three dimensional tomographic velocity structure of upper crust coaxial segment juan de fuca ridge implications for on axis evolution and hydrothermal circulation three dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23 564 ocean bottom seismometer p wave arrivals define systematic lateral variations in seismic structure of the coaxial segment of the juan de fuca ridge jdfr over much of the segment the across axis structure is roughly axisymmetric characterized by a progressive increase in dike velocities moving away from the ridge axis this trend is most apparent in the basal dikes where on axis velocities are about 800 m s slower than those measured elsewhere within the rift valley the on axis sheeted dikes also exhibit ridge oriented azimuthal anisotropy with a peak to peak amplitude of about 600 m s outboard of the rift valley beneath ridge flanks with fault scarps velocities in the upper 1500 m of crust are reduced the maximum amplitude of this anomaly is about 700 m s located near the top of the sheeted dikes variations in the three dimensional velocity model are believed to reflect changes in crustal porosity from which we infer an axisymmetric porosity model for seismic layer 2 of the coaxial segment as the crust ages the evolution of layer 2 porosity could occur in the following way 1 the porosity of zero age on axis dikes is set at formation by the contraction of molten material 2 hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley and 3 normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off axis at the north end of the segment dike velocities are several hundred meters per second slower on average and the across axis structure is lost the transition from a coherent aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the cobb hotspot on the ridge axis the porosity structure we have derived for the coaxial segment suggests an alternative to the usual hydrothermal circulation model of cross axis convection cells a circulation model with along axis convection cells located entirely within the axial valley appears to be more compatible with our data

subsurface tectonic structure between overlapping mid ocean ridge segments our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation the significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level the overlap region between mid ocean ridge segments at 98n on the east pacific rise is characterized by aligned cracks that are structurally more closely related to the propagating ridge segment these aligned cracks which are approximately parallel to the ridge segments provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers especially those with overlap basins covered by volcanic edifices the aligned cracks of the 98039n overlapping spreading center are more similar to the ridge parallel lineaments observed between overlapping axial summit collapse troughs than those found at larger overlapping spreading centers our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related

mechanical properties of shale gas reservoir rocks part 2 ductile creep brittle strength and their relation to the elastic modulus we studied the elastic moduli ductile creep behavior and brittle strength of shale gas reservoir rocks from barnett haynesville eagle ford and fortst john shale in a series of triaxial laboratory experiments we found a strong correlation between the shale compositions in particular the volume of clay plus kerogen and intact rock strength frictional strength and viscoplastic creep viscoplastic creep strain was approximately linear with the applied differential stress the reduction in sample volume during creep suggested that the creep was accommodated by slight pore compaction in a manner similar to instantaneous strain there was more viscoplastic creep in samples deformed perpendicular to the bedding than parallel to the bedding the tendency to creep also correlated well with the static young s modulus we explained this apparent correlation between creep behavior and elastic modulus by appealing to the stress partitioning that occurs between the soft components of the shales clay and kerogen and the stiff components quartz feldspar pyrite and carbonates through a simple 1d analysis we found that a unique relation between the creep compliance and elastic modulus independent of composition and orientation can be established by considering the individual creep behavior of the soft and stiff components that arises from the stress partitioning within the rock this appears to provide a mechanical explanation for why long term ductile deformational properties can appear to correlate with short term elastic properties in shale gas reservoir rocks

novel 3d centimetre to nano scale quantification of an organic rich mudstone the carboniferous bowland shale northern england x ray computed tomography and serial block face scanning electron microscopy imaging techniques were used to produce 3d images with a resolution spanning three orders of magnitude from 7 7 m to 7 nm for one typical bowland shale sample from northern england identified as the largest potential shale gas reservoir in the uk these images were used to quantitatively assess the size geometry and connectivity of pores and organic matter the data revealed four types of porosity intra organic pores organic interface pores intra and inter mineral pores pore sizes are bimodal with peaks at 0 2 m and 0 04 m corresponding to pores located at organic mineral interfaces and within organic matter respectively these pore size distributions were validated by nitrogen adsorption data the multi scale imaging of the four pore types shows that there is no connected visible porosity at these scales with equivalent diameter of 20 nm or larger in this sample however organic matter and clay minerals are connected and so the meso porosity 20 nm within these phases provides possible diffusion transport pathways for gas this work confirms multi scale 3d imaging as a powerful quantification method for shale reservoir characterisation allowing the representative volumes of pores organic and mineral phases to be defined to model shale systems the absence of connected porosity at scales greater than 20 nm indicates the likely importance of the organic matter network and associated smaller scale pores in controlling hydrocarbon transport the application of these techniques to shale gas plays more widely should lead to a greater understanding of properties in the low permeability systems

tectono sedimentary evolution of the bowland basin n england during the dinantian the bowland basin is a ne sw trending sedimentary basin lying between the askrigg bowland high to the n and nw and the central pennine high to the se the basin was initiated in late devonian tournaisian times sediment thickness and facies variations within the dinantian succession show marked asymmetry on both a basin wide and intra basin scale as a result of approximate nw se to n s extension across normal faults thickening of the sedimentary sequence from ne to sw over much of the basin resulted from fault controlled extension along the se margin of the basin with downthrow to the nw giving the basin a half graben character extension occurred in two distinct episodes marked by basin wide debris flow and sedimentary slide slump deposits during the late chadian early arundian the simple half graben became modified by a series of ne sw trending antithetic and nw se trending transfer faults associated with local unconformities and marked lateral thickness and facies variations this interval marks the transition from carbonate ramp to slope sedimentation and a temporary halt in carbonate supply to the basin the second episode of extension the late asbian early brigantian marks the break up of the northern basin margin ending major carbonate deposition within the basin reactivation of these structures at the end of the carboniferous created the ribblesdale fold belt as a result of nw se to n s compression transpression application of this research to areas surrounding the bowland basin has led to the recognition of additional shelf basins the orientation of which suggests control by normal and transfer fault systems discussion and

nontransform offsets of the pacific cocos plate boundary and their traces on the rise flank a multi beam bathymetric reconnaissance of the fast spreading crest of the east pacific rise between 7 n and 2 6 n found the axial ridge to be interrupted by four 4 13 km non transform offsets they have probably persisted since initiation by a small change in spreading direction 3 5 ma at each offset the volcanic rift zones veer 15 toward each other and overlap for 5 25 km around deep basins intervening lateral displacements of the rift zones are too short 2 5 km to break the linear axial volcanoes but cause local bends in their plan shapes some examples have a morphology that mimics the larger intervolcano offsets interpretation of a thorough bathymetric and magnetic survey around the 9 km 5 5 n offset indicates that the mainly one sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small scale jumps of the eruptive zone the northern rift zone is elongating propagating at the expense of the southern one the resulting along axis migration of this and other examples at speeds comparable to spreading rates causes the rise flank scars of the nontransform offsets equivalent to the fracture zones of transform faults to be oblique to the spreading direction these traces are recognizable within the highly lineated fault block terrain of the rise flanks because of the distinctive morphology of the 5 25 km wide bands that accreted at overlapped sections of the rift zones here lineation is less marked and oblique to the spreading direction and volcanic relief is more important than relief caused by faulting the west flank trace of the 5 5 n offset is occupied by a line of volcanoes and shallow volcanic ridges several other volcanic chains previously identified as hot spot traces seem to have grown in similar tectonic settings

three dimensional shallow crustal emplacement at the 9 03 n overlapping spreading center on the east pacific rise correlations between magnetization and tomographic images we report a three dimensional 3 d seismic reflection and tomographic survey conducted at the 9 03 n overlapping spreading center osc on the east pacific rise to understand crustal accretion at this feature inversions of travel time data from 19 ocean bottom hydrophones provide a 3 d image of the shallow velocity structure beneath the nontransform offset and associated discordance zone seismic analysis indicates that layer 2a thickness varies between 100 and 900 m and averages 430 m throughout the study area the heterogeneous upper crustal structure at the osc region contrasts with the simpler symmetric structure flanking the midsegments of the east pacific rise the crust affected by the osc migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond an area of thick layer 2a covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

magnetic and tectonic studies of the dueling propagating spreading centers at 20 40 s on the east pacific rise evidence for crustal rotations we present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20 40 s on the east pacific rise a majority of the data used were collected during a cruise aboard the r v moana wave during which continuous seamarc ii coverage was obtained these data were combined with additional data to produce an anomaly map which extends to anomaly 2 aged crust a three dimensional inversion in the presence of bathymetry was carried out for the area the resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity the results indicate consistent asymmetric spreading faster to the east discontinuous high magnetizations in the discordant zone associated with the discontinuity and southward migration of the feature at a rate of 90 100 mm yr between jaramillo and brunhes time 0 95 to 0 73 ma with slowing during the brunhes to less than 10 mm yr there is also an overlapping jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the nazca to the pacific plate in addition areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector results from this second model support the presence of highly rotated pre brunhes nazca crust within brunhes pacific crust which has been deformed by bookshelf faulting this indicates at least two episodes of crustal transfer from the nazca plate to the pacific plate the discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the nazca plate between the discontinuity and the easter microplate to the south the detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

