raft tectonics the effects of residual topography below a salt de collement abstract some natural examples of thin skinned extensional structures resulting from gravitational gliding on passive margins which display complex listric growth fault patterns are not just controlled by the slope direction examples from the gulf of guinea are presented it is proposed that the inherited topography of the underlying basement can control the initiation of faults oblique to the gliding direction we carried out a series of small scale physical models in order to investigate the effects of residual topography beneath a de collement on faulting in the overlying rafted sedimentary cover the experimental results sustain the hypothesis of structural inheritance and their implications are discussed in the light of natural examples from the gulf of guinea

geochemical and geological characteristics of the es3l lacustrine shale in the bonan sag bohai bay basin china abstract although extensive studies have been conducted on unconventional shale oil resources in recent years little work has been performed on the resource potential of lacustrine shales the es3l lower sub member of the third member of the eocene shahejie formation shale of the bonan sag a set of thick lacustrine deposited sediments offers a good example for furthering the understanding of these rocks geochemical and mineralogical investigations were carried out on a large sample set from the 220 m thick sequence of well luo69 in total 200 core samples were analyzed for total organic carbon toc and rock eval pyrolysis and 421 core samples were analyzed for mineral compositions the es3l shales have a very high toc content with type i lacustrine oil prone kerogen in the early oil generation stage rock eval tmax suppression is evident in the lower part of the es3l member associated with a rise in pi production index with respect to fracability or brittleness the content of brittle minerals in the es3l shale is generally higher than that of marine shales in the usa with higher calcite mineral averaging 51 82 wt and lower clay content averaging 18 86 wt interparticle pores in minerals grains or clay aggregates intraparticle pores intercrystalline dolomite and pyrite framboid and four types of fractures tectonic fractures abnormal pressure fractures clay shrinkage fractures and inter laminar fractures are developed in the es3l shale organic matter pores are not present in the studied samples probably due to lower maturity tectonic fracture may play an important role in hydrocarbon migration and accumulation the results of this study suggest that the es3l shale in the bonan sag has very good shale oil potential especially in the lower section

deep structure of the east mendip hills from gravity aeromagnetic and seismic reflection data beacon hill pericline is the most easterly pericline of the mendip hills whose folds and thrusts were formed during the variscan orogeny by earth movements directed from the south the silurian volcanic rocks making up the core of the pericline are shown to be responsible for the large linear magnetic anomalies present over eastern mendip magnetic anomalies within the radstock coalfield to the north however are related to volcanic horizons in the carboniferous limestone a three dimensional model shows the presence of silurian rocks beneath the spine of the eastern mendip hills seismic reflection sections are used to control two dimensional interpretations of potential field data from beacon hill pericline although the magnetic anomalies are adequately explained by the seismic model the gravity anomalies require the presence of an additional low density body beneath the pericline9s southern flank this body is interpreted as a thrust wedge of old red sandstone emplaced in its present position by a previously unrecognized post variscan normal fault the considerable lateral extent of the structure is demonstrated by the trend of the maxima of the horizontal gradient of the gravity anomalies

geomechanics of hydraulic fracturing microseismicity part 1 shear hybrid and tensile events we investigate the geomechanical behavior of hydraulic fracturing induced microseismicity microseismic events are commonly used to discern stimulation patterns and hydraulic fracture evolution however techniques beyond fracture mapping are required to explain the mechanisms of microseismicity in this series we present an approach to combine seismological and geomechanical techniques to investigate how microseismicity relates to propagating hydrofractures as well as existing natural fractures and faults part 1 describes the first analysis step which is to characterize the microseismic events by their source parameters focal mechanisms and fault plane orientations these parameters are used to determine the mechanical conditions responsible for activation of discrete populations or subpopulations of microseismic events that then can be interpreted in their geological and operational context first we compare microseismic fault plane populations from a mississippian barnett shale texas data set that are determined using a traditional double couple model shear only with a tensile source model hybrid events which may be more suitable for hydraulic fracturing conditions second we employ a new method to distinguish fault planes from auxiliary planes using iterative stress inversion and critical stress instability selection criteria the result is an enhanced microseismic characterization that includes geomechanical parameters such as slip tendency and local activation stress state during the operation using this approach on the barnett shale data two microseismic fault sets are resolved an inclined northeast southwest set with dominant shear and a vertical north south set with more hybrid behavior the results are used in part 2 to further investigate the heterogeneity of the stimulations and to compare models for microseismic activation

geomechanical modeling of hydraulic fracturing why mechanical stratigraphy stress state and pre existing structure matter the increasing exploration and production in unconventional resource plays in the past decade has been accompanied by a greater need for understanding the effectiveness of multistage hydraulic fracturing programs particularly in long 1500 m or 5000 ft subhorizontal boreholes laterals traditional analytical analysis techniques for estimating the size and orientation of fractures induced by fluid injection typically result in predictions of relatively long and planar extension mode i bi wing fractures which may not be representative of natural systems although these traditional approaches offer the advantage of rapid analysis neglect of key features of the natural system e g realistic mechanical stratigraphy pre existing natural faults and fractures and heterogeneity of in situ stresses may render results unrealistic for planning executing and interpreting multimillion dollar hydraulic stimulation programs numerical geomechanical modeling provides a means of including key aspects of natural complexity in simulations of hydraulic fracturing in this study we present the results of two dimensional finite element modeling of fluid injection induced rock deformation that combines a coupled stress pore pressure analysis with a continuum damage mechanics based constitutive relationship the models include both the natural mechanical stratigraphic variability as well as the in situ stress state anisotropy and permit tracking of the temporal and spatial development of shear and tensile permanent strains that develop in response to fluid injection our results show that simple long planar fractures are unlikely to be induced in most mechanically layered natural systems under typical in situ stress conditions analyses that assume this type of fracture geometry may significantly overestimate the reach of hydraulically induced fractures and or effectively stimulated rock volume

fumarole emissions at mount st helens volcano june 1980 to october 1981 degassing of a magma hydrothermal system abstract this study is an investigation of the chemical changes in the mount st helens fumarole gases up to october 1981 the sources of the fumarole gases and the stability of gas species in the shallow magma system these problems are investigated by calculations of element compositions thermodynamic equilibria and magmatic volatile hydrothermal steam mixing models the fumarole gases are treated as mixtures of magmatic volatiles and hydrothermal steam formed by magma degassing and boiling of local waters in a dryout zone near conduit and dome magma the magmatic volatile fraction is significant in fumaroles with temperatures in excess of the magma cracking temperature 700 c i e the temperature below which cracking is induced by thermal stresses during cooling and solidification linear composition changes of the fumarole gases over time appear to be the result of a steady decline in the magmatic volatile mixing fraction which may be due to the tapping of progressively volatile depleted magma the maximum proportion of hydrothermal steam in the fumaroles rose from about 25 35 in september 1980 to around 50 70 by october 1981 fractional degassing of magmatic co 2 and sulfur also contributed to the chemical changes in the fumarole gases the steady chemical changes indicate that replenishment of the magma system with undegassed magma was not significant between september 1980 and september 1981 extrapolations of chemical trends suggest that fumarole gases emitted at the time of formation of the first dome in mid june 1980 were more enriched in a magmatic volatile fraction and contained a minimum of 9 co 2 calculations show h 2 s is the predominant sulfur species in mount st helens magma below depths of 200 m rapid release of gases from magma below this depth is a plausible mechanism for producing the high h 2 s so 2 observed in mount st helens plumes during explosive eruptions this study suggests that dacite andesite volcanos may emit gases richer in co 2 during the earlier episodes of an eruptive cycle and burden the atmosphere with much more h 2 s than so 2 during explosive eruptions

gravity driven thin skinned extension above zechstein group evaporites in the western central north sea an application of computer aided section restoration techniques normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of mesozoic and tertiary strata in the north sea in addition to the previously documented mechanisms of salt withdrawal dissolution and differential sedimentary loading it is recognized that gravity driven thin skinned extension above inclined salt layers has played an important part in north sea basin development commercial section restoration software has been used to facilitate depth conversion restoration and decompaction of seismic sections selected from an interpreted regional database in the western central north sea allowing validation of the interpretations and a graphical and highly quantitative description of salt assisted extension results of this work show that zechstein group evaporites were deposited in shallow sag basins during the permian triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites bedded salt became folded while mobile salt flowed to fill anticline cores since the early jurassic regional tectonic tilting related to post rift subsidence and increasing sedimentary overburden have caused allochthonous mesozoic and tertiary strata to extend by gravity spreading above the mobile salt layer which detaches the allochthon from the underlying autochthonous late palaeozoic rocks concave up listric normal faults sole out in the salt layer propagate into the overlying cover sewuence and have been active at different geological times causing stratal thickening and folding within the allochthon antithetic and synthetic normal faults have developed producing complex upward branching fault systems in map view the listric faults form curvilinear en echelon arrays the faults linked by relay ramps fault blocks are typically 3 7 km wide 2 3 km thick and 7 10 km long salt movement during the jurassic tertiary has been driven by active extension of the cover causing salt to fill potential voids created by fault block rotation thus salt highs occur beneath sites of extension the listric faults generally dip in the same direction as the sub salt surface although there are also some major counter regional faults during extension regional dips have increased up to about 5 which is sufficient for gravity driven extension above a salt layer a total extension of about 6 has occurred the gravity driven thin skinned extension documented in the western central north sea is a phenomenon which can be recognized elsewhere in the north sea basin and can be readily compared with similar phenomena already documented in offshore angola brazil nova scotia and the gulf of mexico

geologic structure controls on initial productions of lower silurian longmaxi shale in south china abstract the lower silurian longmaxi shale is the most important marine shale gas play in south china several hundred wells have been drilled to date of which more than 150 wells have obtained high initial production ip outside of the sichuan basin however wells drilled in the lower silurian longmaxi shale do not reach flow rates that are commercial developing an understanding of the key factors controlling the ips of longmaxi shale is important in screening shale resource plays in this study the toc thermal maturity porosity and gas saturation data obtained from longmaxi shale of southeastern chongqing area outside sichuan basin have been compared with that of sichuan basin the results of this study show that the shales encountered display similar properties including toc 2 thermal maturity 2 5 3 5 eqro and total porosity 3 6 a good correlation between ips and pressure coefficient suggest that the free gas content of gas shale controls on ips porosity and gas saturation of gas shale are the main factors to control its free gas volume the gas saturations in longmaxi shale from high initial production wells in sichuan basin mainly range between 50 and 90 however that of shale in the southeastern chongqing area are decreased to 10 30 this is the directly reason for low ips of wells the contrasting gas saturation of longmaxi shale between sichuan basin and the adjacent fold thrust belts are interpreted to reflect in part differences in sealed condition of fluid system intensive tectonic movements caused the exhumation faulting and fracturing that reduced or destroyed the sealed ability of shale fluid system which are the major factors for enhancing water saturation decreasing fluid pressure and ips finally the study proposes several most favorable zones for longmaxi shale gas exploration and development

the concept of joint saturation and its application two of the major joint driving mechanisms are joint normal stretching and poroelastic shrinkage and these lead to joint sets commonly associated with structural bending and natural hydraulic fracturing respectively regardless of joint driving mechanism joint infilling is a nonhomogeneous poisson process in the presence of stress shadows through probability modeling we show that in all cases joint spacing is best fit with gamma distributions the shape parameter of the best fit gamma distribution to joint spacing data is a quantitative means to assess the extent of saturation which is represented in a new parameter the joint saturation ratio jsr to test the utility of jsr we call upon published structural bending joint data elk basin lilstock and rives plate bending experiment the shape parameters for these well developed structural bending joints are equal to around three corresponding to a jsr of approximately 30p using the same analysis on the spacing of natural hydraulic fractures collected from outcrops in the gas prone devonian sections of the appalachian basin we find that natural hydraulic fractures differ in two aspects from structural bending joints first the joint spacing is proportional to bed thickness in bedded rocks but not in gas shale sections second the joint saturation of natural hydraulic fractures is generally lower than in well developed structural bending joints thus the jsr is a means to distinguish the joint driving mechanism and to represent joint saturation level independent of bed thickness effects it can be used to distinguish natural fractures from drilling induced fractures and to improve the fracture network modeling

centrifuged models of gulf of mexico profiles abstract physical models dynamically scaled in a centrifuge are used to explore the importance of slope progradation in driving internal salt structures much further horizontally than they rise large increments of ductile plastic overburden added at normal gravity values simulated primary clastic sedimentary progradation of a clastic slope over a layer of buoyant fluid simulating salt during load increments at a nominal 800 g in a centrifuge this ductile topographic slope spread over the ductile decollement except for down to basin growth faults all the structures considered to be distinctive in the northern gulf of mexico are simulated in the models that are scaled such that 1 mm and 60 s in the model correspond to 1 1 5 km and 2 6 ma in the prototype ratios of the rate of advance to the rate of rise of the shelf break compared with the rate of advance and increase in relief of salt structures control how much of the deep autochthonous salt is squeezed ahead of the advancing shelf and the paths this salt takes to shallow structural levels the few three dimensional diapiric stocks which developed from many two dimensional mullions in the models spread stock canopies which advanced huge distances before some became tongue canopies by climbing over clastic sediments already aggraded in front of them the models emphasize the significance of ductile spreading of the clastic sediments now being discerned by backstripping

clyde field growth fault secondary detachment above basement faults in north sea the clyde oil field in the southern north sea comprises a jurassic fulmar sand reservoir developed on a fault bounded terrace on the margin of the central graben palinspastic sections were constructed to honor seismic and well data and were repeatedly tested using new techniques for balancing structural sections in extensional regimes cross sectional area and bed length balance clearly demonstrate the presence of both shallow and deep fault detachments seismic remapping has substantiated this model and demonstrates the decoupling of the basement and shallow listric fault sets the new model has significance in terms of variations in reservoir quality and possible reservoir discontinuities resulting from fault seals of regional interest is the possibility of additio al growth fault plays of this type within the north sea basin

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

a magmatic geohygrometer application to mount st helens and other dacitic magmas compositions of andesitic to dacitic melts in equilibrium with plagioclase orthopyroxene and clinopyroxene have been experimentally calibrated as a geohygrometer application to melson s data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of mount st helens reveals systematic dewatering trends ca 4 1980 to 1 1982 h 20 in subliquidus intratelluric melt in pyroclastics and 2 3 1980 to 1 1982 in subliquidus melt in dome building dacites before each pyroclastic event intratelluric crystallization achieved melt h 20 contents of ca 6 at a depth of at least 8 km a necessary condition for crystallization of amphibole dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3 yr cycle from a minimum depth of 4 km in 1980 to 0 35 km in 1982 and pyroclastically erupted comparison of mount st helens compositions to worldwide andesites and dacites suggests that near liquidus arc magmas that crystallize plagioclase and pyroxenes contain l 2 h 2 0 on average whereas magmas in continental margins contain appreciably more h2o

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

structural and tectonic setting of the permo triassic basins of northwest europe abstract the permo triassic basins of nw europe formed by i late orogenic spreading of the variscan mountain belt eventually leading to ocean floor formation in the mid atlantic ii continental rifting propagating south from the arctic and iii thermal subsidence following carboniferous volcanic activity in the north sea spreading values in the northernmost north sea west of shetland and west and southwest of britain are relatively high but the sediments are dominantly continental the permo triassic rifting affected lithosphere thickened during caledonian and variscan orogenesis however subsidence associated with the arctic rifting together with global sea level changes allowed marine incursions into the north sea producing the large zechstein salt basins interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across britain and the north sea this fault pattern formed the basic framework for subsequent jurassic and cretaceous basin formation

local crustal extension at mount st helens washington seismicity and the orientation of fault planes from focal mechanisms indicate that mount st helens is located at a dextral offset along the st helens seismic zone shz earthquake swarms occurring in this offset are related to volcanic eruptions because motion on the shz is in a right lateral strike slip sense this dextral offset creates extension within a volume of the crust between the offset fault segments this offset geometry is similar to that of geothermal areas along the san andreas fault system we apply a model derived from these geothermal areas to mount st helens and find that the major differences between mount st helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments l to the seismogenic depth h at mount st helens this ratio is 1 whereas in the geothermal areas the ratio is 1 we propose that when l h 1 as at mount st helens the regional minimum principal stress does not completely dominate the small volume under extension and as a consequence the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where l h 1 late quaternary volcanic vents near mount st helens strike northeast similar to the strike of a set of pre quaternary faults and intrusive rocks that are mapped north of the volcano in addition the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress we infer that the spatial position of mount st helens is controlled by the junction of the right stepping offset of the shz with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust the sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of right lateral shear measured from geodetic lines around mount st helens during june and july 1980

tof sims and electron microprobe investigations of zoned magmatic orthopyroxenes first results of trace and minor element analysis with implications for diffusion modeling zoned phenocrysts in volcanic rocks potentially provide an archive of magmatic processes as a crystal grows and comes into contact with different melt batches the chemical and textural signature of this journey is recorded within its crystal lattice the timescale of some magmatic processes can be investigated through the relaxation of chemical gradients across crystal growth zones through the application of diffusion modeling techniques one of the current limitations to diffusion modeling is the spatial and analytical resolution of the chemical profile that conventional techniques such as electron probe microanalyzer epma dynamic secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms can achieve here for the first time we present time of flight tof sims tof sims data for zoning of orthopyroxene crystals from the may 1982 eruption of mount st helens volcano u s a and cross calibrate these data between backscattered electron images and epma tof sims has the advantage of being able to achieve micrometer to nanoscale spatial resolution of major elements as well as analyses of light elements such as li and trace and minor elements na k and ni at concentrations that cannot be achieved by epma provided that convolution overlap effects and polyatomic mass inferences are carefully considered with tof sims analyses we identified zoning of li on a spatial scale ca 5 10 m that would be inaccessible to most other conventional analytical techniques preliminary results indicate that li a fast diffusing element may be introduced to the crystals in the minutes hours or days prior to eruption and may provide insights into pre eruptive magmatic processes thus tof sims has the potential to be a powerful tool for obtaining minor and trace element profiles across compositional interfaces within crystals at high spatial resolution

petrology of crystal clots in the pumice of mount st helens march 19 1982 eruption significant role of fe ti oxide crystallization abstract a petrographic mineralogical and chemical description of the dacite pumice and of the contained mafic crystal clots from the march 19 1982 eruption of mount st helens is presented the crystal clots are interpreted as cumulates and the phenocryst phases in the pumice are not fully representative of phases fractionating at depth the crystallization sequence is inferred to be olivine plus plagioclase followed by pyroxenes and late stage amphibole magnetite and ilmenite crystallized early providing a mechanism for minimal iron enrichment as estimated from fe ti oxide and two pyroxene geothermometers crystallization temperatures were approximately 1000 c at oxygen fugacities approximately one order of magnitude higher than the nno buffer the parental magma was more mafic than dacite and had a comparatively low water content as the magma evolved to dacite the water content increased to a value in excess of 4 wt and amphibole began to crystallize

a latest cretaceous hotspot and the southeasterly tilt of britain the southeasterly regional dip of england and wales is usually ascribed to miocene movements loss of up to 2 km of cover from northern england shown by apatite fission track analysis has been related to uplift and erosion induced by a mantle hotspot but its effects were far more widespread than considered hitherto outcrop patterns throughout much of england wales and eastern ireland can be related to a hotspot beneath the irish sea uplift occurred within the late maastrichtian and erosion continued into the late palaeogene after post oligocene down faulting of the irish sea the modern drainage was initiated on the eroded surface unconformities provide the best means of dating orogenic movements in some localities the time interval between the youngest rocks preserved beneath the unconformity and the oldest preserved above may be small enough to enable the dating of fold movements to be determined with some precision thus in southeastern britain the major e w structures of the hampshire basin the weald and the london basin can be dated as miocene as rocks of oligocene age are involved in the folding in the hampshire basin and pliocene sands rest in erosional pockets of the chalk of se england because fold movements tend to affect large areas of the crust at discrete intervals of time the southeasterly tilt of britain evident from the fact that a nw se transect from anglesey to london crosses outcrops of rocks getting progressively younger from the precambrian of anglesey to the tertiary of the london basin has

multiplets their behavior and utility at dacitic and andesitic volcanic centers 1 multiplets or groups of earthquakes with similar waveforms are commonly observed at volcanoes particularly those exhibiting unrest using triggered seismic data from the 1980 1986 mount st helens msh eruption we have constructed a catalog of multiplet occurrence our analysis reveals that the occurrence of multiplets is related at least in part to the viscosity of the magma we also constructed catalogs of multiplet occurrence using continuous seismic data from the 2004 eruption at msh and 2007 eruption at bezymianny volcano russia prior to explosions at msh in 2004 and bezymianny in 2007 the multiplet proportion of total seismicity mpts declined while the average amplitudes and standard deviations of the average amplitude increased the life spans of multiplets time between the first and last event were also shorter prior to explosions than during passive lava extrusion dome forming eruptions that include a partially solidified plug like msh 1983 1986 and 2004 2008 often possess multiplets with longer life spans and mpts values exceeding 50 conceptually the relatively unstable environment prior to explosions is characterized by large and variable stress gradients brought about by rapidly changing overpressures within the conduit we infer that such complex stress fields affect the number of concurrent families mpts average amplitude and standard deviation of the amplitude of the multiplets we also argue that multiplet detection may be an important new monitoring tool for determining the timing of explosions and in forecasting the type of eruption

addition of magmatic volatiles into the hot spring waters of loowit canyon mount st helens washington usa geochemical studies on cold meteoric waters post 1980 hot spring waters fumarole emissions from the dacite dome and volcanic rocks at mount st helens msh from 1985 to 1989 show that magmatic volatiles are involved in the formation of a new hydrothermal system hot spring waters are enriched in 18o by as much as 2 and display enrichments in d relative to cold waters a well defined isotopic trend is displayed by the isotopic composition of a 400 c fumarole condensate collected from the central crater in 1980 33 d 6 18o of condensate samples collected on the dome and of cold meteoric and hot spring waters the trend indicates that mixing occurs between local meteoric water and magmatic water degassing from the dacite dome between 30 and 70 magmatic water is present in the dome fumarole discharges and 10 magnatic water has been added to the waters of the hydrothermal system relations between cl so4 and hco3 indicate that the hot spring waters are immature volcanic waters formed by reaction of rocks with waters generated by absorption of acidic volcanic fluids in addition the b cl ratios of the spring waters are similar to the b cl ratios of the fumarole condensates 0 02 values of 13c in the hco3 of the hot springs 9 5 to 13 5 are similar to the magmatic value at msh 10 5 and the 3he 4he ratio relative to air in a hot spring water is 5 7 suggesting a magmatic origin for this component

geomechanics of hydraulic fracturing microseismicity part 2 stress state determination we investigate the hydraulic fracturing process by analysis of the associated microseismicity in part 1 we recognized double couple and hybrid microseismic events and their fault plane orientations critical stress instability and stress inversion techniques were used to assess fracture activation conditions in part 2 we apply results from the tensile source model to investigate how activated faults relate to the stress state and geologic setting we assess potential mechanisms for induced microseismicity including leakoff and diffuse pressurized fracture network flow stress shadowing adjacent to large parent hydraulic fractures and crack tip stress perturbations data are from the mississippian barnett shale texas and include microseismic events from sequential pumping stages in two adjacent horizontal wells that were recorded in two downhole monitor wells as well as operations wellbore derived stress and natural fracture data results point to activation of inclined faults whose orientation is dominantly northeast southwest and vertical north south faults the activation stress states for a range of modeling scenarios show stress rotation decreased mean stress and increased deviatoric stress this stress state cannot be explained by sidewall leakoff in the stress shadow region adjacent to hydrofractures but is consistent with hybrid and shear activation obliquely ahead of pressurized fractures information about hydrofracture evolution and operationally related dynamic stress change is obscured by geomechanical heterogeneity that is likely geologic in nature the most compelling observation is that the most highly misoriented microseismic faults occur in the same vicinity as a carbonate dominated submarine fan feature that was previously expected to act as a minor fracture barrier

three dimensional geometry and kinematics of experimental piggyback thrusting the three dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box strike parallel geometric elements simulated include lateral ramps eyed sheath folds splays and thrust thrust interference fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum eroding technique a kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport despite the steady forward advance of a rear wall the piggyback wedge accreted episodically recording different strain domains in longitudinal cross sections strain partitioning in single layers by bed length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth

analysis of swarms of high frequency seismic events at nevado del ruiz volcano colombia january 1986 august 1987 development of a procedure abstract the determination of signal properties like frequency amplitude signature of the sequence of arrivals of p and s phases and the subsequent computation of hypocenter parameters are the steps of a procedure in the analysis of swarms of high frequency seismic events from nevado del ruiz volcano which could be useful in other volcanic areas the use of this procedure on ruiz data led to the determination of various seismic source volumes which together with hypocenters of non swarmed events and the identification of geological features such as faults which cross the area alignment of domes and volcanic edifices and the record of low frequency seismic events not centered in the area of the active vent suggest the existence of a magma chamber of a rather complex geometry this hypothesis would suggest that magma emplaced in zones of weakness faults that affect the area may act as the source of seismic activity this demands a revision of the methodology used in the deformation measurements and in the analysis of released energy and event occurrence too the relations between hypocenters sources location and relative energy of the swarms as well as the general volcanic activity tremor ash and gas emission deformation could turn out to be a possible means to assist in the identification of premonitory activity and thus be relevant to the process of monitoring and forecasting

relationship between basement linked and gravity driven fault systems in the ukcs salt basins abstract extension of basement below significant detachment layers such as salt results in the formation of structures in the cover strata above the detachment layer in two ways thick skinned cover structures evolve to balance basement stretching these are linked to the basement faults via detachment in the salt layer or are directly linked to the basement structure depending on the amount of basement fault displacement relative to salt layer thickness basement extension also results in rotation of major basement fault blocks tilting the detachment layer and precipitating gravity driven thin skinned slip of the cover involving detached extensional faulting and folding detachment layers consisting specifically of salt play two roles in facilitating the development of thin skinned systems salt constitutes the detachment layer and also lends buoyancy to the cores of anticlinal folds reducing resistance to shortening the detachment layer may become tilted away from the basement fault scarp resulting in synthetic slip systems with the same polarity as the underlying basement fault or the detachment may be tilted toward the basement scarp giving antithetic slip systems of the opposite polarity in both cases the slides may terminate downdip in detachment folds and or reverse faults both types of thin skinned system will interact with those cover structures directly balancing basement extension and all of these components must be considered during section restoration tilt associated with 3d basement fault geometry e g relay zones may result in decoupled cover faults locally trending at high angles to the trend of underlying basement faults contrasting strain rates of basement faulting and related cover slip can lead to continuation of cover structural evolution after cessation of basement extension giving for example cover faults and folds of lower cretaceous age in the central north sea though basement extension had occurred in the upper jurassic synthetic systems may include footwall derived olistostromes emplaced above basement hangingwalls these commonly occur above basement relay ramps on segmented basin margins examples from the central graben and channel basin are discussed examples of antithetic systems from the irish sea central graben and a field example from the bristol channel basin are discussed

fault distribution and timing in the central irish sea basin well data analysis and the interpretation of 2d and 3d seismic reflection data provide valuable insights into the distribution and timing of fault activity within the central irish sea basin cisb structural and stratigraphic relationships have been used to constrain the timing of fault movements and to interpret the mapped fault patterns in terms of the tectonic evolution of the area four main fault trends are identified at the top lower triassic sherwood sandstone group level i a ne sw fault trend that parallels the basin bounding faults and is believed to be of mesozoic age ii a pervasive system of north south trending faults that cross cut the earlier ne sw trending faults which manifests evidence of later mesozoic extension followed by post oligocene transpressional fault reactivation iii a nne ssw trending steeply dipping fault set iv a wnw ese trending conjugate extensional set that formed perpendicular to the nne ssw trending transpressional faults during late tertiary dextral shearing early tertiary axial centred basin inversion and regional exhumation have resulted in the elevation of the sherwood sandstone reservoir to shallow structural levels within the basin continued fault reactivation into late tertiary time has resulted in the compartmentalization of mapped structural closures and suggests that trap integrity is a major exploration risk factor in the cisb the central irish sea basin cisb is a ne sw trending graben located between the kish bank basin to the nw and the st george s channel and cardigan bay basins to the se fig la the cisb forms part of a linked system of mesozoic basins between the uk and ireland which also

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

the tectonic evolution of the north celtic sea and cardigan bay basins with special reference to basin inversion abstract the morphology and structural evolution of the north celtic sea basin ncsb and cardigan bay basin cbb are in large part controlled by reactivation of basement features of caledonian and possibly variscan origin from earliest triassic until maastrichtian times sedimentation was almost continuous initially in a rapidly subsiding rift system and finally in a progressive passive sag phase at the onset of the alpine orogeny both basins experienced spectacular inversion along readily identifiable axes the inversion takes the form of a compressive reactivation and reversal of the pre existing growth faults and a broad regional upwarping of the cretaceous depocentre the amount of inversion varies depending on the location within the basin but values in excess of 1000 m are possible by utilising regional seismic data the timing of inversion can be estimated as occurring between maastrichtian and eocene times which correlates with the so called laramide pulse of the alpine orogeny

eruption prediction aided by electronic tiltmeter data at mount st helens telemetry from electronic tiltmeters in the crater at mount st helens contributed to accurate predictions of all six effusive eruptions from june 1981 to august 1982 tilting of the crater floor began several weeks before each eruption accelerated sharply for several days and then abruptly changed direction a few minutes to days before extrusion began each episode of uplift was caused by the intrusion of magma into the lava dome from a shallow source causing the dome to inflate and eventually rupture release of magma pressure and increased surface loading by magma added to the dome combined to cause subsidence just prior to extrusion

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

salt tectonics in the norwegian danish basin a study is presented of salt tectonics in a 28 000 km2 area in the norwegian danish basin of the north sea the basin is mainly of permian to early jurassic age and contains sediments which locally reach 8 km in thickness zechstein upper permian salt occurs widely in the basin and has given rise to widespread and intense salt tectonics using seismic reflection sections with a total length of 14 000 km as well as borehole data 86 salt structures have been mapped within the study area the floor of the basin that is the base of the zechstein salt and the faults affecting this floor have also been mapped it is therefore possible to investigate the association of faults and salt structures and hence to ascertain to what extent faulting of the basin floor may have initiated salt movement various lines of investigation and reasoning have been developed to arrive at an estimate of the number of structures for which salt movement is likely to have been initiated by faulting of the basin floor it is found that only for a limited number of salt structures probably about five out of a total of 86 structures the salt movement is likely to have been initiated by faulting the very large majority of the salt structures originated independent of faulting the mechanism responsible for the initiation of salt movement in this majority has also been studied it is found that these salt structures can be divided into two groups one group comprises about 37 salt structures which are randomly distributed nearly all of them are situated where the present basin floor is relatively flat and at its deepest they are thought to be the result of classical halokinesis that is the autonomous upward movement under the influence of gravity of mobile salt at locations where the sedimentary overburden at the time of initial movement was locally less heavy a second group comprises about 43 individual structures which are aligned along nine so called trend lines it is first shown that these trend lines are independent of faults and fault trends in the basin floor the origin of these trends is then discussed by considering two alternative hypotheses for the mechanism responsible for the initiation of salt movement along these trend lines the first hypothesis assumes that salt ridges in the subsurface are initially pushed up by a prograding sedimentary overburden the second hypothesis assumes that initial ridges of mobilized salt are formed along a palaeo depth contour which migrates up the flank of the subsiding basin both hypotheses assume that the salt ridges thus formed can subsequently develop into individual salt structures which produce the observed pattern of trend lines a critical analysis of these two alternative hypotheses leads to the conclusion that the second one involving the concept of a palaeo depth contour which migrates up flank is the more acceptable the mechanism involved may be characterized as a special type of halokinesis namely as halokinesis constrained to operate along a line

geometric models of listric normal faults and rollover folds existing geometric models allow master fault shapes to be constructed given the shape and heave or displacement of a deformed marker horizon in the hanging wall these models assist in projecting normal faults to depth where the fault geometry is poorly constrained by available seismic data currently it is unclear which model best predicts the actual relation between rollover and fault geometries to address this deficiency the relative accuracies of the constant heave constant displacement constant bed length slip line and inclined shear constructions are evaluated using clay model analogs and seismic examples each of the geometric models predicts a different fault shape and depth to detachment for the same rollover shape because different hanging wall deformat on mechanisms are assumed a construction is developed for determining the hanging wall geometry from a known fault geometry this construction is based on the inclined shear model and geometric constraints imposed by material displacement paths viewed from different reference frames fixed to the hanging wall and footwall blocks in clay models the inclined shear antithetic shear greek alpha 20 degrees and slip line constructions produce the best agreement between actual and modeled fault and marker bed shapes in rollovers for subsurface examples the inclined shear model antithetic shear greek alpha 20 40 degrees and the constant displacement model predict fault trajectories that are the most similar to the positions and shapes of the master fault segments interpreted fr m offshore norway and gulf of mexico seismic lines the shear angle required in the inclined shear model can often be estimated from the orientation of minor faults in the hanging wall the shear angle may also be determined through iterative modeling in which the shear angle is systematically varied and the resultant model fault shape is compared to the shape of the shallow segment of the master fault interpreted from the seismic record

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

upper crustal structure of the mount hood oregon region as revealed by time term analysis seismic refraction data with a dense areal distribution were collected to study the seismic structure of mount hood and the surrounding region this area is typical of cascade volcanoes and is geologically quite complex the prime goals of this project were to search for velocity variations in the upper crustal rocks and to determine if the velocity of these rocks is anisotropic a new system including 100 remote recording units was developed to facilitate the collection of data in this type of survey the data collected in this study reveal a large variation in velocity and thickness of the uppermost crustal rocks that is probably typical of the high cascade province a regional structural pattern surrounding mount hood where there is a marked thinning of low velocity near surface rocks suggests that the present edifice of mount hood lies on top of a much larger structure possibly the roof of a large batholith that was emplaced prior to the eruption of the volcanic rocks that form the modern mountain a straightforward time term analysis of the data reveals this structure when the time terms are used to correct the travel time observations for the variation in structure under each station the remaining set of residuals indicates a variation of travel time with azimuth while a systematic error in the time term reduction cannot be ruled out this variation is probably the result of anisotropic velocity structure in the underlying refractor the alignment of the direction of maximum velocity with the direction of maximum principal stress and the decrease in anisotropy with depth suggest that the anisotropic velocity structure is related to the opening and closing of fractures in the present stress field the direction of maximum velocity is about n25 w and the degree of anisotropy appears to vary with depth from about 3 near the surface to zero at depths below 8 km

hydrogen isotope evidence for extrusion mechanisms of the mount st helens lava dome from october 1980 to october 1986 the dacite lava dome at mount st helens grew through the accumulation of more than 15 separate lobes during extrusive episodes that lasted from 1 to 24 days except for an anomalous period of continuous growth throughout most of 19831 over these six years the growth rate was generally volume predictable that is the amount of lava added to the dome in a given eruption was roughly proportional to the time since the preceding extrusion2 these well documented eruptions were separated by repose intervals lasting from several months to more than a year2 in an attempt to identify the triggering mechanisms for these periodic eruptions we have used hydrogen isotope analyses to determine water content and deuterium content for 18 samples of the mount st helens dome dacite these isotope data the first ever collected from an active lava dome suggest a steady state process of magma evolution combining crystallization induced volatile production in the chamber with three different degassing mechanisms closed system volatile loss in the magma chamber open system volatile release during ascent and kinetically controlled degassing upon eruption at the surface the data suggest that future dome building eruptions may require a new influx of volatile rich magma into the chamber

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

the gravity anomaly of the st george s channel basin southern irish sea a possible explanation in terms of salt migration gravity profiles across the st george9s channel basin show a marked discrepancy between the observed anomalies and the anomalies calculated using a model based upon an interpretation of seismic data this difference may be explained in terms of salt migration away from the centre towards the flanks of the basin evidence for such a migration path is provided by the seismic data an excess of salt on the se side of the basin has intruded into a major fault giving rise to a salt wall the gravity models suggest that the basin reaches a thickness of 10 km near its centre but that permian and carboniferous sediments are possibly thin or absent

faulting and fracturing in shale and self sourced reservoirs introduction production from self sourced reservoirs relies on natural and induced fracturing for permeability and conductance of hydrocarbons to the producing wellbores thus natural or induced fracturing is often a key to success in unconventional reservoir plays on the other hand fractures may compromise seals and large or well connected fractures or faults may cause undesirable complications for unconventional reservoirs natural and induced fractures are influenced by 1 mechanical stratigraphy 2 pre existing natural deformation such as faults fractures and folds and 3 in situ stress conditions both natural and as modified by stimulation and pressure depletion this special issue of the aapg bulletin elucidates some of these structural geologic and geomechanical controls understanding the occurrence and controls on natural and induced faulting and fracturing in self sourced reservoirs is a key component for developing effective approaches for exploiting self sourced reservoirs

high precision relative locations of earthquakes at mount st helens washington several earthquake sequences have occurred at mount st helens as clusters of shallow events with nearly identical waveforms multiplets we present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques a multiplet is studied in pairs of events called doublets each doublet is made up of a reference event and one other event from the multiplet the cross spectrum of a short window beginning just before the p arrival is used to compute differences in arrival times between seismograms from the same station for each doublet these time differences computed with a precision of 1 2 ms are then used to locate each earthquake relative to the reference event engineering explosions with well known locations are used to test the relocation technique explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion the relocation technique is applied to earthquakes preceding the september 10 1984 and the may june 1985 dome building eruptions of mount st helens forty events that occurred during a 12 hour period in september 1984 are located in a volume approximately 30 m in diameter beneath the lava dome the earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume in the may june 1985 eruption we recognize three multiplet sets that took place successively in time the multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit

field examples and numerical modelling of oversteps and bends along normal faults in cross section abstract oversteps and bends along cross sections through small scale numerical modelling distinct element analysis has been carried out to simulate the patterns of deformation at oversteps and bends area increase pull apart development and localised stress concentration occur at extensional oversteps and bends stress concentration and a tendency for compaction are typical of contractional oversteps and bends such results are compatible with the structures observed in the field a sensitivity analysis of the modelling has been carried out to investigate the effects of variations in mechanical parameters on deformation although models with weaker properties are more deformable the overall geometry of deformation varies little with variations in mechanical parameters

explosive tephra emissions at mount st helens 1989 1991 the violent escape of magmatic gas following storms from 24 august 1989 until 18 june 1991 mount st helens produced at least 28 shallow explosion like seismic events with signatures similar to those produced by gas explosions on the dome during the mid 1980s at least six were accompanied by violent emission off non juvenile tephra ejection of blocks of rock nearly 1 km from the vent and avalanching of debris off the north side of the dome four produced no emission of tephra and the remainder occurred when the crater could not be observed although later crater visits found no new tephra deposits all six confirmed emissions and most although not all other seismic events took place hours to days after storms over time periods ranging from 2 to 15 days statistical goodness of fit tests give a probability of 1 x 10 2 to 1 x 10 5 that the precipitation prior to emissions was fortuitously greater than normal similar tests for precipitation prior to all 28 events give probabilities of 3 x 10 2 to 1 x 10 3 the short delay between storms and emissions suggests that the events that follow storms originate at very shallow depth probably within the dome itself although the exact causal mechanism is not known it is speculated that slope instability or accelerated growth of cooling fractures following storms may have released gas trapped within or at the base of the dome of the events that did not follow storms three were located seismically between 1 4 and 2 1 km depth thus the events as a group are not confined to shallow depth and probably record gas transport from a deeper magmatic source a deep source is also suggested by an unprecedented 4 yr long increase in background seismicity at 2 to 9 km depth that peaked in 1989 and 1990 calculations indicate that the energy content of gas exsolved from the crystallizing conduit was probably sufficient to power the emissions

control of mechanical stratigraphy on bed restricted jointing and normal faulting eagle ford formation south central texas outcrops of the middle eagle ford formation in south central texas reveal well developed joint networks in subhorizontal competent carbonate chalk beds and less well developed networks in interlayered incompetent calcareous mudrock beds northeast striking bed perpendicular joints in competent beds have the longest trace lengths and are abutted by northwest striking joints all observed joints terminate vertically in incompetent beds normal faults are common but less abundant than joints dominantly dip north northwest or southeast and are abutted by the joint sets and thus predated jointing the faults cut multiple competent and incompetent beds providing vertical connectivity across mechanical layering products of hybrid and shear failure the dip of these faults is steep through competent beds and moderate through incompetent beds resulting in refracted fault profiles with dilation and calcite precipitation along steep segments fluid inclusions in fault zone calcite commonly contain liquid hydrocarbons rare two phase fluid inclusions homogenized between about 1 40 and 58 c and 2 90 and 100 c suggesting trapping of aqueous fluids at elevated temperatures and depths on the order of 2 km 6562 ft fluid inclusion and stable isotope geochemistry analyses suggest that faults transmitted externally derived fluids these faults likely formed at depths equivalent to portions of the present day oil and gas production from the eagle ford play in south texas faults connect across layering and provide pathways for vertical fluid movement within the eagle ford formation in contrast to vertically restricted joints that produce bed parallel fracture permeability these observations elucidate natural fractures and induced hydraulic fracturing within the eagle ford formation

failure of cap rock seals as determined from mechanical stratigraphy stress history and tensile failure analysis of exhumed analogs the sedimentologic and tectonic histories of clastic cap rocks and their inherent mechanical properties control the nature of permeable fractures within them the migration of fluid through mm to cm scale fracture networks can result in focused fluid flow allowing hydrocarbon production from unconventional reservoirs or compromising the seal integrity of fluid traps to understand the nature and distribution of subsurface fluid flow pathways through fracture networks in cap rock seals we examine four exhumed paleozoic and mesozoic seal analogs in utah we combine these outcrop analyses with subsidence analysis paleoloading histories and rock strength testing data in modified mohr coulomb griffith analyses to evaluate the effects of differential stress and rock type on fracture mode relative to the underlying sandstone reservoirs all four seal types are low permeability heterolithic sequences that show mineralized hydraulic extension fractures extensional shear fractures and shear fractures burial history models suggest that the cap rock seal analogs reached a maximum burial depth 4 km 2 5 mi and experienced a lithostatic load of up to 110 mpa 15 954 psi median tensile strength from indirect mechanical tests ranges from 2 3 mpa 334 psi in siltstone to 11 5 mpa 1668 psi in calcareous shale analysis of the pore fluid factor through time shows changes in the expected failure mode extensional shear or hydraulic extension and that failure mode depends on a combination of mechanical rock properties and differential stress as expected with increasing lithostatic load the amount of overpressure that is required to induce failure increases but is also lithology dependent

the south celtic sea bristol channel basin origin deformation and inversion history abstract the south celtic sea bristol channel basin is a relatively narrow and elongated sedimentary trough located within the northern part of the hercynian fold belt and filled with permo triassic redbeds to jurassic marine sediments its present configuration is the result of a three fold sequence of tectonic phases initiated during the late palaeozoic the origin of the basin appears to be related to localised extensional reactivation of hercynian thrust zones during the permian and early triassic followed by regional subsidence during the late triassic and jurassic uplift of the basin margins together with dextral strike slip movements in an east west sense and large scale erosion during the late jurassic to early cretaceous resulted in the present elongated configuration of the lower mesozoic basin regional subsidence from aptian time onwards resulted in widespread deposition of a relatively thin section of wealden elastics and upper cretaceous chalk unconformably overlying folded pre cretaceous strata sedimentation was interrupted during the early tertiary by a series of tectonic events that included emplacement of the lundy igneous mass during the early eocene and mild basin inversion during the oligo miocene inversion was accompanied by dextral reactivation of the nw se oriented sticklepath fault zone the total amount of uplift probably not exceeding 350 m reflects the rather stable configuration of the underlying lithospheric crust as indicated by seismic reflection and refraction data

salt flow and diapirism related to extension at crustal scale abstract the analysis of the structure and history of salt diapirs in large ancient evaporitic basins such as the southeastern north sea basin suggests that diapirism is controlled by crustal scale extension laboratory experiments on small scale models designed to simulate configurations comparable to the southeastern north sea basin show that diapirs which grow in a sedimentary cover during basement extension can develop close to basement normal faults or at a significant distance from them if the displacement on the basement faults is transfered horizontally along the salt layer for given values of salt viscosity strain rate and friction coefficient in brittle rocks the pattern of salt diapirism in a particular area depends on the ratio of cover thickness to salt thickness as well as the absolute thicknesses of cover and salt layers the experiments give criteria for identifying different types of diapirs and provide models of horizontal flow in a salt layer undergoing extension and diapir development some dynamic implications of the experiments are applied to the southeastern north sea basin

deformation monitoring at mount st helens in 1981 and 1982 for several weeks before each eruption of mount st helens in 1981 and 1982 viscous magma rising in the feeder conduit inflated the lava dome and shoved the crater floor laterally against the immobile crater walls producing ground cracks and thrust faults the rates of deformation accelerated before eruptions and thus it was possible to predict eruptions 3 to 19 days in advance lack of deformation outside the crater showed that intrusion of magma during 1981 and 1982 was not voluminous

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

the 14 february 1981 elk lake washington earthquake sequence a magnitude 5 5 ml earthquake occurred on 14 february 1981 near elk lake in the southern washington cascade range it was the largest earthquake to occur in washington since the 1965 seattle earthquake mb 6 5 the epicenters of 1050 located aftershocks mcoda 0 delineate a fault zone 6 km long north south by 3 km wide that extends from 5 to 12 km in depth immediately after the main shock aftershocks occurred at generally deeper depths to the north and south indicating that the break propagated bilaterally and downward first motion patterns for the main shock and 84 aftershocks mcoda 2 0 indicate predominantly right lateral strike slip faulting along a nearly vertical plane striking essentially north south in agreement with the alignment of epicenters earthquake activity at elk lake for the 7 yr period to 1980 was low nine months before the main shock an earthquake swarm began just southeast of what was to become the present aftershock zone the swarm lasted 2 months and was followed by 7 months of quiescence before two immediate foreshocks occurred on 11 february 1981 the overall pattern of seismicity observed throughout the elk lake sequence corresponds well to that predicted by an asperity model examination of the magnitude frequency relation b indicates that there was no significant temporal variation in b throughout the swarm main shock after shock sequence however b is significantly higher on the east side of the fault zone than on the west side for the entire aftershock sequence b 0 77 0 05 this earthquake sequence is important for three reasons 1 the main shock is the largest earthquake to occur in the pacific northwest since 1965 2 the sequence has been long and aftershocks are numerous and 3 its occurrence indicates that the potential seismic hazard in southwest washington must be reevaluated

allochthonous salt structure and stratigraphy of the north eastern gulf of mexico part ii structure abstract the kinematic evolution of allochthonous salt in the north eastern gulf of mexico proceeds in three stages 1 since its middle jurassic deposition the louann salt was loaded by sediments causing episodic basinward movement of salt ultimately leading to large concentrations of salt masses in a slope environment by the end of the lower cretaceous 2 a regime of starved sedimentation during late cretaceous and early oligocene is responsible for the stabilization of these early salt accumulations 3 with renewed rapid accumulation of sediments during the neogene and pliocene extensive allochthonous salt tongues and sheets formed by gravity spreading within the younger sediments of the slope autochthonous salt allochthonous salt and detached allochthonous salt are typical stages of evolution major down to the basin growth faults separate allochthonous salt sheets from their original feeder stocks extension along down to the basin master growth faults is compensated mainly by salt withdrawal and partly by basinward shortening

possible variscan thrusting beneath the somerton anticline somerset vibroseis seismic data were acquired by goal petroleum plc during 1982 in the vicinity of the somerton anticline somerset these data show the simple form of the anticline but in addition show a southward dipping discontinuous seismic event beneath gravity modelling demonstrates that this event may be correlated with an e w trending gravity gradient which can be followed for approximately 150 km the dipping seismic event may represent a variscan thrust plane which dips to the ssw in contrast to similar planes seen to the east which dip to the sse the variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within and to the east of the mendip hills the polden hills fault runs sub parallel to the proposed thrust plane beneath the somerton anticline and may be linked to a mesozoic reactivation of the postulated thrust

post eruptive seismic activity of mount cameroon cameroon west africa a statistical analysis abstract recent seismological studies of the cameroon volcanic line show that mt cameroon is the most active centre so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986 the network was reinforced with temporary stations up till 1987 and the local seismicity was studied here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations four swarms lasting 9 to 14 months are identified at intervals of 2 3 years most earthquakes are felt intensity and magnitude respectively less than vi mm and 5 during the first three swarms and a few during repose periods the main focal regions are the northwest and southeast flanks the bimbia and bioko regions in the south of the volcano hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities the subcrustal events are observed only in the southeast flank they are the most regular earthquakes with a monthly frequency of 9 to 15 events they are characteristic earthquakes with magnitude 2 8 0 1 between 1984 and 1992 their yearly mean time interval between successive events range from 50 to 86 hours for that period their occurrence can be modelled as a stationary renewal process with a 3 day period but the analysis of variance shows possible significant differences among yearly means a weibull s distribution confirms that the time intervals between successive deep events are not independent and in 1993 a swarm of deep earthquakes is recorded hence a non loglinear magnitude frequency relation the deep seismicity is thought to be associated with a zone of weakness perhaps a magmatic conduit and may have some close relationship with the magmatic activity

synthesis of salt tectonics in the southern north sea uk abstract a structural model encompassing the southern north sea basin west of the central graben has been developed this model consists of a rift system affecting the post salt section around the basin margin and a large area of detached compressional buckle folds within the basin this pattern is initially a response to gravity sliding of the post salt section on the salt within the basin during the late triassic to late jurassic a close relationship between the location and trend of the peripheral graben system and basement structures in the pre salt is noted pre jurassic extension across the peripheral graben systems was balanced by the sum of fault heaves at the pre salt rotliegend level and shortening across salt cored buckle folds in the post salt section salt pillows and swells passively infilled the cores of these gravity induced buckle folds cretaceous and tertiary inversion involved basin tilt and renewed movement on basin bounding basement faults notably reverse movements did not propagate from basement structures up into the peripheral graben systems the post salt sedimentary section experienced gravity spreading in response to inversion related uplift resulting in syn inversion extensional faulting in the sole pit high where the mesozoic section was thickest this extension combined with a loss of fault heave in the pre salt section was balanced by amplification of salt cored buckle folds in the centre of the basin in the context of the model described here salt pillows represent passive infill of thin skinned compressional buckle folds which later amplified during thick skinned basement shortening crestal collapse of such folds occurs via normal faulting accompanied by reactive diapirism such reactive diapirs establish conduits through which salt may leak leading to pillow deflation and ultimately conduit preservation as a salt wall flanked by rim synclines in areas where the buckle folds were emergent the salt structures described here are related to cover folds and faults which in turn reflect episodes of basin extension tilting and inversion hence individual salt structures can be said to be only remotely connected with regional intraplate stresses

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

the geometry of planar domino style normal faults above a dipping basal detachment abstract geometrical analysis of planar domino style normal faults rooted into a dipping basal detachment fault allows derivation of equations which relate 1 horizontal extension within the upper plate 2 dip of the detachment 3 final fault dips 4 rotation that faults and beds undergo and 5 net slip on domino style faults past geometrical models have focused on extremely idealized and non unique geometries in which domino style faults are parallel and similar cut off points are all at the same elevation after faulting this corresponds to evenly spaced domino style faults above a horizontal detachment considering non parallel faults and dipping detachments introduces unique geometries which allows calculation for example of permissible depth ranges to the detachment horizontal extension varies significantly 1 with dip of the detachment and 2 for synthetic and antithetic cases in which the domino style faults dip in the same or opposite direction respectively as the detachment when other factors e g rotation and fault dips are held constant horizontal extension greatly increases for moderate detachment dips and moderately decreases for antithetic detachment dips as compared to the horizontal detachment case this is important because the synthetic case has been widely reported

seismic precursors to the mount st helens eruptions in 1981 and 1982 six categories of seismic events are recognized on the seismograms from stations in the vicinity of mount st helens two types of high frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers medium and low frequency earthquakes occur at shallow depths less than 3 kilometers within the volcano and increase in number and size before eruptions temporal changes in the energy release of the low frequency earthquakes have been used in predicting all the eruptions since october 1980 during and after eruptions two types of low frequency emergent surface events occur including rockfalls and steam or gas bursts from the lava dome

mesozoic regional tectonics and south viking graben formation evidence for localized thin skinned detachments during rift development and inversion abstract a regional thin skinned model of south viking graben development is presented in which zechstein marginal evaporites and interpreted salt have acted as a key de collement horizon along which mechanical decoupling of post salt triassic and younger strata from pre salt basement has occurred the western margin of the south viking graben defines an asymmetrical late jurassic half graben controlled by episodic extension along a nne ssw trending eastward dipping fault system an initial minor phase of rifting in the triassic is suggested by the slight westward thickening and associated divergence of interpreted triassic sequences at depth the scarcity absence of upper triassic and lower jurassic sequences is believed to result from a subsequent phase of uplift which affected the region in the mid cimmerian attributed to the development of a thermal dome at the future site of a jurassic rift triple junction to the southwest the onset of large scale extension from the late callovian onwards marked the major phase of half graben formation and created a regional hanging wall tilt which locally caused gravity gliding of post salt section to the wnw with resultant low angle extensional faulting upslope and salt cored buckling downslope the late jurassic half graben was subsequently tightened by a phase of thin skinned inversion in the latest jurassic early cretaceous associated with back steepening of the western margin fault system with resulting reactivation of the extensional detachment system the effects of this inversion have been noted in many parts of the northern north sea within the south viking graben shortening estimates vary between 14 and 18 and are expressed on seismic data by localized hanging wall folds and the back steepening of faults along the western margin fault complex of the graben together with out of graben thrusting up the hanging wall dip slope variation in both the extension and inversion mechanisms appears to be controlled by interpreted nw se transfer systems which compartmentalize the graben into three subgraben within which differential zechstein basin development occurred

planar detaching faults in the southern horn graben danish north sea the danish horn graben is located in the south eastern north sea and suffered fault controlled differential subsidence during the triassic within the horn graben several faults cut the triassic succession but detach at or close to the top pre zechstein surface an e w striking trend of planar detaching faults is mapped using 2 d seismic sections and the displacement on the faults is analysed with respect to timing and spatial distribution of the displacement the spatial distribution of the displacement is analysed using the fault analysis projection system badley earth sciences whereas the timing is analysed by reconstructing the seismic sections and thus obtaining the spatial distribution of displacement at different times the occurrence of the detaching planar faults appears to be controlled by the difference in basal friction across the pinch out line of the mobile zechstein sediments a dipping detachment surface is necessary to create a driving mechanism but is of minor importance in initiating the faulting the timing of the faulting is controlled by a general change in tectonic pattern enabling a n s extension of the sediments covering the top pre zechstein surface

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

deep earthquakes beneath mount st helens evidence for magmatic gas transport small magnitude earthquakes began beneath mount st helens 40 days before the eruption of 20 march 1982 unlike earlier preeruption seismicity for this volcano which had been limited to shallow events less than 3 kilometers many of these earthquakes were deep between 5 and 11 kilometers the location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 march eruption than prior to any of the earlier dome building eruptions the depth time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir

the role of seismic attributes in understanding the hydraulically fracturable limits and reservoir performance in shale reservoirs an example from the eagle ford shale south texas as the importance of self sourcing reservoirs continues to increase it is more important than ever to evaluate rock properties that contribute to productive wells it has become increasingly evident that in order to maximize potential returns an integrated approach to shale play characterization is necessary to identify productive areas numerous criteria exist to characterize ultra low permeability shale reservoirs and their associated resource potential these include measures of organic richness thermal maturity lithologic heterogeneity and formation brittleness the latter a descriptor of the geomechanical rock properties can play a significant role in overall well performance and is commonly a key productivity driver thus an understanding of the mechanical properties of the target section is fundamental for high grading prospective areas well placement design and hydraulic stimulation effectiveness observation of geomechanical attributes extracted from seismic data in the eagle ford shale captures changing mechanical properties indicative of strike oriented lithologic facies changes using acoustic logs core and three dimensional seismic data we assess the mechanical contrast between eagle ford facies units and their effect on well performance we use three dimensional seismic data to map the structure and facies distribution in an area where identification of reservoir facies is a major challenge to development drilling in this study we demonstrate how young s modulus and density inverted from three dimensional seismic data prove as effective discriminators for the purpose of identifying and mapping facies changes and establishing the hydraulically fracturable limits in areas where effective stimulation and proppant embedment in the formation during pressure drawdown is a concern the result is an interpretation that identifies and uses the mechanical changes from three dimensional seismic data attributes associated with the brittle carbonate rich eagle ford facies to predict both the reservoirs hydraulically fracturable limits as well as the variability in well performance associated with proppant embedment the changes in mechanical properties of the eagle ford facies are important in high grading productive intervals in these ultra low permeability rocks we believe we can apply this method to other shale reservoirs where rock mechanics may play an important role

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory