hydrogeologic role of geologic structures part 1 the paradigm abstract grouting to reduce fracture permeability is one option for minimizing ground water inflow to a large acid producing lead zinc mine in fractured metamorphic rock in north idaho for grouting to reduce mine water inflow effectively the hydrogeologic characteristics of the various scales of structurally controlled fracturing must be identified and a conceptual model of the ground water flow system must be developed this paper is the first of two papers which use fracture mapping geologic structural mapping and a series of underground hydraulic stress tests to develop a conceptual model of structurally controlled ground water flow in the vicinity of the mine these data were collected in an effort to identify order within the structurally controlled spatial permeability distribution geologic structural discontinuities ranging from joints to faults that extend for several miles form a geologic structural hierarchy in the rock surrounding the mine the tiers of the hierarchy control ground water flow into the mine at different scales the most prominent faults control ground water inflow on the scale of the entire mine various levels of hydraulic continuity are evident within the rock mass bounded by two of the most prominent faults the highest level of hydraulic continuity appears to be associated with a set of sub parallel steeply dipping faults minor faults joints and relict bedding planes to a limited extent connect the fractures of this set and form a lower level of hydraulic continuity the next lower level of hydraulic continuity within the hierarchy is related to a major fault that is characterized in drill core by abundant gouge the hydraulic continuity of the matrix within the unfractured quartzite is the lowest level within the hierarchy these levels constitute the components of the order within the spatial permeability distribution that we have interpreted from the structural and hydraulic stress test data

episodic and simultaneous illitization in oil bearing brent group and fulmar formation sandstones from the northern and southern north sea based on illite k ar dating size fractions 0 4 and 0 4 1 0 m of brent group sandstones from the northern north sea contain mostly illite smectite mixed layers with kaolinite whereas the same size fractions of fulmar formation sandstones from the south central north sea consist of illite smectite mixed layers with minor chlorite transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking the k ar data of the fractions less than 0 4 m of brent group samples plot on two arrays in a 40ar 36ar vs 40k 36ar diagram that have isochron characteristics with ages of 76 5 4 2 and 40 0 1 5 ma and initial 40ar 36ar ratios of 253 16 and 301 18 respectively for the fulmar formation samples the data points of the fractions less than 0 2 and less than 0 4 m also fit two isochrons with ages of 76 6 1 4 and 47 9 0 5 ma and initial 40ar 36ar ratios of 359 52 and 304 2 respectively some of the coarser 0 4 1 0 m fractions also plot on the two isochrons but most plot above indicating the presence of detrital components more than 0 4 m the almost identical ages obtained from illite type crystals of sandstones with different deposition ages that are located about 600 km 373 mi apart record two simultaneous illitization episodes these events were not induced by local burial conditions but are related to episodic pressure and or temperature increases in the studied reservoirs probably induced by hydrocarbon injection this interpretation is indirectly supported by notably different k ar illite ages from cores of a nearby reservoir at hydrostatic pressure illite is not as well crystallized as expected for potential crystallization temperatures above 160c measured by fluid inclusion determinations in both the northern and south central north sea the two illite generations remain unaffected after crystallization despite continued burial suggesting notably higher crystallization temperatures than those estimated from geothermal gradients no recent illite crystallization or alteration is recorded in the k ar ages despite a dramatic regional acceleration of the subsidence in the southern north sea

diagenesis in north sea hpht clastic reservoirs consequences for porosity and overpressure prediction upper jurassic clastic reservoirs of the fulmar formation in the central north sea possess anomalously high porosities for their present day depth of burial reservoirs with the highest overpressures have the highest porosities possess less macroquartz cement and have significant secondary porosity quartz cementation in hpht high pressure high temperature reservoirs has been inhibited by a combination of factors high overpressure limited fluid movement presence of early grain coating cements high pore fluid salinity and possibly petroleum migration secondary porosity has contributed to reservoir quality with an average of 4vol extra porosity created quantitative prediction of porosity would require an improved depositional model for the fulmar accurate thermal and pressure modelling and detailed knowledge of field filling and leakage histories theoretical calculations indicate that diagenetic reactions occurring in the fulmar formation smectite illitisation and quartz cementation did not generate significant overpressure because seal permeabilities were too high and the rate of volume increase associated with the reactions too small therefore diagenetic reactions can effectively be ignored when modelling overpressure generation in the central north sea although cementation will affect rock permeability and rates of fluid dissipation

light element distributions n b li in baltic basin bentonites record organic sources abstract distributions of nitrogen boron and lithium were studied in illite smectite from ordovician silurian k bentonites in the baltic basin these trace elements are linked to thermal maturation of organic matter creating fluid signatures that are distinct from those in regionally buried sediments the fluid chemistry is ideally recorded in authigenic illite within the k bentonite because b substitutes in tetrahedral sites of illite li in octahedral sites and n is fixed in the interlayers as nh 4 three nanometric size fractions of illite 10 among these size fractions relate mainly to changes in fluid composition during illite growth bentonite volcanic ash layers mark stratigraphic timelines while illitization of the bentonite k bentonite varies as a function of burial and tectonic temperature gradients in the central baltic basin bentonites were deposited across a carbonate platform near estonia grading to deep basin shales near denmark and poland compositional variations in illite reflect fluids from different sources high n illite 2000 ppm in the southwest basin transitions to high b 250 ppm and high li 100 ppm illite to the northeast these trends follow a thermal gradient decreasing from southwest 200 c to northeast 90 c isotope ratios of b and li show opposing trends in samples across this thermal gradient the bisotopes decrease from southwest to northeast 18 to 4 while li isotopes increase from 5 to 32 the cambrian alum shale a regional hydrocarbon source rock was studied as a potential source of b and li as more than 50 of its b and li was associated with organic matter with increasing thermal maturity illite 11b decreased from 0 to 13 while 7li increased from 20 to 5 these opposing trends are similar to those observed in the k bentonites this suggests that organic sources of n b and li dominate fluids in bentonites from the southwestern baltic basin whereas basement derived fluids dominate in the northeastern baltic basin these trace elements record paleofluid changes during illite crystal growth and can identify pathways of chemically distinct oil and gas related fluids

microstructural investigation of the interaction and interdependence of cataclastic and plastic mechanisms in feldspar crystals deformed in the semi brittle field abstract plagioclase feldspar an 50 single crystals from the hogarth range nsw australia have been deformed in the brittle to semi brittle regime at 500 800 c 90 300 mpa effective pressure and strain rate between 10 5 and 10 6 this gem quality feldspar is both macroscopically and microscopically devoid of any microstructure other than very rare growth twins so all microstructures produced by experimental deformation can be easily identified the specimens were deformed to failure which occurred at strains of

effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading a grain based universal distinct element code model was developed to generate a deformable polygonal grain like structure to simulate the microstructure of brittle rock it takes into account grain scale heterogeneity including microgeometric heterogeneity grain scale elastic heterogeneity and microcontact heterogeneity the microgeometric heterogeneity can be used to match the grain size distribution of the rock the discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy the modeling approach was evaluated using lac du bonnet granite and a aspo diorite the microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading the crack initiation stress was found to be controlled primarily by the microscale geometric heterogeneity while the microcontact heterogeneity controlled the strength characteristics the effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined

regional shortening fabrics in eastern north america far field stress transmission from the appalachian ouachita orogenic belt paleozoic carbonates of cratonic eastern north america comprise the footwall of the appalachian ouachita fold and thrust belt and contain a layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the appalachian ouachita thrust transport direction in carbonates of the thrust belt proper restored width 400 km and within carbonates up to 1700 km into the foreland giving a pre thrusting sedimentary prism 2100 km wide through which compressive orogenic stresses were transmitted the shortening strain magnitudes 6 and the inferred calcite twinning differential stress magnitudes 90 mpa decrease exponentially away from the orogenic front calcite twinning strain patterns in other adjacent tectonic provinces such as the grenville laramide keweenawan rift and newark basin are distinct from the twinning strains preserved in the cratonic paleozoic carbonates appalachian orogen far field stresses calcite microstructures

rb sr and k ar dating of clay diagenesis in jurassic sandstone oil reservoir north sea rb sr and k ar isotopic determinations were combined with scanning electron microscopy sem observations and x ray diffractometry xrd controls to date the diagenetic formation of clay minerals from oil bearing sandstones of the middle jurassic brent sandstone in the north sea the separated clay fractions often contain detrital components especially k feldspars which are reduced to sizes smaller than 0 4 m either by intense in situ alteration or by destructive sample preparation direct rb sr and k ar isotopic dating of diagenetic illite therefore was not possible isotopic data obtained on mixtures of detrital and newly formed components showed however that illite formed about 40 to 45 ma this value also was obtained directly on clay fractions separated by a gentle method of disaggregation illitization is related to emplacement of hydrocarbons and associated waters into the formation aqueous and hydrocarbon fluids simultaneously trapped in quartz overgrowth inclusions indicate late silicification contemporaneous with this fluid migration minimum trapping temperatures of at least 110 c for these fluid inclusions are too high for the present burial depth hot fluids are assumed to have originated in deep source rocks beneath the brent reservoirs and migrated upward during the middle eocene

sedimentary evolution of a palaeozoic basin and ridge system the middle and upper devonian of the ahnet and mouydir algerian sahara the ahnet and mouydir regions of southern algeria are part of one of the world s largest almost undeformed exposures of palaeozoic rocks which exemplify a hitherto poorly known early variscan development of a devonian basin and ridge system this area includes a series of intracratonic basins along the northern margin of the west african craton which consists from w to e of the reggane basin azel matti ridge ahnet basin foum belrem ridge and mouydir basin the depositional and palaeogeographic interpretation is based on 71 sections in this region which for the first time were biostratigraphically calibrated by means of conodonts goniatites and brachiopods the structural evolution during devonian times was probably controlled by reactivation of ancient n s to nw se running faults in the precambrian basement which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf a hiatus during emsian times indicates widespread emergence during this interval the entire area was flooded during the earliest eifelian when the first vestiges of the azel matti ridge become evident by stratigraphic condensation the palaeogeographic differentiation is most apparent during the givetian when a shoal with reduced carbonate sedimentation was established on the azel matti ridge passing towards the west and east into basinal environments of the reggane and ahnet basins respectively the foum belrem ridge is distinguished by increased subsidence during the early givetian and by revived uplift during the late givetian in the mouydir basin further east up to 1000 m of shales were deposited during the givetian the early frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites these lithologies occur repeatedly already during the middle devonian and document intermittent anoxic conditions the basin and ridge topography is levelled by the shallowing up sequence of up to 1400 m thick upper frasnian and famennian shales which grade into a deltaic sequence of uppermost famennian tournaisian sandstones the up to now only vaguely discriminated lithostratigraphic formations of the devonian have been biostratigraphically defined in suitable type sections