crustal fissuring and its relationship to magmatic and hydrothermal processes on the east pacific rise crest 9 12 to 54 n the deep towed argo i optical acoustical vehicle and a geographic information system gis have been used to establish the abundance widths and spatial distribution of fissures as well as the relative age distribution of lavas along the narrow 500 m wide axial zone of the east pacific rise epr from 9o12 to 9o54 n on a second order scale 78 km long wider but less numerous fissures are found in the northern portion of the survey area this changes to narrower more abundant fissures in the south a profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike near 9o49 9o35 and 9o15 n where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle filling and covering older fissures on a fourth order scale 5 15 km long the mean density of fissuring on a given segment is greater where relative axial lava age is greater fissure density also correlates with hydrothermal vent abundance and type increased cracking toward segment tips is observed at the second order scale whereas fourth order segments tend to be more cracked in the middle cracking on a fourth order scale may be driven by the propagation of dikes rather than by the far field plate stresses the above relations constrain the model of haymon et al 1991 in which individual fourth order segments are in different phases of a volcanic hydrothermal tectonic cycle

spatial patterns of sediment accumulation on a holocene carbonate tidal flat northwest andros island bahamas abstract to characterize spatial patterns of sedimentation and analyze the morphology of part of the modern tidal flats of northwest andros island in the bahamas this study integrated remote sensing geographic information systems gis and carbonate sedimentology the fundamental data are a landsat tm image that has been classified to create a thematic map of eight subfacies interpreted to represent a distinct tidal flat subenvironment such as adjacent marine exposed levee beach ridge pond and algal marsh spatial statistics of the thematic map characterize the patterns of sediment accumulation quantitative analysis highlights several interesting results concerning subfacies character and distribution 1 of the eight mapped subfacies low algal marsh is most widespread representing 27 5 of the total area whereas exposed levee beach ridge is the least widespread accounting for 10 of the area 2 the patches of different subfacies have different shape complexities with low algal marsh high algal marsh and mangrove ponds being the least complex and exposed levee beach ridge being the most complex 3 markov chain analysis suggests that lateral transitions between different subfacies are highly ordered and 4 frequency distribution of subfacies patch area and lacunarity gap size distribution data exhibit power law relationships over several orders of magnitude consistent with fractal characteristics the fractal nature of patch size and gaps between facies illustrate that on this tidal flat neither the size nor the spatial distribution of subfacies has a characteristic scale this statistical behavior is consistent with the presence of self organization or emergence of pattern in the absence of a template or external forcing the statistical self organization on the tidal flat is the cumulative expression of local processes but it becomes apparent only through analysis of the whole system these results are inconsistent with models suggesting that tidal flats include a migrating complex of randomly distributed randomly sized subenvironments ancient successions that include random patterns may reflect the more pronounced influence of forces external to the sedimentary system instead of an absence of those forces

exploring the effect of meso scale shale beds on a reservoir s overall stress sensitivity to seismic waves abstract intra reservoir sub seismic shale beds with a thickness range from 1 10 m are present in most clastic reservoirs they are often studied to investigate their effect on fluid flow and reservoir performance here it is found that shales with such thicknesses can also strongly affect the effective elastic wave behaviour of the composite reservoir particularly when observed with frequent time lapse surveys more specifically for reservoirs experiencing pressure depletion the effective impedance of the reservoir interval is usually expected to harden however our results indicate that shales can reduce this hardening effect or perhaps unexpectedly soften the overall impedance the overall in situ stress sensitivity of the reservoir is reduced below that predicted from taking laboratory core plug measurements of sand stress sensitivity alone these predictions are based on a combination of geomechanical and pressure diffusion phenomena that are in turn controlled by the shale thickness permeability and the mechanical properties as sub seismic shale layers of approximately 1 m thickness take less than three months to pressure equilibrate whilst thicker shale layers of 10 m can take over 20 years in the context of repeated seismic surveying our predictions require accurate knowledge of the shale properties and statistics and hence a good description of the sedimentology based on our default property values it appears that reduced or anomalous stress sensitivity is likely to be more important for 4d projects with frequent acquisitions of 3 12 months but is of less concern when seismic is repeated on conventional time periods of every 5 10 years the critical set of parameters required to carry out accurate calibration of these predictions is not yet fully available from published literature

experimental investigation of matrix permeability of gas shales predicting long term production from gas shale reservoirs has been a major challenge for the petroleum industry to better understand how production profiles are likely to evolve with time we have conducted laboratory experiments examining the effects of confining stress and pore pressure on permeability experiments were conducted on intact core samples from the barnett eagle ford marcellus and montney shale reservoirs the methodology used to measure permeability allows us to separate the reduction of permeability with depletion because of the resultant increase in effective confining stress and the increase in permeability associated with knudsen diffusion and molecular slippage also known as klinkenberg effects at very low pore pressure by separating these effects we are able to estimate the relative contribution of both darcy and diffusive fluxes to total flow in depleted reservoirs our data show that the effective permeability of the rock is significantly enhanced at very low pore pressures because of the slippage effects we use the magnitude of the klinkenberg effect to estimate the effective aperture of the flow paths within the samples and compare these estimates to scanning electron microscopy image observations our results suggest effective flow paths to be on the order from tens of nanometers in most samples to 100 200 nm in a relatively high permeability eagle ford sample finally to gain insight on the scale dependence of permeability measurements the same core plugs were crushed and permeability was again measured at the particle scale using the so called gas research institute method the results show much lower permeability than the intact core samples with very little correlation to the measurements on the larger scale cores

effects of tectonism and magmatism on crack structure in oceanic crust a seismic anisotropy study we analyzed 25 675 traveltime residuals from a three dimensional seismic tomographic inversion to investigate crack induced seismic anisotropy in the upper oceanic crust the study covered two regions with contrasting levels of magmatic activity on the western limb of the 9 n overlapping spreading center on the east pacific rise the level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region in contrast we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation strongly anisotropic dikes with inferred narrow and water saturated cracks provide important along axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions furthermore a significant clockwise rotation 20 30 of fast directions occurs in both regions with increasing depth such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest

crack induced seismic anisotropy in the oceanic crust across the east pacific rise 9 30 n the seismic anisotropy of the shallow oceanic crust across the east pacific rise 9 30pn is studied with p wave refraction data collected during a controlled source three dimensional tomography experiment the anisotropy is indicated by a cos 2a pattern of travel time residuals where a is the receiver to shot azimuth the travel time data are best fit by a model containing 4 anisotropy from 500 m to 1 km depth below the sea floor 2 from 1 to 2 km and 0 below 2 km depth the upper 500 m of crust is not independently constrained by the data since ray paths at these depths are oriented near vertically the fast direction of p wave propagation is aligned along a trend oriented 6 10 counterclockwise from the rise axis the results are consistent with the presence of vertically aligned cracks that form on or very near the rise axis are oriented nearly parallel to the rise trend penetrate to less than 2 km depth and provide 6 10 of the total pore space in the shallow crust in comparison with similar studies across slow and intermediatespreading ridges there is no apparent spreading rate dependence of the magnitude of anisotropy fl 2001 published by elsevier science b v

some effects of compaction and geological time on the pore parameters of argillaceous rocks graphical statistics have been applied to the pore size distribution curves of argillaceous rocks to characterize the changes in pore parameters that result from compaction and geological time the most striking characteristic of recently deposited sediment is the high variability in mean pore size and in the sorting and skewness of the pore system the mean pore size ranges from 15 to 980 nm sorting ranges from very well sorted to poorly sorted and skewness varies from systems in which small pores predominate over large ones to systems in which large pores predominate this high variability in pore structure represents the many environmental and mineral related variables that affect the pore system of newly deposited sediment the mean pore size of shales decreases with increasing compaction and approaches a limiting value of about 3 5 nm at depth within a geological time span of 50 m y and or depth of burial of about 1200 m most sediments have reached an irreversible well sorted pore size distribution early diagenetic processes apparently affect the skewness of pore systems more than compaction such that within about 50 m y the pore system is negatively skewed with small pores predominating over large sediments buried to a depth of 500 m or less exhibit a porosity range of 40 85 below 500 m porosity decreases linearly with burial depth no correlation exists between the surface area of shale pore systems and depth of burial geological age and the pore parameters mean pore size sorting and skewness

deep tow studies of the overlapping spreading centers at 9 03 n on the east pacific rise the deeply towed instrument package of the scripps institution of oceanography was used to study for the first time the fine scale structure of an overlapping spreading center osc system the 9 03 n osc on the east pacific rise epr the eastern and western limbs of this osc system which are 8 km apart and overlap by 27 km show marked differences away from the tips the spreading centers are highly tectonized and differ greatly from the ridge tips the western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area in both cases the ridge tip fissures are 10 15 oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively the eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through going trace of the east pacific rise in contrast the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis some of the very few tectonic features observed in the overlap basin may reflect the existence of a short lived shear couple due to the interaction of the two overlapping spreading centers sediment cover over the whole area and especially in the overlap basin is relatively thick supporting the idea of a time averaged deficit in the magmatic budget at this osc we suggest that the 9 03 n osc developed where two magmatic pulses independent in space and time and propagating along the strike of the east pacific rise away from the loci of melt emplacement failed to meet misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing osc geometry independent lines of evidence derived from the three dimensional inversion of the magnetic field at the 9 03 n osc sempere et al 1984 from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field sempere and macdonald 1986 and from geochemical data langmuir et al 1986 support our interpretation of the deep tow data we suggest that deviations from axial linearity of the spreading centers devals langmuir et al 1986 and small nonoverlapping offsets snoos batiza and margolis 1986 are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on oscs and some saddle points i e devals snoos are all small rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the east pacific rise

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

the rise flank trails left by migrating offsets of the equatorial east pacific rise axis sea beam and magnetic surveys of the young 1 ma rise flank around nontransform rise crest offsets at 5 5 n 4 9 n 3 4 n 2 0 n and 2 8 s mapped distinctive trails of obliquely lineated highly magnetic crust that spreads from the characteristically curving overlapping rift zones a deep tow survey around the small 1 5 km 3 9 n offset found only normal rise flank terrain near bottom deep tow observations at the medium size 9 km 5 5 n offset and the large 27 km 2 0 n offset provided more detailed information on the formation of crust in the offset trails and of the structure of tectonically modified crust at interrift rise crest sites patches of interrift crust which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets are periodically shed from the axial zone together with abandoned portions of overlapped rift zone to become important elements of the rise flank trails the oblique azimuths of the paired fracture zone trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25 125 mm yr net movement of the 27 km 2 8 s offset has been slower with alternating periods of rapid northward and southward migration information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial east pacific rise epr whose trails were less thoroughly mapped most offsets of the southern pacific cocos boundary have migrated south and most on the northern pacific nazca boundary have migrated north the patterns are not readily explained by the inferred distribution of mantle upwellings along the epr axis

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

seafloor photo geology and sonar terrain modeling at the 9 n overlapping spreading center east pacific rise 1 a fundamental goal in the study of mid ocean ridges is to understand the relationship between the distribution of melt at depth and seafloor features building on geophysical information on subsurface melt at the 9 n overlapping spreading center on the east pacific rise we use terrain modeling dsl 120a side scan and bathymetry photo geology jason ii and whoi towcam and geochemical data to explore this relationship terrain modeling identified four distinct geomorphic provinces with common seafloor characteristics that correspond well to changes in subsurface melt distribution visual observations were used to interpret terrain modeling results and to establish a relative seafloor age scale calibrated with radiometric age dates to identify areas of recent volcanism on the east limb recent eruptions in the north are localized over the margins of the 4 km wide asymmetric melt sill forming a prominent off axis pillow ridge along the southern east limb recent eruptions occur along a neovolcanic ridge that hugs the overlap basin and lies several kilometers west of the plunging melt sill our results suggest that long term southward migration of the east limb occurs through a series of diking events with a net southward propagation direction examining sites of recent eruptions in the context of geophysical data on melt distribution in the crust and upper mantle suggests melt may follow complex paths from depth to the surface overall our findings emphasize the value of integrating information obtained from photo geology terrain modeling lava geochemistry and petrography and geophysics to constrain the nature of melt delivery at mid ocean ridges

upper crustal velocity structure beneath the central lucky strike segment from seismic refraction measurements we present a three dimensional velocity model of the upper crust around the central volcano of the lucky strike segment mid atlantic ridge the model constructed from a 3 d array of air gun shots 37 5 m spacing along line and 100 m between lines to ocean bottom seismometers fired during a 3 d seismic reflection survey shows an off axis velocity increase 1 km s a low velocity region within the median valley and a low velocity anomaly underneath the lucky strike volcano our observations indicate a porosity decrease of 1 9 corresponding to a velocity increase of 0 5 1 km s over a distance of 8 km from the ridge axis 0 7 ma and a porosity decrease of 4 11 corresponding to a velocity increase of 2 km s between a depth of 0 5 and 1 75 km below seafloor a sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis we favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion lava sheeted dike boundary and active extension faults fractures and fissures the porosity variation with depth probably depends on pore space collapse hydrothermal alteration and a change of stress accommodation the off axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

permeability of shale by the beam bending method abstract the beam bending method permits measurement of liquid permeability in the nanodarcy range in a few minutes to a few hours this technique has been applied successfully to determine the permeability as well as the viscoelastic properties of isotropic materials with low permeability such as gels porous glass and cement paste the method has been extended to measure transversely anisotropic materials such as sedimentary rock to find the permeability parallel and perpendicular to the bedding in this study measurements have been made on a set of shales from varying depths and locations in the continental united states the measured permeabilities range 0 009 400 nanodarcies nd 10 21 m 2 the permeability in the direction parallel to the bedding orientation was larger than that perpendicular to the bedding orientation by a factor ranging from 1 2 to 6 this is the first instance of using the beam bending method to measure the permeabilities of shale in different orientations the measured permeabilities were compared to the kozeny carman and katz thompson models the pore geometry parameters used in the models such as the pore size distribution characteristic pore diameters porosity and tortuosity were measured using mercury intrusion porosimetry mip gravimetry and electrical conductivity respectively the measured permeability values match better with the predictions from the katz thompson equation

magmatic processes and segmentation at a fast spreading mid ocean ridge detailed investigation of an axial discontinuity on the east pacific rise crest at 9 37 n 1 geophysical and petrological boundaries on mid ocean ridges provide ideal locations to study the relationships between magmatic tectonic and hydrothermal processes alvin based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the east pacific rise epr crest between 9 36 n and 9 38 n this ridge crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough asct that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area the ascts overlap by 3 km and are offset in a right lateral sense by 0 45 km near bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval when combined with the inferred age and structure of the seafloor basalt geochemistry and hydrothermal fluid chemistry these temporal changes suggest active southward propagation of the eastern asct and show that the western asct was unaffected by the recent magmatic event numerous extinct hydrothermal vents and older looking lava flow surfaces suggest waning of magmatic activity in the western asct young looking lava flows within or proximal to the eastern asct have anomalously high mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9 46 52 n epr region we propose that the young looking lava flows in the eastern asct are related to the 1991 eruption data show that the 9 37 n axial discontinuity marks a magmatic and hydrothermal boundary along the epr ridge crest and we argue that it be classified as a third order discontinuity this result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9 37 n discontinuity

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

shear wave splitting and crustal anisotropy at the mid atlantic ridge 35 n 1 shear wave splitting observed in microearthquake data at the axis of the mid atlantic ridge near 35 n has a fast polarization direction that is parallel to the trend of the axial valley the time delays between fast and slow s wave arrivals range from 35 to 180 ms with an average of 90 ms and show no relationship with ray path length source to receiver azimuth or receiver location the anisotropy is attributed to a shallow distribution of vertical fluid filled cracks aligned parallel to the trend of the axial valley joint modeling of the shear wave anisotropy and coincident p wave anisotropy results using recent theoretical models for the elasticity of a porous medium with aligned cracks suggests that the crack distribution that causes the observed p wave anisotropy can account for at most 10 ms of the shear wave delay most of the shear wave delay thus likely accrues within the shallowmost 500 m seismic layer 2a and the percent s wave anisotropy within this highly fissured layer is 8 30 isolated fluid filled cracks at 500 m to 3 km depth that are too thin or too shallow to be detected by the p wave experiment may also contribute to the shear wave delays the joint analysis of p and s wave anisotropy is an important approach for constraining the crack distributions in the upper oceanic crust and is especially suited for seismically active hydrothermal systems at slow and intermediate spreading mid ocean ridges

3d quantification of mineral components and porosity distribution in westphalian c sandstone by microfocus x ray computed tomography abstract microfocus x ray computed tomography ct measurements were performed on deltaic sandstone samples from the diagenetically altered westphalian c strata campine basin north east belgium in order to collect three dimensional data on mineral quartz kaolinite ankerite etc and porosity distribution the acquired ct data were compared with point counting results of stained impregnated thin sections to qualify the sensitivity of ct comparison between techniques shows positive results with regard to the quantification of main mineral phases such as quartz and ankerite and porosity distribution however major discrepancies exist when studying the clay minerals this can be explained by the influence of microporosity associated with these clay minerals resulting in partial volume effects

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions