proterozoic accretionary tectonics at the southern margin of the archean wyoming craton the proterozoic cheyenne belt at the southern margin of the archean wyoming craton consists of strongly deformed lithotectonic blocks bound by major mylonite zones from north to south and structurally lowest to highest these blocks include 1 archean crystalline basement and associated early proterozoic miogeoclinal rocks 2 an amphibolite grade orthogneiss terrane of unknown age containing probable rift related mafic intrusive rocks 3 a 1750 1790 ma marginal basin succession consisting of upper amphibolite grade pelitic and volcano genie schist associated peraluminous granite and minor ultramafic rocks and 4 a 1750 1790 ma intermediate to mafic plutonic metamorphic complex interpreted as the deep roots of an island arc system the earliest deformation d1 produced a synmetamorphic penetrative horizontal transposition foliation and associated recumbent folds tectonic blocks were juxtaposed along major mylonite zones during d2 macroscopic anil microscopic structures associated with d2 indicate northward thrusting along low angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the wyoming craton at about 1750 ma thrusting occurred at minimum temperatures of 475 c and produced an inverted metamorphic gradient in pelitic rocks north of the belt mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of dextral strike slip d3 between 1750 and 1400 ma cataclasis d4 and brecciation associated with the laramide orogeny d5 locally overprint earlier structures oblique convergence between an island arc and the archean continental nucleus may explain accretion crustal shortening and subsequent strike slip the phanerozoic accretionary events of eastern and western north america provide an analog for this model presence of similar lithologies and shear zones south of the cheyenne belt suggest that the southern margin of the wyoming craton may have been a long lived zone of crustal accretion

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

imaging permeability structure within the highly permeable carbonate earth inverse theory and experiment in most oil reservoir rocks the squirt flow characteristic frequency fsq range of 10 100 khz is much less than the biot characteristic frequency f0 range of 1 mhz 1 ghz in contrast carbonate aquifers have very high permeability 10 3000 d and the two characteristic frequencies are reversed f0 from 10 hz to 10 khz and fsq from 1 mhz to 100 mhz the principal objective of this paper is to develop a pilot inverse method for robust imaging of the high permeability structure within carbonate aquifers an analytical approximate model called the super k model is developed the super k model coincides numerically with the combined biot and squirt flow mechanism model when permeability is higher than 100 md and the frequency is lower than 100 khz the super k regime in the super k regime the pore fluid is always relaxed so that the attenuation due to the biot mechanism is roughly four times larger than that of the biot model also empirical equations are developed that relate the stiffness and rigidit

structural analysis of a ductile brittle precambrian shear zone in the sierra madre wyoming western extension of the cheyenne belt abstract existing plate tectonic models for the cheyenne belt a proterozoic suture at the southern margin of the archean wyoming craton are based largely on data from the medicine bow mountains southern wyoming this paper tests the hypothesis that a major structural and geochronological discontinuity in the sierra madre of southern wyoming is the western extension of the cheyenne belt the sierra madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones the eastern segment records early ductile thrusting ca 1750 ma and later dextral strike slip faulting as revealed by macrostructural microstructural and quartz fabric analysis structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the medicine bow mountains the correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting c slip in synkinematic granites in both ranges isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive the western segment of the sierra madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone kinematic analysis within the cataclastic zone suggests that it formed during north directed thrusting east striking south dipping thrust faults and associated overturned to the north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone this spatial association of north vergent structures and their apparent kinematic compatibility suggests that they are genetically related two major northwest striking cataclastic dextral strike slip in the eastern sierra madre appear to merge with the cataclastic thrust fault these cataclastic faults are interpreted as parts of a north vergent thrust tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north it is not known whether this deformational event occurred in response to continued post suture convergence to the south or if it represents a separate and distinct precambrian event the prominent bend in the sierra madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

