the critical slip distance for seismic faulting experimentally based friction laws1 2 have been found to predict virtually the entire range of observed behaviour of natural faults3 4 these laws contain a critical slip distance l which plays a key role in determining the degree of fault instability the size of the zone of earthquake nucleation the frictional breakdown width and the proportion of pre and post seismic slip to co seismic slip in laboratory measurements l is found to be about 10 5m but modelling results show that it must be about 10 2m if natural earthquake behaviour is to be simulated the discovery that fault surfaces are fractal over the scale range 10 5 105 refs 5 6 even for faults with large net slip has confused the problem of scaling this parameter from laboratory experiments to natural faults because fractal surfaces have no characteristic length here i show that geometrically unmated fractal surfaces when in contact under a normal load develop a characteristic length in their contact because long wavelength apertures close under load whereas short wavelength apertures may remain open this critical distance may be identified with l and calculations based on fault topography data show that at seismogenic depths it will be in the range anticipated from the earthquake modelling studies

tectonometamorphic evolution of an intracontinental orogeny inferred from p t t d paths of the metapelites from the rehamna massif morocco new petrographic and microstructural observations mineral equilibria modelling and u pb monazite geochronological studies were carried out to investigate relationship between deformation and metamorphism across the rehamna massif moroccan variscan belt in this area typical barrovian muscovite to staurolite zones developed in cambrian to carboniferous metasedimentary rocks that are distributed around a dome like structure first assemblages are characterized by the presence of locally preserved andalusite followed by prograde evolution culminating at 6 kbar and 620 c in the structurally deepest staurolite zone rocks this barrovian sequence was subsequently uplifted to supracrustal levels heterogeneously reworked at greenschist facies conditions which was followed locally by static growth of andalusite indicating heating to 2 5 4 kbar and 530 570 c the 206pb 238u monazite age of 298 3 4 1 ma is interpreted as minimum age of peak metamorphic conditions whereas the ages of 275 8 1 7 ma and 277 0 1 1 ma date decompression and heating at low pressure in agreement with previous dating of permian granitoids intruding the rehamna massif the prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust s1 horizontal schistosity the horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing n s compression the re heating of the massif follows late massive e w shortening refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of permian granitoids we argue that metamorphic evolution of the rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental gondwanan plate the tectonometamorphic history of the rehamna massif is put into palaeozoic plate tectonic perspective and late carboniferous reactivation of devonian early carboniferous basins formed during stretching of the north gondwana margin and formation of the palaeotethys ocean the inherited heat budget of these magma rich basins plays a role in the preferential location of this intracontinental orogen it is shown that rapid transition from lithospheric stretching to compression is characterized by specific ht type of barrovian metamorphism which markedly differs from similar barrovian sequences along palaeozoic plate boundaries reported from variscan europe this article is protected by copyright all rights reserved

tectonic feedback and the earthquake cycle the occurrence of cyclical instabilities along plate boundaries at regular intervals suggests that the process of earthquake causation differs in some respects from the model of elastic rebound in its simplest forms the model of tectonic feedback modifies the concept of this original model in that it provides a physical interaction between the loading rate and the state of strain on the fault two examples are developed a central chile and b mexico the predictions of earthquake hazards for both types of models are compared

veins in the lockport dolostone evidence for an acadian fluid circulation system the orientation and spatial distribution of veins in the lockport dolostone attest to a fluid circulation system active during the acadian orogeny in western new york and southern ontario outcrops east of the clarendon linden fault zone are cut by a prominent east northeast systematic calcite filled vein set whereas these systematic veins are absent west of the fault zone except in two quarries systematic veins display distinct characteristics the mean vein orientation rotates clockwise from 067 in the east to 086 farther west veins do not propagate into the basal lockport group and calcite vein 18 o pdb values are significantly lighter to the east of the clarendon linden fault zone maximum horizontal stress s h trend lines drawn parallel to the strike of the systematic veins are incompatible with s h inferred from alleghanian plateau and other post paleozoic structures however because east west s h trend lines are compatible with an acadian tectonic event in western new england our interpretation is that systematic veins in the lockport group are a cratonward signature of the acadian orogeny

microfabric and microshear evolution in deformed till abstract the bed deformation model of glacier movement requires that ice masses shear their beds to high strains for homogeneous basal tills that can result from such deformation however there are no sedimentological criteria for estimating strain magnitude thus a large ring shear device was used to shear two basal tills and their particle fabrics and microshears were studied at various strains up to 108 sand particle fabrics in a longitudinal flow plane became steady at shear strains of 7 39 steady fabrics were moderately strong with s1 eigenvalues of 0 71 0 74 these values are smaller than those from experiments with pebble sized particles perhaps reflecting the greater probability of sand particles colliding with larger grains fabrics parallel to flow strengthened progressively indicating that particles did not reorient with their long axes transverse to flow r1 and r2 microshears developed and persisted to the highest strains attained r1 shears became more pervasive with strain and oriented closer to the shear plane movement along both sets of shears provides an explanation for the plunge of the steady state sand fabric approximately 10 upglacier these experiments provide a basis for estimating till deformation magnitude but results will require cautious application to the geologic record where deformation processes may be ambiguous

origin of minerals in joint and cleat systems of the pottsville formation black warrior basin alabama implications for coalbed methane generation and production coalbed methane is produced from naturally fractured strata in the lower pennsylvanian pottsville formation in the eastern part of the black warrior basin alabama major fracture systems include orthogonal fractures which consist of systematic joints in siliciclastic strata and face cleats in coal that strike northeast throughout the basin calcite and minor amounts of pyrite commonly fill joints in sandstone and shale and less commonly cleats in coal joint fill calcite postdates most pyrite and is a weakly ferroan coarse crystalline variety that formed during a period of uplift and erosion late in the burial history pyrite forms fine to coarse euhedral crystals that line joint walls or are complexly intergrown with calcite stable isotope data reveal large variations in the carbon isotope composition of joint and cleat fill calcite 10 3 to 24 3 peedee belemnite pdb but only a relatively narrow range in the oxygen isotope composition of this calcite 16 2 to 4 1 pdb negative carbon values can be attributed to 13c depleted co2 derived from the oxidation of organic matter and moderately to highly positive carbon values can be attributed to bacterial methanogenesis assuming crystallization temperatures of 2050c most joint and cleat fill calcite precipitated from fluids with 18o ratios ranging from about 11 to 2 standard mean ocean water smow uplift and unroofing since the mesozoic led to meteoric recharge of pottsville strata and development of freshwater plumes that were fed by meteoric recharge along the structurally upturned southeastern margin of the basin influxes of fresh water into the basin via faults and coalbeds facilitated late stage bacterial methanogenesis which accounts for the high gas content in coal and the carbonate cementation of joints and cleats diagenetic and epigenetic minerals can affect the transmissivity and storage capacity of joints and cleats and they appear to contribute significantly to interwell heterogeneity in the pottsville formation in highly productive coalbed methane fields joint and cleat fill calcite have strongly positive 13c values whereas calcite fill has lower 13c values in fields that are shut in or abandoned petrographic analysis and stable isotope geochemistry of joint and cleat fill cements provide insight into coalbed methane reservoir quality and the nature and extent of reservoir compartmentalization which are important factors governing methane production

occurrence of minerals within fractures and matrix of selected bowen and ruhr basin coals abstract late permian age samples of black coal from the coeval bandanna formation and baralaba coal measures of queensland s bowen basin as well as carboniferous coal from germany s ruhr basin were investigated this paper presents scanning electron microscope sem analysis of the mode of occurrence of minerals within these coals and characterisation of the natural fractures present within the samples the coal samples located proximal to orogenic fronts in both basins are structurally complex with multiple generations of mineralisation evident stable isotope analyses of carbonates from selected samples indicate that methanogenesis related co2 reduction has occurred at two of the bowen basin sampling locations ankerite precipitated from a magmatic fluid or basinal brine is associated with significant sulphide mineralisation within brecciated ruhr coal proximal to major fault intersections with anticlines and has been meteorically altered to ferroan calcite and sulphates in coal from immediately below the carboniferous cretaceous unconformity similar alteration has occurred within a bowen basin coal sample close to the present day land surface variation of mineralisation within both the cleats and the coal matrix occurs on a fine scale with crystals of multiple mineral species often being present within micrometres of each other

jointing within the outer arc of a forebulge at the onset of the alleghanian orogeny the oldest joint set in devonian rocks of western new york state has an atypical ns strike and predates regionally more abundant nw striking and ene striking joints driven by hydrocarbon related fluid decompression the ns joints originated in higher modulus diagenetic carbonate and were driven initially by a different mechanism either joint normal stretching and or thermoelastic contraction the origin of these joints in higher modulus carbonate concretions indicates the presence of a tensile stress produced by uniform regional extensional strain upper devonian shale hosting the ns joints crops out in that area of the appalachian basin where a morrowan erosional unconformity marks the region of maximum upward lithospheric flexure of an early alleghanian forebulge the ns strike of these early joints points to a forebulge stretching axis oriented approximately east west and associated in time and space with crustal loading that drove both the northfieldian orogeny and the underplating of the bronson hill anticlinorium in new england ultimately subsidence of the morrowan forebulge buried the upper devonian shale succession to the oil window during the latter part of the alleghanian tectonic cycle resulting in the propagation of fluid driven nw and ene trending joints in black shale

the last glacial maximum in the north sea basin micromorphological evidence of extensive glaciation despite a long history of investigation critical issues regarding the last glacial cycle in northwest europe remain unresolved one of these refers to the extent timing and dynamics of late devensian weichselian glaciation of the north sea basin and whether the british and scandinavian ice sheets were confluent at any time during this period this has been the result of the lack of the detailed sedimentological data required to reconstruct processes and environment of sediments recovered through coring this study presents the results of seismic sedimentological and micromorphological evidence used to reconstruct the depositional processes of regionally extensive seismic units across the north sea basin thin section micromorphology is used here to provide an effective means of discriminating between subglacial and glacimarine sediments from cored samples and deriving process based interpretations from sediment cores on the basis of micromorphology critical formations from the basin have been reinterpreted with consequent stratigraphic implications within the current stratigraphic understanding of the north sea basin a complex reconstruction is suggested with a minimum of three major glacial episodes inferred on at least two occasions during the weichselian devensian the british and scandinavian ice sheets were confluent in the central north sea whilst micromorphology can provide much greater confidence in the interpretation of late quaternary offshore stratigraphic sequences it is noted that a much better geochronology is required to resolve key stratigraphic issues between the onshore and offshore stratigraphic records copyright 2006 john wiley sons ltd

joint sets that enhance production from middle and upper devonian gas shales of the appalachian basin the marine middle and upper devonian section of the appalachian basin includes several black shale units that carry two regional joint sets j1 and j2 sets as observed in outcrop core and borehole images these joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter when present together earlier j1 joints are crosscut by later j2 joints in outcrops of black shale on the foreland northwest side of the appalachian basin the east northeast trending j1 set is more closely spaced than the northwest striking j2 set however j2 joints are far more pervasive throughout the exposed devonian marine clastic section on both sides of the basin by geological coincidence the j1 set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field shmax because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east northeast fracture stimulation from vertical wells intersects and drains j2 joints horizontal drilling and subsequent stimulation benefit from both joint sets by drilling in the north northwest south southeast directions horizontal wells cross and drain j1 joints whenever present then staged hydraulic fracture stimulations if necessary run east northeast i e parallel to the j1 set under the influence of the contemporary tectonic stress field thereby crosscutting and draining j2 joints

kinematic history of the narragansett basin massachusetts and rhode island constraints on late paleozoic plate reconstructions the formation of the narragansett basin of rhode island and massachusetts and the polyphase deformation geometry of the pennsylvanian age basin sediments can be explained by collision and strike slip interactions between an avalonian microcontinent the european north american plate and the african plate during the late paleozoic sinistral movement between an avalonian microcontinent and the european north american plate resulted in formation of pull apart basins subsequently africa moved toward north america along an east west trending dextral megashear zone and collided with the eastern margin of southern north america the narragansett basin is located on the avalonian microcontinent near the suture with the north american continent and in the transition zone between the megashear and the collision tectonic regimes the basin shape stratigraphy and sedimentary relationships can be explained by the coalescence of two composite grabens which formed at different times the thicker northern graben is only affected by one deformation whereas the southern graben shows evidence of four distinct phases of postrifting deformation the first two coaxial folding deformations of the basin sediments are attributed to the closure of the basin complex during collision of africa and north america the third and fourth deformations are interpreted as sinistral r riedal and dextral shears respectively associated with the east west trending megashear system a prograde barrovian metamorphic front started in the western portion of the southern graben slightly after the first deformation and migrated northward and eastward where it is synchronous with the second deformation

alleghanian tectono thermal evolution of the dextral transcurrent hylas zone virginia piedmont u s a abstract the hylas zone is the northernmost segment of the eastern piedmont fault system and lies in the grenville goochland terrane virginia like the other major segments the nutbush creek zone the hollister zone and the modoc irmo zone the hylas zone experienced late paleozoic dextral transcurrent shearing early deformation in the zone was ductile and produced type i and type ii s c mylonites under amphibolite grade metamorphic conditions later deformation was at the brittle ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons the feldspar exhibits a strain dependent sequence of microstructures including kink bands antithetic extensional microfaults pull apart bends in the microfaults by development of transverse fractures and microboudinage correlation of mineral mechanical response with a thermal decay curve based on isotopic mineral ages yields a temporally constrained deformation history ductile dextral shearing occurred subsequent to the intrusion of the 330 ma petersburg granite and passed into the brittle ductile transition by approximately 260 ma dextral faulting terminated before 240 ma the appalachian dextral transcurrent faulting event therefore continued through permian in some areas

repeated advance and retreat of the east antarctic ice sheet on the continental shelf during the early pliocene warm period abstract diatom analysis of a sediment core recovered at iodp site u1358 on the continental shelf off the adelie coast indicated that the lower section of the core contained an assemblage dating back to the thalassiosira innura zone of the lower pliocene that ranges from 4 2 to 5 12 ma based on lithological descriptions at both a macro and micro scale of this early pliocene part of the core four facies were interpreted from the diamictons representing the progressive advance and retreat of the grounding line over the site facies 1a and 1b contain a distinct directional signal from the orientation of the a axis of clasts with several phases of fabric development along with both brittle and ductile deformation features that are common in sediments that have been subglacially deformed facies 1c and 1d are finely laminated and were deposited in open marine conditions the four facies within the depositional model provide for the first time direct evidence for ice advancing across the shelf adjacent to the wilkes subglacial basin on at least four occasions separated by three periods of open marine conditions indicating retreat of grounded ice inland of the site during a warmer than present early pliocene the times of open marine conditions are correlated with previous findings from the neighbouring rise sites that also indicated an oscillating ice margin this has significant implications because firstly it suggests a dynamic east antarctic ice sheet eais that is probably far more sensitive to climatic and oceanic forcing even during relatively short time periods than had previously been thought secondly it suggests that proxies used to interpret the advance and retreat of the grounding line from the rise can be linked with direct evidence of grounding line migration from the shelf it also has important implications for the future behaviour and sensitivity of the eais under present continuing warming conditions together with results from the rise this paper provides a crucial ice extent target for a new ice sheet model of this region during the pliocene

linkages between east antarctic ice sheet extent and southern ocean temperatures based on a pliocene high resolution record of ice rafted debris off prydz bay east antarctica 1 ice rafted debris mass accumulation rates ird mar at a drill site on the antarctic continental margin are investigated to evaluate the linkages between east antarctic ice sheet extent and southern ocean temperatures in the early to mid pliocene odp site 1165 is within 400 km of the antarctic coastline and in the direct pathway of icebergs released by the amery ice shelf the amery ice shelf is the largest ice shelf in east antarctica and it buttresses the lambert glacier drainage system which accounts for 14 of the outflow from the east antarctic ice sheet ird mar were low during peak southern ocean warming in the early pliocene after a brief precursor a tenfold increase in ird mar at 3 3 ma marks the termination of the early pliocene ice sheet minimum coincident with the m2 glacial for the mid pliocene a strong correlation exists between the high amplitude signal in the lr04 benthic stack and ird mar suggesting linkages between east antarctic ice extent global ice volume and deep water temperatures the ird record at site 1165 provides evidence of greater sensitivity of the lambert glacier amery ice shelf system to southern ocean warming than is currently predicted by ice sheet models which may relate to uncertainties in the understanding of ocean heat uptake poleward heat transport and ice sheet ocean interactions

mineral occurrence and its impact on fracture generation in selected qinshui basin coals an experimental perspective abstract the effect of mineral matter on the fracture evolution was investigated for lower permian and upper pennsylvanian coal samples from the qinshui basin of china optical microscopy scanning electron microscopy with energy dispersive x ray spectrometry sem eds and x ray computed tomography x ray ct were used to investigate the composition and modes of occurrence of minerals within the coals and to characterize the natural and induced fractures present within the samples the spatial distribution of mineral matter investigated by x ray ct demonstrates that the variation of mineralization within the cleats fractures occurred at fine scales and most of minerals were scattered in coal core samples the original fractures normally were present as two sets one set was almost parallel to the bedding plane and the other was almost perpendicular to the bedding plane for created fractures the difference in density between the matrix and mineral matter may weaken the cohesion of coals and it could explain why the created fractures propagate along the junction of minerals and organic coal matrix the internal fracture porosities varied from 0 06 to 20 69 which indicates a strong internal heterogeneity of the coals after a stress is applied the inclination of the main shear fractures ranges from 55 to 95 using energy analysis the fracture energy appears to represent nearly half of the total dissipated energy which can be correlated with the fracture density and orientation the formation and distribution of shear fractures is related to the spatial distribution of mineral matter and the overall mineral content these investigations serve to characterize hydro fracturing and pulverized coal generation during coalbed methane development

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

seismic velocity modelling of the carboneras fault zone se spain abstract the carboneras fault zone forms part of a major strike slip fault system in se spain striking ne sw and accommodating up to 40 km displacement it affects basement metamorphic rocks and unconformably overlying upper miocene sediments and volcanic rocks high resolution shallow seismic tomographic sections were made across the fault zone in two localities from the same areas fault rocks and their wallrocks were collected for laboratory seismic velocity measurements the laboratory data were corrected for the substantial effects of near surface crack damage by combining these results with geological cross sections forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ground truth the inferences made from them these velocity depth relationships matched moderately well with those extracted from the in situ tomography results aspects of the in situ seismic sections matched features on the forward modelled sections but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool

is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of north america on the appalachian plateau of new york ene striking joints parallel the mid continent contemporary stress field and do not correlate with other structures joints within ohio indiana michigan and west virginia also parallel the mid continent maximum horizontal compression as indicated by hydraulic fracture measurements in northern indiana where the contemporary stress field deviates significantly from the orientation of the mid continent stress the attitude of the joints deviates accordingly in addition to joints structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid continent stress field from this i conclude that these joints are mode i cracks whose orientation corresponds to the contemporary tectonic stress field in itself the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds

micromorphological criteria for discriminating subglacial and glacimarine sediments evidence from a contemporary tidewater glacier spitsbergen abstract sedimentological and micromorphological evidence is presented from four sites within st jonsfjorden western spitsbergen sedimentological and geomorphological evidence is used to distinguish sites comprising undisturbed glacimarine sediments from glacially over ridden glacimarine sediments and subglacial diamicts impregnated thin sections are used to define associations of micromorphological characteristics that allow diamicts formed under these different conditions to be discriminated in situ glacimarine deposits display winnowed textures dropstone structures and distinct vertical orientations of sand grains whilst subglacial conditions result in structures indicative of deformation under moderate to high stress regimes these criteria may help to determine the origin of ancient and quaternary diamicts of questionable subglacial or glacimarine origin

acadian dextral transpression and synorogenic sedimentary successions in the appalachians the successive seboomook littleton northern appalachians and catskill pocono central appalachians clastic wedges suggest oblique convergence and southwestward migration of acadian orogeny beginning in early devonian and continuing into early mississippian wrench fault movement in maritime canada coincided with deposition of all but the earliest components of the catskill pocono clastic wedge and continued into the pennsylvanian contrasts between a thin lower to middle devonian shallow shelf facies in the alabama appalachian fold thrust belt and a time equivalent thick shallowing upward sedimentary to volcanic succession in the adjacent talladega slate belt are interpreted to reflect a wrench fault basin a wrench fault setting for devonian rocks in alabama integrated with manifestations of oblique convergence during the acadian orogeny in the central and northern appalachians can be accommodated in dextral transpression along the entire length of the acadian appalachian orogen

forecast model for large and great earthquakes in southern california an earthquake instability model is used to compute theoretical fault slip and ground surface deformation before possible future earthquakes on the san andreas fault in southern california the quasistatic model represents the earth s crust by an elastic half space and the san andreas imperial cerro prieto and san jacinto faults by flat rectangular surfaces whose strikes approximate the actual local strikes the seismogenic zone of the san andreas fault is represented by a brittle strain softening patch extending from the ground surface to 11 km depth and from parkfield to the salton sea 500 km the strength of the patch varies along strike and unstable failures of parts of the patch are earthquake analogs failed areas heal immediately after instability for the model to reproduce the lengths slips and times of earthquakes since 1080 a d reported by k sieh and others the patch must be divided into five sections along strike the sections have alternately low and high strength with the two strongest sections being roughly coincident with the major bends of the san andreas fault ten to twenty years before unstable slippage of patch areas portions of the patch bottom begin to fail stably aseismically the stable failure causes accelerating fault slip at depth and at the fault trace as well as anomalous changes in lengthening rates of trilateration lines in the model simulation the next two sections to fail unstably in the future are just north of the salton sea and just south of parkfield rupture lengths are about 100 km and 40 km at favorable locations precursory anomalies of fault slip and trilateration line length become large enough to be detected in field data a few years before each earthquake

quasi static modelling of stress histories during the earthquake cycle precursory seismic and aseismic stress release summary a functional form describing non linearity in the stress history during the quasistatic phase of anelasticity in the final stages of semi brittle failure is presented semi brittle failure is known to occur in laboratory rock samples and the existence of earthquake precursors indicates that the earth s upper crust also exhibits an anelastic response in the final stages of the buildup to dynamic failure the functional form of the stress history depends on the amount of localized aseism stress relaxation on a fault nucleation patch and the amount of relatively more distributed seismic damage around the fault measured by a catalogue of earthquakes or acoustic emissions aseismic stress relaxation on the nucleation patch is modelled assuming slip occurs behind a crack front which is growing due to subcritical crack growth by the mechanism of stress corrosion under these conditions it is impossible to model correctly the observed duration of intermediate term earthquake precursors given realistic estimates of the size of the nucleation patch 0 1 1 km and the stress corrosion index n where 10 n 50 therefore intermediate term earthquake precursors must result from more distributed seismic or aseismic damage coupled with fluid transport in a volume much larger than that of the nucleation patch in terms of seismic moment or seismic energy release the precursory seismic damage in the quasi static anelastic phase is dominated by the largest magnitudes for the usual case of b 2c 3 the smaller events dominate the remote stress drop in terms of the inferred power law exponent d 3b c of the fault length distribution d 2 corresponds to distributed damage and d 2 and strain softening and shear localization occurs when d 2

quantitative prediction of fractures using the finite element method a case study of the lower silurian longmaxi formation in northern guizhou south china abstract natural fractures have long been considered important factors in the production of gas from shale reservoirs because they can connect pore spaces and enlarge transport channels thereby influencing the migration accumulation and preservation of shale gas industrial level shale gas production has been initiated in the lower silurian longmaxi formation in northern guizhou south china however it is important to quantitatively predict the distribution of natural fractures in the lower silurian shale reservoirs to locate additional sweet spots in northern guizhou in this study data obtained from outcrops cores thin sections field emission scanning electron microscope fe sem images and x ray diffraction xrd were used to determine the developmental characteristics and controlling factors of these fractures correlation analysis indicated that the mechanical parameters of the longmaxi shale are mainly related to the total organic carbon toc quartz clay calcite and dolomite contents the spatial variations in the mechanical parameters of the longmaxi shale were determined based on the spatial variations in the toc and mineral contents then a heterogeneous geomechanical model of the study area was established based on interpretations of the fault systems derived from seismic data and acoustic emission ae experiments performed on samples of the relevant rocks the paleotectonic stress fields during the yanshanian period were obtained using the finite element method fem finally a fracture density calculation model was established to analyze the quantitative development of fractures and the effects of faults and mechanical parameters on the development of fractures were determined the results suggest that the main developmental period of tectonic fractures in the longmaxi formation was the early yanshanian period during this time the horizontal principal stress conditions were dominated by a se nw trending 135 315 compressional stress field and the longmaxi formation experienced a maximum tectonic stress of 110 120 mpa this simulated paleotectonic stress field was mainly controlled by faults and the contents of toc quartz clay calcite and dolomite at different positions along the same fault the degree of fracture development varies significantly overall the distribution of fractures in the longmaxi formation can be used to optimize well deployment and provides a basis for the future exploration of shale gas

dynamic behaviour of the east antarctic ice sheet during pliocene warmth warm intervals within the pliocene epoch 5 33 2 58 million years ago were characterized by global temperatures comparable to those predicted for the end of this century1 and atmospheric co2 concentrations similar to today2 3 4 estimates for global sea level highstands during these times5 imply possible retreat of the east antarctic ice sheet but ice proximal evidence from the antarctic margin is scarce here we present new data from pliocene marine sediments recovered offshore of adelie land east antarctica that reveal dynamic behaviour of the east antarctic ice sheet in the vicinity of the low lying wilkes subglacial basin during times of past climatic warmth sedimentary sequences deposited between 5 3 and 3 3 million years ago indicate increases in southern ocean surface water productivity associated with elevated circum antarctic temperatures the geochemical provenance of detrital material deposited during these warm intervals suggests active erosion of continental bedrock from within the wilkes subglacial basin an area today buried beneath the east antarctic ice sheet we interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the east antarctic ice sheet was sensitive to climatic warmth during the pliocene

a geologic history of the north central appalachians part 2 the appalachian basin from the silurian through the carboniferous the north central appalachians occupy a critical position within the 3000 km long appalachian orogen lying southwest of the boundary between the central and northern appalachians cnab the one billion year long history of tectonic activity in the north central appalachians includes the assembly and breakup of a late proterozoic supercontinent the creation and evolution of the appalachian orogen during the paleozoic and the mesozoic transformation of the active orogen into a passive margin during pangea9s disassembly an important part of the middle and late paleozoic history is the formation and growth of the appalachian basin an enormous elongate continental basin lying cratonward of the active appalachian internides along laurentia9s eastern margin the appalachian basin developed out of the taconic orogeny the second of the four orogenies that formed and modified the character of the appalachian orogen prior to the taconic orogeny the eastern margin of laurentia consisted of a broad carbonate shelf facing the theic ocean the taconic orogeny obduced various theic components including microcontinents magmatic arcs and accretionary prisms onto the carbonate shelf the resulting taconic highlands formed a topographic barrier between theia and the craton9s interior because tectonic activity in the internides continued more or less throughout the remainder of the paleozoic the appalachian basin never had a direct connection with theia the coarse grained molasse from the newly uplifted taconic highland spread westward over most of the basin during the early silurian carbonate deposition gradually encroached eastward over the basin as the siliciclastic input from the southeast waned a resurgence in the earliest late silurian perhaps related to orogenesis in newfoundland and the maritimes expanded the clastic wedge somewhat before long however carbonate deposition once again dominated most of the north central basin for the remainder of the silurian and into the early devonian the early to middle devonian acadian orogeny began introducing siliciclastic material into the eastern part of the appalachian basin thereby ending the largely paralic environment that persisted from the late silurian a number of deltas formed around local sediment input centers during the middle devonian continued uplift and or orogenesis must have continued into the late devonian because a vast amount of terrigenous sediment was introduced into the appalachian basin to form the vast upper devonian catskill delta although the principal tectonic activity appears to have centered in new england the large volume of sediment input suggests that orogenesis may have extended southward to the north central appalachians however direct evidence of actual acadian deformation and metamorphism is lacking at this latitude the north central appalachian basin underwent a significant change during the carboniferous prior to this time the shoreline shifted laterally through transgressions and regressions largely as a function of sediment input and regional subsidence in the carboniferous vertical movements in the basin became more important during the early carboniferous much of the mauch chunk delta was eroded in the central and western parts of the basin at the end of the early carboniferous an unconformity truncated progressively older rocks to the north toward new york new sediment transport patterns and depositional environments within the basin during the late carboniferous probably reflect significant topographic and tectonic changes not only in the hinterland but in the basin itself perhaps in conjunction with climatic changes extensive interfluvial swamps on a lower delta plain accumulated thick organic deposits which would become coal this delta plain graded southwestward into tidal and marine environments the alleghany orogeny in the early permian interrupted deposition in much of the north central appalachian basin and profoundly altered its structure

models of aftershock occurrence summary the omori formula for aftershock activity can be shown to be a consequence of each of two models of a complex earthquake in the first case aftershocks are assumed to be caused by subsequent slip on asperities of a fault which are locked during the fracture in the main shock in the second aftershocks are caused by catastrophic coalescence of nearby small fractures with the fracture surface of the main shock hence the first source model describes aftershock activity in the interior of a main shock rupture zone while the second describes aftershock activity near the outer boundary of a main shock rupture zone stress corrosion cracking is assumed to be the physical cause of the quasistatic growth of the main fracture zone at the expense of the area of the locked patches in the first model and is also the cause of the growth of the small satellite fractures in the second

deformation conditions for fracturing in the middle devonian sequence of the central appalachians during the late paleozoic alleghenian orogeny in the central appalachians fluid inclusion microthermometry and oxygen and carbon stable isotope analysis vein minerals from the middle devonian shale section show that fluid conditions pressure temperature and composition are constantly changing during deformation and vary spatially across the fold thrust belt the earliest fractures in the region formed prior to folding early during the alleghenian orogeny as the rocks were buried into the oil generation window they contain minerals that contain degraded hydrocarbon inclusions and basinal brine inclusions during multiple vein reopening events later mineral stages contain increasingly more mature hydrocarbon fluids late quartz mineralization is pervasive and typically contains the high temperature brine inclusions the vein opening history is related to changes in fluid connectivity associated with 1 burial by over thrusting and or syntectonic depositional loading and or 2 folding during uplift and erosion initial fracture formation and fluid trapping depths range from 3 5 km 2 2 mi in the plateau province and along the appalachian structural front to 4 5 to 5 0 km 2 8 to 3 1 mi in the valley and ridge province late stage fracturing and fracture reopening is related to the maximum syntectonic burial which varies from about 4 km 2 5 mi in the plateau to over 11 km 6 8 mi in the valley and ridge fractures in the valley and ridge and western pennsylvania plateau provinces cannot be categorized into the simple and classification model burial history modeling indicates that fractures forming within and near the end of the oil window were nnw and nw striking not ene striking fractures

early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the alleghanian orogeny early ene striking joints present coordinates within both pennsylvanian coal and devonian black shale of the central and southern appalachians reflect an approximately rectilinear stress field with a dimension 1500 km this appalachian wide stress field awsf dates from the time of joint propagation when both the coal and shale were buried to the oil window during the 10 15 m y period straddling the pennsylvanian permian boundary the awsf was generated during the final assembly of pangea as a consequence of plate boundary tractions arising from late stage oblique convergence where maximum horizontal stress s h of the awsf was parallel to the direction of closure between gondwana and laurentia after closure the awsf persisted during dextral slip of peri gondwanan microcontinents when sh appears to have crosscut plate scale trans current faults at around 30 sup o following 10 m y of dextral slip during tightening of gondwana against laurentia the awsf was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline shaped alleghanian map pattern seen today

structure of the kingston orocline in the appalachian fold thrust belt new york the trend of the post taconic appalachian fold thrust belt changes at the city of kingston in the new york recess to the southwest of kingston fold thrust structures trend northeast southwest whereas to the north of kingston fold thrust structures trend due north to north northeast this paper discusses the structural geometry and origin of this change in trend detailed mapping 1 9 600 suggests that the fold thrust belt in the vicinity of kingston can be divided into three structural domains northern central and southern in the northern domain megascopic folds are open significant faults do not intersect the present day ground surface ordovician strata below the taconic unconformity are not extensively thrust over post unconformity strata and shortening is less than 5 in the southern domain shortening across the fold thrust belt is 30 numerous faults intersect the ground surface and ordovician strata are significantly thrust over postunconformity strata in the central domain there are numerous accommodation faults there are locally two noncoaxial spaced cleavages and strike slip lineations overprint dip slip lineations on many slip surfaces to explain the contrasts between domains we suggest that the bend at kingston is an intersection orocline created when northeast southwest trending structures of the kittatiny shawangunk segment of the appalachian fold thrust belt overprinted and reoriented older north south trending structures of the hudson valley fold thrust belt calcite twin strain gauge measurements indicate that there was less than 2 penetrative tangential stretching accompanying the development of this orocline

earthquake precursors in japan precursor time and detectability abstract earthquake precursor data amounting to 418 in number accumulated in japan are analyzed most data are obtained under the nation wide earthquake prediction programme the precursor time t versus main shock magnitude m relationship is different for different groups of precursors a linear relation such as log t a bm where a and b are constants seems to hold for precursor disciplines such as land deformation revealed by geodetic survey and tide gauge observation ground tilt observed by a water tube tiltmeter anomalous seismic activity and geomagnetic field change although a similar relationship seems to hold for the disciplines of earth currents radon concentration underground water etc the precursor time is substantially smaller for these disciplines than for the former disciplines the above characteristics of precursor time suggests that precursors closely connected to the dilatancy generation seem to be observed at an earlier stage of seismogenetic activity precursors probably associated with diffusion of underground water into the dilatant region seem to follow for a number of precursor disciplines such as foreshock ground tilt observed by a pendulum tiltmeter resistivity by a variometer and so on no dependence of t on m is found the relationship between m and the logarithmic epicentral distance d measured in kilometers is different from discipline to discipline relying on empirical and theoretical studies of coseismic strain step it turns out that precursors found by geodetic and geomagnetic observations seem to reflect a crastal strain of the order of 10 7 10 6 precursors obtained by a resistivity variometer seem to represent a strain of the order of 10 9 10 8 precursors of other disciplines seem to correspond to a strain of 10 8 10 7 it is clear that precursors can be detected further from the epicenter as the main shock magnitude becomes larger in some favourable cases a precursor can be observed even where the epicentral distance is several times larger than that of the epicentral area

pore pressure changes induced by slip on permeable and impermeable faults pore pressure changes due to a ramp introduction of slip on permeable and impermeable faults in a fluid saturated rock mass are calculated for the purpose of evaluating water well level fluctuations the calculations demonstrate the importance of coupling between deformation and fluid diffusion at observation points less than 5 4ct0 where c is the diffusivity and t0 is the rise time the decay of pore pressure in the results here is due entirely to fluid mass diffusion an approach that neglects diffusion and assumes that the pore pressure is proportional to the mean normal stress would predict a ramp pore pressure response at distances greater than 5 4ct0 the pore pressure decays so slowly that the neglect of diffusion may be appropriate for both permeable and impermeable faults the pore pressure decays more rapidly for shorter slip zone lengths and longer rise times however the pore pressure change calculated for the impermeable fault is larger particularly for observation points near the fault and decays less rapidly than for the permeable fault these differences suggest that fault permeability can be a significant factor in the response of water wells near faults and care should be used in inferring details of the slip distribution if hydrologic conditions are not known these results are applied to a water well level change observed by lippincott et al a satisfactory fit to the data is obtained by uniform slip over a fault length of about 1 5 km and a rise time of 8 hours although the slip magnitude is not well constrained by the fit the range of possible values includes the 0 5 to 1 0 cm inferred by lippincott et al using a different approach

revisiting the hubbert rubey pore pressure model for overthrust faulting inferences from bedding parallel detachment surfaces within middle devonian gas shale the appalachian basin usa abstract both bedding parallel slickensides and cleavage duplexes are forms of mesoscopic scale detachment faulting populating black marcellus and geneseo burket and intervening gray mahantango shales of the middle devonian a section known for abnormal pore pressure below the appalachian plateau the abundance and the orientation of slickensides and cleavage duplexes in the more organic rich black shale relative to gray shale suggests that maturation related abnormal pore pressure facilitates detachment a mesoscopic manifestation of the hubbert rubey pore pressure model for overthrust faulting the former are discrete slip surfaces whereas the latter consists of nested anastomosing slip surfaces either cutting through bedding or on disrupted bedding surfaces stacked as mesoscopic versions of thrust duplexes cleavage duplexes are between a few cm and over 1 m thick with their hanging walls commonly transported toward the appalachian foreland regardless of local limb dip cleavage duplexes are most common near the stratigraphic maximum flooding surface the organic rich section most prone to develop maturation related pore pressure in the middle devonian gas shales bedding parallel slickensides are somewhat more evenly distributed in the black shale but also found in overlying gray shale in both black and gray shales slickensides are more abundant on the limbs of folds an indication of pore pressure related flexural slip folding on the macroscopic scale the pine mountain block of the southern appalachian mountains was enabled by a basal detachment cutting along the upper devonian chattanooga black shale which has a thermal maturity sufficient for the generation of abnormal pore pressure the pine mountain block is a large scale overthrust showing little evidence of collapse of the hinterland side a credible example of a pore pressure aided overthrust fault block of the type envisioned by the hubbert rubey model

a reinterpretation of the precursory seismic b value anomaly from fracture mechanics summary a new model is presented which can explain the major temporal fluctuations in seismic b value in terms of the underlying physical processes of time varying applied stress and crack growth under conditions of constant strain rate the model predicts two minima in the b value separated by a temporary maximum of inflexion point a corollary being that a single broad maximum would be expected in scattering attenuation these fluctuations in b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence we present preliminary results from controlled laboratory experiments and report recent field observations both of which are consistent with the predicted form and in particular exhibit the distinctive second short term anomaly which reaches a value bc 0 5 during the critical phase

timing of recent accelerations of pine island glacier antarctica 1 we have used interferometric synthetic aperture radar insar data and sequential landsat imagery to identify and temporally constrain two acceleration events on pine island glacier pig these two events are separated by a period of at least seven years 1987 1994 the change in discharge between two flux gates indicates that the majority of the increase in discharge associated with the second acceleration originates well inland 80 km from the grounding line an analysis indicates that changes in driving stress consistent with observed thinning rates are sufficient in magnitude to explain much of the acceleration

biogenic facies in the antarctic glacimarine environment basis for a polar glacimarine summary abstract the association of glacial deposits diamicts or tillites with biogenic and or chemically derived sedimentary rocks in the ancient record is one which continues to challenge interpretation an analogous association can be found on portions of the east antarctic continental shelf where relict glacial and glacimarine diamictons are overlain by a varied and complex suite of modern biogenic deposits carbonate bioclastic material can be found in two distinct zones and its deposition may have been contemporaneous with partial glacial expansion onto the shelf a nearshore carbonate facies consists of in situ and partially reworked barnacles bryozoans ostracods pelycepods gastropods planktonic and benthonic foraminifera a siliceous biogenic component is intermixed within the shallow water 500 m siliceous facies of pelagic hemipelagic and sediment gravity flow origin is found within inner shelf depressions and consists predominantly of diatom frustules and radiolaria these accumulations of siliceous ooze and mud clearly post date glacial recession from the shelf and contain laminated cross laminated and massive interbeds which are accumulating at rates of 0 3 cm yr the extreme polar climate inhibits meltwater production and terrigenous influx into marine basins this allows for the accumulation of biogenic rich material which is also favored by the high seasonal productivity inherent in the south polar oceans ancient analogs to these modern deposits exist and their association with glacimarine strata could be used to infer polar climatic settings and glacial retreat that is related to eustatic sea level rather than regional warming

continental shelf drift deposit indicates non steady state antarctic bottom water production in the holocene a late quaternary current lain sediment drift deposit over 30 m in thickness has been discovered on the continental shelf of east antarctica in an 850 m deep glacial trough off george vth land radiocarbon dating indicates that a period of rapid deposition on the drift averaging 290 cm kyr occurred in the mid holocene between about 3000 and 5000 yr before present slower rates of around 10 cm kyr during the past 0 3000 yr and from 5000 to about 13000 yr bp coincides with deposition of bioturbated ice rafted debris ird rich sandy mud under an energetic bottom current regime in contrast the mid holocene 3000 5000 yr bp sediments are fine grained laminated to cross laminated with minimal ird content and are contemporaneous with a period of warmer marine conditions with less sea ice production this pattern suggests that bottom currents were weaker than present day in the mid holocene and that the rate of dense bottom water production was reduced at that time this scenario is consistent with the hypothesis of non steady state rates of antarctic bottom water production through the holocene as recently proposed by broecker and his colleagues

diagenetic mineralization in pennsylvanian coals from indiana usa 13c 12c and 18o 16o implications for cleat origin and coalbed methane generation cleats and fractures in southwestern indiana coal seams are often filled with authigenic kaolinite and or calcite carbon and oxygen stable isotope ratios of kaolinite calcite and coalbed co2 were evaluated in combination with measured values and published estimates of 18o of coalbed paleowaters that had been present at the time of mineralization 18omineral and 18owater values jointly constrain the paleotemperature of mineralization the isotopic evidence and the thermal and tectonic history of this part of the illinois basin led to the conclusion that maximum burial and heat sterilization of coal seams approximately 272 ma ago was followed by advective heat redistribution and concurrent precipitation of kaolinite in cleats at a burial depth of 1600 m at 78 5 c post paleozoic uplift the development of a second generation of cleats and subsequent precipitation of calcite occurred at shallower burial depth between 500 to 1300 m at a lower temperature of 43 6 c the available paleowater in coalbeds was likely ocean water and or tropical meteoric water with a 18owater 1 25 versus vsmow inoculation of coalbeds with methanogenic co2 reducing microbes occurred at an even later time because modern microbially influenced 13c enriched coalbed co2 i e the isotopically fractionated residue of microbial co2 reduction is out of isotopic equilibrium with 13c depleted calcite in cleats

pennsylvania salient of the appalachians a two azimuth transport model based on new compilations of piedmont data new compilations of geometry and tectonic transport in the pennsylvania salient suggest that these features continue uninterrupted from the piedmont through the valley and ridge provinces these data are the basis for a new two azimuth transport model for origin of the curvature edge geometry of an eocambrian craton and a thickened stratigraphy acted as templates for alleghanian regional decollement first as reading prong n35 w motion followed by n68 w blue ridge motion both displacements involved some drag rotation and overprinting in a central zone this model avoids the problem of minimal tangential stretching explains the anthracite basins as trailing edge graben and basin structures against the autochthonous pocono plateau and produces the nittany juniata culmination as an overprinted pileup of duplexes in the central zone the model of detachment followed by changing azimuth of motion of a megadecollement sheet may apply to other salients throughout the world

coal cleat domains and domain boundaries in the allegheny plateau of west virginia regional face cleats cutting pennsylvanian and permian coal seams in the allegheny plateau of west virginia can be divided into domains separated by boundaries that are sharply defined along most of their lengths domains and domain boundaries are established based on cleat trends number of regional cleat sets and relative chronology of cleat set development regional cleats of each set show a common trend and abutting relationships between multiple sets within a domain describe a progressive history of changing stress fields and cleat development boundaries dividing the in situ directions of horizontal principal stress are coincident with cleat domain boundaries suggesting a common and persistent controlling factor uniform trends of face cleats within domains and ab upt changes in cleat signature across domain boundaries can be spatially related to regional basement structures and a depositional hinge line within the coal bearing and underlying mississippian rocks in addition fold structures in coal and underlying sedimentary rocks across the allegheny plateau commonly terminate or change trend abruptly at joint domain boundaries in some cases regions of common fold trends in coal bearing rocks are contained within specific domains face cleat trends commonly differ from joint trends in rocks immediately bounding coal seams however one domain boundary in coal can be traced into mississippian rocks through the unconformity at the base of the pennsylvanian section in this case mississippian and pennsylvanian joint trends differ it follows tha joint domain boundaries can extend downward through rocks of different lithologies as well as coincide with conformable and unconformable stratigraphic boundaries

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

fractures in mudrocks advances in constraining timing and understanding mechanisms abstract recent advances in fracture timing techniques have led to new insights about sequences and mechanisms of fracture formation in mudrocks methods to constrain fracture timing in mudrocks include i field evidence including crosscutting relationships and evidence of compaction ii petrographic evidence including signs of soft sediment deformation and diagnostic mineral assemblages or textures iii stable isotopic evidence iv thermometric dating using burial history modelling and v radiometric age dating in this contribution we briefly review each method with salient examples from the literature many documented examples of mudrock hosted fractures formed during early burial a non intuitive finding that demands a re assessment of our assumptions about what types of materials can develop opening mode fractures in general the style and distribution of fractures varies dramatically based on fracture timing highlighting the importance of constraining timing for predicting fracture patterns and interpreting their mechanisms including the role of fluid flow

tectonic evolution of the rehamna metamorphic dome morocco in the context of the alleghanian variscan orogeny structural and 40ar 39ar geochronological investigations of the rehamna massif meseta moroccan variscan belt provide new constraints on the tectonic evolution of the alleghanian variscan orogen during the upper paleozoic three main tectonic events have been recognized 1 southward thrusting of an ordovician sequence over the proterozoic basement its cambrian sedimentary cover and the overlying devono carboniferous basin this event caused subhorizontal shearing and prograde barrovian metamorphism of the buried rocks 2 continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated e w this was responsible for synconvergent detachment of the ordovician upper crustal sequence the timing of these two episodes is constrained to 310 295 ma by cooling and metamorphic amphibole and mica ages 3 a nw wnw convergence in a direction orthogonal to the previous one and characterized by the accretion of the rehamna dome to the continental basement in the east based on 40ar 39ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40ar 39ar ages from greenschist facies mylonite the timing of this event falls between 295 and 280 ma the end of the variscan orogeny in the moroccan meseta is constrained by the 40ar 39ar cooling age of a posttectonic pluton dated at 275 ma the tectonic events highlighted in morocco coincide with the late variscan alleghanian tectonic evolution of southern europe and north america and can be correlated with the global reorganization of plates that accompanied suturing of pangaea at around 295 ma

continental shelf record of the east antarctic ice sheet evolution seismo stratigraphic evidence from the george v basin abstract the late quaternary ice sheet ice shelf extent in the george v basin east antarctica has been reconstructed through analyses of chirp sub bottom profiles integrated with multi channel seismic data and sediment cores four glacial facies related to the advance and retreat history of the glaciated margin have been distinguished facies 1 represents outcrop of crystalline and sedimentary rocks along the steep inner shelf and comprises canyons once carved by glaciers facies 2 represents moraines and morainal banks and ridges with a depositional origin along the middle inner shelf facies 3 represents glacial flutes along the middle outer shelf facies 4 is related to ice keel turbation at water depths 775 m with drape sheet and mound characters and numerous undisturbed sub bottom sub parallel reflectors facies md1 b a fluted sediment drift deposit at water depth

minimal antarctic sea ice during the pliocene antarctic sea ice concentration at ocean drilling program sites 1165 64 380 s 67 219 e and 1166 67 696 s 74 787 e was lower than today through much of the pliocene the low sea ice concentration is evident from the proportion of the diatom eucampia antarctica with intercalary valves eucampia index this sea ice proxy was calibrated by using modern diatom data obtained from core top samples and winter sea ice concentration data september average through 1979 1987 the modern relationship is expressed as a binomial generalized linear model modern sea ice model this model was applied to the pliocene eucampia index within a 95 tolerance interval obtained from bootstrap estimates the results indicate that reduced winter sea ice concentrations persisted through much of the pliocene and at times were 78 and 61 relatively less concentrated than today at sites 1165 and 1166 respectively 2005 geological society of america

seismo stratigraphic analysis of the wilkes land continental margin east antarctica influence of glacially driven processes on the cenozoic deposition abstract multichannel seismic reflection data were collected from the wilkes land continental margin and particularly focusing on the lower rise area by the international wilkes basin glacial history wega project existing lower resolution seismic data also are used to understand the relationship between the basement and the sedimentary cover and to correlate the evolution of the rise and the shelf on the basis of the acoustic character and the internal geometry we recognise nine different sequences named wl s1 wl s9 from the deepest to the shallowest within the sedimentary section sequence wl s1 represents syn rift sediments deposited within tilted blocks of sedimentary crystalline and volcanic basement sequences wl s2 wl s9 represent the post rift sediments that show quite a complex architecture sequence wl s2 is a well developed wedge downlapping onto the break up unconformity sequences wl s3 wl s7 are parts of a margin prograding aggrading sequence during which large sediment ridges were deposited on the slope and rise in particular sequences wl s5 wl s7 began with the deposition of a turbidite fan system sequence wl s5 that developed in a braided channel levee complex wl s6 wl s7 dominated by downslope flows indirect correlation of the sequences wl s4 wl s6 with dsdp 269 about 600 km north of the study area suggests an early late miocene age sequences wl s8 and wl s9 represent the smoothing and filling of the pre existing morphology in a generally low energy environment with reduced bottom current activity during this time most of the sediment is deposited on the shelf and slope while the rise is generally starved differences in downslope current energy and in sediment load are interpreted to reflect variation of the terrigenous input from the shelf to the rise through time we suggest that channel levee systems on the rise reflect an increase in sediment supply from the continental shelf during the early miocene marking the growth of wet based temperate ice sheet on the near continent the youngest sequences on the rise and shelf reflect decreased sediment supply to the rise that may be related to the transition from a temperate to a polar glacial system

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

the alleghanian southern appalachian piedmont a transpressional model alleghanian dextral transpression produced multiple strike slip shear zones forming a contractional mega duplex in the central and southern appalachian piedmont structural upwelling along the zones is identifiable as crustal arching on seismic reflection profiles that roughly corresponds to the piedmont gravity high the structural upwelling coincides both temporally and spatially with areas of peak alleghanian plutonism and metamorphism the compositionally bimodal plutons generally occur in or around the fault zones and are isotopically primitive geobarometry of cognate xenocryst inclusions of coexisting plagioclase and pyroxene from a granitoid indicates 8 11 kbar or 31 to 42 5 km depth to proposed source region this depth roughly corresponds to the pre erosional location of multiple reflections beneath the arches on seismic profiles that are interpreted as the moho the granitoids are proposed to have formed by partial melting of the lowermost crust through heat from mafic intrusions and decompression resulting from crustal arching beneath the strike slip faults the granitoids ascended along the strike slip faults and were emplaced in or near them the crustal arching and melting processes appear to be important mechanisms in the development of strike slip orogens

horizontal slip along alleghanian joints of the appalachian plateau evidence showing that mild penetrative strain does little to change the pristine appearance of early joints abstract some alleghanian joints in black shales of the geneseo and middlesex formations of the catskill delta complex finger lakes district new york slipped horizontally up to 8 cm horizontal slip is measured by the offset of ene striking joints alleghanian joints striking 330 350 display a right lateral slip with an average value of 1 9 cm while joints striking 004 010 slip in the left lateral sense with an average value of 1 3 cm the maximum horizontal stress sh driving this slip falls between 350 and 004 the orientation of local alleghanian layer parallel shortening as indicated by both disjunctive and pencil cleavage by commonality of orientation we infer that slip on alleghanian joints is driven contemporaneously with layer parallel shortening if so the offset ene striking joints predate the alleghanian stress field these observations mean that both pre alleghanian and early alleghanian joints persist through a period of penetrative strain

microfabric analysis by manual and automated stereological procedures a methodological approach to antarctic tillite fabric analysis is commonly used to infer former movement directions of diamictons however analysis techniques are typically time consuming and partly subjective stereological analysis provides an alternative for objective determination of the preferred orientation of particles the ability of manual and automated stereological procedures to determine the preferred orientation of particles 120 4000 m in tillite is assessed using core samples from mount feather mcmurdo dry valleys antarctica orthogonal sets of vertical thin sections were subjected to directed secant analysis involving the determination of the number of intersections between particle outlines and a rotating series of parallel lines the manual analysis reveals a weakly developed three dimensional tilt angle plunge of 68 this reflects the depth averaged subglacial deformational structure of the deposit because the preferred orientation signal is weak in the automated set up this signal is obscured by the effect of digitization hampering the reliable assessment of the orientation direction the study shows microfabric variability within the studied cored section that is interpreted as a shear fabric reliable macrostructural information that might constrain ice flow directions requires a larger number of randomly or systematically drawn core samples from which averaged microstructural fabrics can be derived

ultrasonic imaging of the onset and growth of fractures within partially saturated whitby mudstone using coda wave decorrelation inversion using active ultrasonic source survey data coda wave decorrelation cwd time lapse imaging during the triaxial compression of whitby mudstone cores provides a 3 d description of the evolution and

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

northern ancestry for the goochland terrane as a displaced fragment of laurentia the ancestral position of the goochland terrane an isolated block containing mesoproterozoic crust in the appalachian piedmont of eastern virginia during grenvillian orogenesis has direct bearing on the breakup of rodinia ages lithology geochemistry and pb and nd isotope compositions of the montpelier anorthosite and state farm gneiss of the goochland terrane allow correlation with the grenvillian roseland anorthosite and associated igneous suites of the blue ridge farther west in virginia in contrast extensive metapelites which underwent devonian metamorphism and intrusion and amphibolites of the goochland terrane lack equivalents in the blue ridge dextral slip faults which bound the goochland terrane as well as dextral faults farther north suggest large scale translation prior to late paleozoic reaccretion ages of extension related a type granitoids within the virginia blue ridge 735 680 ma suggest physical separation from compositionally similar granitoids in the goochland terrane 630 588 ma reading prong 602 ma and manhattan prong 563 ma pre iapetan restoration of the goochland terrane northeastward of the blue ridge and outboard of the reading prong accounts for its affinities to the blue ridge unique anorthosites manhattan prong abundant amphibolites and reading prong ca 600 ma magmatism thus similar to the translation of madagascar out of africa the goochland terrane was rifted from laurentia marooned in oceanic crust and then dextrally translated southward 500 km prior to late paleozoic reaccretion documentation of large scale dextral translation of eastern laurentia relative to its fragments and other cratons during the neoproterozoic and paleozoic may assist in locating far traveled pieces of the grenville orogen for reconstruction of rodinia

fluid inclusions in veins from the middle devonian shales a record of deformation conditions and fluid evolution in the appalachian plateau fluid inclusion microthermometric analysis of multiple vein sets from the middle devonian shales provides insight into deformation conditions and fluid evolution in the appalachian plateau during and since the alleghanian orogeny the data illustrate a dynamic system in which deformation conditions and fluid compositions changed continually during basin subsidence in response to orogeny and postorogenic uplift and denudation two types of inclusions are present in the veins 1 hydrocarbon inclusions that contain ch4 c2h6 co2 higher hydrocarbons more than two carbon atoms and bitumen and 2 two phase aqueous inclusions that contain very saline up to 27 wt nacl equivalent nacl cacl2 brines the veins formed in response to stresses associated with the alleghanian orogeny during subsidence of the middle devonian shale section in the central plateau province each vein set contains inclusions of hydrocarbons that are thermally more mature than those in the previously formed set the earliest vein set cfjv1 cross fold joints and veins 1 formed during the onset of hydrocarbon generation at a depth of 2 3 km and contains inclusions that are composed of degraded bitumen the next youngest vein set cfjv2a formed under lithostatic pore fluid pressure conditions at a depth of at least 2 3 km and contains primary inclusions with a condensate like composition cfjv2b also formed under lithostatic conditions at a depth of 3 0 km and contains primary ch4 c2h6 co2 inclusions the younger cfjv3a formed at at least 3 9 km depth under lithostatic conditions and contains primary ch4 co2 inclusions the youngest vein set strike parallel sjv strike joints and veins formed during maximum burial or early during uplift of the middle devonian shale section sjv along with reopened cross fold joints and veins contains barite with nearly pure ch4 inclusions and two phase aqueous inclusions the presence of barite in all of the vein sets is a result of the breaching of the middle devonian shale overpressure seal in response to relaxation of alleghanian compressive stresses and the influx of surrounding brines into the shale section the trapping of fluids in the barite continued throughout the uplift history because of microfracturing

earthquake magnitude recent research and current trends abstract the magnitude recommendations adopted by the iaspei assembly at zurich in 1967 have had both a stabilizing and a stimulating effect on magnitude determinations and related research from 1967 onwards one magnitude research paper has appeared on the average almost every week thus making this parameter the most studied one in seismology new facilities and more accurate methods e g concerning instrumental equipment and interpretation techniques have made it possible to improve earlier achievements the application of magnitude scales has been extended in all respects e g with regard to epicentral distances focal depths wave types and wave periods magnitude frequency relations have become the most investigated equations within seismology observationally as well as theoretically they have wide applications e g for estimating the maximum magnitudes of future earthquakes an important item in earthquake prediction the magnitudes provide significant information on other source parameters such as wave energy fault length seismic moment relations between different types of magnitude yield valuable information on source properties for instance relations between magnitudes based on body and surface waves are used for efficient discrimination between earthquakes and underground explosions it is our purpose to review the magnitude development in the post zurich period 1967 1980 partly for geologists tectonophysicists and engineers who need an overview partly for seismologists who need an introduction to an overwhelmingly comprehensive literature

cleats and their relation to geologic lineaments and coalbed methane potential in pennsylvanian coals in indiana abstract cleats and fractures in pennsylvanian coals in southwestern indiana were described statistically analyzed and subsequently interpreted in terms of their origin relation to geologic lineaments and significance for coal permeability and coalbed gas generation and storage these cleats can be interpreted as the result of superimposed endogenic and exogenic processes endogenic processes are associated with coalification i e matrix dehydration and shrinkage while exogenic processes are mainly associated with larger scale phenomena such as tectonic stress at least two distinct generations of cleats were identified on the basis of field reconnaissance and microscopic study a first generation of cleats that developed early on during coalification and a second generation that cuts through the previous one at an angle that mimics the orientation of the present day stress field the observed parallelism between early formed cleats and mapped lineaments suggests a well established tectonic control during early cleat formation authigenic minerals filling early cleats represent the vestiges of once open hydrologic regimes the second generation of cleats is characterized by less prominent features i e smaller apertures with a much less pronounced occurrence of authigenic mineralization our findings suggest a multistage development of cleats that resulted from tectonic stress regimes that changed orientation during coalification and basin evolution the coals studied are characterized by a macrocleat distribution similar to that of well developed coalbed methane basins e g black warrior basin alabama scatter plots and regression analyses of meso and microcleats reveal a power law distribution between spacing and cleat aperture the same distribution was observed for fractures at microscopic scale our observations suggest that microcleats enhance permeability by providing additional paths for migration of gas out of the coal matrix in addition to providing access for methanogenic bacteria the abundance distribution and orientation of cleats control coal fabric and are crucial features in all stages of coalbed gas operations i e exploration and production understanding coal fabric is important for coal gas exploration as it may be related to groundwater migration and the occurrence of methanogenic bacteria prerequisite to biogenic gas accumulations likewise the distribution of cleats in coal also determines pathways for migration and accumulation of thermogenic gas generated during coalification

fluid evolution in fracturing black shales appalachian basin opening mode veins in cores drilled from the mudrocks overlying and underlying the major silurian salt decollement in the appalachian plateau tioga and lawrence counties pennsylvania have mineralogic and isotopic compositions generally matching those of their host mudrocks suggesting opening and filling amid little cross stratal fluid motion calcite and most trace minerals probably entered the veins via dissolution reprecipitation from nearby host rock consistent with this interpretation are the observations that 1 trace minerals within the veins including quartz pyrite and dolomite are invariably also present within the layers hosting the veins with vein cement minerals generally reflecting the abundance and solubility of minerals in the host rock and 2 carbon and oxygen isotopic compositions of vein filling calcite are similar to those of calcite within the host rock with vein filling 18o slightly depleted and 13c slightly enriched modeling the fluid isotopic evolution assuming vein opening and filling amid immobile connate formation water accounts for these minor but systematic differences which are attributable to increasing temperature and hydrocarbon maturation an exception to the above trend is barite which despite its low solubility is systematically enriched in veins with respect to the host rock it is unclear whether barite precipitation resulted from the influx of external fluids perhaps deriving from silurian salt or from barium mobilized at depth from local clays or organic material

structural controls on fractured coal reservoirs in the southern appalachian black warrior foreland basin abstract coal is a nearly impermeable rock type for which the production of fluids requires the presence of open fractures basin wide controls on the fractured coal reservoirs of the black warrior foreland basin are demonstrated by the variability of maximum production rates from coalbed methane wells reservoir behavior depends on distance from the thrust front far from the thrust front normal faults are barriers to fluid migration and compartmentalize the reservoirs close to the thrust front rates are enhanced along some normal faults and a new trend is developed the two trends have the geometry of conjugate strike slip faults with the same 1 direction as the appalachian fold thrust belt and are inferred to be the result of late pure shear deformation of the foreland face cleat causes significant permeability anisotropy in some shallow coal seams but does not produce a map scale production trend

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

tracking the burial and tectonic history of devonian shale of the appalachian basin by analysis of joint intersection style the most pervasive systematic joints hosted by devonian black shale of the appalachian plateau include east northeast joints and younger northwest striking cross fold joints both sets were driven exclusively by fluid pressure generated as a consequence of hydrocarbon related maturation supplemented by subsequent tectonic compaction during the alleghanian tectonic cycle in the more deeply buried proximal region of the catskill delta joints of both sets cross cut however in stratigraphically equivalent black shale of the distal shallower region of the delta to the west 25 of the joint intersections are crosscutting whereas 75 of the intersections are defined by east northeast joints curving into and abutting cross fold joints east northeast joints in the distal shale propagated early but were neither as long nor as pervasive as similarly oriented joints in the more deeply buried proximal delta deposits however these same joints are much better developed more closely spaced stratigraphically lower in the distal shale succession where essentially all intersections are crosscutting it is likely that relatively high contact stress an effective stress on east northeast joints in the more deeply buried parts of the delta enabled the unimpeded transmission of elastic stress concentration at the tip of cross fold joints across the older joints in the distal delta exhumation of the devonian shale sequence following establishment of the contemporary stress field in middle tertiary time to within 1 km of the earth s surface resulted in renewed propagation of east northeast joints the propagation path of the reactivated joints curved into and terminated against cross fold joints at angles of 60 90 the curving to termination intersection style is a manifestation of crack tip stress field blunting as a consequence of slippage along the interface of cross fold joints enabled by exhumation induced relaxation of contact stress on these latter joints it is noteworthy that the orientation of the densely formed east northeast joints parallel to the maximum horizontal stress direction of the contemporary lithospheric stress field likely imparts a meaningful permeability anisotropy to these hydrocarbon source rocks

fractal characteristics of active normal faults an example of the eastern gulf of corinth greece abstract we present a model proposing the evolution of segmented normal faults in a currently extended area allowing an explanation of strong earthquakes hosted by short faults active normal faults accommodating the current extension in the gulf of corinth result from the linkage of smaller component faults well exposed fault zones in the sofiko range are arranged as pairs of graben bounding faults and thus our structural analysis has implications for the evolution of active normal faults as well as graben evolution a basal strip of fresh bedrock at the base of fault scarps suggests continuous seismic or aseismic creep thus the structural analysis of component faults that show a power law length distribution describes an evolved fault zone of particular interest is that faulting in this region is self similar between limits distribution is characterized by a fractal dimension of d 1 and the limits in our study are between 10 m mapping resolution and double the maximum displacement of each fault zone component segments of any compartmentalized fault show a length to maximum displacement ratio of two basin architecture and structural analysis suggest that the nucleation points of faults are detectable over the entire evolution of the fault zone all analyzed faults evolved by an independent nucleation model morphotectonic and kinematic data as well as fractal dimension of trace length population plots all suggest that fault zones defining north and south dipping conjugates are active at least over the late stages of basin evolution although this study was focused on a relatively small area this pattern of fault growth appears to be applicable to the entire fault population in the gulf of corinth and is thus an important model for normal faults

integrating seismic reflection and geological data and interpretations across an internal basement massif the southern appalachian pine mountain window usa the southern appalachian pine mountain window exposes 1 1 ga grenvillian basement and its metasedimentary paleozoic cover through the allochthonous inner piedmont the issue of whether the crustal block inside the window was either transported above the master appalachian late alleghanian decollement or is an autochthonous block that was overridden by the decollement has been debated for some time new detrital zircon geochronologic data from the cover rocks inside the window suggest this crustal block was derived from gondwana but docked with laurentia before the alleghanian event reprocessed deep seismic reflection data from west central georgia pr and poststack noise reduction amplitude variation analysis and prestack depth migration indicate that a significant band of subhorizontal reflections occurs almost continuously beneath the window collinear with the originally recognized decollement reflections north of the window a marked variation in the decollement image from strong and coherent north of the window to more diffuse directly beneath the window is likely a partial consequence of the different geology between the inner piedmont and the window the more diffuse image beneath the window may also result from imaging problems related to changes in topography and fold of cover i e signal to noise ratio two alternative tectonic models for the pine mountain window can partially account for the observed variation in the decollement reflectivity 1 the pine mountain block could be truncated below by a relatively smooth continuation of the decollement the window would thus expose an allochthonous basement duplex or hors block thrust upward from the south along the late proterozoic rifted continental margin 2 the window represents localized exhumation of autochthonous basement and cover along a zone of distributed intrabasement shearing directly beneath the window either model is viable if only reflector geometry is considered model 1 is favored if both geometry and kinematics of blue ridge piedmont thrust sheet emplacement are incorporated in either model the southern margin of the window merges to the west with the iapetan early alleghanian central piedmont suture which juxtaposes north american affinity piedmont rocks to the north and exotic panafrican rocks of the carolina avalon terrane to the south immediately south of the window this suture dips southward and merges in the lower crust with the late alleghanian suture joining the appalachians with gondwana

tectonic implications drawn from differences in the surface morphology on two joint sets in the appalachian valley and ridge virginia surface morphology distinguishes two joint sets in the virginia west virginia appalachians and points to a difference in rupture velocity between the two sets the earlier joint set j1 propagated prior to or during fault related folding and was subsequently tilted as folds grew to have steeper limb dips j1 is characterized by a rougher surface displaying a symmetrical plumose pattern created during the propagation of a simple rupture front in ceramics such morphology is the product of a faster rupture under relatively high stress intensity k i k ic where k ic indicates critical stress intensity this relatively fast joint growth is consistent with an effective tensile stress sustained by oblique plate convergence at the onset of the alleghanian orogeny the later joint set j2 propagated normal to the allegheny front in a subvertical orientation independent of local bed dip a multilobed rupture leaves a smooth surface on j2 joints by analogy with ceramics such a surface is indicative of slower propagation by subcritical crack growth k i k ic this slow joint growth is consistent with the slow generation of effective tensile stress arising from hydrocarbon maturation postdating fold growth during the waning stages of the alleghanian orogeny

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

a new microstructural mapping methodology for the identification analysis and interpretation of polyphase deformation within subglacial sediments although integrated clast macro and microfabric studies have been used to provide evidence of palaeo ice flow directions to discriminate between diamictons deposited in differing sedimentary environments and to model the response of tills to subglacial deformation such studies have typically only examined the bulk fabric developed within the diamicton largely ignoring the potential for several phases of fabric development within these typically polydeformed deposits we present a new graphical method for the identification description and analysis of clast microfabrics in polydeformed tills this method utilises high resolution digital scans of orientated thin sections imported into a standard computer graphics package to measure the orientation of long axes of detrital skeleton grains included within the till these data are then used to identify the main clast microfabrics present within the till allowing the delineation of clast microfabric domains in which elongate grains share a similar preferred orientation this process leads to the identification characterisation and most importantly the interrelationships between the various generations of microfabrics the relationships between these clast microfabrics and other microstructures e g plasmic fabrics turbate structures shears folds faults etc can also be examined providing a detailed microfabric microstructural map of the thin section clast long axis orientation data collected for different parts of a thin section are plotted on a series of rose diagrams allowing the variation in clast microfabric orientation to be displayed graphically a new set of terminology for the description of clast microfabrics in tills which is based upon the system used by metamorphic petrologists is also presented three case studies of the fabrics present within subglacial sediments of pleistocene age as well as the present are used as examples of how to apply this methodology

fracture nucleation process in intact rocks abstract yukutake h 1992 fracture nucleation process in intact rocks in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 247 257 we produced tomographic images for cross sections of granite samples by measuring the elastic wave velocity along 20 ray paths at high sampling rates during triaxial compression tests the dynamic fracturing process just before ultimate fracture was investigated from the rapid changes in the associated tomographic images it was found that microcracks concentrated locally in several regions as axial load was increased and the velocity then recovered in some of the intensively microcracked regions 6 60 s before fracture subsequently the amplitude of the elastic waves through the most intensively microcracked region also recovered in a few cases the frequency of the elastic wave through the most intensively microcracked region became extremely high we consider that fracture nucleated in the intensively microcracked regions and infer that several macrocracks occur due to an abrupt coalescence of microcracks in the fracture nucleated region a crack length in the process zone for brittle fracture was evaluated from the stress drop observed just before fracture and the published value of the shear fracture energy for westerly granite we expect from the results of both tomography and stress drops that subsequent instability occurred when one of larger macrocracks formed by the coalescence of the macrocracks reached a critical length in the most intensively nucleated region

correlation between magnetic anisotropy and fabric for devonian shales on the appalachian plateau the magnetic anisotropy of devonian black shale samples was measured from two cores drilled in the appalachian plateau the mineralogy of the shales is predominantly clay with small quantities of quartz and minor amounts of opaques and chlorite magnetite is the predominant ferromagnetic mineral present in the samples the magnetic fabric was measured at both room temperature and liquid nitrogen temperature and is dominated by a well defined bedding vertical compaction and a lesser defined magnetic lineation measurements of the anisotropy of magnetic susceptibility ams at liquid nitrogen temperature which enhances the paramagnetic contribution in the rock showed a strong increase in both the bulk susceptibility and susceptibility differences this increase suggests that the ams is controlled by the paramagnetic minerals particularly the clays and chlorite strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry good correlations have been found 1 between the orientation of the magnetic lineation and the long axes of the chlorite crystals and 2 between the degree of magnetic foliation and the amount of vertical compaction the magnetic lineation also agrees well with the direction of seismic anisotropy over the plateau the anisotropy of the anhysteretic remanence which expresses the anisotropy due to the ferromagnetic component in the rocks shows a weaker correlation with the amount of vertical compaction a weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post dates the main alleghanian orogeny the magnetic anisotropy of the devonian shales mirrors the compaction and tectonic fabric on the appalachian plateau

late paleozoic transcurrent tectonic assembly of the central appalachian piedmont recent investigations in south eastern pennsylvania and northern maryland have demonstrated a major anastomosing strike slip shear system the pleasant grove huntingdon valley shear system emerges from beneath the coastal plain cover at trenton new jersey and extends to the area west of baltimore maryland where it is overlain by the culpepper mesozoic rift basin the sense of offset across this system is dextral in the susquehanna river region and north of the shear zone the rocks of the octoraro formation contain evidence for two metamorphisms and deformations prior to strike slip shearing whereas south of the shear zone the peters creek formation contains evidence for only one the discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen although conclusive ages for the strike slip deformation do not exist at this time the timing of deformation is loosely constrained where the shear system crosscuts known taconian structures in the piedmont comparison of deformation style with other regions in the appalachian suggests the pleasant grove huntingdon valley shear system is related to alleghanian transcurrent tectonics in the piedmont palinspastic reconstruction of the pleasant grove huntingdon valley shear system reveals fundamental problems in current tectonic models for the central appalachian piedmont a minimum of 150 km of dextral offset is proposed for the pleasant grove huntingdon valley shear system based on reconstruction of the cambrian ordovician shelf edge between northern maryland and southeastern new york displacement of this magnitude can account for the previously proposed tectonic models that portray a failed iapetan rift block and microcontinent that contains the baltimore grenvillian massifs even though a history of early orthogonal collision is preserved within discrete structural blocks transcurrent shearing has greatly influenced the distribution of those blocks models not including the strike slip component of tectonic assembly need serious reconsideration as evidence grows that the magnitude of orogen parallel displacement is equal to or larger than the orthogonal component

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form