foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

thermal anomaly profiles inferred from fluid inclusions near extensional and strike slip faults of the liaodong bay subbasin bohai bay basin china implications for fluid flow and the petroleum system abstract different types of faulting may greatly impact the flow of hot fluids and related thermal anomalies which may substantially affect the migration and accumulation of petroleum however little is known about the effects of fault types in this study 64 core samples were collected from localities controlled by extensional and strike slip faults in the liaodong bay subbasin bohai bay basin and fluid inclusions in these samples were used as thermal indicators of paleo fluids the results show strong thermal anomalies near the extensional and strike slip faults due to the flow of hot fluids the highest anomalies were recorded at localities proximal to the faults 134 4 c at a distance of 375 m from an extensional fault and 145 5 c at a distance of 262 5 m from the strike slip fault however these anomalies are all local and decrease with increasing distance from the faults up to a maximum distance of approximately 3000 m compared with those near the strike slip fault the anomalies near the extensional faults decrease more slowly and typically exhibit higher anomalies at similar distances which indicates that the extensional faults are better pathways for the flow of hot fluids the thermal effects of hot fluids have enhanced the maturation of organic matter and shifted the threshold depth for petroleum generation upward by approximately 300 m such effects on source rock maturation decrease with increasing distance from the faults up to a maximum distance of approximately 3000 m most ro values near the extensional faults are clearly higher than those near the strike slip fault thus the extensional faults play a more efficient role in the enhancement of source rock maturation and are interpreted as preferential pathways for the migration of generated hydrocarbons in the liaodong bay subbasin

sedimentology stratigraphy and geochronology of the proterozoic mazatzal group central arizona quartzite conglomerate and shale of the mazatzal group record the filling of a prot erozoic intra arc basin in central arizona u pb ages of zircons from rhyolite ash flow tuff indicate that deposition began at 1701 2 ma basal deposits of the newly defined pine creek conglomerate formed in an alluvial fan setting synchronous with the final phase of extrusive rhyolite volcanism and active faulting after volcanic activity ceased shallower slope braided stream environments developed in early deadman quartzite time subsequent marine transgression produced shallow subaqueous deposits of the upper part of the deadman quartzite these were overlain by prodelta sediments of the maverick shale indicating a phase of basin deepening finally the mazatzal peak quartzite was deposited as the basin shoaled again first to shallow marine and subsequently to fluvial conditions the quartzite is diagenetic quartz arenite originally deposited as lithic arenite and lithic arkose the provenance was initially restricted and earliest sediment was derived mainly from the subjacent red rock group when tectonic activity ceased how ever the surrounding highlands were planed down by erosion and detritus from a wider variety of source rocks was funneled into the basin this included contributions from arc related supracrustal rocks of the payson ophiolite and east verde river formation and finally a granitic basement input detrital quartz in the lower part of the mazatzal group is largely monocrystalline and volcanic in origin polycrystalline quartz associated with detrital feldspar and of probable continental affinity is concentrated in the uppermost parts of the sequence the life span of the intra arc basin was on the order of 30 m y from the formation of the payson ophiolite at 1 73 ga to the deposition of the upper mazatzal peak quartzite sometime after 1 70 ga the pre mazatzal red rock group represents the last stages of volcanic arc activity and the mazatzal group records the transition from orogenic to nonorogenic sedimentation in central arizona

tectonic implications of finite strain variations in baraboo interval quartzites ca 1700 ma mazatzal orogen wisconsin and minnesota usa abstract variably deformed baraboo interval quartzites 1750 1630 ma are now correlated by detrital zircon ages holm d schneider d coath c d 1998 age and deformation of early proterozoic quartzites in the southern lake superior region implications for extent of foreland deformation during final assembly of laurentia geology 26 907 910 and were analyzed using finite strain techniques which document a se nw tectonic shortening orientation from the baraboo syncline south to the flambeau syncline north fabric interpretations using bedding and the principal strain axis orientations indicate there was a mix of layer parallel and layer normal synfolding shortening in the plane of tectonic transport se nw in the thrust belt finite strain magnitudes increase from the foreland barron sioux quartzites south toward the baraboo syncline electron backscatter device ebsd fabric diagrams pole figures of quartz optic axes also demonstrate an increasingly penetrative strain to the south anisotropy of low field ac magnetic susceptibility ams as a strain proxy preserve kmin measurements in the baraboo syncline quartzites that are sub horizontal e w and intersect bedding ams measurements in underlying rhyolite on opposite limbs of the baraboo syncline are clean appear to be primary magmatic as kmax intersects bedding but require a complex folding interpretation that is inconsistent with field kinematic indicators the flat lying foreland barron and sioux quartzites preserve a mix of results quartzites with no strain finite strain and ebsd measurements or small strains that preserve vertical shortening ams measurements in the foreland preserve kmin axes that are sub horizontal and parallel to the regional shortening orientation in the adjacent thrust belt the mazatzal orogen produced a thrust belt that is orders of magnitude wider 500 km and of higher metamorphic grade than most thrust belts where both the thrust belt and foreland are quartzites

regional framework structural and petroleum aspects of rift basins in niger chad and the central african republic c a r abstract this paper overviews the regional framework tectonic structural and petroleum aspects of rifts in niger chad and the c a r the data base is from mainly proprietary exploration work consisting of some 50 000 kilometres of seismic profiles 50 exploration wells one million square kilometres of aeromagnetics coverage and extensive gravity surveys there have been 13 oil and two oil and gas discoveries a five phased tectonic history dating from the pan african orogeny 750 550 my b p to the present suggests that the western central african rift system wcas with its component west african rift subsystem was and central african subsystem cas formed mainly by the mechanical separation of african crustal blocks during the early cretaceous among the resulting rift basins in niger chad and the c a r seven are in the was grein kafra tenere tefidet termit bongor and n dgel edgi and three doba doseo and salamat are in the cas the was basins in niger and chad are all extensional and contain more than 14 000 m of continental to marine early cretaceous to recent clastic sediments and minor amounts of volcanics medium to light oil 20 api 46 api and gas have been discovered in the termit basin in reservoir source and seal beds of late cretaceous and palaeogene age the most common structural styles are extensional normal fault blocks and transtensional synthetic and antithetic normal fault blocks the cas doba doseo and salamat are extensional to transtensional rift basins containing up to 7500 m of terrestrial mainly early cretaceous clastics heavy to light oil 15 39 api and gas have been discovered in doba and doseo basins source rocks are early cretaceous lacustrine shales whereas reservoirs and seals are both early and late cretaceous dominant structural styles are extensional and transtensional fault blocks transpressional anticlines and flower structures the existence of a total rift basin sediment volume of more than one million cubic kilometres with structured reservoir source and seal rocks favours the generation migration and entrapment of additional significant volumes of hydrocarbons in many of these basins

clay minerals as geo thermometer a comparative study based on high spatial resolution analyses of illite and chlorite in gulf coast sandstones texas u s a phyllosilicates are among the most important stable minerals within the earth s crust their use as geo thermometer bears great potential for application to the thermal history of rocks within the stability range of layered silicates and was tested here a high resolution analytical technique combining focused ion beam fib milling and analytical electron microscopy aem analysis has been applied to a series of sandstone core samples from the gulf coast texas u s a the nanoscale compositional variations of k deficient mica and chlorite flakes show that rim compositions are the most likely to approach equilibrium compositions whereas core compositions may be relict especially for illite like phases these rim analyses were used to test existing empirical or thermodynamically formulated thermo baro meters against maximum temperatures which are perfectly constrained for the selected samples as they were measured in situ during drilling 100 230 c and 300 1200 bars the results show that most of the empirical models overestimate the temperature while thermodynamic models yields reasonable estimates for diagenetic to anchizonal conditions especially if the fe3 content is taken into account this study clearly shows that phyllosilicates thermometry is reliable when combined with an analytical technique giving access to the fine scale compositional variations that may represent local equilibration whereas using micrometric compositional analysis precludes trustworthy application of such thermometers

intermittent mixing processes occurring before plinian eruptions of popocatepetl volcano mexico insights from textural compositional variations in plagioclase and sr nd pb isotopes volcan popocatepetl has explosively erupted in plinian style at least five times in the last 23 000 years extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene and the occasional presence of xenocrysts of cr rich fe ti oxides and mg rich olivines and pyroxenes indicate that magma mixing has been a major process affecting the magmatic system the nearly invariant composition of the erupted products andesitic dacitic suggests however that mixing is not acting alone and must be balanced by assimilation and or crystallization to investigate the magmatic processes that have modified the plinian magmas textural and compositional variations and growth rates in plagioclase were used to approximate the frequency of mixing events affecting each magma systematic analysis of sr nd and pb isotopes was carried out on plagioclase pyroxene and pumice matrix glass to constrain the extent of assimilation of upper crustal rocks additionally a series of phase equilibrium experiments were carried out to constrain the depth where such mixing and assimilation occurred we find that magma was stored at one of two different depths beneath popocatepetl with magma mixing acting in both reservoirs mixing frequency and the relative impact on mineral compositions and textures has varied with time assimilation of calcareous rocks underneath popocatepetl has not been pervasive and does not contribute significantly to the evolution of the plinian magmas the similar compositions of magmas with diverse mixing histories suggest that fractional crystallization and possibly assimilation of deep crust takes place at depth and that intermediate magmas ascend into the upper crust already differentiated

fracture network analysis of the st eustache quarry quebec canada for groundwater resources management a detailed structural survey has been conducted on a fractured sedimentary rock formation in the st eustache quarry quebec canada to supplement a hydrogeological study the two main types of dis

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

systematics of chlorite alteration at the phelps dodge massive sulfide deposit matagami quebec hydrothermal alteration at the phelps dodge archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages quartz chlorite albite epidote or sericite quartz chlorite or sericite chlorite or sericite and chlorite talc or stilpnomelane alteration at constant al was initiated by hydration addition of fe mg silicification and leaching of ca na which produced al saturated chlorites with a wide range of fe fe mg ratios within the assemblage quartz chlorite albite epidote or sericite with continued addition of fe and less mg combined with extensive leaching of quartz the chlorite became al undersaturated quartz chlorite talc assemblage and ultimately formed a rock composed essentially of chlorite the chlorite talc or stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone chlorites in the si al mg fe system form a rectangular solid solution plane with limited substitution of al for si and extensive substitution of fe for mg in si al space the solid solution plane is outlined by al saturation presence of other al rich mineral and al undersaturation presence of al poor or al free mineral boundaries al iv substitution for si in the chlorites increases from 2 3 to 3 01 formula units based on 8 si al iv with fe fe mg whereas al iv occupancy decreases from 2 88 to 2 65 the valence charge balance is compensated by octahedral site vacancies or by the conversion of fe super 2 to fe super 3 the al iv content of chlorite also increases with temperature chlorites on the al saturated boundary of the solid solution field form a potentially useful geothermometer t degrees c 106 al iv 18 al iv is corrected for changes with the fe fe mg ratio as al iv corrected al iv 0 7 fe fe mg the fe fe mg ratios of bulk rock and contained chlorite correlate well birefringence colors of the chlorite change from green to brown violet and berlin blue as fe fe mg changes from 0 18 to 0 64 the fe fe mg variation apparently depends largely on the proportions of mg rich seawater and fe rich hydrothermal fluid in the altering brine

hot fluid flows around a major fault identified by paleothermometric studies tim mersoi basin niger abstract faults are mostly considered as low permeability zones due to their frequent sealing but they can also contribute significantly to fluid movements in sedimentary basins and subsequently to temperature anomalies in favorable cases paleo temperatures and pressures can be determined from fluid inclusion and chlorite thermometry using this approach we estimate the temperature gap between incoming fluids and host rocks along the arlit fault a major fault affecting the whole series of continental siliciclastic formations from paleozoic to late jurassic in the tim mersoi basin niger strong alteration halos and uranium deposits occur along this regional north south trending fault as well as a secondary network of ne sw faults sandstone porosity is partially reduced by authigenic quartz and fe rich trioctahedral chlorites the chlorites precipitated directly from the solutions under reducing conditions prior to uranium oxide deposition the temperatures and pressures are estimated to be around 115 150 c and 80 120 bars respectively and are remarkably similar in all formations from the carboniferous to the jurassic these temperatures are much higher around 60 80 c than those estimated at maximum burial and document hot fluid flow along the major fault this thermal anomaly is inferred to be related to the opening of the atlantic ocean during the cretaceous before the onset of the main exhumation stages during the mid tertiary this study documents the role of faults as significant conduits for fluid flow and their role in the mass and heat transfer related to the formation of economic deposits

fold axis parallel rotation within the laramide derby dome fold wind river basin wyoming usa abstract derby dome a doubly plunging anticline 7 3 km on the eastern flank of the wind river range wyoming trends nw se in response to the regional ne sw directed shortening of the cretaceous eocene laramide orogeny mesozoic sediments are exposed around the fold hinge above an east dipping thrust fault that offsets archean crystalline rocks at depth stress and strain ellipsoidal data were determined through the measurement of mechanically twinned calcite in limestones triassic alcova through j k morrison formation rocks 13 samples calcite cements 5 samples and synfolding calcite veins 16 samples around the northern half of the fold on the outer limbs of the fold the maximum shortening strain axis 3 5 15 nevs in the limestones and cements is sub horizontal layer parallel and normal ne sw to the fold hinge reflecting regional sevier laramide shortening this regional layer parallel strain fabric is rotated into a fold axis parallel orientation nw se near the fold hinge indicating that significant rotations occurred during folding synfolding calcite veins of varying orientations also preserve a local sub horizontal hinge parallel shortening strain 4 0 17 nevs suggesting that the regional laramide stress and strain field was locally rotated into parallelism with the fold during shortening and displacement on the underlying thrust fault in both the country rock cement and vein data sets the strain overprint noise nevs increases toward the fold hinge inferred differential stress magnitudes are also higher for the vein calcite than for the country rock limestones or cements and there is no interpretable pattern around the fold avg 560 bars range of 240 2000 bars fracture measurements n 74 in different lithologies have different orientations on each side of the adjacent dallas dome fold suggesting layer parallel rotation during folding or active fracturing occurred uniquely on each fold limb

numerical simulation and evaluation of groundwater resources in a fractured chalk aquifer a case study in zinder well field niger groundwater is often a unique water source for agriculture industry and domestic use for local inhabitants in semi arid african continent owing to shortage of surface water and rainfall it is of significance to quantify and assess the groundwater resources for local sustainable development in order to correctly evaluate groundwater resources of zinder well field with fractured media in zinder niger a conceptual model of hydrogeology was set up by a geological and hydrogeological survey for simulating the real conditions a large multi well pumping test with designed pumping rate was conducted in addition a two dimensional finite element transient flow model was developed to simulate groundwater flow at the site results show that generally good agreements have been found between the simulated and observed hydrogeology heads for the test the hydraulic parameters were obtained from the pumping test a 11 year prediction of groundwater resources has been made under three exploitation schemes respectively and it indicates that the well field can offer 7 000 m3 day when the aquifer remains confined an optimal scheme of exploitation is provided

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

chloritization of the hydrothermally altered bedrock at the igarape bahia gold deposit carajas brazil the igarape bahia gold deposit has developed from weathering of a near vertical hydrothermal cu au mineralization zone the unweathered bedrock composed of chlorite schists is mainly metamorphosed basalts pyroclastic and clastic sedimentary rocks and iron formation contents and fe fe mg ratios of chlorites increase from distal country rock towards the mineralization zone which can be attributed to different water rock ratios and locations in a hydrothermal system in the hydrothermal system high salinity fluids convected through basin floor rocks stripping metals from the recharge zones with precipitation in discharge zones the chlorite with lower fe fe mg ratios indicates alteration by relatively unreacted mg rich fluids occurring within recharge zones by contrast the chlorite with higher fe fe mg ratios in the mineralization zone formed from solutions rich in fe mn au cu h2s and sio2 within a discharge zone the iron formation could also be formed within the discharge zone or on the basin floor from the fe rich fluids the distal country rock with less chlorite content is a hydrothermal product at low water rock ratios whereas the proximal country rock and the host rock with more chlorite content formed at high water rock ratio conditions the al iv contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 c with temperatures slightly increasing from distal country rock towards the mineralization zone

the first stage in the formation of the uranium deposit of arlit niger role of a new non continental organic matter abstract the uranium deposits of arlit are located in fluvial sandstone rich in organic matter of continental origin type iii deposited in palaeochannels previous studies on arlit deposits documented that organic matter of continental origin type iii was the main contributor to the reduction of uranium from the oxidation state of uranium vi u vi to uranium iv u iv this study shows that migrated oils of non continental origin strongly contributed to the genesis of uranium mineralization the petrographic analyses showed that i solid bitumen resulting from the polymerization of migrated oils is present in abundance in the porosity of the sandstone and fractures at the level of the channels of the tarat formation tamou takriza southern taza and guezouman formation mine sector of akouta and ii uranium mineralization is mainly associated with this migrated organic matter currently expressed in the form of solid bitumen the association occurs at an infra micrometric scale in the solid bitumen the geochemical analysis rock eval py gc ms and gc ms and the analysis of different indicators and biomarkers n alkanes pr ph steranes indicate the presence of a second organic matter with a different signature to type iii the oils present in the porosity of the sandstone of the guezouman and tarat formations confirm that channels of these two formations are the traps of the oil migration

deformation mechanisms in the southeastern ramp region of the pine mountain block tennessee a field and petrofabric study of the southwestern part of the pine mountain thrust sheet shows that most deformation features may be attributed to one of two processes the first layer parallel compression is responsible for a a layer parallel maximum shortening direction in the mechanically twinned fabric of the limestone and dolomite country rock b transport parallel extension fractures and joints c transport normal stylolite surfaces and d some small thrusts the second bending at thrust ramp hinges is responsible for a non bedding parallel maximum shortening directions in mechanically twinned calcite from fracture fillings b transport normal extension fractures and joints and c small normal faults possible explanations for the failure of calcite in the country rock to twin in response to bending at the ramp include both 1 an early imposed pre thrusting calcite twin strain hardening and 2 deformation of the country rock by competing mechanisms such as pressure solution small normal faults are parallel to the strike of the thrust sheet and seem to have evolved from strike parallel extension fractures in response to continued extension both small thrusts and stylolite surfaces are rare in outcrop there seems to be no major change in deformation of rocks which have traveled through different fractions of the total ramp length so long as the rocks have passed onto the ramp

a tectonic model for the location of palaeozoic ring complexes in air niger west africa the fir region in niger is one of the largest peralkaline granite provinces in the world in addition to the granites some plutons are characterized by an abundance of plagioclase rich cumulates gabbros troctolites and anorthosites with subordinate metaluminous granites and quartz syenites these anorogenic ring complexes were intruded along a 400 km zone near the 9 e meridian the province formed during a major magmatic event newly dated at around 407 8 ma and comprises 28 plutons which range from 0 8 to 65 km in diameter and show no correlation between distribution and age remote sensing over the whole fir massif demonstrates the existence of two main trends of faults or lineaments n50 e n90 e and n120 e n150 e autocorrelation analysis of the ring complexes reveals three large parallel high density strips orientated n20 e based on the structural and geological setting coupled with map analysis a new tectonic model for the location of the complexes is presented we propose a dextral n5 e shear zone model where the emplacement of the ring structures is controlled by n20 e dextral riedel shear r with a secondary n50 e tensional gash t this tectonic model implies a control by lithospheric structures for the emplacement of the ring complexes as well as a relationship between a transtensional tectonic regime and intra plate alkaline magmatism the new geochronological data on the ayir massif allow us to derive a silurian devonian palaeomagnetic pole representative for gondwana which fits data for the other continents better than previous estimates

a multidisciplinary fractured rock characterization study at raymond field site raymond ca abstract a dedicated field site was developed and a suite of experiments were conducted in the sierra nevada foothills near the town of raymond ca to develop and test a multi disciplinary approach to the characterization of groundwater flow and transport in fractured rocks a wealth of geologic hydrologic and geophysical data was collected at the site using a variety of unique tools a cluster of nine approximately 90 m deep boreholes were drilled at the site in a v shaped pattern with an angle of 60 the boreholes are spaced 7 5 15 30 and 60 m from the central borehole various geophysical and hydrologic tests were conducted in and between these boreholes integration of cross hole radar and seismic tomography borehole flow surveys and images from a new digital borehole scanner indicated that groundwater flow is mainly confined to a few sub horizontal fracture zones a unique suite of hydraulic tests were conducted in which three to four intervals in each of the nine boreholes were isolated using pneumatic packers some 130 injection tests were conducted and more than 4100 cross hole transient pressure measurements were obtained a computer algorithm was developed to analyze such massive interference data systematically as a result of the analysis an image of the fracture connections emerged which is consistent with the geophysical data high precision tiltmeters were effective in remotely characterizing the preferential flow path several radial convergent tracer tests were conducted by injecting a mixture of several conservative tracers and one sorbing tracer deuterium fluorescein lithium bromide and polystyrene micro spheres some differences between the breakthrough curves are observed which may be due to possible differences among so called conservative tracers some characterization tools were found to be more effective than others in locating flowing fractures however no single tool was almighty characterization of fractured rock is extremely challenging and requires a stepwise and well thought approach which is basically a good old scientific approach prediction of transport based on the characterization results is even more challenging and one should always bear in mind that it is virtually impossible to uniquely characterize a fractured rock

relationship between remagnetization magnetic fabric and deformation in paleozoic carbonates abstract we have studied the magnetic fabric of paleozoic carbonates from numerous sites in the north american midcontinent for comparison with paleomagnetic behavior and calcite twinning strains sites carrying a kiaman remagnetization have significantly lower anhysteretic remanent magnetization arm anisotropy less than 3 than other sites minimum axes of arm ellipsoids are distributed in a vertical plane trending approximately se nw this trend is very similar to the orientation of alleghenian layer parallel shortening as inferred by mechanical twinning in calcite sites lacking a kiaman overprint invariably have steeply oriented minimum arm axes and relatively strong anisotropy whereas the most strongly remagnetized sites have nearly horizontal minima and weak anisotropy these observations suggest that diagenetic authigenic magnetites formed independently of any pre existing fabric of precursor sulphides and host rock and that the magnetic fabric of the whole midcontinent was at least partially affected by alleghenian syn orogenic activity

closing of the midcontinent rift a far field effect of grenvillian compression the midcontinent rift formed in the laurentian supercontinent between 1109 and 1094 ma soon after rifting stresses changed from extensional to compressional and the central graben of the rift was partly inverted by thrusting on original extensional faults thrusting culminated at about 1060 ma but may have begun as early as 1080 ma on the southwest trending arm of the rift the crust was shortened about 30 km on the southeast trending arm strike slip motion was dominant the rift developed adjacent to the tectonically active grenville province and its rapid evolution from an extensional to a compressional feature at ca 1080 ma was coincident with renewal of northwest directed thrusting in the grenvllle probably caused by continent continent collision a zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere stresses transmitted from the grenville province utilized this weak zone to close and invert the rift

early proterozoic central plains orogen a major buried structure in the north central united states a major early proterozoic orogen central plains more than 1000 km long and at least 500 km wide has been delineated in the subsurface of nebraska kansas and missouri as a result of a new compilation of precambrian basement drill data in the northern midcontinent the orogen is composed of metamorphic and granitoid rocks in the ringe 1 63 1 80 ga and it is interpreted on the basis of isotopic ages rock types and structure as reflected by magnetic and gravity anomalies as being the eastward extension of the early proterozoic foldbelt exposed in basement uplifts in colorado and southeastern wyoming it truncates the southern margin of the superior archean craton and its marginal early proterozoic orogens the penokean 1 83 1 90 ga and the trans hudson 1 85 1 95 ga the pattern of intersecting proterozoic mobile belts marginal to archean cratons is comparable to that in other continents such as western australia and southern africa

from field data to fracture network modeling an example incorporating spatial structure this paper describes a technique for processing field data for a fracture network model which accounts for the observed spatial variability this has been done by generating a network subregion by subregion where the properties of each subregion are predicted through geostatistics once the geometry of a particular realization is specified flow through the network is studied we develop the method for a two dimensional analysis based on data from fanay augeres a uranium mine in france we plan to extend the analysis to three dimensions and compare the results with in situ test results in particular we have focused on the data collected in a long section of a drift where fractures have been mapped and steady state permeability tests have been performed in 10 boreholes in order to generate fractures in a statistically heterogeneous region we first divided the region into statistically homogeneous subregions in each subregion and for each fracture set we must specify the areal fracture density and the orientation length and aperture distributions we divided the fractures into five sets based on tectonic history and observed that for each set fractures spaced close together tended to have similar orientations this was built into the simulation an estimate of the aperture distribution for each set was made by assuming that the hydraulic apertures of the fractures intersecting a well test zone were proportional to the fracture opening observed in the core data input to the geostatistical analysis for each set consisted of 16 values of mean length and density of fractures in each 5 m by 2 m section of the drift wall the results of the simulation are tables of values for mean length and fracture density for each of the five sets the value of density simulated for each subregion was used directly to determine the number of fractures to be generated of that set in the subregion the value of mean length was used as the mean of the length distribution in the subregion the local coefficient of variation for length was the same in every subregion using the data described above we generated a 100 m by 100 m fracture network in a series of 100 statistically homogeneous subregions the fractured region generated has a total of 65 740 fractures a 70 m by 70 m region was isolated for directional permeability testing results show that the system is barely connected about 0 1 of the fractures essentially control permeability in the language of percolation theory this mesh is close to the critical density for which infinite clusters of fractures are found problems remaining include that 1 the analysis should be extended to three dimensions 2 the geostatistical simulation and network simulation should be combined rather than used as two separate operations and 3 the nature of flow through single fractures at fanay augeres should be investigated

cenozoic oxidation episodes in west africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies imouraren deposit tim mersoi basin northern niger uranium u deposits can undergo oxidizing events subsequent to which the u may be redistributed or precipitated under distinct mineralogical forms occurrences of u6 minerals can be the result of either i in situ oxidation and alteration of previous u4 minerals or ii direct precipitation from u6 rich solutions samples from imouraren the largest u6 ore deposit known in the world located in the tim mersoi basin northern niger have been studied to determine the nature distribution and timing of occurrences of u and related minerals the host rock is composed of sandstones and analcime rich siliciclastic rocks of jurassic age the u ore is largely dominated by acicular hexavalent u minerals and by a very small amount of uraninite a comprehensive geochemical model that describes the origin and geochemical setting of this style of economically important mineralization has been developed the petrographic and mineralogical studies show evidence of in situ uraninite alteration into uranophane and metatyuyamunite documented by textural relationships and chemical analyses using sem and electron microprobe the textural relationship between the u4 and u6 minerals is supported by the near identical ree signatures of the primary uraninite and related secondary u minerals ion microprobe u pb dating from 21 to 1 ma confirms that primary uraninites were altered to u6 minerals under supergene conditions during cenozoic oxidation events in western africa in relation with the well known stages of intense continental weathering that have affected western africa from the early cenozoic to recent times

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial

diagenetic chloritization of feldspars in sandstones abstract detrital microcline and albite are commonly replaced diagenetically by chlorite in sandstones of the visingso group upper proterozoic southern sweden authigenic quartz and in some cases albite and illite are associated with the chlorite the sem examinations suggest that chloritization of feldspar has usually been preceded by dissolution of the replaced feldspar grain fe 2 and mg2 which are needed for the chloritization have partly been derived from the detrital fe mg silicates mainly biotite fe oxide pigment and fe ti oxide grains whereas si4 and al3 are supplied mostly by the altered feldspars the k released from microcline and na from albite were mostly incorporated in authigenic illite and albite respectively

roles of fault structures and regional formations on co2 migration and distribution in shallow saline aquifer in green river utah abstract understanding co2 migration and distribution in fault systems is essential to evaluate long term secure co2 storage and prevent hazardous effects caused by co2 leakage to elucidate the role of the fault system on subsurface co2 migration and leakage processes a two dimensional multi phase transport model was constructed to represent little grand wash lgw and salt wash sw faults where naturally originating co2 is being leaked to the surface according to simulation results buoyant co2 leaked through various pathways including the faults themselves fault offsets and damaged caprock because of both fault systems and caprocks serving as barriers multiple trapped co2 plumes were developed in this region presence of trapped co2 plumes in the subsurface is supported by multiple field observations e g elevated soil co2 fluxes travertines and co2 driven cold water geysers co2 springs adjacent to both lgw and sw faults sensitivity studies were conducted with different permeabilities for faults and caprock various co2 source locations and differing fault parameters e g fault throw and cutoff angle which affected subsurface co2 distribution including size shape and location of trapped co2 plumes finally such trapped co2 plumes have played a key role in the development of co2 driven cold water geysers in these regions

regional hydrogeology of the silurian and ordovician sedimentary rock underlying niagara falls ontario canada abstract due to concern over the potential for widespread groundwater contamination in the sedimentary rock underlying the niagara falls area this study was done to investigate the hydrogeology of the silurian and ordovician stratigraphy underlying the upper niagara river and the eastern niagara peninsula seven boreholes up to 150 m deep were drilled instrumented with multiple packer casing tested for permeability sampled for inorganic and organic solutes and monitored for hydraulic head to provide data for a conceptual model of regional groundwater flow results show that there are at least three distinct groundwater flow regimes in the bedrock the uppermost regime consists of fracture zones in the guelph and lockport formations within which hydraulic conductivity hydraulic head measurements and geochemical analyses indicate active groundwater circulation primarily discharging towards the niagara gorge and escarpment underlying the lockport formation are an overpressured high hydraulic head regime in the clinton upper cataract lower queenston formation and an underpressured low hydraulic head regime in the lower cataract upper queenston formation in both regimes geochemical analyses and permeability measurements indicate very old and saline groundwater which probably has undergone minimal migration since pre pleistocene time the implication based on the study so far is that potential groundwater contamination below the bottom of the lockport formation is probably not significant in the niagara falls area except adjacent to the niagara gorge where vertical permeability in the lower flow regimes may be enhanced

a major late jurassic fluid event at the basin basement unconformity in western france 40ar 39ar and k ar dating fluid chemistry and related geodynamic context the malm lower cretaceous period is often considered to represent a period of time markedly devoid of crustal deformations and of heat and mass transfers in western france although numerous evidences of such transfers are reported in varied basins from western iberia to western africa migrating evaporitic derived brines and seawater interacted with both the sedimentary cover and the basement in the poitou high located at the boundary between aquitaine and paris basins occurring after sedimentation this activity induced significant alterations of the sediments by dolomitization and silicification and the basement by adularization and locally by concentration of f ba pb zn assemblages that are typical of carbonate hosted metal deposits thus metals were leached from metamorphic granitic basement already weathered and peneplaned during triassic time and were redeposited by brines during this main fluid event the main objective of this study was to decipher the timing of this major fluid heat mass transfer on the basis of 40ar 39ar and k ar dating of epigenetic minerals having similar paragenetic and geochemical signatures over an extended area from the atlantic ocean coast to the french massif central a late jurassic fluid stage 146 156 ma significantly younger than what is generally reported in western europe was obtained this fluid event occurred within a segment of a continental margin that was located away from main rift zones and far more than 500 km from major deformation areas such as the main rift or orogenic zones all over western europe these results yield a new concept based on wide wavelength relationships between fluid movements and geodynamic events affecting both the sedimentary basins and their basements

nanoscale chemical zoning of chlorite and implications for low temperature thermometry application to the glarus alps switzerland abstract a combination of adapted chlorite thermometry and high spatial resolution analytical techniques tem edx fib shows that the low grade metamorphic chlorites of the glarus alps central alps switzerland do not record the peak metamorphic conditions as commonly assumed in previous studies of this reference area for low grade metamorphism chlorites have rather recorded several stages of the retrograde path through an intracrystalline nanometric scale compositional zoning the consistency of the nanoscale zoning patterns observed both within sample and along the transect suggests that local equilibrium was achieved at this spatial scale and maintained during growth or re equilibration applying recent thermobarometers we highlight that chlorites recorded a distinct behaviour between the northern and southern part of the studied transect the south of the glarus area displays a regular p t exhumation path from 3 0 0 2 kbar and 310 20 c for maximum p t estimated with conventional thermometers to 0 8 kbar and 220 c according to chlorite crystal core analysis and to 0 3 kbar and 190 c according to chlorite crystal rim analysis on the contrary the north of the glarus area shows an apparent break in the exhumation with p estimates from chlorite crystal rims 1 3 kbar higher than from chlorite crystal cores 0 8 kbar even if the absolute pressure values are fraught with large uncertainty their contrasting core rim pattern is not model dependent we try to correlate these new thermobarometric results with independent data to refine the exhumation scenario of the north helvetic flyschs confirming differential uplift along the transect this reappraisal of glarus chlorite thermometry demonstrates that an adapted thermometry analysis protocol opens new prospects for investigating the evolution of low grade metamorphic terranes

sevier laramide deformation of the continental interior from calcite twinning analysis west central north america abstract paleozoic mesozoic carbonates that cover cratonic western north america contain a regional layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the sevier thrust transport direction e w in carbonates of the idaho wyoming portion of the thrust belt and within carbonates as far as 2000 km into the plate interior the inferred calcite twinning differential stress magnitudes generally decrease across the thrust belt and decrease exponentially away from the orogenic front into the craton synorogenic calcite cements and veins preserve a distinct twinning deformation history in the thrust belt twinning strains commonly record local out of transport piggyback strain events with high differential stresses

p t deformation fe3 fe2 mapping at the thin section scale and comparison with xanes mapping application to a garnet bearing metapelite from the sambagawa metamorphic belt japan quantitative x ray maps of composition from a chlorite k white mica albite quartz and garnet bearing thin section from a sambagawa blueschist facies metapelite were combined with a multiequilibrium calculation method to calculate a p t fe3 fe2 deformation map at the millimetre scale the studied sample was chosen because elongated chlorite crystallization tails pressure shadows rimmed by phengite are present which is an appropriate assemblage for the quantification of the p t evolution chlorite temperature and fe3 content maps were calculated by successive iterations for each pixel analysis of fe3 until convergence of the four chlorite quartz h2o equilibria that can be written using the fe and mg amesite clinchlore daphnite and sudoite chlorite end members the calculated map of fe2 fe3 in chlorite is in good qualitative agreement with the in situ mapping of this ratio using xanes x ray absorption near edge structure techniques the temperature map indicates that high temperature chlorite zones with low fe3 contents alternate with lower temperature zones and higher fe3 contents in the crystallization tail late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite 250 c showing fe3 fetotal up to 0 4 groups of chlorite and mica pixels were then identified based on compositional and structural criteria and a p tdeformation map was calculated using representative analyses of these groups the calculated p tdeformation map suggests that in contrast to chlorite the composition of most mica did not change significantly during exhumation mica reequilibrated in late ew shear bands only ew shearing was already active at 0 1 gpa 500 c which corresponds to the peak temperature and probably pressure conditions at reduced redox conditions the intensity of deformation probably decreased with decrease in temperature to 350 400 c at this temperature a second main deformation event corresponding to a further ew stretching occurred and was still active below 250 c and more oxidizing conditions these results indicate that the scale at which p t data can be obtained is now close to the scale of observation of structural geologists a close link between deformation and mineral reaction is therefore possible at the microscopic scale which provides information about the relationship between deformation and mineral reactivity the modalities of deformation with time and the p t conditions at which it occurred

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

late paleoproterozoic climate tectonics and metamorphism in the southern lake superior region and proto north america evidence from baraboo interval quartzites abstract red supermature quartzites of the baraboo interval of the lake superior region contain detrital zircon that ranges in age from 1782 to 1712 ma deposition clearly occurred after the geon 18 penokean orogeny these late paleoproterozoic sedimentary rocks consist largely of quartz kaolinite or pyrophyllite and hematite detrital feldspar and muscovite are rare or absent their chemical index of alteration ranges from 96 8 to 98 6 among the most chemically mature clastic sediments in the geological record the quartzites are underlain by mature feldspar free paleosols accounting for the absence of feldspar in the overlying sediments and indicating the presence of first cycle quartzose detritus such physical and chemical characteristics imply that late paleoproterozoic deposition in the lake superior region occurred in a stable tectonic setting with subdued topographic relief in a warm humid climate folding and low grade metamorphism of the quartzites is thought to reflect 1630 ma foreland

cratonic sedimentation during the proterozoic an anorogenic connection in wisconsin and the upper midwest certain proterozoic quartzites including baraboo barron and sioux and associated metasedimentary rocks in the upper midwest represent a distinct epicontinental depositional environment new data indicate that argillite chert iron formation micaceous conglomerate and rhyolite are much more abundant and significant in this sequence than previously reported the entire group of rocks was deposited during one period in a series of tectonic episodes magmatism and uplift related to epeirogeny none of these episodes included overt rifting sedimentation began on subsiding continental crust during the waning stages of anorogenic magmatism 1760 m y ago following deposition a regional event 1630 m y ago is interpreted as a time of uplift and mild deformation with no magmatic expression in the region epeirogenic tectonism was culminated in the widespread intrusion of 1500 m y old alkalic plutonism which metamorphosed and further deformed the sedimentary rocks and surroundings

dating multistage paleofluid percolations a k ar and 18o 16o study of fracture illites from altered hercynian plutonites at the basement cover interface poitou high france isotopic age determination of fluid events in basement fractures is of prime importance for the reconstruction of paleo hydraulic regimes however material from fracture or wall rock is not generally easily dated in the case of fractures filled by minerals such as phyllosilicates it can be expected that subsequent fluid circulation may cause partial alteration or recrystallization of the previous phyllosilicates disturbing the isotope signature using a specific methodology including the extraction process without contamination of the finest 0 2 m authigenic fractions identification of clay fractions by x ray diffraction xrd scanning electron microscopy and electron microprobe analysis of stable isotope composition d h 18o and k ar dating fluid event ages can be estimated this methodology was applied to the northwestern margin of the french massif central where basinal brines have interacted with the sedimentary cover infra toarcian formations and the fractured hercynian crystalline basement during periods of unknown age precipitates from brines sealed most basement fractures as carbonates and a recrystallization of early hercynian phengite chlorite into illite and illite smectite minerals occurred in the earlier fractures xrd patterns show that the clay fraction is dominated by illite and ordered mixed layer minerals having a relatively high amount of illite i s r3 in the shallowest samples beneath the paleo surface a late assemblage is identified as a mixture of i s minerals a part of them being characterized by significant amounts of swelling layers i s r0 the d values of the clay fractions are fairly constant around 50 10 but 18o values of the same clay fractions display a wide range from 8 to 18 smow the study of the size fractions between 2 and 0 2 m reveals a correlation between k ar ages and 18o values which cannot be related to a change in the relative amounts of clay minerals deep fractures at depths of 570 and 923 m are characterized by 18o values around 10 3 0 4 smow and old ages ranging from 253 to 272 ma the finest fractions 0 2 m of clays located at shallow level 300 m beneath the paleo surface yield the highest 18o values 15 7 18 2 and the youngest k ar dates 188 to 198 ma these dates are likely to record the extensional episode and thermal anomaly related to the rifting of central atlantic ocean c 190 ma the other important thermal extensional episode which affected the aquitaine basin namely the opening of the gascogne gulf at c 120 ma is not detected by the k ar method on the clay fractions we believe that resetting of clay type k ar ages linked to recrystallization was favoured in the upper part of the basement where the ancient hercynian clay minerals were destabilized by subaerial low temperature alteration during permo triassic times from a methodologic point of view the analytical characterization of several size fractions of clays from reactivated fractures using both stable isotope geochemistry and k ar isotope geochemistry can be considered as a powerful tool for dating fluid events when no suitable material for conventional methods is available

symmagmatic folding of the base of the bergell pluton central alps abstract evidence for magmatic submagmatic and solid state deformation in tonalite granodiorite and country rocks found at the deep seated floor 22 26 km of the bergell pluton demonstrates that final emplacement and crystallization occurred during regional deformation of the pluton and the underlying country rocks after northward emplacement over the country rocks but before complete crystallization the floor of the pluton was folded during simultaneous n s shortening and e w stretching this is evidenced by synmagmatic folds with e w striking nearly vertical axial planes and by regional east plunging stretching lineations in the country rocks which are parallel to the regional scale fold axes and the magmatic mineral lineations in the pluton opposite senses of shear from the well foliated occasionally mylonitic contact suggest that deformation was mostly accomplished by pure shear synmagmatic deformation is related to late stage n s shortening of the alpine orogen and shows that the still partially molten pluton responded to low differential stress very much like the country rocks deformed in the solid state at high temperatures post emplacement tilting associated with backthrusting along the insubric mylonites led to the exposure of the pluton s floor at its present day western margin

fluid expulsion from overpressured basins implications for pb zn mineralisation and dolomitisation of the east midlands platform northern england abstract our understanding of burial diagenesis within carbonates is often limited by poor constraints on available fluid volumes and geochemistry however regional stratigraphic and burial history data are often readily available using these data to develop numerical models which couple sedimentological and hydrological basin evolution we estimate the volumes and geochemistry of fluids that were available to drive dolomitisation and pb ore genesis within the carboniferous derbyshire platform of northern england current conceptual models of these processes invoke tectonic release of burial induced overpressure developed within adjacent dinantian basins as a drive for reactive fluid flow to the platform our simulations show that compaction driven flow may lead to a supply of fluids that is more complex in its temporal evolution than may be expected spatial variations in the rate of fluid expulsion from different sediments lead to a staggered delivery of fluids from different sources rapid fluid expulsion within deeper sediments leads to a downwards decreasing pressure gradient that subsequently draws down fluid from within overlying sediments thus early fluid supply to the platform is sourced from the deep basins while later fluid supply descends from above the platform as well as from the sides we suggest that such a flow development may have important implications for the relative timing and distribution of a sequence of diagenetic products within the platform this hypothesis is tested using volume estimates from our simulations we conclude that although this staggered fluid supply model may be applicable generically it appears that it is only valid for explaining mineralisation in the derbyshire platform fluid volumes supplied to the platform are insufficient to explain dolomitisation our simulations are supported by a sensitivity analysis that identifies that compaction driven flow in this system is strongly controlled by the rate of burial and sediment permeabilities within the compacting basins

midcontinent u s fault and fold zones a legacy of proterozoic intracratonic extensional tectonism the u s continental interior midcontinent contains numerous fault and fold zones seismic and drilling data indicate that some of these zones first formed as proterozoic eocambrian rift faults but the origin of most remains enigmatic we propose that the enigmatic fault and fold zones also began as proterozoic eocambrian normal faults we base our hypothesis on the following 1 enigmatic zones parallel known rifts 2 the structural style of enigmatic zones mirrors the structural style of known rifts 3 the map pattern of some enigmatic zones e g the la salle deformation belt of illinois resembles the map pattern of contemporary rifts and 4 it is easier to rupture an intact craton by normal faulting than by reverse or strike slip faulting these zones along with known rifts represent the legacy of widespread extensional tectonism that brittlely broke up the craton into fault bounded blocks prior to deposition of phanerozoic platform cover once formed midcontinent fault and fold zones remained weak allowing cratonic blocks to jostle relative to one another during the phanerozoic thereby inverting faults and creating transpressional or transtensional structural assemblages localizing seismicity and channeling or releasing ore generating fluids

the autocorrelation function an image processing tool for fabric analysis abstract the use of the autocorrelation function acf is proposed as a method for fabric and or strain analysis its chief purpose is to extend fabric analysis to fine grained materials such as clays micrites etc or to materials where the determination of individual grain shapes is tedious or impossible the main advantage of the method is that no segmentation of the image prior to analysis is necessary in other words any bias associated with identifying grain boundaries and the like is avoided in as much as the recognition of grains or grain boundaries is a matter of scale the acf can be used without loss of generality whenever the grain size is small in comparison with the size of the analysed area the method is computer based it requires a scanner for the digitization of the images and sufficient storage space and computing power for digital image processing the method uses half tone images as input these can be obtained from thin sections acetate peels polished surfaces photographic negatives or positives etc the acf is a two dimensional function that is related to the average shape that occurs in the fabric it is scaled with respect to the original image and allows simultaneous analysis of small and large features the acf enables a a quantitative description of the fabric in general terms b an efficient grain size determination and c if strain is a valid interpretation of the fabric the derivation of the finite strains in the plane of observation in the first part of the paper the acf method is introduced by way of a few numerically generated fabrics and compared to other methods of fabric and strain analysis in the second part an application to three samples of experimentally deformed solnhofen limestone is demonstrated

experimental nucleation and growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces abstract laboratory experiments were performed to investigate high temperature 100 150 c authigenic formation of clay coatings smectites chlorites on clean feldspar and quartz surfaces artificial formation waters were used with mg concentrations in the range of north sea brines experiments were run for 21 50 days in brines composed of magnesium chloride and sodium carbonate adjusted to circum neutral ph the experiments suggest that the silica activity is the main factor determining if grain coating smectites or chlorite form the clay minerals form easily on both clean quartz and feldspar surfaces but chlorite coatings were formed only on feldspar surfaces while smectite coatings were formed on both feldspar and quartz the chlorite morphology varies between honeycomb edge to face and rosette patterns while all smectite formed with honeycomb like textures the clay coatings produced in this study are morphologically similar to naturally occurring diagenetic clay minerals in natural sediments and sedimentary rocks the formation of clay coatings is promoted by pre existing clay drapings on mineral grains the growth substrates used in this study were not coated with such a natural precursor materials this suggests that the nucleation of clay coatings in nature may be possible on clean quartz and feldspar surfaces but pre existing clay minerals may impact the allowed levels of supersaturations and thereby the formation of the grain coating phases

two early proterozoic successions in central wisconsin and their tectonic significance geological mapping of 7 500 km 2 of previously virtually unmapped precambrian rocks in central wisconsin suggests the presence of two early proterozoic sedimentary volcanic plutonic successions evidence for the two successions includes structural stratigraphic metamorphic and lithologic relations that are consistent throughout the area the older succession unconformably overlies archean rocks it is characterized by amphibolite facies metamorphism folding about west to northwest plunging isoclinal f 1 fold axes with more open coaxial f 2 folding basal quartzite and pelitic sediments of the older succession are overlain by volcanic rocks and intruded by syntectonic tonalite granodiorite and trondhjemite plutons the younger succession consists primarily of a greenschist facies calc alkaline basalt rhyolite sequence that unconformably overlies the older amphibolite facies rocks structures in the greenschist sequence trend northeasterly with fold axes that plunge easterly the plutonic rocks are dominantly syntectonic to late tectonic granites and quartz monzonites with only minor tonalite in most areas the two successions have been juxtaposed along major faults the older amphibolitic sequence may correlate with the chocolay group farther north in the lake superior region and the younger green schist rocks with the menominee and or baraga groups if this correlation is correct the deformation metamorphism and intrusion of the green schist succession would have occurred during the penokean orogeny metamorphism to amphibole rank isoclinal folding and tonalite intrusion of the older succession would be a major pre penokean erogenic event within the early proterozoic that may correlate with the post chocolay pre menominee break in northern michigan wisconsin and minnesota we suggest that this earlier event has been unrecognized and that it may be a significant part of the early proterozoic history of the lake superior region

geophysical evidence of cretaceous volcanics in logone birni basin northern cameroon central africa and consequences for the west and central african rift system abstract detailed analyses and interpretation realized by combining existing 2d reflection seismic and gravity magnetic data of the logone birni basin lbb in the west and central african rift system wcas have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin the volcanic activity in the lbb is restricted to the cretaceous period three main volcanic episodes are identified and are associated to the neocomian late albian and cenomanian turonian rifting phases respectively the volcanic bodies within the lower cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the upper cretaceous bodies are morphologically expressed in the forms of dykes and sills the volcanic activity was more intense in the western region of the central lbb zina sub basin along the cameroon nigeria border whereas it was scanty and scattered in the other parts of the basin the main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults although wcas is associated with large amount of crustal extension and minor volcanism the intense volcanic activity observed in lbb during the cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north south and southwest by the lake chad poli and chum triple junctions tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere thus providing conduits for magma migration

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

a new chlorite geothermometer for diagenetic to low grade metamorphic conditions the evolution of chlorite composition with temperature and pressure serves as basis to a number of chlorite chemical thermometers for which the oxidation state of iron has been recognised as a recurrent issue especially at low temperature t a new chlorite geothermometer that does not require prior fe3 knowledge is formulated calibrated on 161 analyses with well constrained t data covering a wide range of geological contexts and tested here for low t chlorites t 350 c and pressures below 4 kbar the new solid solution model used involves six end member components the mg and fe end members of al free chlorite s sudoite and amesite and so accounts for all low t chlorite compositions ideal mixing on site is assumed with an ordered cationic distribution in tetrahedral and octahedral sites applied to chlorite analyses from three distinct low t environments for which independent t data are available gulf coast texas saint martin lesser antilles toyoha hokkaido the new pure fe2 thermometer performs at least as well as the recent models which require an estimate of fe3 content this relief from the ferric iron issue combined with the simple formulation of the semi empirical approach makes the present thermometer a very practical tool well suited for for example the handling of large analytical datasets provided it is used in the calibration range t 350 c p 4 kbar

closed form flow model of a damped slug test in a fractured bedrock borehole summary an existing closed form model is modified to describe the damped response of groundwater in a fractured bedrock borehole with variable apertures and dips to a slug test the existing theory which requires single sized horizontal fractures is accurately calibrated by slug test data from three uncased bedrock boreholes in the dedham granite and an observation well screened just below the contact surface with a till drumlin apertures and dips vary however so the ability of the modified theory to accommodate different sizes and inclinations improves upon the physical validity of its predecessor when fracture information accompanies slug test data geophysical logs identify a large number and dip of fractures in the uncased boreholes in the dedham granite in this regard a lognormally distributed horizontal aperture calibration of the slug tests in the uncased boreholes retains the accuracy of the single size model and yields aperture statistics more consistent with literature values the slug test in the screened observation well is accurately calibrated with the modified horizontal theory for discrete two sizes based upon the average fracture spacing found in the uncased boreholes all four results yield comparable compressibility estimates which depend on fracture spacing but not size or dip the calibrated aperture size and calculated fracture porosity and permeability decrease with length of the borehole into the dedham granite the measured dip and aperture for flowing and nonflowing fractures in one of the boreholes accurately calibrates the modified theory the inclusion of dip reduces the calibrated permeability because of the increased ellipsoidal area at the interface of the borehole and the inclined fractures

strain partitioning in the helvetic thrust belt of eastern switzerland from the leading edge to the internal zone abstract the strain and strain partitioning in the helvetic thrust belt are analysed in order to determine their variation across the belt from the external thin skinned portion to the internal thick skinned portion and in order to begin the detailed mechanical analysis of the entire fold and thrust belt coarse grained limestones of jurassic and cretaceous age were chosen for analysis because they can be traced from the external to the internal zones the strain within a bed is partitioned into three components transgranular pressure solution microfaults veins twinning granular and intragranular exclusive of twinning crystal plasticity associated with recrystallization transgranular strain was very important in the early history of the helvetic nappes giving way to twinning at a later stage under tectonic burial twinning strain was early in the infrahelvetic complex giving way to intracrystalline deformation at the later stage of burial the transition from transgranular to intragranular dominated strain occurred at a vitrinite reflectance value of 3 5 significantly below the greenschist grade of metamorphism in a large scale fold at the leading edge of the helvetic nappes transgranular strains were associated with the buckling stage of the fold and twinning occurred during fold tightening in more internal parts of the helvetic nappes twinning strains are also late with regard to transgranular strains but here their pattern corresponds to the regional finite strain pattern in the internal zone the infrahelvetic complex twinning seems to have been an early event and records an initial layer parallel shortening followed by regional internal deformation with intragranular strains

detrital zircon ages from early proterozoic quartzites wisconsin support rapid weathering and deposition of mature quartz arenites abstract the ages of more than 200 detrital zircons from two proterozoic quartzites the baraboo and hamilton mounds quartzites from central wisconsin have been measured utilizing a laser ablation icpms technique both quartzites contain penokean 1 83 1 89 ga and archean components as well as a younger post penokean component the 69 detrital zircons from hamilton mounds contain major pre penokean age populations at 3 2 2 9 and 2 55 ga penokean ages at 1895 1853 and 1811 ma and a post penokean population at 1772 ma most detrital zircon ages are similar to previously measured ages of locally underlying gneisses and intrusive rocks of the marshfield terrane the 147 detrital zircons from two samples of the baraboo quartzite show a grossly similar age distribution with populations at 1754 and 1850 ma and 2 55 ga however the zircon populations in the baraboo quartzite differ from those of hamilton mounds in having a large population at 1920 ma a broad distribution of ages from 2 0 to 2 4 ga a