estimating permeability of shale gas reservoirs from porosity and rock compositions it is challenges for effectively estimating the permeability of shale gas reservoirs by traditional models however the study on this area is lacked and deficient we proposed a method for predicting permeability of shale gas reservoirs from porosity and rock compositions including mineralogy and organic matter content which is applicable to laboratory data and downhole measurements first two sets including 38 samples from the longmaxi formations were tested for porosity permeability grain density total organic carbon toc content mineralogical compositions and low temperature nitrogen adsorption we used the kozeny s equation to calculate the specific surface area which was viewed as the effective specific surface in shale formations through comparing with the specific surface from low temperature nitrogen adsorption further the effective specific surface was found to be positively correlated with clays pyrite and toc contents while negatively with quartz feldspar and carbonates then an

normal fault growth and fault related folding in a salt influenced rift basin south viking graben offshore norway abstract three dimensional seismic data were analysed to reconstruct the structural and stratigraphical development of a salt influenced rift basin and thus gain an understanding of the relationships between normal fault growth salt tectonics and the evolution of syn rift depocentres the sleipner basin south viking graben northern north sea is ca 30 km long by 8 km wide and is bound to the east by a major extensional fault zone sleipner fault zone two types of fault related fold are identified within the basin 1 a fault parallel monocline interpreted as an extensional forced fold which formed through the upward propagation of the sleipner fault zone through ductile evaporites of the zechstein supergroup and 2 three fault perpendicular salt cored anticlines that compartmentalise the basin into four sub basins and are related to displacement gradients along strike of the sleipner fault zone detailed seismic stratigraphic analysis of pre and syn rift stratal units reveals a complex interplay between fault growth and salt movement which strongly controlled the evolution of syn rift depocentres during the early syn rift a series of depocentres separated along strike by the fault perpendicular folds were offset into the axis of the basin ca 3 4 5 km to the west of the sleipner fault zone by the fault propagation fold later in the rift event the influence of the fault perpendicular folds depleted resulting in a larger interconnected depocentre that shifted into the immediate hangingwall of the fault as the surface of the fault propagation fold was breached the results of this study have implications for normal fault growth and sedimentary depocentre development in salt influenced rift basins and contribute to the general understanding of the controls on salt migration

permeability prediction in chalks the velocity of elastic waves is the primary datum available for acquiring information about subsurface characteristics such as lithology and porosity cheap and quick spatial coverage ease of measurement information of permeability can be achieved if sonic velocity is used for permeability prediction so we have investigated the use of velocity data to predict permeability the compressional velocity from wireline logs and core plugs of the chalk reservoir in the south arne field north sea has been used for this study we compared various methods of permeability prediction from velocities the relationships between permeability and porosity from core data were first examined using kozeny s equation the data were analyzed for any correlations to the specific surface of the grain sg and to the hydraulic property defined as the flow zone indicator fzi these two methods use two different approaches to enhance permeability prediction from kozeny s equation the fzi is based on a concept of a tortuous flow path in a granular bed the sg concept considers the pore space that is exposed to fluid flow and models permeability resulting from effective flow parallel to pressure drop the porosity permeability relationships were replaced by relationships between velocity of elastic waves and permeability using laboratory data and the relationships were then applied to well log data we found that the permeability prediction in chalk and possibly other sediments with large surface areas could be improved significantly using the effective specific surface as the fluid flow concept the fzi unit is appropriate for highly permeable sedimentary rocks such as sandstones and limestones that have small surface areas

early proterozoic metasediments from north central colorado metamorphism provenance and tectonic setting early proterozoic metasediments from the big thompson canyon area in north central colorado increase in metamorphic grade from the biotite zone to the sillimanite zone representing a low pressure facies series that probably developed in response to a steep geotherm over a thermal high high grade rocks are enriched in k 2 o al 2 o 3 fe 2 o 3 mgo tio 2 cr sc cs rb ba and light ree and are depleted in sio 2 cao and zr relative to low grade rocks these differences chiefly reflect differences in the original sandstone shale ratio detrital grains in medium and coarse metasediments are quartz quartzite and feldspar and they suggest a recycled orogen provenance in which granitic rocks and quartzites dominate in the source the abundance of shale in the original succession also attests to the importance of granite in the source major and trace element contents of the rocks constrain the input of mafic to dacitic detritus to less than a few percent ree patterns in all rocks are similar to phanerozoic shales and cratonic sandstones the big thompson canyon metasediments may have been derived from an uplifted collisional orogen in southern wyoming in which unroofed proterozoic granitic rocks and quartzites of the snowy pass supergroup served as the principal sources

