age in the northern ungava peninsula quebec canada in the archean superior province diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite facies conditions coeval with voluminous granitic magmatism the paleoproterozoic narsajuaq arc contains evidence for contemporaneous magmatism transpressional deformation and granulite facies metamorphism prior to its collision with the superior province basement in both plutonic domains regional compositional layering is defined by 1 metre to kilometre scale alternation of generally well foliated tonalite and quartz diorite bodies and 2 centimetre to kilometre scale variably deformed granitic veins and sheets that lie parallel to layering tectonic foliation in the host rocks syn tectonic intrusion of a substantial portion of the veins along extension fractures sub parallel to layering foliation i e at high angle to the regional shortening direction is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures compositional layering generated and or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

the interrelationship between magmatic and ore forming hydrothermal processes in the oman ophiolite the semail ophiolite of oman contains numerous gossans minor sulfide showings and ferromanganoan deposits as well as several massive sulfide deposits the largest of which 8 x 10 6 metric tons is the deposit of lasail detailed field mapping of the lavas and shallow intrusions around this deposit in conjunction with petrographic studies of their metamorphic assemblages enables the ore forming hydrothermal activity to be related to the location and evolution of an underlying magma chamber it can be demonstrated that the deposit formed in the short time interval between the eruption of a lava unit the geotimes unit at a marginal ocean basin spreading axis and the eruption of a second lava unit the lasail unit at an off axis or arc seamount moreover it was the seamount not the spreading axis magma chamber which drove the ore forming hydrothermal convection providing both a near surface heat source and by virtue of its intrusive pressure a localized fracture system on its flanks subsequent eruptions from this same magma chamber then provided a protective capping for the deposit the orientations and distributions of andesitic cone sheets show that the magma chamber lay some 1 5 km from the deposit itself and was in fact a magma cupola developed on the flanks of a much larger magma chamber similar though less detailed studies of other deposits in oman indicate that only very small mound deposits lie within the spreading axis lavas themselves the majority of deposits lie at the boundary between the spreading axis and seamount lavas and another group of deposits is related to the magma chambers that fed a third submarine lava unit the alley unit also associated with the construction of the seamounts these results may provide useful constraints for future mineral exploration programs in the oman and perhaps also in other ophiolite complexes and on the ocean floor

natural hydraulic fracturing of tight gas sandstone reservoirs piceance basin colorado natural fractures form preferred pathways for basinal fluid flow and associated heat and mass transport in gas sandstone reservoirs with low matrix permeability fractures provide flow pathways between organic rich source and reservoir layers during gas charge and between matrix pores hydraulic fractures and the well bore during production while the formation of natural fractures has previously been associated with gas generation and pore fluid pressure increase through a process referred to as natural hydraulic fracturing other driving mechanisms such as stress changes by tectonic or exhumation processes remained viable alternatives to test whether these mechanisms contributed to fracture development we investigated the spatial and temporal distribution of fracture formation and its relationship to gas generation migration and charge in sandstone of the cretaceous mesaverde group across the entire production interval on a basinwide scale using fluid inclusion microthermometry of crack seal fracture cement formed concurrently with fracture opening we observed temperature trends that when compared with temperature evolution models of the formation date fracture formation between 41 and 6 ma in the northern and between 39 and 6 ma in the southern piceance basin the onset of fracture formation 20 30 m y prior to maximum burial eliminates changes in stress state associated with exhumation as a mechanism for triggering the onset of fracture formation instead calculated paleo pore fluid pressures of 40 90 mpa 5800 13 000 psi during fracture opening and the presence of methane rich inclusions in fracture cement suggest that fracture formation was aided by high pore fluid pressures during gas generation in organic rich shales and coals and associated charging of adjacent and interlayered sandstone reservoirs a 10 20 m y age progression in the onset of fracture formation from deeper to shallower horizons of the mesaverde group is consistent with gas generation and onset of fracture formation activated by burial temperature with limited upward migration of gas at this stage of reservoir evolution this age progression with depth is inconsistent with fracture formation triggered by changes in stress conditions associated with tectonic or structural processes expected to affect the entire formation synchronously our observations are thus most consistent with fracture formation by natural hydraulic fracturing in response to gas generation in interbedded source layers and reservoir charge based on widespread observations of fractures with similar structural and diagenetic attributes we consider natural hydraulic fracture formation in response to thermocatalytic gas generation to be a fundamental mode of brittle failure in otherwise structurally quiescent basins

the gold content of volcanogenic massive sulfide deposits volcanogenic massive sulfide deposits contain variable amounts of gold both in terms of average grade and total gold content with some vms deposits hosting world class gold mines with more than 100 t au previous studies have identified gold rich vms as having an average gold grade expressed in g t exceeding the total abundance of base metals expressed in wt however statistically meaningful criteria for the identification of truly anomalous deposits have not been established this paper presents a more extensive analysis of gold grades and tonnages of 513 vms deposits worldwide revealing a number of important features in the distribution of the data a large proportion of deposits are characterized by a relatively low gold grade 2 g t with a gradual decrease in frequency towards maximum gold grades defining a log normal distribution in the analysis presented in this paper the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of vms deposits based on gold grade especially when comparing deposits within individual belts and districts the geometric mean gold grade of 513 vms deposits worldwide is 0 76 g t the geometric standard deviation is 2 70 g t au in this analysis deposits with more than 3 46 g t au geometric mean plus one geometric standard deviation are considered auriferous the geometric mean gold content is 4 7 t au with a geometric standard deviation of 26 3 t au deposits containing 31 t au or more geometric mean plus one geometric standard deviation are also considered to be anomalous in terms of gold content irrespective of the gold grade deposits with more than 3 46 g t au and 31 t au are considered gold rich vms a large proportion of the total gold hosted in vms worldwide is found in a relatively small number of such deposits the identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in vms at the district scale the gold rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes the gold rich vms are commonly associated with transitional to calc alkaline intermediate to felsic volcanic rocks which may reflect a particularly fertile geodynamic setting and or timing e g early arc rifting or rifting front at the deposit scale uncommon alteration assemblages e g advanced argillic aluminous strongly siliceous or potassium feldspar alteration and trace element signatures may be recognized e g au ag as sb bi hg te suggesting a direct magmatic input in some systems

geochemical variability of the oman ophiolite lavas relationship with spatial distribution and paleomagnetic directions 1 the multielemental composition of 31 lavas sampled in the extrusive section of the oman ophiolite was determined by inductively coupled plasma source mass spectrometry icp ms this study allowed us to define clear geochemical criteria to characterize the different lavas types in oman most of the oman ophiolite extrusive sequence is composed of lavas of composition similar to present day morb with the exception of slight nb ta negative anomalies v1 magmatism v1 lavas display ree patterns and zr hf ratios in the range of morb zr hf 33 15 38 7 in contrast the overlaying v2 lavas which outcrop only in the northern part of the ophiolite are ree depleted relative to v1 and display low zr hf ratios 23 6 30 5 v2 lavas may be further classified into two sub groups v2 type i lavas display a continuous decrease from hree to lree the upper v2 type ii lavas are more depleted in hree and mree but they are enriched in lree they are also distinguished by their low nb ta ratios 10 53 11 65 relative to other oman basalts v1 12 5 14 v2 type i 12 35 14 4 as for n morb melting of an upwelling mantle beneath an oceanic spreading centre formed v1 lavas v2 resulted from fluid enhanced melting of previously depleted mantle residual after v1 extraction this process probably occurred at different melt rock ratios thus resulting in the two v2 sub groups and was enhanced in the northern part of the ophiolite v3 lavas overlie the v1 v2 sequence in the salahi area they display incompatible element rich patterns lree mree hree and a high nb ta ratio typical of within plate basalts in the different studied areas the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data

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

the death of an accretion zone as evidenced by the magmatic history of the sumail ophiolite oman abstract the ophiolitic crustal sequence of the salahi block in the northern part of oman shows a complex magmatic history the oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates isotropic gabbros a sheeted dike complex and a first tholeiitic lava series v1 depleted in compatible elements the second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite v2 directly linked as can be seen from field evidence to more differentiated high level intrusive plutons this volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements a third volcanic event v3 exhibiting a transitional character enriched in lree s caps the ophiolite these v3 lavas are separated from the v2 unit by a 15 m thick pelagic sedimentary section a geotectonic evolution of the sumail ophiolite is proposed on the basis of field mineralogical and geochemical data after the oceanic accretion phase that gave birth to the main ophiolitic sequence a second stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode such a model accounts well for the depleted character of the v2 lavas the compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second stage melts which are then emplaced in the crust and partly extruded the v3 volcanism postdates the intraoceanic detachment of the ophiolite but precedes its obduction onto the arabian continental margin it is interpreted as an intraplate seamount type magmatism

the subduction initiation rule a key for linking ophiolites intra oceanic forearcs and subduction initiation we establish the subduction initiation rule sir which predicts that most ophiolites form during subduction initiation si and that the diagnostic magmatic chemostratigraphic progression for sir ophiolites is from less to more hfse depleted and lile enriched composi tions this chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during si the magmatic chemostratigraphic progression for the izu bonin mariana ibm forearc and most tethyan ophiolites is specifically from morb like to arc like volcanic arc basalts or vab boninites or bon because si progressed until establishment of a mature subduction zone morb like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of si the contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early si but continued melting results in a depleted harzburgitic residue that is progressively metasomatized by fluids from the sinking slab subsequent partial melting of this residue yields typical ssz like lavas in the latter stages of si if si is arrested early e g as a result of collision morb only ophiolites might be expected consequently morb and ssz only ophiolites may represent end members of the si ophiolite spectrum the chemostratigraphic similarity of the mariana forearc with that of ophiolites that follow the sir intimates that a model linking such ophiolites oceanic forearcs and si is globally applicable

orthogonal cross joints do they imply a regional stress rotation abstract orthogonal cross joints extend across intervals between systematic joints in brittle sedimentary strata and abut the systematic joints at about 90 angles these joints typically form a ladder like pattern if viewed on a bedding surface a common interpretation is that orthogonal cross joints define the orientation of the regional stress field during their formation least compressive stress perpendicular to the joints it follows that they indicate a rotation of regional principal stresses by 90 after the formation of the systematic joints using a three dimensional boundary element code poly3d we considered a simple geologic case of vertical systematic fractures developing in horizontal strata under a triaxial remote load with the maximum principal tensile stress being horizontal and perpendicular to the strike of the fractures the intermediate principal stress being horizontal and parallel to the strike of the fractures and the least principal tensile stress i e maximum compressive stress being vertical the results show that the local maximum principal stress is first perpendicular and then parallel to the strike of the systematic fractures as the ratio of fracture spacing to height changes from greater than to less than a critical value when the horizontal remote principal stress ratio the ratio of the intermediate remote principal stress to the maximum remote principal stress under the sign convention of positive for tensile stresses is greater than a threshold value 0 2 thus the fracturing process changes from infilling of systematic fractures to the formation of orthogonal cross fractures this provides an alternative mechanism for the formation of orthogonal cross joints that does not require a systematic rotation of the regional stress field by 90 the critical spacing to height ratio for the local principal stress switch is independent of the least remote principal stress i e overburden it increases nonlinearly with increasing ratio of the horizontal remote principal stresses and decreases nonlinearly with increasing poisson s ratio of the material

moist morb axial magmatism in the oman ophiolite the evidence against a mid ocean ridge origin the oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid oceanic ridge basalt morb its internal structure is regarded by many as being directly comparable to ocean lithosphere from the east pacific rise however there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn because the morb like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites to some this implies that the entire ophiolite formed above a subduction zone others maintain that it formed at a true open ocean mor and that the water required to generate the arc like magmas derived from descending near axis hydrothermal fluids or from ancient subduction a popular compromise posits that the axial suite formed at a true mor and the later magmatism documents the initiation of obduction we test these models by reexamining the morb like character of the early axial lavas and dikes we show that fractionation trends require the presence of water at concentrations significantly higher than any open ocean morb instead trends are identical to those of backarc basin and intraoceanic forearc volcanics by showing that the entire ophiolite formed in a hydrous system we rule out all models in which the oman ophiolite was generated at an open ocean mor instead it formed at a submarine spreading center above a probably newly initiated subduction zone

geothermometry and geobarometry of overpressured lower paleozoic gas shales in the jiaoshiba field central china insight from fluid inclusions in fracture cements the wufeng longmaxi organic rich shales host the largest shale gas fields of china this study examines sealed fractures within core samples of the wufeng longmaxi shales in the jiaoshiba shale gas field in order to understand the development of overpressures in terms of magnitude timing and burial in wufeng longmaxi shales and thus the causes of present day overpressure in these paleozoic shale formations as well as in all gas shales quartz and calcite fracture cements from the wufeng longmaxi shale intervals in four wells at depth intervals between 2253 89 m and 3046 60 m were investigated and the fluid composition temperature and pressure during natural fracture cementation determined using an integrated approach consisting of petrography raman spectroscopy and microthermometry many crystals in fracture cements were found to contain methane inclusions only and aqueous two phase inclusions were consistently observed alongside methane inclusions in all cement samples indicating that fluid inclusions trapped during fracture cementation are saturated with a methane hydrocarbon fluid homogenization temperatures of methane saturated aqueous inclusions provide trends in trapping temperatures that th values concentrate in the range of 198 5 c 229 9 c 196 2 c 221 7 c for quartz and calcite respectively pore fluid pressures of 91 8 139 4 mpa for methane inclusions calculated using the raman shift of c h symmetric stretching v1 band of methane and equations of state for supercritical methane indicate fluid inclusions trapped at near lithostatic pressures high trapping temperature and overpressure conditions in fluid inclusions represent a state of temperature and overpressure of wufeng longmaxi shales at maximum burial and the early stage of the yanshanian uplift which can provide a key evidence for understanding the formation and evolution of overpressure our results demonstrate that the main cause of present day overpressure in shale gas deposits is actually the preservation of moderate high overpressure developed as a result of gas generation at maximum burial depths

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

competent unit thickness variation in detachment folds in the northeastern brooks range alaska geometric analysis and a conceptual model detailed measurements of six map scale detachment folds in the northeastern brooks range alaska document significant variations in structural thickness of the competent lisburne limestone thickness variations occur mainly by parasitic folding and penetrative strain and may be controlled by differences in mechanical stratigraphy relative thicknesses of the competent and incompetent units and structural relief of the underlying basement the geometry of these detachment folds is not consistent with key assumptions of existing geometric and kinematic models such as constant competent unit thickness or constant detachment depth we propose a new model that allows both competent and incompetent unit thicknesses to vary throughout the fold this model allows a more realistic geometric description of some natural detachment folds than previous models but the number of variables makes unique reconstruction of specific fold geometries or kinematics difficult comparison of models with natural folds demonstrates that significant error can result if shortening estimates are based on models that incorrectly assume constant competent unit thickness or constant detachment depth use of surveying techniques to quantify map scale fold geometry can provide better reconstructions of fold geometry better shortening estimates and information to constrain kinematic reconstructions

metalliferous sediments within lava sequences of the sumail ophiolite oman mineralogical and geochemical characterization origin and evolution abstract lens shaped siliceous pelagic sediments and metalliferous deposits sampled in the wadi salahi block of the sumail ophiolite are interbedded within the successive lava flows of the extrusive sequence petrological mineralogical and geochemical characteristics of these sediments allow their oceanic environment of deposition and their early diagenetic and later hydrothermal transformations to be defined the sediments are primary radiolarian oozes deposited in a highly productive and open equatorial pelagic realm the biogenic components are mixed with hydrothermal metallic compounds similar to those found in the axial zone of modern active ridges the deposits are subsequently transformed by various oceanic hydrothermal events which are expressed by specific secondary parageneses the sedimentary lenses from the lower lava sequence are epidotized whereas the uppermost lenses from the upper lava unit contain pumpellyite some peculiar lenses are not affected by these oceanic metamorphisms for instance the deposits closely associated with the zuhan orebody which are fe and silica rich with scarce sulfates or the manganiferous sediments located at the top of the lower lava sequences which were earlier lithified by the deposition of the overlying lava flows nowhere in the whole sedimentary section described were detrital or volcaniclastic remains found disproving the possibility that these sediments may have been deposited in a seamount or island arc environment

multistory duplexes with forward dipping roofs north central brooks range alaska the endicott mountains allochthon has been thrust far northward over the north slope parautochthon in the northern brooks range progressively younger units are exposed northward within the allochthon to the south the incompetent hunt fork shale has thickened internally by asymmetric folds and thrust faults northward the competent kanayut conglomerate forms a duplex between a floor thrust in hunt fork and a roof thrust in the kayak shale to the north the competent lisburne group forms a duplex between a floor thrust in kayak and a roof thrust in the siksikpuk formation both duplexes formed from north vergent detachment folds whose steep limbs were later truncated by south dipping thrust faults that only locally breach immediately overlying roof thrusts within the parautochthon the kayak lisburne and siksikpuk equivalent echooka formation form a duplex identical to that in the allochthon this duplex is succeeded abruptly northward by detachment folds in lisburne these folds are parasitic to an anticlinorium interpreted to reflect a fault bend folded horse in north slope basement with a roof thrust in kayak and a floor thrust at depth these structures constitute two northward tapered internally deformed wedges that are juxtaposed at the base of the allochthon within each wedge competent units have been shortened independently between detachments located mainly in incompetent units the basal detachment of each wedge cuts upsection forward northward to define a wedge geometry within which units dip regionally forward these dips reflect forward decrease in internal structural thickening by forward vergent folds and hindward dipping thrust faults

fracturing and fluid flow in a sub decollement sandstone or a leak in the basement crack seal texture within fracture cements in the triassic el alamar formation ne mexico shows that the fractures opened during precipitation of quartz cements later overlapping calcite cements further occluded pore space previous workers defined four systematic fracture sets a oldest to d youngest with relative timing constrained by crosscutting relationships quartz fluid inclusion homogenization temperatures are higher within set b 148 20 c than in set c 105 12 c these data and previous burial history modelling are consistent with set c forming during exhumation fluid inclusions in set c quartz have higher salinity than those in set b 22 9 v 14 2 wt nacl equivalent respectively and set c quartz cement is more enriched in 18 o 20 2 v 18 7 vsmow under most assumptions about the true temperature during fracture opening the burial duration the amount of cement precipitated and fluid flow patterns it appears that the fracture fluid became depleted in 18 o and enriched in 13 c this isotopic evolution combined with increasing salinity suggests that throughout fracture opening there was a gravity driven influx of fluid from upsection jurassic evaporites which form a regional decollement fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading rather than increases in fluid pressure

fracture paragenesis and microthermometry in lisburne group detachment folds implications for the thermal and structural evolution of the northeastern brooks range alaska the distribution character and relative age of fractures in detachment folded mississippian pennsylvanian lisburne group carbonates and overlying permian triassic clastic rocks in the northeastern brooks range of northern alaska provide important clues to the thermal and deformational sequence experienced by these rocks although paleothermal indices in the host rock limit the conditions of folding to temperatures equal to or less than 280c field and petrographic relationships suggest that different fracture sets formed at different times during the deformational history of the rocks providing a record of deformation under changing temperature and pressure conditions these rocks probably initially entered the oil generation window 80 140c during the early cretaceous formation of the colville basin via thrust loading by the brooks range to the south regional fractures formed during this time as a result of high pore pressures and low in situ differential stresses shortening in these rocks related to the advancing northeastern brooks range fold and thrust belt began during the late cretaceous to early tertiary early phases of detachment folding were via flexural slip with associated fracturing with continued shortening and growth of detachment folds structural thickening resulted in deeper burial of the bottom part of the deforming wedge early fold related fractures were subsequently overprinted by penetrative strain during peak folding at temperatures of approximately 280c continued shortening resulted in uplift and erosional unroofing at approximately 60 ma late fold related fractures formed at about 150c subsequent uplift of the thickened wedge through 60c occurred after about 25 ma late pervasive extension fractures related to unroofing and or regional stresses formed at relatively shallow depths and low temperatures overprinting all the earlier fractures and penetrative structures

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

syn to late orogenic quartz veins marking a retrograde deformation path in a slate belt examples from the high ardenne slate belt belgium abstract an integrated structural petrographic and microthermometric methodology has been applied on syn to late orogenic quartz veins from the high ardenne slate belt belgium to define their relative timing the quartz precipitates which do not show any mutual cross cutting relationships represent the meso scale brittle accommodation during fold initiation amplification and locking crystal plastic deformation structures and p t trapping conditions indicate that the different processes accommodating folding occurred in a progressive manner along a retrograde deformation path associated with the gradual exhumation of the slate belt from ca 7 5 to 6 km depth successive veining occurred from peak metamorphic conditions ca 300 c and 190 mpa measured in extrados veins to subsequent lower p t conditions in the periphery of a lenticular vein ca 275 c and 180 mpa late orogenic saddle reef filling ca 245 c and 160 mpa and the core of a lenticular vein ca 220 c and 150 mpa open cavity growth and fluid assisted brecciation indicate that veining occurred under overpressured fluid conditions it is therefore fair to assume that the high ardenne slate belt acted at least episodically as a mid crustal overpressured fluid reservoir not only in the earliest and latest stages but also during the main stage of orogeny

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

in situ exhumation from bedrock of large rounded boulders at the giant s causeway northern ireland an alternative genesis for large shore boulders mega clasts abstract very large boulders mega clasts are found on some coasts the size and position of the boulders has been used to suggest that contemporary marine processes acting within their normal spatial and energy range are unlikely to have moved them explanations for the presence of such boulders include transport by infrequent very high energy marine processes storms or tsunamis mass movement from backing cliffs transport by ice or exhumation from glacial deposits this paper advances an alternative explanation which does not involve transport by any of the marine or glacial processes or gravity it is proposed that in a very specific geological and topographic setting large boulders are exhumed in situ by storm waves acting on heavily weathered jointed basalts eventually wave action liberates residual blocks from the deeply weathered matrix these liberated boulders will be mobile only if they lie within wave competence and the larger ones will remain as stationary residuals the same in situ weathering processes followed by removal of the friable matrix material debris by wave action also progressively round the boulders consequently despite their appearance of being transport rounded the larger boulders have not transported at all in specific locations the assumption that the presence and rounding of such large clasts in the shore zone can be attributed to marine transport can lead to erroneous interpretations of very high energy storm wave or tsunami activity

evolving mechanical stratigraphy during detachment folding abstract the combination of a complex lithostratigraphy and shortening can lead to a mechanical stratigraphy that changes character and mode of deformation during detachment folding in the detachment folded carbonates of the carboniferous lisburne group of northern alaska a complex lithostragraphy led to a mechanical stratigraphy that evolved during folding with mechanical unit thickness decreasing as deformation progressed and the fold tightened newly forming mechanical boundaries changed the dominant deformation behavior and subsequent geometry during folding of a given horizon during early folding flexural slip dominated in units comprised of thick bedded competent lithologies while flexural flow dominated in units composed of thinner bedded incompetent lithologies after fold lock up homogeneous flattening dominated in the less competent mechanical units the final result was a fold with a complex geometry and a suite of fractures and other structures indicative of both flexural slip flexural flow folding and homogeneous flattening

calculated volume and pressure changes during the thermal cracking of oil to gas in reservoirs 1 the rising temperature that accompanies increasing burial depth converts oil in a reservoir into thermal gas a consideration of hydrogen balance during this cracking shows that approximately 3000 cubic ft 85 cubic meters of gas at standard temperature and pressure is generated from each barrel of oil in addition a graphitic residue is precipitated if the volume relationships among oil thermal gas and the graphitic residue are combined with data for gas solubility in pore water and gas nonideality z factor then pressure can be calculated for any degree of thermal cracking these calculations show that in an effectively isolated system pressures would become very high and could considerably exceed the rock load so that fracturing must occur causing pressure b eed off and loss of gas the lithostatic gradient 1 0 psi ft or 22 6 kpa m is reached after only about 1 0 of the oil is cracked if the reservoir system remains open i e at hydrostatic pressure and is initially filled with oil that is subsequently cracked to gas then roughly 75 of the gas will be lost or the reservoir volume must effectively increase in size for example by moving the gas water contact downward

integrated reservoir characterization and simulation of a shallow light oil low temperature reservoir umiat field national petroleum reserve alaska umiat field in northern alaska is a shallow light oil accumulation with an estimated original oil in place of more than 1 5 billion bbl and 99 bcf associated gas the field discovered in 1946 was never considered viable because it is shallow in permafrost and far from any infrastructure modern drilling and production techniques now make umiat a more attractive target if the behavior of a rock ice and light oil system at low pressure can be understood and simulated the umiat reservoir consists of shoreface and deltaic sandstones of the cretaceous nanushuk formation deformed by a thrust related anticline depositional environment imparts a strong vertical and horizontal permeability anisotropy to the reservoir that may be further complicated by diagenesis and open natural fractures experimental and theoretical studies indicate that there is a significant reduction in the relative permeability of oil in the presence of ice with a maximum reduction when connate water is fresh and less reduction when water is saline a representative umiat oil sample was reconstituted by comparing the composition of a severely weathered umiat fluid to a theoretical umiat fluid composition derived using the pedersen method this sample was then used to determine fluid properties at reservoir conditions such as bubble point pressure viscosity and density these geologic and engineering data were integrated into a simulation model that indicate recoveries of 12 15 can be achieved over a 50 yr production period using cold gas injection from five well pads with a wagon wheel configuration of multilateral wells

epr microplates a model for the oman ophiolite structural mapping in the southern half of the oman ophiolite has revealed a palaeoridge organization with similarities to an epr microplate forming in superfast spreading conditions a nw trending propagator was rapidly opening in a lithosphere no more than 1 myr older and itself created in a ne sw ridge system the nw trending propagator underlined by small mantle diapirs was active or dying when detached as part of a future ophiolite local thrusting of the future ophiolite was also initiated very early between 1 and 5 myr after ridge accretion it was accompanied by a 40 rotation within this time lapse such an activity supports the comparison of the oman palaeoridge system with an oceanic microplate and provides evidence to suggest the existence of thrusts at active microplates

multiple episodes of cenozoic denudation in the northeastern brooks range fission track data from the okpilak batholith alaska the northeastern brooks range of alaska is a complex mesozoic to cenozoic northward verging fold and thrust belt in response to regional compression shortening in the upper crust has occurred thr

fault zone processes and fluid history in austin chalk southwest texas faults are commonly the largest fractures in a fracture network forming the backbone of the permeability system in low permeability rocks faults in outcrops of the austin chalk an important oil and gas reservoir in south texas louisiana and westernmost mississippi exemplify deformation and fluid histories of subseismic scale faults in mechanically layered chalk dominated strata exposures of near horizontal austin chalk in southwest texas contain northwest and southeast dipping normal faults with exposed heights of meters to tens of meters and displacements of centimeters to decimeters local fault dips are steep to vertical through chalk and limestone beds and moderate through marl mud rock and clay rich ash beds producing refracted fault profiles failure surface characteristics indicate various failure modes including tensile hybrid shear and locally compactive shear behavior depending on the host lithology dilational zones associated with steep fault segments host calcite veins crack seal textures in fault zone veins record repeated reactivation of refracted fault profiles fluid inclusion and stable isotope geochemistry analyses of fault veins indicate formation at depths of 1 4 to 2 9 km 4593 to 9514 ft and possibly of 4 2 km 13 780 ft fluids include locally sourced saline waters and externally sourced waters and oil in rocks that did not reach oil maturation conditions inclusions of 30 api to greater than 50 api oil are found in veins 30 km 19 mi updip from the eagle ford formation oil zone these results illustrate the migration of oil and other fluids within subseismic scale austin chalk faults and fundamental control of lithology on fault zone architecture

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

loading configurations and driving mechanisms for joints based on the griffith energy balance concept abstract using the griffith energy balance concept to model joint propagation in the brittle crust two laboratory loading configurations serve as appropriate analogs for in situ conditions the dead weight load and the fixed grips load the distinction between these loading configurations is based largely on whether or not a loaded boundary moves as a joint grows during displacement of a loaded boundary the energy necessary for joint propagation comes from work by the dead weight i e a remote stress when the loaded boundary remains stationary as if held by rigid grips the energy for joint propagation develops upon release of elastic strain energy within the rock mass these two generic loading configurations serve as models for four common natural loading configurations a joint normal load a thermoelastic load a fluid load and an axial load each loading configuration triggers a different joint driving mechanism each of which is the release of energy through elastic strain and or work the four mechanisms for energy release are joint normal stretching elastic contraction poroelastic contraction under either a constant fluid drive or fluid decompression and axial shortening respectively geological circumstances favoring each of the joint driving mechanisms are as follows the release of work under joint normal stretching occurs whenever layer parallel extension keeps pace with slow or subcritical joint propagation under fixed grips a substantial crack normal tensile stress can accumulate by thermoelastic contraction until joint propagation is driven by the release of elastic strain energy within the earth the rate of joint propagation dictates which of these two driving mechanisms operates with faster propagation driven by release of strain energy like a dead weight load acting to separate the joint walls pore fluid exerts a traction on the interior of some joints joint propagation under fluid loading may be driven by a release of elastic strain energy and or work by the pore fluid during poroelastic contraction again propagation velocity dictates the driving mechanism with critical to supercritical joint propagation accompanying fluid decompression and subcritical crack propagation taking place under a constant fluid drive during axial shortening a jointing process called axial splitting may occur following wing crack growth from an initial crack tilted relative to the maximum compressive stress here the joint driving mechanism may be either the work of a remote stress i e a dead weight or the release of elastic strain energy i e fixed grips

fracture controlled paleohydrology in a map scale detachment fold insights from the analysis of fluid inclusions in calcite and quartz veins abstract this study uses fluid inclusions in quartz and calcite veins to characterize the fracture controlled paleohydrology of a map scale evaporite cored detachment fold in the sierra madre oriental of northeastern mexico field observations indicate that the veins are tectonic in origin and that they formed in a general sequence that corresponds to four broad stages of progressive folding we collected samples from each vein stage in various structural and stratigraphic positions across the fold after determining the mineral paragenesis and origin of inclusions in each sample we used standard microthermometric techniques to measure the homogenization temperatures t h salinities and eutectic temperatures of the available two and three phase aqueous inclusions neither t h salinity nor eutectic temperature varied systematically with inclusion origin or mineral type the data are also not significantly correlated with structural position or vein type but are strongly partitioned by stratigraphy suggesting that the area was a vertically stratified hydrologic system consisting of three regional paleohydrostratigraphic units an upper unit comprised the indidura formation through difunta group and is characterized by t h near 150 c and salinity t h near 150 c and salinities near 12 or 22 wt nacl equivalent the lower unit comprised the zuloaga and la casita formations and is characterized by an average t h near 225 c and salinities in three groups near 12 22 or 36 wt nacl equivalent the preservation of multiple distinct fluid types in many veins suggests that fracture development during folding and uplift created a second paleohydrologic system that overprinted the vertically stratified system fracture controlled fluid migration in this second system occurred in periodic repeated and spatially variable pulses

geothermal regime and hydrocarbon kitchen evolution of the offshore bohai bay basin north china the offshore bohai bay basin located in the central bohai bay basin north china one of the most petroliferous basins in china in this article based on formation testing temperature drill stem tests and bottom hole temperature data 80 thermal gradient values at the depth interval of 0 to approximately 3000 m 9843 ft in the offshore bohai bay basin were obtained the basinwide average thermal gradient is 31 8 4 6c km based on the above thermal gradient data and the corresponding average weighted thermal conductivity data 80 measured terrestrial heat flow values were obtained these values range from 33 5 to 84 0 mw m2 with an average value of 60 8 8 7 mw m2 the heat flow and thermal gradient distribution in this region generally show higher values in the uplifts and lower ones in the sags a thermal history derived from vitrinite reflectance and apatite fission track data indicates that paleogene cooling occurred after a period of much higher paleogeothermal gradient 3854c km tectonic subsidence analysis reveals that the area experienced initial synrift subsidence during the paleogene followed by subsequent thermal subsidence since the neogene thermal and tectonic subsidence histories of this area are of great significance to petroleum exploration and hydrocarbon resource assessment because they bear directly on issues of petroleum source rock maturation the maturation and hydrocarbon expulsion histories of the paleogene shahejie 3 formation e2s3 which is the most important source rock in the offshore bohai bay basin are modeled results show that the shahejie 3 formation is in a high mature stage at the present day and the bozhong and qikou sags are the most important kitchens the huanghekou sag became the third most important hydrocarbon kitchen in the early neogene based on this hydrocarbon kitchen evolution oil and gas mainly accumulated after 12 ma the evolution of kitchen areas may provide new insights for the understanding of the oil and gas exploration potential of the offshore bohai bay basin

fault and fracture systems in a fold and thrust belt an example from bolivia this paper presents an outcrop analog study of fractures in low porosity sandstones in the subandean thrust belt we analyze the evolution of fault and joint systems in these sandstones quantify their density along the structural trend and identify the main factors controlling their variability we show that faults and joints occur at different scales in a hierarchical fashion as a consequence of progressive shearing the first generation is an orthogonal set of joints one parallel and the other perpendicular to the bedding azimuth shearing along these joints transformed them into small faults and created new sets of fractures oblique to the bedding attitude linkage of these small faults facilitated the formation of larger faults with significant strike slip offset shearing along bedding planes created subvertical splay joints that induced the formation of conjugate normal faults in this thrust belt subordinate strike slip and normal faults are concomitant products of compressive deformation this study documents a hierarchical correspondence between spacing of structural heterogeneities and stratigraphic architecture we measured spacings of joints and outcrop scale faults along the backlimb of the abra del condor anticline we subdivided the structural discontinuities into four main groups joints small faults intermediate faults and fault zones spacing of joints small faults and intermediate faults has a lognormal distribution whereas spacing of fault zones shows a normal distribution the mean of these distributions is about the same as the thickness of the confining stratigraphic intervals therefore spacing and dimensions of joints and faults have a first order relationship to the thickness of the confining stratigraphic sequences

volcanostratigraphic controls on the occurrence of massive sulfide deposits in the semail ophiolite oman the semail ophiolite in oman is capped by up to 2 km of basaltic andesitic lavas that host copper dominant cyprus type volcanogenic massive sulfide vms deposits this study identifies multiple volcanostratigraphic horizons on which the deposits are situated based on characterization of footwall and hanging wall lavas from 16 deposits or deposit clusters comparison of field and petrographic features compositions of igneous clinopyroxenes and whole rock geochemical signatures permits classification of the lavas within a modified version of the established regional volcanostratigraphy four extrusive units host deposits geotimes earliest lasail alley and boninitic alley latest the latter was previously known only at few localities but this study reveals its regional extent and significance as a host for vms deposits the geotimes and lasail units represent late cretaceous ocean spreading ridge and related off axis volcanic environments respectively the alley and boninitic alley units represent younger subduction related volcanism prior to coniacian santonian obduction of the ophiolite our results show that vms deposits occur on or near the geotimes lasail and geotimes alley contacts as well as entirely within the geotimes lasail alley and boninitic alley units highest cu grades tend to occur in deposits lying on or within the geotimes whereas highest au grades occur in deposits within the boninitic alley in contrast to earlier studies we conclude that essentially every horizon marking a hiatus in lava deposition in the semail ophiolite i e contacts between the four major eruptive units and umbers and sedimentary chert layers within the units has exploration potential for cu au vms deposits

paleostress directions from the preferred orientation of closed microfractures fluid inclusion planes in sandstone east texas basin u s a abstract closed extensional microfractures are useful indicators of paleostress directions in shallowly buried quartz cemented quartz arenites of the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 670 m thick that was deposited in the gradually subsiding northern gulf of mexico basin the closed microfractures are syndiagenetic fluid inclusion rich planes that have an ene strike that is subparallel to contemporary quartz filled extension veins and regional normal faults indicating that closed microfractures contain the maximum paleostress direction these microstructures are potentially useful for mapping paleostress trajectories in passive margin and platform settings particularly where samples are obtained from core previous experimental studies of microfracture closure suggest that microfractures evolved into fluid inclusion planes by dendritic quartz precipitation in east texas microfracture closure may have occurred at temperatures as low as 85 c and depths as shallow as 1500 m

natural fractures in carbonate rich tight oil reservoirs from the permian lucaogou formation southern junggar basin nw china insights from fluid inclusion microthermometry and isotopic geochemistry abstract natural fractures within tight reservoirs form preferred pathways for hydrocarbon charge and provide space for hydrocarbon preservation however the mechanism that controls the formation of natural fractures in tight reservoirs remains controversial this study investigates natural fractures within carbonate rich tight oil reservoirs from the permian lucaogou formation in the jimsar sag southern junggar basin drill core and image log observations show that fractures in the lucaogou formation can be categorized into two types bedding parallel fractures and inclined fractures the carbon and oxygen isotopic compositions of calcite cements in the bedding parallel fractures range from 0 6 to 6 0 13cpdb and from 15 5 to 8 9 18opdb respectively which are similar to those of the organic rich host rock this suggests that the bedding parallel fractures likely formed in a closed fluid system fluid inclusion analyses of the aqueous inclusions in calcite cements from the bedding parallel fractures record homogenization temperatures of 91 5 c 130 4 c indicating that they formed at the cretaceous to cenozoic during when the lucaogou formation reached peak oil generation stage therefore the bedding parallel fractures were most likely generated by hydrocarbon generation overpressure the presence of hydrocarbon inclusions in bedding parallel fracture cements supports natural fracturing by high pore fluid pressure the 13cpdb and 18opdb of calcite cements within the inclined fractures vary from 3 6 to 9 6 and from 11 0 to 2 9 respectively which are close to those of the carbonate rich parent rock this also supports a closed fluid system scenario the relatively lower fluid inclusion trapping temperatures 70 5 c 109 7 c suggest that they were likely generated by decrease of confining stress during uplift processes therefore both an increase in pore fluid pressure and a decrease in confining stress contribute to the formation of natural fractures in the lucaogou formation tight oil reservoirs

thermal maturity patterns in alaska implications for tectonic evolution and hydrocarbon potential nearly 10 000 vitrinite reflectance and conodont color alteration index determinations from sedimentary rocks in alaska were used to produce a thermal maturity map of rocks exposed at the surface and to evaluate subsurface thermal maturity relations in the colville and cook inlet basins rocks exposed at the surface of the tertiary interior basins and in the aleutian forearc and backarc basins uniformly are of very low thermal maturity indicating that these basins are at or near maximum burial have seen little uplift and exhumation and are probably thermally immature with respect to hydrocarbon generation in contrast many sedimentary basins show elevated levels of thermal maturity at the surface with the highest values at basin margins this geometry suggests a patt rn of greater uplift along basin margins possibly reflecting isostatic readjustments as crustal loads are removed by erosion we investigated thermal maturity relations in three sedimentary basins colville cook inlet and kandik in more detail thermal maturity patterns in the colville basin are broadly asymmetric indicating systematic differential uplift ranging from a minimum of no uplift in the north point thomson area to 9 13 km of uplift and exhumation in the central brooks range even greater uplift further to the south is indicated by the presence of greenschist facies and higher grade metamorphic rocks this pattern may reflect the deflexing of the lithosphere subsequent to the principal episode s of crustal convergence and thickening these patterns further suggest a similar thermal history for the proximal colville basin and the northern foothills belt suggesting the possibility of hydrocar on accumulations in the foothills thermal maturity isograds within the brooks range cut major thrust faults indicating that maximum burial postdated the principal phases of thrusting in contrast isograds in the foothills belt to the north are warped broadly by local structure indicating continued north south shortening subsequent to maximum burial such deformation could have remobilized hydrocarbons in early traps a broad southward extension of thermally immature rocks in the central portions of the foothills belt suggests relatively young east west shortening parallel to the strike of the orogene a feature that to date has not been included in regional tectonic syntheses alternatively the thermal maturity pattern could be explained by tectonically unrelated episodes of uplif in the eastern and western parts of the brooks range in the cook inlet basin vitrinite reflectance isograds also are indicative of relatively greater uplift at the basin margins than at the basin center which appears to be presently at its maximum burial depth uplift in the cook inlet basin may reflect compression along the faults bounding the basin relatively high thermal maturity along the western margin of the basin also may reflect magmatic heat sources from the alaska peninsula aleutian volcanic arc the seldovia arch a major structural feature of the end page 1874 basin does not appear to deform vitrinite reflectance isograds implying that deformation on that structure ceased prior to maximum burial in the kandik basin a thermal maturity anomaly thermally mature younger rocks in fault contact with thermally immature older rocks provides clues to the nature and timing of east west thrusting mesozoic foreland basin deposits associated with thrusting buried paleozoic rocks of the easternmost part of this fold and thrust belt to relatively shallow depths driving potential hydrocarbon source rocks into the oil generation window the western foreland basin deposits were overridden by advancing thrusts and tectonically buried to as deep as 10 km these disparate thermal domains are juxtaposed along the glenn creek fault which may represent a terrane boundary in east central alaska

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

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

style controls and timing of fold and thrust deformation of the jago stock northeastern brooks range alaska involvement of the devonian jago stock in cenozoic fold and thrust deformation of the northeastern brooks range illustrates the influence of a relatively small isolated crystalline body on the mechanical stratigraphy and subsequent deformatioml behavior of an otherwise layered sedimentary package the small size of the stock allowed it and the structurally coupled overlying mississippian kekiktuk conglomerate to deform nonpenetratively as a horse in a regional duplex in contrast to the semiductile behavior of the nearby but much larger okpilak batholith shear was localized in the upper part of the stock and the conglomerate due to partial detachment of the overlying carboniferous lisbume group north vergent thrust related folds formed in the mechanically layered lisburne group carbonates instead of the symmetrical unfaulted detachment folds more typical of the region because an underlying regional detachment horizon in the mississippian kayak sme is depositionally absent over the stock unusually competent contact metamorphosed pre mississippian metasedimentary rocks were thrust over the stock and its cover because a ramp formed at the edge of the stock ad cut upsection through the lisburne group due to the absence of kayak shale a ar13 r age of foliated white mica indicates thrusting of the stock by 61 ma fission track ages indicate cooling at 44 and 28 ma these ages indicate a cooling history that implies 11 h of unroofing since 61 ma only 1 5 km of which can be explained by the inferred duplex structure the remining 9 5 km of unroofing is most likely due to subduplex structural thickening above a deep regional detachment

stratigraphic architecture and fracture controlled dolomitization of the cretaceous khami and bangestan groups an outcrop case study zagros mountains iran this work has been carried out as part of a joint study between norsk hydro now statoil and nioc national iranian oil company

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

petrochemistry and origin of sub ophiolitic metamorphic and related rocks in the oman mountains the semail ophiolite complex of the northern oman mountains was obducted during cenomanian to campanian upper cretaceous times onto the arabian continental margin during this time span schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice whole rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid ocean ridges a comparison of trace element geochemistries however suggests that the protoliths were not equivalent to the haybi alkaline and transitional tholeiites found in the underlying structural slice or to the semail ophiolite volcanic rocks which lie structurally higher in the sequence it is therefore suggested that the amphibolites may have been derived from jurassic or lower cretaceous oceanic basalts not recognized elsewhere in the oman mountains structurally beneath the amphibolites within the metamorphic sheet greenschist facies metasedimentary rocks including quartzites and marbles are interbanded with metabasite horizons containing actinolite and less commonly blue crossitic amphiboles it is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement and were accreted to the base of the ophiolite during progressive underthrusting