pan african displaced terranes in the tuareg shield central sahara concepts developed in the recently published model of the air region eastern tuareg shield niger africa as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the tuareg shield 500000 km 2 the 23 terranes recognized have their own lithological metamorphic magmatic and tectonic characteristics and are separated by subvertical strike slip megashear zones that can be traced for hundreds of kilometres or by major thrust fronts some of these boundaries have ophiolitic assemblages or molassic deposits the tuareg shield was shaped and partly accreted during the pan african orogeny 750 550 ma but not as a homogeneous body

illite and hydrocarbon exploration illite is a general term for the dioctahedral mica like clay mineral common in sedimentary rocks especially shales illite is of interest to the petroleum industry because it can provide a k ar isotope date that constrains the timing of basin heating events it is critical to establish that hydrocarbon formation and migration occurred after the formation of the trap anticline etc that is to hold the oil illite also may precipitate in the pores of sandstone reservoirs impeding fluid flow illite in shales is a mixture of detrital mica and its weathering products with diagenetic illite formed by reaction with pore fluids during burial k ar ages are apparent ages of mixtures of detrital and diagenetic end members and what we need are the ages of the end members themselves this paper describes a methodology based on mineralogy and crystallography for interpreting the k ar ages from illites in sedimentary rocks and for estimating the ages of the end members

dynamic recrystallization processes in plagioclase porphyroclasts abstract recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the jotun complex south norwegian caledonides the deformation took place at a temperature of approximately 700 c and at pressures below 900 mpa two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations the crystallographic orientation and grain shapes for the two populations are interpreted as soft and hard orientations of the dominant slip system 010 001 with respect to the kinematic reference system the orientations of porphyroclasts and recrystallized grains were measured with a u stage microscope analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts in type 1 porphyroclasts soft orientation the misorientation is inferred to be caused by subgrain rotation due to the activation of the 010 001 slip system resulting in a host control orientation relationship whereas in type 2 porphyroclasts hard orientation no dominant misorientation relationship could be identified the slip systems of the dislocations inferred from transmission electron microscopy tem analysis correspond well with the results of the u stage misorientation analysis in type 2 porphyroclasts hard orientation micro fracturing was observed in the tem whereas microfractures are absent in type 1 porphyroclasts soft orientation micro fracturing is likely to be the mechanism for nucleation of new grains in type 2 porphyroclasts resulting in a no host control orientation relationship between porphyroclasts and recrystallized grains

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

fluid percolations in a fault zone a study of fluid inclusion planes in the st sylvestre granite northwest massif central france abstract a method for relating fluid flow evolution direction of fluid percolation to the bulk brittle network microfracture system at each state in a sequence of deformational events is presented fluids percolating through cracks are generally trapped as fluid inclusion planes alignment of fluid inclusions the method involves the use of fluid inclusions not only as a tool for the establishment of the physico chemical conditions under which fluids were entrapped but also as a microstructural marker of the geometry of permeability the study shows that through the determination of the paleopressures for each stage of fluid migration the chronology of the cracks and p t conditions prevailing during each crack formation can be established this methodology has focussed on a granitic example from the french massif central but may be applied to other geological environments

the role of microcracking and grain boundary dilation during retrograde reactions department of geology university of southampton southampton oceanography centre southampton so14 3zh uk email ajb1 soc soton ac ukveined metamorphic rocks are common and give evidence of fracture controlled fluid flow however the processesinvolved in fluid infiltration in domains between fractures is rarely well established reaction products concentrated atgrain boundaries give an indication that diffusion of elements along grain boundaries may be important and linearfluid inclusion trails provide evidence of grain scale microcracking despite these common observations the relativeimportance of grain boundary processes and microcracking at different metamorphic grades is not well characterised etheridge et al 1983 developed a metamorphic porosity model comprising grain boundary bubbles and tubules ofwhite white 1981 but also incorporating dilatant grain boundaries and intra grain tensile cracks variations in thelocal granular stress state will initiate transient micro cracking and grain boundary dilatancy promoting enhancedporosity permeability and grain scale fluid infiltration this in turn causes retrogression the extent to which it devel ops being influenced by the degree of connectivity of the dilatant cracks and grain boundaries and whether for particu lar p t x conditions the infiltrating fluid is at equilibrium or not with the host being infiltrated with reference to examples of greenschist facies retrogression from troms norway and south harris scotland this study establishes that the processes of microcracking and grain boundary dilatancy are highly heterogeneous atthin section scale in examples where reaction products are not related to mutual contact of specific phases it is shownthat the development of reaction products varies according to grain boundary orientation and density of microcracks an example from norway shows a quartz rich garnet mica schist where a centimetre scale brittle fracture has in duced extensive microcracking and fluid infiltration in the adjacent matrix fluid inclusions defining healed microcracks perpendicular to schistosity have h

controls on reservoir quality in exhumed basins an example from the ordovician sandstone illizi basin algeria abstract petroleum exploration in many north african intracratonic basins targets early paleozoic sandstones as the primary reservoir objective these sandstones are often characterized by highly variable reservoir quality 0 0001 1000 md and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential the objective of this study is to characterize the primary controls on reservoir quality in an ordovician field in the illizi basin of algeria through detailed core and petrographic analysis and establish if variations in thermal history across the field have a material impact on reservoir quality the best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1 fibrous illite these lithologies are most commonly found within the high energy tidally reworked post glacial facies sandstones of the uppermost ordovician succession observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial this interpretation is supported by field wide numerical modelling of sandstone diagenesis this study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability thermal history therefore is an important consideration in reservoir quality studies in exhumed basins where variations in present day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play

upper greenschist facies intragrain deformation of albite in mylonitic meta pegmatite and the influence of crystallographic anisotropy on microstructure formation abstract we studied the deformation behaviour of albite from permian meta pegmatite in cretaceous upper greenschist facies shear zones from the austroalpine matsch unit in the eastern alps italy sodium feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid crustal conditions in grains with different angular relations between their 010 planes and the kinematic frame the studied rocks were deformed at c 500 c in localized shear zones with well characterized top w shear kinematics supposedly during the cretaceous upper greenschist facies tectonometamorphic event microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing dissolution precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine grained albite with some compositional variability ab96 98 and ab89 91 in cracks new albite precipitates form aggregates with straight segments of high angle grain boundaries nearly 120 dihedral angles and only a poor or no orientation relation to the hosting clast intragranular kinking is related to continuous lattice rotation of up to 15 by a misorientation axis close to albite 100 and the formation of subgrain boundaries with maximum misorientations of 7 synthetic microshear zones supposedly nucleated on pre existing cracks and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone new microstructural and textural data from mylonitic permian meta pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation

dislocation generation slip systems and dynamic recrystallization in experimentally deformed plagioclase single crystals abstract three samples of gem quality plagioclase crystals of an60 were experimentally deformed at 900 c 1 gpa confining pressure and strain rates of 7 5 8 7 10 7 s 1 the starting material is effectively dislocation free so that all observed defects were introduced during the experiments two samples were shortened normal to one of the principal slip planes 010 corresponding to a hard orientation and one sample was deformed with a schmid factor of 0 45 for the principal slip system 001 010 corresponding to a soft orientation several slip systems were activated in the soft sample dislocations of the 001 010 and 110 001 system are about equally abundant whereas 110 111 and 101 in 131 to 242 are less common in the soft sample plastic deformation is pervasive and deformation bands are abundant in the hard samples the plastic deformation is concentrated in rims along the sample boundaries deformation bands and shear fractures are common twinning occurs in close association with fracturing and the processes are clearly interrelated glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation grain boundaries of tiny defect free grains in healed fracture zones have migrated subsequent to fracturing these grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization nucleation via small fragments can explain a non host controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi brittle to plastic transition

acoustic emission study of microfracturing during the cyclic loading of westerly granite a rectangular prism of westerly granite was deformed in a uniaxial cycling experiment acoustic emission ae activity was recorded using eight transducers in real time hypocenters of 116 events were determined using a standard least squares approach event clustering was definitely evident and could easily be assessed using a stereographic projection of hypocenters this clustering was observed at volumetric strains well below 0 03 the distribution of hypocenters changed from cycle to cycle suggestive of a spatial migration the cumulative distribution of hypocenters documents the strongly anisotropic development of dilatancy in westerly granite once the sample underwent the initial deformation and a zone of weakness was established subsequent events clustered in and about the initial zone volumetric strain differential stress acoustic emission event rate and cumulative numbers were continously measured during the experiment the cumulative number of events per cycle generally decreased with the number of cycles the majority of events occurred upon loading those that occured during unloading were few in number and not as systematically related to stress as were those for loading in general waveform quality deteriorated with increased cycling and in later cycles waveforms typically displayed low frequency components in initial onsets followed by a more typical higher frequency arrival

geological link between fluid inclusions dilatant microcracks and paleostress field seismic investigations using three component instruments have demonstrated shear wave splitting in widely different geological and tectonic environments shear wave analysis suggests that the inferred anisotropy can best be explained if the rocks contain distributions of subparallel subvertical fluid filled microfractures which are aligned according to the current stress field the crampin extensive dilatancy anisotropy eda model geological evidence for widespread fluid activity in the earth s crust supports this hypothesis and suggests that the myriads of planes of microscopic secondary fluid inclusions which characterise most geological materials represent fossil eda cracks field studies confirm a strong spatial relationship between the orientation of fluid inclusion arrays and the paleodirections of maximum compressive stress at elevated temperatures and pressures the healing of fluid filled cracks in the crust of whatever origin is considered to be a fairly rapid process moreover the inclusions which form as a result of crack healing respond to changes in temperature pressure and stress by changing shape and or volume this indicates a high degree of fracture compliance sufficient perhaps to react to rapid changes in the current stress field and associated shear wave splitting though individual inclusions typically 20 m will have no effect on the propagation of seismic waves with wavelengths of tens to hundreds of meters the rock volume sampled by the waves will be rendered anisotropic if the overall average of the inclusions shows a preferred orientation thus fluid inclusions may constitute one of the principal types of fluid filled microcrack described by the eda hypothesis

melt assisted interior to margin switch from dislocation to diffusion creep in coarse grained plagioclase evidence from a deformed anorthosite pluton abstract at bolangir eastern india massive anorthosites in the pluton interior grade into anorthosites with outward dipping margin parallel foliation neighboring the pluton margin sheets and veins of ferrodiorites residual melts of anorthosite crystallization concordant and discordant to the margin parallel foliation suggest pluton deformation at near magmatic conditions t 950 c and p 6 12 kbar in the pluton interior the larger than centimeter sized magmatic plagioclase grains are replaced by aggregates of smaller 100 600 m dynamically recrystallized internally strained grains with un equilibrated boundaries neighboring the pluton margin poly sized 200 2500 m plagioclase grains in anorthosites are of two types strain free rectangular shaped plagioclase grains with high energy an richer margins indenting neighboring plagioclase grains formed by diffusion creep whereas unstrained end to end touching euhedral plagioclase grains showing tilling represent magmatic flow textures the pluton interior to margin switch in plagioclase deformation from grain boundary migration accommodated dislocation creep to grain boundary diffusion creep is attributed to the increasing melt fraction melt crystal ratio during syn deformation pluton emplacement plagioclase grains in the bolangir pluton are significantly coarser compared to plagioclase aggregates

brittle ductile microfabrics in naturally deformed cordierite evidence for significant short term strain rate variations abstract brittle and ductile microfabrics in cordierite from a migmatite were studied by universal stage method and electron probe micro analysis the microfabrics record crystal fracturing and ductile deformation of one deformation event at 570 c and 380 mpa but at variable strain rates the cordierite microfabrics evolve in four stages 1 at strain rates of 10 7 s 1 brittle micro shear zones form producing rotated fine grained cordierite crystal fragments 2 at decreasing strain rates 001 010 and 100 subgrain boundaries develop and fluid circulation leads to partial cordierite breakdown and oriented growth of sillimanite staurolite magnetite and quartz along shear planes 3 subsequent to deformation and during continuous infiltration of fluids the same minerals grow randomly along shear planes and cordierite recrystallizes statically 4 during continuous annealing and fluid infiltration the recrystallized cordierite grains coarsen and sillimanite staurolite magnetite and quartz form along the grain boundaries brittle high temperature and high strain rate microfabrics followed by ductile high temperature deformation at lower strain rates and or by recovery as recorded in the cordierite studied may be more common in naturally deformed rocks than typically assumed this is evidence for and may be used as an analytical tool to detect paleo seismic events

characteristics of microcracks in amphibolite and granulite facies grade rocks from southeastern pennsylvania our observations of the microcracks in a small suite of samples of amphibolite and granulite facies rocks of variable composition show that 1 most cracks are partly or completely sealed 2 the minerals that seal microcracks are unrelated to phase equilibria associated with granulite facies metamorphic conditions and 3 the present crack porosity is related to bulk composition rather than to metamorphic grade all observed cracks open healed sealed appear to have formed in response to tectonic stresses associated with retrograde metamorphism

dating of shallow faults new constraints from the alptransit tunnel site switzerland methods for dating shallow faults in the earth9s crust are still evolving and their reliability remains controversial based on results from well preserved fault gouge samples from the alptransit deep tunnel site in switzerland we propose a simplified and readily applicable approach to investigate and date clay rich gouge zones we collected 10 samples covering a north south section of 15 km all sampled faults are developed in orthogneiss host rock have a rather consistent approximately northwest southeast strike are mainly normal and are related to northeast southwest orogen parallel extension during alpine exhumation fine grained clay separates from the fault gouges consist of illite 1m 1m d and 2m 1 polytypes smectite and chlorite with minor amounts of feldspar in some sample fractions the k ar ages for

implications of k ar ages of whole rock and grain size fractions of metapelites and intercalated metatuffs within an anchizonal terrane white mica bearing fractions ranging in grain size from 0 4 m to 6 3 20 m were separated from metapelites and intercalated metatuffs of the eastern rheinisches schiefergebirge frg the stratigraphic age of these rocks is middle devonian eifelian and they contain detrital material of northwestern provenance old red continent probably mainly derived from the caledonian orogen folding in the carboniferous was associated with cleavage formation and an apparently synkinematic anchizonal metamorphism apparent k ar ages of metapelite fractions display a marked positive correlation with grain size illustrating the detrital influence which is diminished with decreasing grain size and increasing metamorphism determined by illite crystallinity contrasting grain morphologies observed by sem enable the interpretation of apparent age grain size relationship for coarse fractions the anticipated lack of detrital mica in metatuffs is confirmed by the fairly consistent apparent k ar ages determined for the coarser than 0 63 m size fractions which date the anchizonal metamorphism at ca 330 ma comparison of metatuff and metapelite apparent ages suggests that the extent of rejuvenation in the latter was largely dependent on grain size rejuvenation was also somewhat controlled by the degree of anchizonal metamorphism as suggested by differences in k ar results of metapelites which were metamorphosed at variable anchizonal conditions fractions 0 63 m from upper anchizonal metapelites record ca 330 ma ages similar to those of the 0 63 20 m sizes in metatuffs together those results confirm the limited applicability of conventional k ar dating on bulk clay fractions 2 m of very low grade anchizonal metamorphic rocks in dating metamorphic events and concomitant cleavage formation

micromechanisms of shear zone propagation at the brittle viscous transition our investigation of progressively strained rock samples from the margins of greenschist facies shear zones utilizes a space for time approach to reveal how the mylonitic overprint of metapsammitic and pelitic host rocks at the cap de creus involved brittle fracturing we present a set of microscale observations indicating that microfractures formed immediately prior to or coevally with a fine grained mylonite microfracturing dominated early stages of strain localization on the scale of the shear zones on the microscale centimeter long fractures facilitated strain softening by allowing enhanced fluid access thereby accelerating the dynamic recrystallization of quartz and a metamorphic reaction of biotite as these two processes produce a polyphase matrix of small dislocation poor grains that eventually form an interconnected rheologically weak phase fractures become inactive this represents a strain dependent brittle viscous transition we outline this transition in a conceptual model for the rheological evolution of mid crustal shear zones

reaction induced weakening of plagioclase olivine composites abstract the localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions in order to study the effects of solid solid mineral reactions on plastic deformation of rocks we have investigated the shear deformation of plagioclase olivine composites during the reaction plagioclase olivine orthopyroxene clinopyroxene spinel garnet microstructures of plagioclase olivine composites were studied after shear deformation experiments in a griggs apparatus experiments were performed on anorthite forsterite an fo and labradorite forsterite lab fo composites at 900 c confining pressures between 1000 1600 mpa and with constant shear strain rates of 5 10 5 s 1 in absence of reaction lab fo composites are stronger than pure olivine and labradorite end members that deform with a high temperature plasticity mechanism lab fo composites strain harden due to the inhibition of extensive recrystallisation by interphase boundaries in an fo composites the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine grained size

reconstructing fluid flow events in lower triassic sandstones of the eastern paris basin by elemental tracing and isotopic dating of nanometric illite crystals lower to middle triassic sandstones from eastern paris basin were buried to a maximum depth of 2500 m at a paleo temperature of about 100 c they contain extensive amounts of authigenic platy and filamentous illite particles similar to those reported in reservoirs generally buried at 3000 to 5000 m and subjected to temperatures of 120 to 150 c to evaluate this unexpected occurrence such sandstones were collected from drill cores between 1825 and 2000 m depth and nanometric sized sub fractions were separated the illite crystals were identified by xrd observed by sem and tem analyzed for their major trace rare earth elements and oxygen isotope compositions and dated by k ar and rb sr illite particles display varied growth features in the rock pore space and on authigenic quartz and adularia that they postdate tem eds crystal chemical in situ data show that the illite lath fiber and platelet morphologies correspond at least to two populations with varied interlayer charges between 0 7 and 0 9 in the former and between 0 8 and 1 0 in the latter the fe fe mg ratio being higher in the platelets except for the deeper conglomerate the paas normalized ree patterns of the illite crystals are bell shaped enriched in middle rees ca carbonates and ca phosphates were detected together with illite in the separates these soluble components yield 87sr 86sr ratios that are not strictly in chemical equilibrium with the illite crystals suggesting successive fluids flows with different chemical compositions the k ar data of finer 0 05 m illite separates confirm two crystallization events at 179 4 4 5 and 149 4 2 5 ma during the early and late jurassic the slightly coarser fractions contain also earlier crystallized or detrital k bearing minerals characterized by lower 18o values the 18o of the finest authigenic illite separates tends to decrease slightly with depth from 18 2 0 2 to 16 3 0 2 suggesting different but contemporaneous crystallization conditions deeper in the section the illite platelets and filaments crystallized in changing physical chemical crystallization conditions induced by fluids flows through the host rock pore system these flow events were probably driven by repetitive rifting episodes of the north atlantic ocean although located several hundreds kilometers away from eastern paris basin and or by fracturing events in the nearby basement of the vosges massif complex relationships between geodynamical events thermal anomalies and advective fluids confirm that remote tectonic activities can impact quiescent basins even if located far from tectono thermal activities by discrete and long distance fluid flows

replacement reactions and deformation by dissolution and precipitation processes in amphibolites the deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition fluid content pressure and temperature these mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation here we show that fracturing and fluid flow triggered coupled dissolution precipitation and dissolution precipitation creep processes which were responsible for the formation of a mylonitic microstructure in amphibolites this fabric is developed over a crustal thickness of 500 m in the lower seve nappe scandinavian caledonides amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence the albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the 001 cleavage planes and display lobate edges with embayments and peninsular features two albite oligoclase generations bright in cathodoluminescence resorbed and overgrew the porphyroclasts sealing the fractures electron backscattered diffraction shows that the two albite oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them these two albite oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation the amphibole crystals experienced a similar microstructural evolution as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c axis no crystal plasticity is observed in either mineral species plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies 600 c 0 75 0 97 gpa our results demonstrate that at middle to lower crustal levels the presence of h2o rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution precipitation and favours deformation by dissolution precipitation creep over dislocation creep in plagioclase and amphibole

hydrogeologic role of geologic structures part 2 analytical models abstract this paper is the second of two papers that address the influence of geologic structures on ground water flow at various scales in fractured rocks the ultimate purpose of this research is to investigate the feasibility of grouting preferentially permeable zones as a strategy to minimize the production of acid mine drainage in underground hard rock mines in which the major permeability is structure and fracture controlled the aim of grouting is to reduce permeability around mined out openings to minimize the rate of inflow of ground water into such openings via the structurally controlled preferentially permeable pathways a series of hydraulic stress tests were conducted to help characterize the role of geologic structures in controlling the ground water flow system in the vicinity of the bunker hill mine in north idaho the results of these tests indicate that most of the ground water that flows from the underground drillholes used for hydraulic stress testing is derived from a few discrete structurally produced fracture zones that are more or less connected through smaller scale fractures four types of analytical models are considered as a means of analyzing the results of multiple drillhole hydraulic stress tests as follows cross hole equivalent porous media double porosity equivalent porous media a solution to flow in and around a single vertical fracture leaky equivalent porous media partial penetration the estimation of hydraulic coefficients in complex fractured rock environments involves the combined application of a number of deterministic analytical models the models to be used are selected dependent on the location of the drawdown observations relative to the water producing zone and the length of the test the result of the tests can be related to the permeability hierarchy discussed in our first paper

the shift of the α β transition temperature of quartz associated with the thermal expansion of granite at high pressure abstract measurements are presented of volume changes in granite during room temperature compression to 100 200 and 300 mpa confining pressure followed by temperature increase to 900 c comparison with thermal expansion and compressibility data for the constituent minerals allows changes in porosity to be estimated under confining pressure porosity is found to decrease with heating to 200 c through expansion of the minerals into cracks which are thought to be related to the geological cooling history of the rock between 200 c and 840 c porosity increases as a result of differential thermal expansion of the constituent minerals but crack opening is increasingly suppressed at higher confining pressures extrapolation of the results indicates that differential thermal expansion can no longer cause crack opening in dry granite at confining pressures in excess of 450 mpa the quartz transition temperature in granite is marked by a kink in the thermal expansion curve of the rock and it is found to increase by 60 c 70 c per 100 mpa confining pressure as opposed to the published value of 26 c per 100 mpa for single crystals of quartz equations are presented which allow calculation of the effects of confining pressure and temperature on the stresses and displacements in and around a spherical inclusion embedded in a matrix of different elasticity and thermal expansion the theory together with a simple self consistent model for granite accounts semiquantitatively for the observations of thermal expansion and the effect of confining pressure thereon and for the observed transition temperatures for quartz in granite

diagenetic evolution of clay minerals in oil bearing neogene sandstones and associated shales mahakam delta basin kalimantan indonesia the clay fraction of the mahakam delta basin eastern kalimantan indonesia consists of mixed layer illite smectite kaolinite dickite detrital illite and chlorite kaolinite formed early as vermicular particle aggregates and thick dickite booklets formed late in pore spaces the dickite particles probably grew in deeper intervals of the sequence in contact with brines acidified by decarboxylation of organic matter during dissolution of k feldspar and upward migration of potassium in the handil field on the nearshore anticline evolution of the mixed layer illite smectite is characterized by a decrease in expandability with depth in the tunu field located on the next offshore anticline variable amounts of expandable layers suggest a different evolution the conversion process of smectite to illite layers in the mixed layers depends on the lithology of the host rocks transformation in the shales and dissolution precipitation in the sandstones sample location also played a role in the conversion process because the types of illite clays in the two locations differ when they are compared at similar degrees of evolution present day temperatures and paleotemperatures vitrinite reflectance and illitization of the mixed layer illite smectite all suggest that organic matter and clay particles evolved differently in the handil field but similarly in the tunu field integration of the analytical results obtained here and comparison with a two dimensional numerical model suggest that hydrocarbon generation took place in the deeper synclinal zones of the sequence and that oil migrated upward with brines probably inducing most of the illitization in the upper sequence both models emphasize the role of faults in channelizing fluid flow during the final stages of evolution of the basin

a new technology for the characterization of microfractured reservoirs test case unayzah reservoir wudayhi field saudi arabia this article presents a test case of a new technology using artificially enhanced anisotropy of magnetic susceptibility referred to here as eams for the characterization of microfractured reservoirs these are reservoirs in which microfractures are essential to porosity and or permeability a conventional geological characterization is costly time consuming and difficult to quantify in terms of assessing fracture impact on porosity and permeability therefore an efficient and effective method is required to characterize these microfractures and to determine their contribution to porosity and permeability the eams technology which we developed and tested allows rapid analysis that bridges reservoir geology and engineering using petrography the margin of error to detect microfractures that impact porosity and or permeability is 43 however it requires three times the sampling rate of the new eams technology the lower part of the unayzah reservoir unayzah b c in the wudayhi field saudi arabia where fractures were studied and microfractures are known to impact reservoir performance is used to develop and verify the eams technology the results show that eams derived microfracture fabric strikes east northeast west southwest consistent with that obtained by geological means the effective porosity profile obtained from eams tests is similar to that of the conventionally acquired porosity open microfractures in tested samples increase mean values of reservoir effective porosity by 36 50 in unayzah b c the occurrence of connected microfractures is estimated to cause an increase in average permeability of 75 in unayzah b c this is in agreement with the fact that wells in microfractured unayzah b c have 4 5 14 times the productivity of wells as nonfractured sections of this reservoir a maximum permeability trend of northeast southwest permeability anisotropy is detected the implementation of the eams technology in other fields with microfractured reservoirs will directly impact operational and simulation effort

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

diagenesis of quartz in the upper proterozoic kaimur sandstones son valley central india abstract the upper proterozoic kaimur sandstones in central india are quartz sublithic and lithic arenites cemented by quartz illite and hematite diagenetic quartz occurs in five modes syntaxial overgrowths fracture healings aggregates of small euhedral crystals quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources such as silica dissolved in meteoric waters stylolitization clay mineral diagenesis and the alteration of detrital silicates studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation

emplacement related microstructures in the margin of a deformed pluton the san jose tonalite baja california mexico abstract in the san jose tonalite pluton baja california mexico interior magmatic fabrics give way progressively to subparallel solid state fabrics with s c structures near the margins the outer rim marginal northern unit approximately 1 km thick is inferred to have been deformed during the emplacement of the inner parts of the magma chamber the rocks show a transition from strongly deformed at the outer edge to progressively less deformed towards the inner edge of the marginal northern unit a small amount of melt appears to have been present during the deformation as indicated by 1 apparent contact melting between some grains of plagioclase and 2 coarse grained aggregates and single grains of quartz k feldspar and or more sodic plagioclase in fracture controlled openings in primary plagioclase crystals the more sodic plagioclase typically grew as rims on the walls of the openings in twin continuity with the primary plagioclase the deformed rocks have a prominent foliation marked mainly by aggregates of biotite with contributions by hornblende and titanite grains as well as apparently recrystallized aggregates of quartz and or feldspar at least some of which may have originated as zones of fragmentation many primary grains of plagioclase hornblende biotite and titanite show evidence of brittle deformation and fragmentation the shapes of aggregates in the folia relationships between incipient folia and residual primary grains and occurrences of many folia in narrow zones transecting strong minerals plagioclase and hornblende suggest that brittle processes may have controlled at least some of the foliation development however evidence of strain induced grain boundary migration and bulge nucleation of new grains of quartz and plagioclase indicates that dislocation creep also played a part in the deformation recrystallization of plagioclase together with intrusion of the pluton at greenschist facies ambient temperatures confirms high temperature deformation with heat derived from the cooling magma

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

hercynian compressional deformations of the ahnet mouydir basin algerian saharan platform far field stress effects of the late palaeozoic orogeny n s to nw se trending faults and reverse faults occur within the palaeozoic ahnet mouydir basin of the algerian saharan platform located to the east of the west african pan african suture zone deformation and stratigraphic analysis show that this basin underwent a nne ssw to ene wsw shortening at about the carboniferous permian transition or more probably during the early permian a brief review of the more or less synchronous deformations registered in the neighbouring regions and more specifically around the west african craton illustrates the mechanical coupling between intraplate tectonics and the hercynian plate margins orogenies

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

constraining burial history and petroleum charge in exhumed basins new insights from the illizi basin algeria intracratonic sag basins commonly have relatively simple tectonic histories however later tectonic activity involving exhumation can make reconstructing the burial history a challenging task this is important because the relative timing of hydrocarbon generation and trap formation can be a key factor in risk assessment if trap formation postdates peak hydrocarbon generation exploration plays are typically downgraded mechanisms for charge in such exhumed basins are critical factors for understanding exploration risk this study uses data collected from an ordovician gas condensate field in the illizi basin of algeria to document the charging of a trap formed or modified during exhumation of the basin following maximum burial integrated analysis of sonic compaction data thermal history indicators and stratigraphic well data was used to constrain the burial and thermal history of the region hydrocarbon generation in the lower silurian source rock is interpreted to have occurred during the carboniferous prior to hercynian exhumation and during the late cretaceous early eocene maximum burial prior to eocene exhumation structural reconstructions indicate that the field was initially located on the southern flank of a long lived intrabasinal paleozoic paleohigh the large low relief structural closure that defines the present day accumulation formed as a result of northward tilting of the illizi basin during eocene uplift of the hoggar massif the study demonstrates that the timing of trap formation at the ordovician field postdates the main local hydrocarbon generation events within the basin suggesting that alternative hydrocarbon charge mechanisms are required this study indicates considerable potential to charge updip traps on the flanks of exhumed petroliferous basins via redistribution of the preexisting hydrocarbons within the basin

lithosphere strength inferred from fracture strength of rocks at high confining pressures and temperatures abstract a model is proposed for the strength of the crust or lithosphere in which the peak strength is in the brittle regime but not on the brittle ductile transition the model is based on experimental evidence which suggests that the fracture mechanism changes from ordinary brittle fracture to a high pressure type when the compressive strength equals the frictional strength the model assumes that a high pressure type of fracture is dominant in the earth s lower crust except where very high pore pressures exist it takes account of the scale effect on the strength of rocks the strength of the high pressure regime is estimated as a function of depth or pressure and temperature using experimental results derived at high confining pressures and temperatures in the upper part of the brittle zone where the frictional strength is lower than the compressive strength byerlee s friction law is used in the ductile regime the steady state creep law is used two cases of lithospheric strength dry and wet hydrostatic water pressure respectively are evaluated for thrust strike slip and normal faulting mechanisms with the exception of a wet lithosphere and a normal faulting mechanism the strength between the frictional and creep regimes is approximately constant with depth although the peak strength is in this part

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

potassium argon timing of episodic mica and illite crystallization in highly indurated hassi messaoud algeria hydrocarbon bearing sandstones nanometer to micrometer mica and illite separates of indurated cambrian and ordovician oil bearing sandstones from the hassi messaoud field algeria were extracted x rayed observed by scanning and transmission electron microscopy and k ar dated electron microscope observations revealed typical euhedral shapes for the mica to illite particles of most size fractions almost no odd shaped detrital crystals were detected the combined results document several generations of mineralogical and morphological identical mica to illite crystals that could not be differentiated by the traditional identification methods illite and mica genesis was multiphased with crystallization episodes at 340 10 ca middle mississippian 280 10 ma ca early permian and 170 10 ma ca middle jurassic younger than the stratigraphic age of the host rocks which is incompatible with a detrital origin the two older mica ages confirm that the hydrocarbon generation and emplacement had to start after the variscan tectonothermal event and before exhumation of the meta sediments the younger k ar ages at 135 to 110 ma ca early cretaceous relate to further crystallization episodes whereas those at circa 295 265 and 210 ma probably correspond to variable mixtures of the older and younger mica to illite end members three average k ar values are statistically significant the oldest at 340 10 ma corresponds to the start of the variscan tectonic activity and the intermediate at 280 10 ma sets its end both episodes probably modifying the reservoir capacities of the potential hydrocarbon host rocks the ages at 170 10 ma identify a further diagenetic activity characterized by illitization of dickite type precursors in local reservoirs these younger ages could correspond to the hydrocarbon charge into reservoirs which stopped diagenetic illitization at a present day depth of approximately 4000 m 13 000 ft

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out