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

stratigraphy and depositional setting of the proterozoic snowy pass supergroup southeastern wyoming record of an early proterozoic atlantic type cratonic margin metasedimentary rocks in the medicine bow mountains and sierra madre are divided into four groups the 3 km thick phantom lake metamorphic suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late archean granites the 2 5 km thick deep lake group unconformably overlies the phantom lake suite and late archean granites and contains fluvial sediments including radioactive quartz pebble conglomerates and glaciomarine deposits both successions are intruded by large sills of tholeiitic gabbro the 4 5 km thick lower libby creek group is inferred to be in thrust fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta this succession is intruded by the 2 000 m y old gaps intrusion and comagmatic tholeiitic to weakly alkalic dikes the 3 km thick upper libby creek group is bounded by a thrust fault below and by the cheyenne belt above and contains carbonates and marine slates the early proterozoic deep lake lower libby creek and upper libby creek groups collectively are named the snowy pass supergroup lithologies and stratification sequence in the well preserved medicine bow mountain section suggest transgressive miogeoclinal sedimentation during the early proterozoic paleocurrent data indicate that fluvial then deltaic sedimentation of the deep lake and lower libby creek groups took place on a southwest dipping paleoslope parallel to the inferred south cratonic boundary of the wyoming province this and a few west directed paleocurrents suggest a continental or microcontinental block to the south bounding sedimentation in a northeast elongated basin a rift setting for deposition of these units explains the transgressional character of the sediments the deltaic sedimentation with paleocurrents parallel to the cratonic boundary and the 120 bend in the cheyenne belt between the medicine bow mountains and the sierra madre the upper libby creek group is interpreted to represent open marine conditions following separation of the two continental blocks tholeiitic sills in the deep lake group and tholeiitic to weakly alkalic dikes in the libby creek group are thought to be related to basaltic igneous activity associated with compound early proterozoic rifting between 2 300 and 2 000 m y ago

changes of shear moduli in carbonate rocks implications for gassmann applicability in laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions in particular these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation this change in shear modulus puts gassmann s assumption of a constant shear modulus into question and also explains why velocities predicted with the gassmann equation can be lower or higher than measured velocities

ultrasonic velocities of north sea chalk samples influence of porosity fluid content and texture we have studied 56 unfractured chalk samples of the upper cretaceous tor formation of the dan south arne and gorm fields danish north sea the samples have porosities of between 14 and 45 and calcite content of over 95 the ultrasonic compressional and shear wave velocities vp and vs for dry and water saturated samples were measured at up to 75 bar confining hydrostatic pressure corresponding to effective stress in the reservoir the porosity is the main control of the ultrasonic velocities and therefore of the elastic moduli the elastic moduli are slightly higher for samples from the south arne field than from the dan field for identical porosities this difference may be due to textural differences between the chalk at the two locations because we observe that large grains i e filled microfossils and fossil fragments that occur more frequently in samples from the dan field have a porosity reducing effect and that samples rich in large grains have a relatively low porosity for a given p wave modulus the clay content in the samples is low and is mainly represented by either kaolinite or smectite samples with smectite have a lower p wave modulus than samples with kaolinite at equal porosity we find that ultrasonic vp and vs of dry chalk samples can be satisfactorily estimated with gassmann s relationships from data for water saturated samples a pronounced difference between the vp vs ratios for dry and water saturated chalk samples indicates promising results for seismic amplitude versus offset analyses

an early proterozoic volcanic arc succession in southeastern wyoming the green mountain formation of early proterozoic age in the sierra madre range of southeastern wyoming comprises a bimodal mafic and felsic volcanic assemblage the rocks which are chiefly breccias agglomerates flows and volcaniclastic sediments represent both subaerial and submarine eruptions and in part were redeposited in fluvial and nearshore marine environments volcanic rocks are clearly calc alkaline in character and share a large number of geochemical features in common with continental margin arcs or evolved oceanic arc systems the low mg numbers and ni contents of the basalts require 30 40 olivine fractional crystallization and the high contents of the most incompatible elements high la sm n ratios and low zr nb ratios require an undepleted or enriched mantle source geochemical data are consistent with an origin for the felsic volcanics and associated encampment river granodiorite by shallow fractional crystallization of calc alkaline basalt in a tectonic setting similar to modern ar

growth stabilization and reactivation of proterozoic lithosphere in the southwestern united states growth of proterozoic continental lithosphere in the southwestern united states involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca 1 74 1 70 and 1 65 1 60 ga prograde metamorphism accompanied orogenic assembly and peak metamorphic conditions outlasted deformation regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m y after assembly regions of low metamorphic grade were not uplifted substantially after assembly we suggest that 1 relatively thin lithospheric fragments were assembled into isostatically stable normal thickness continental lithosphere 2 assembly did not erase lithospheric scale heterogeneities 3 the present juxtaposition of different crustal levels reflects differential uplift related to 1 4 1 1 ga tectonomagmatic activity and 4 the boundaries between different lithospheric blocks were repeatedly reactivated from precambrian through tertiary time

tectonic delamination and subcrustal imbrication of the precambrian lithosphere in northwestern canada mapped by lithoprobe seismic reflection profiling of the precambrian lithosphere in northwestern canada has produced images of delamination structures tectonic wedges at crustal scale to about 30 km depth and for the first time at lithospheric scale to about 100 km depth and has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere coupled with observations of similar structures in thrust and fold belts delamination and lower plate imbrication are persistent features at many scales

in suspect terrane provenance of the late archean phantom lake metamorphic suite sierra madre wyoming the 2 68 ga phantom lake metamorphic suite of the sierra madre is a volcanogenic volcaniclastic and siliciclastic sequence that may have been deposited on or near the margin of the wyoming province or alternatively it may represent part of an exotic block accreted onto the southern margin of the wyoming province the metamorphosed supracrustal rocks of the phantom lake metamorphic suite along with quartzofeldspathic gneisses and granitoids of similar age have light rare earth element lree enriched ree patterns with little to no eu anomaly these patterns are comparable to those of modern oceanic arc rocks and sediments both supracrustal and metaigneous rocks have radiogenic initial end from 4 5 to 2 5 and nd crustal residence ages between 2 7 and 3 0 ga it is proposed that these juvenile rocks were part of an intra oceanic arc system formed beyond the influence of detritus from the wyoming province and subsequently were accreted onto the southern wyoming province following intrusion of grani

carbonate rocks matrix permeability estimation carbonate rocks store half of the world s proven oil reserves genesis and postdepositional diagenetic processes define the porous network topology and the matrix permeability this study compiles a database of porosity specific surface mercury porosimetry and permeability values extracted from published sources and complements the database through a focused experimental study specific surface and porosity combine to estimate the pore size dsur permeability versus dsur data cluster along a single trend with a slope of 2 in a log log scale which is in agreement with the kozeny carman model discordant data points correspond to samples with dual porosity or broad pore size distributions with long tails where flow channels along larger interconnected pores indeed the detailed analysis of all the porosimetry data in the database shows that permeability correlates best with the pore size d80 that is the 80th percentile in pore size distributions once again the best fit is a power function in terms of d80 2 analogous to kozeny carman the prediction uncertainty using d80 is one order of magnitude and has the same degree of uncertainty as more complex models and analyses this observation suggests an irreducible uncertainty of one order of magnitude in permeability estimation from index properties such as porosity mercury porosimetry and specific surface probably resulting from specimen preparation effects inherent physical differences in permeation versus invasion and difficulties in data interpretation these estimates of permeability are most valuable when specimens are limited to small sizes such as cuttings

the application of high resolution 3d seismic data to model the distribution of mechanical and hydrogeological properties of a potential host rock for the deep storage of radioactive waste in france abstract in the context of a deep geological repository of high level radioactive wastes the french national radioactive waste management agency andra has conducted an extensive characterization of the callovo oxfordian argillaceous rock and surrounding formations in the eastern paris basin as part of this project an accurate 3d seismic derived geological model is needed the paper shows the procedure used for building the 3d seismic constrained geological model in depth by combining time to depth conversion of seismic horizons consistent seismic velocity model and elastic impedance in time it also shows how the 3d model is used for mechanical and hydrogeological studies the 3d seismic field data example illustrates the potential of the proposed depth conversion procedure for estimating density and velocity distributions which are consistent with the depth conversion of seismic horizons using the bayesian kriging method the geological model shows good agreement with well log data obtained from a reference well located closest to the 3d seismic survey area modeling of the mechanical parameters such as shear modulus young modulus bulk modulus indicates low variability of parameters confirming the homogeneity of the target formation callovo oxfordian claystone 3d modeling of a permeability index ik seis computed from seismic attributes instantaneous frequency envelope elastic impedance and validated at the reference well shows promising potential for supporting hydrogeological simulation and decision making related to safety issues

linked sequence stratigraphic and structural evolution of propagating normal faults two distinct phases in the structural evolution of normal faults can be identified in the miocene gulf of suez rift 1 an initial growth fold stage when the fault is a buried structure and 2 a subsequent surface faulting stage during the growth fold stage strata thin and become truncated toward the fault zone and are rotated and diverge away from the buried fault into growth synclines in contrast once the fault breaks surface strata form a divergent wedge which is rotated and thickens into the fault the two tectono stratigraphic styles also occur contemporaneously along the length of a single fault segment growth folding characterizes deformation around the ends of fault segments where the fault is blind whereas the center of fault segments are characterized by surface faulting these observations suggest that marked along strike variation in stratal surfaces and facies stacking patterns will occur in depositional sequences in areas of normal faulting

kinematic history of the cheyenne belt medicine bow mountains southeastern wyoming the proterozoic cheyenne belt marks the southern boundary of the archean wyoming province within the medicine bow mountains it consists of strongly deformed lithologically distinct blocks bounded by mylonite zones detailed field and petrographic investigations of the boundary augmented by study of microscopic kinematic indicators allow us to refine existing models for tectonic development of the belt northward thrusting under amphibolite facies conditions along low angle mylonite zones emplaced successively deeper crustal blocks over supracrustal rocks of the wyoming province mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of distributive dextral strike slip we suggest oblique convergence between an island arc and the wyoming craton as a possible mechanism for both deformational events accretionary events in eastern and western north america which involved initial convergence followed by strike slip provide a phanerozoic analog for our model

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

geometry of proterozoic sutures in the central rocky mountains from seismic reflection data cheyenne belt and farwell mountain structures 1 seismic reflection data show 40 s dipping reflectors that extend from the cheyenne belt cb archean proterozoic suture to 20 km depths and profound differences in crustal reflectivity across the suture zone which is itself imaged as an interwedging boundary archean crust has a moho reflection at 40 km and abundant upper crustal reflectivity proterozoic crust has no moho reflection and is distinctly less reflective reflective zones south of the cheyenne belt include 40 s and n dipping reflections that extend from the farwell mountain zone to 20 km depths these are interpreted to be a complex cryptic suture zone between the 1 78 1 76 ga green mountain block and 1 75 1 72 ga rawah block

relating p wave attenuation to permeability to relate p wave attenuation to permeability we examine a three dimensional 3 d theoretical model of a cylindrical pore filled with viscous fluid and embedded in an infinite isotropic elastic medium we calculate both attenuation and permeability as functions of the direction of wave propagation attenuation estimates are based on the squirt flow mechanism permeability is calculated using the kozeny carman relation we find that in the case when a plane p wave propagates perpendicular to the pore orientation q 90 1 attenuation is always higher than when a wave propagates parallel to this orientation q 0 1 the ratio of these two attenuation values q 90 1 q 0 1 increases with an increasing pore radius and decreasing frequency and saturation by changing permeability varying the radius of the pore we find that the permeability attenuation relation is characterized by a peak that shifts toward lower permeabilities as frequency decreases therefore the attenuation of a low frequency wave d

estimating permeability of carbonate rocks from porosity and vp vs we present a method for predicting permeability from sonic and density data the method removes the porosity effect on the ratio vp vs of dry rock and it addresses the specific surface as an indirect measure of permeability we look at ultrasonic data porosity and the permeability of 114 carbonate core plugs in doing so we establish an empirical relationship between the specific surface of the solid phase as calculated by kozeny s equation and vp vs linearly transformed to remove the porosity effect one must view the specific surface derived by using kozeny s equation as an effective specific surface because kozeny s equation only holds for homogeneous rock with interconnected pores the ratio vp vs of dry rocks on the other hand seems to be controlled by the true specific surface pointing to an inherent limitation in the method the 114 carbonate plugs originate in three geological settings and comprise 83 calcitic and 31 dolomitic samples their depositional texture varies from mud do

quartz fabric transition in a cooling syntectonic granite hermitage massif france abstract the hermitage granite situated in the northwest massif central france is a syntectonic hercynian leucogranite emplaced along an active transcurrent shear zone during emplacement and cooling the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands c s mylonites increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline a slip and rotation recrystallization migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction we discuss relevant deformation mechanisms and rheological implications for syntectonic plutons in particular we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid state flow

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

porosity permeability relationships in miocene carbonate platforms and slopes seaward of the great barrier reef australia odp leg 194 marion plateau this paper reports a series of 700 porosity permeability analyses and supporting petrographic and sedimentologic descriptions from early to late miocene carbonate strata cored on the marion plateau offshore from north eastern australia during ocean drilling program odp leg 194 the samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform top facies but also include 79 plugs from deeper water slope to hemipelagic drift facies outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones the large degree of short range heterogeneity typical of these strata and the better porosity permeability correlation of dolostones than limestones the platforms have experienced widely varying calcite cementation dolomitization and dissolution but show little clear evidence of meteoric diagenesis suggesting that subaerial exposure may have played little role in porosity permeability evolution permeability for given porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud rich samples dolomitization tends to reduce the variation of permeability for given porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system as a result there are no differences in permeability porosity trends for different dolostone textures whether dominated by intercrystalline vuggy or preserved intergranular pore types two platform top sites separated by only 5 km display a major lateral variation in dolostone porosity permeability characteristics within the youngest dolostone units this difference is interpreted as reflecting a relatively windward current facing setting of the site with the overall higher permeability for given porosity site 1199 that led to less muddy depositional facies greater cementation and lesser grain dissolution pore geometry parameters measured by petrographic image analysis confirm that the windward dolostones have pores that are both larger and less intricate than dolostones comprising the more current protected location

new models for paleoproterozoic orogenesis in the cheyenne belt region evidence from the geology and u pb geochronology of the big creek gneiss southeastern wyoming the disputed age of the deep crust of the colorado province is central to hypotheses for paleoproterozoic crustal growth in the region we studied the high grade big creek gneiss southeastern wyoming as a potential exposure of pre 1780 ma basement rocks new geologic mapping and u pb geochronological data indicate that the big creek gneiss exposes a deeper but coeval level of the green mountain arc relative to the predominantly supracrustal section to the west the big creek gneiss is composed of supracrustal rocks a ca 1780 ma green mountain arc correlative bimodal intrusive suite a ca 1763 ma extensional bimodal intrusive suite and widespread ca 1630 ma pegmatitic leucogranite the mafic member of the younger bimodal suite is documented here for the first time u pb zircon ages from migmatite leucosomes indicate penetrative deformation of the big creek gneiss at ca 1750 ma we find that the postarc intrusive suite is mantle involved implying a second period of crustal growth shortening postdates arc magmatism by 20 m y implying that termination of arc magmatism and accretion were separate events finally criteria previously used to constrain the polarity of subduction for the green mountain arc are not reliable we propose two competing models 1 southward dipping green mountain arc subduction present coordinates with slab breakoff related magmatism following arc accretion or 2 northward dipping subduction with extensional postarc magmatism in both models high temperature deformation coincides with accretion along the cheyenne belt and extensional magmatism is an important component of crustal growth we prefer the northward dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks

constraints on arabian plate motion and extensional history of the red sea analysis of the geologic and geophysical data from the western and northern margin of the arabian plate combined with data from the red sea gulf of aden leads to an interpretation of arabian plate motion that supports an episodic extensional history in the red sea and gulf of aden phase 1 extension began in medial to late eocene and was caused by the nw propagation of the reoriented central indian ridge spreading center it stretched and thinned continental crust along numerous wrench and normal faults that reactivated preexisting structures first phase extension stopped when the arabian plate terminally sutured to eurasia in the medial miocene and halted northward movement of arabia relative to africa the combined african arabian plates moved north more slowly constricting and thickening continental crust in the bitlis zagros sutures while the red sea remained technically quiescent and accumulated salt by the early pliocene continued northward movement was accommodated by lateral extrusion of large continental wedges north of the bitlis zagros sutures this allowed arabia to move northward faster than africa and reopen the red sea where phase 2 extension was expressed by seafloor spreading extension within the red sea gulf of aden rift system has been controlled by the interactions and effects of two factors 1 convergent processes between the north margin of the arabian plate and the eurasian plate to the north and 2 northwest propagation of extension associated with the medial eocene reorganization of the central indian spreading center

evidence for an inverted metamorphic gradient associated with a precambrian suture southern wyoming an inverted metamorphic gradient associated with the northern mylonite zone of the cheyenne belt a deeply eroded precambrian suture in southern wyoming has been documented within metasedimentary rocks of the early proterozoic snowy pass supergroup metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite garnet and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone a minimum temperature increase of approximately 100 c over a km wide zone is required for this transition parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the snowy pass supergroup field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite grade rocks over rocks of the snowy pass supergroup along the northern mylonite zone mineral equilibria and garnet biotite geothermometry on synkinematic mineral assemblages within the snowy pass metasedimentary rocks indicate deformation at minimum temperatures of 480 c and pressures of 350 400 mpa 3 5 4 0 kbar this implies tectonic burial or upper plate thickness of 13 15 km the narrow character of metamorphic zonation and microtextures within the snowy pass supergroup which indicate late synkine matic growth of garnet and staurolite preclude rotation of pre existing isograds by folding as a mechanism for development of the inverted gradient conductive transport of heat from the upper into the lower plate across the originally low angle thrust is insufficient to produce the necessary temperatures in the lower plate shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated up dip advection of metamorphic fluids is a feasible but unproven mechanism for heat transport the possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present

the moho of north america a brief review focused on recent studies abstract recent national programs in canada lithoprobe and the u s earthscope are providing vast quantities of data and many new scientific insights concerning crustal and lithospheric mantle structure and evolution more modest but significant results are also being produced in and about mexico this paper primarily focuses on results published since 2005 and presents an updated moho map of north america this overview of crustal structure does reveal a number of interesting observations for example two buried distinct crustal blocks have been discovered in the past 20 years one was caught in a precambrian suture zone in canada and the other is buried under the gulf coast region and is still of unknown origin the precambrian assembly of the continent has been documented by the mapping of numerous suture zones and shallow portions of subducted slabs have been preserved in several cases not surprisingly the internal structure of these suture zones is complex and highly variable isostatic adjustment has been shown to be a complex process involving the lithospheric mantle not just crustal structure variations specifically the correlation of crustal thickness with elevation is only approximate at best several crustal blocks that have been stable over very long periods of time possess unusually thick crust whose lower portion is usually fast vp 7 0 km s rifting has been shown to affect the crust in a variety of ways ranging from the massive magmatic modification that produced the mid continent rift system to the broad region of extension in the basin and range province crustal structure along the active modern western margin of north america is highly complex as would be expected as a result of transitions between subduction and transform faulting and variations in volcanism and intraplate magmatism we clearly still have a lot to learn about the structure and evolution of this continent

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

style and sequence of deformation during extensional fault propagation folding examples from the hammam faraun and el qaa fault blocks suez rift egypt kilometre scale fault parallel folds are identified adjacent to normal faults in the oligo miocene suez rift egypt and are interpreted to have formed in response to fault propagation folding above upward propagating blind faults the geometry scale and distribution of secondary structures within the folds and their cross cutting relationships with the master faults allow the style and sequence of deformation during fault propagation folding to be established and suggest that during the initial stages of folding the proto footwall underwent extension which was accommodated by layer parallel slip in encasing mudstone horizons and linked normal faulting and block rotation in carbonate and sandstone units the proto hanging wall also contains dominantly extensional normal faults although locally where the master fault had a convex into the footwall map view trace reverse faulting and fracturing occurred secondary structures adjacent to the master fault were not all active simultaneously but initiated and died at different stages during the evolution of the fault propagation fold the results of this study confirm many key predictions of numerical and physical analogue models but also highlight several important controls on the evolution of fault propagation folds in extensional settings which existing models cannot capture such as the influence of the map view trace of the propagating fault and lateral variations in cover stratigraphy lithology and strength on the style and magnitude of secondary deformation

gravity driven crustal shortening in failed rifts it is proposed that gravity driven crustal shortening caused by the potential energy of marginal uplifts adjacent to thinned crust in a rift zone occurred immediately after rifting ceased in both the gulf of suez and north sea rifts compressional stresses of the order of 1 2 mpa would have been generated which were relieved by finite contractional strains of the order of 2 5 shortening of this magnitude has been observed as post rift contractional structures in the western sinai gulf of suez and the viking graben north sea these observations are compatible with a physical model for stresses generated in the crust due to lateral variations in crustal thickness this model helps to explain some apparently anomalous structures within rifts such as tight hanging wall folds some aspects of fault block rotation footwall uplift and tilting and also has important implications for the formation of unconformities and the redistribution of sediments in rifts the phenomenon of gravity driven crustal shortening in failed rifts may have occurred in other major intra cratonic rifts

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

quantification of pore structure and its effect on sonic velocity and permeability in carbonates carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude traditional pore type classifications describe these pore structures but are inadequate for correlations to the rock s physical properties we introduce a digital image analysis dia method that produces quantitative pore space parameters which can be linked to physical properties in carbonates in particular sonic velocity and permeability the dia parameters derived from thin sections capture two dimensional pore size domsize roundness aspect ratio ar and pore network complexity poa comparing these dia parameters to porosity permeability and p wave velocity shows that in addition to porosity the combined effect of microporosity the pore network complexity and pore size of the macropores is most influential for the acoustic behavior combining these parameters with porosity improves the coefficient of determination r2 velocity estimates from 0 542 to 0 840 the analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small complicated pores estimates of permeability from porosity alone are very ineffective r2 0 143 but can be improved when pore geometry information poa r2 0 415 and domsize r2 0 383 are incorporated furthermore results from the correlation of dia parameters to acoustic data reveal that 1 intergrain and or intercrystalline and separate vug porosity cannot always be separated using sonic logs 2 p wave velocity is not solely controlled by the percentage of spherical porosity and 3 quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

permeability and acoustic velocity controlling factors determined from x ray tomography images of carbonate rocks carbonate reservoir rocks exhibit a great variability in texture that directly impacts petrophysical parameters many exhibit bi and multimodal pore networks with pores ranging from less than 1 m to several millimeters in diameter furthermore many pore systems are too large to be captured by routine core analysis and well logs average total porosity over different volumes consequently prediction of carbonate properties from seismic data and log interpretation is still a challenge in particular amplitude versus offset classification systems developed for clastic rocks which are dominated by connected intergranular unimodal pore networks are not applicable to carbonate rocks pore geometrical parameters derived from digital image analysis dia of thin sections were recently used to improve the coefficient of determination of velocity and permeability versus porosity although this substantially improved the coefficient of determination no spatial information of the pore space was considered because dia parameters were obtained from two dimensional analyses here we propose a methodology to link local and global pore space parameters obtained from three dimensional 3 d images to experimental physical properties of carbonate rocks to improve p wave velocity and permeability predictions results show that applying a combination of porosity microporosity and 3 d geometrical parameters to p wave velocity significantly improves the adjusted coefficient of determination from 0 490 to 0 962 a substantial improvement is also observed in permeability prediction from 0 668 to 0 948 both results can be interpreted to reflect a pore geometrical control and pore size control on p wave velocity and permeability

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations