anisotropy of magnetic susceptibility and p wave velocity in core samples from the taiwan chelungpu fault drilling project tcdp the anisotropy of magnetic susceptibility ams and anisotropy of p wave velocity apv were characterized in core samples retrieved from the tcdp hole a at depths ranging from 600m to 1400m thus cutting across the thrust fault that ruptured during the 1999 chi chi earthquake our data show that the ams was similar for all the samples regardless of the lithologic contrasts with a magnetic fabric characteristic of weakly deformed sedimentary rocks that had undergone layer parallel shortening this robust observation applies to both the siltstones and sandstones at all depths investigated in contrast the apv data reflect fundamental differences between the elastic fabrics in the two facies while apv of the siltstone samples is controlled by a preferred orientation of the matrix minerals the apv of the sandstones is greatly influenced by microcrack porosity with a preferred orientation parallel to the maximum principal direction of paleostress and tectonic deformation these ams and apv data obtained in tcdp hole a at locations not directly impacted by the coseismic rupture provide a first order petrofabric framework for the characterization of hydromechanical anisotropy in the chelungpu fault system

influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from mt etna volcano subjected to elevated pressure abstract we report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from etna italy results were obtained for dry and saturated samples deformed under triaxial compression during each test the effective pressure was first increased up to 190 mpa to investigate the effect of pre existing crack closure on seismic properties then the effective pressure was unloaded down to 20 mpa a pressure which mirrors the stress field acting under a lava pile of approximately 1 5 2 km thick and deviatoric stress was increased until failure of the specimens using an effective medium model the measured elastic wave velocities were inverted in terms of two crack densities i the crack density of the pre existing thermal cracks and v the crack density of the stress induced cracks in addition a link was established between elastic properties elastic wave velocities vp and vs and permeability using a statistical permeability model our results show that the velocities increase with increasing hydrostatic pressure up to 190 mpa due to the closure of the pre existing thermal cracks this is interpreted by a decrease of the crack density i from 1 to 0 2 the effect of pre existing cracks closure is also highlighted by the permeability evolution which decreases of more than two orders of magnitude under deviatoric loading the velocities signature is interpreted in the first stage of the loading by the closure of the pre existing thermal cracks however with increasing deviatoric loading newly formed vertical cracks nucleate and propagate this is clearly seen from the velocity signature and its interpretation in term of crack density from the location of the acoustic emission sources and from microstructural observations this competition between pre existing cracks closure and propagation of vertical cracks is also seen from the permeability evolution and our study shows that mechanically induced cracks has lesser influence on permeability change than pre existing thermal cracks

lithological control on shear wave velocity anisotropy in core samples from the taiwan chelungpu fault drilling project abstract the present study provides laboratory constrains on the in situ source of shear wave birefringence in siltstone and sandstone lithologies from the taiwan chelungpu fault drilling project tcdp bench top measurements at ambient pressure and temperature are performed across the end faces of vertically drilled samples while the polarization angle of the source and receiver is rotated from 0 to 360 in azimuth shear wave birefringence in siltstone is low 3 5 and controlled by low angle 30 bedding dip the sandstone samples exhibit moderate 15 shear wave anisotropy due to a network of vertical microcracks with normal about the bedding strike direction as related studies infer that the microcracks responsible for shear wave birefringence in sandstone are also open at depth our results suggest that anisotropy in borehole measurements can simply be accounted for by the observed siltstone and sandstone microstructures i e without invoking larger scale objects nor randomly oriented cracks fractures subjected to triaxial stress state

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

mica deformation fabrics and the seismic properties of the continental crust abstract seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica however naturally deformed micaceous rocks commonly show composite e g s c fabrics it is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields an outcrop analogue for granitic mid crustal deformed zones is used here to calibrate the seismic response against both parameters seismic responses are modelled using crystallographic preferred orientations for polymineralic micaceous granitic gneisses measured using electron back scatter diffraction the sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation so called rock and fabric recipes the maximum p and s anisotropy are calculated at 16 6 and 23 9 for single foliation gneisses but for mixed i e s c foliation gneisses these values reduce to 5 8 and 7 5 respectively furthermore mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy this effect coupled with the geographical orientation of fabrics in nature can generate substantial variations in the orientation and magnitude of seismic anisotropy especially for shear waves as measured for the continental crust using existing receiver function and teleseismic near vertical incidence methods thus maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth varying deformation kinematics and tectonic decoupling

elastic parameters of west bohemian granites under hydrostatic pressure the west bohemian seismoactive region is situated near the contact of the moldanubian bohemian and saxothuringian units in which a large volume is occupied by granitoid massifs the spatial distribution of p wave velocities and the rock fabric of five representative samples from these massifs were studied the p wave velocities were measured on spherical samples in 132 independent directions under hydrostatic pressure up to 400 mpa using the pulse transmission method the pressure of 400 mpa corresponds to a depth of about 15 km in the area under study the changes of p wave velocity were correlated with the preferred orientations of the main rock fabric elements i e rock forming minerals and microcracks the values of the p wave velocity from laboratory measurements on granite samples fit the velocity model used by seismologists in the west bohemian seismoactive region

laboratory approach to the study of elastic anisotropy on rock samples the experimental approach hardware and software to the study of the elastic anisotropy of rocks on spherical samples under hydrostatic pressure up to 400 mpa is discussed a substantial innovation of the existing measuring system and processing methods enabled us to make a detailed investigation and evaluation of the kinematic as well as dynamic parameters of elastic waves propagating through anisotropic media the innovation is based on digital recording of the wave pattern with a high sampling density of both time and amplitude several options and results obtained with the innovated laboratory equipment are presented

constraints on the tectonic development of the eastern gulf of mexico provided by magnetic anomaly data boundaries between oceanic and transitional crust in the eastern gulf of mexico have been delineated using detailed accurately navigated high quality aeromagnetic data these data indicate that considerably more oceanic crust may be present than previously suggested by other geophysical data especially seismic reflection data the n s extent of oceanic crust substantially increases westward from approximately 280 km near 87 w to roughly 390 km near 90 w indicating the proximity 10 of the pole of rotation variations in the amount of oceanic crust together with discontinuities in the magnetic anomaly patterns interpreted as the traces of fossil transform faults or fracture zones have been used to obtain a rough estimate of the location of the pole of rotation 24 n 81 5 w for the seafloor spreading phase of opening the magnetic data have been used to develop a model for the opening of the gulf and to identify some geometrical relationships between its conjugate continental margins based upon the estimated pole position the eastern gulf is interpreted to have opened in an approximately nne ssw direction with the counterclockwise rotation of yucatan away from north america the seafloor spreading phase of movement accounts for about 25 of the counterclockwise motion of yucatan rotating the yucatan block clockwise by this amount to restore it to its predrift position places the campeche bank adjacent to the mississippi trough louisiana shelf further clockwise rotation of yucatan by an additional 30 35 about this pole restores extended crust to its original thickness and is consistent with rotations of yucatan predicted from paleomagnetic data from chiapas

75 plus years of anisotropy in exploration and reservoir seismics a historical review of concepts and methods the idea that the propagation of elastic waves can be anisotropic i e that the velocity may depend on the direction is about 175 years old the first steps are connected with the top scientists of that time people such as cauchy fresnel green and kelvin for most of the 19th century anisotropic wave propagation was studied mainly by mathematical physicists and the only applications were in crystal optics and crystal elasticity during these years important steps in the formal description of the subject were made at the turn of the 20th century rudzki stressed the significance of seismic anisotropy he studied many of its aspects but his ideas were not applied research in seismic anisotropy became stagnant after his death in 1916 beginning about 1950 the significance of seismic anisotropy for exploration seismics was studied mainly in connection with thinly layered media and the resulting transverse isotropy very soon it became clear that the effect of layer induced anisotropy on data ac

elastic anisotropy of core samples from the taiwan chelungpu fault drilling project tcdp direct 3 d measurements and weak anisotropy approximations summary the study of seismic anisotropy has become a powerful tool to decipher rock physics attributes in reservoirs or in complex tectonic settings we compare direct 3 d measurements of p wave velocity in 132 different directions on spherical rock samples to the prediction of the approximate model proposed by louis et al based on a tensorial approach the data set includes measurements on dry spheres under confining pressure ranging from 5 to 200 mpa for three sandstones retrieved at a depth of 850 1365 and 1394 metres in tcdp hole a taiwan chelungpu fault drilling project as long as the p wave velocity anisotropy is weak we show that the predictions of the approximate model are in good agreement with the measurements as the tensorial method is designed to work with cylindrical samples cored in three orthogonal directions a significant gain both in the number of measurements involved and in sample preparation is achieved compared to measurements on spheres we analysed the pressure dependence of the velocity field and show that as the confining pressure is raised the velocity increases the anisotropy decreases but remains significant even at high pressure and the shape of the ellipsoid representing the velocity or elastic fabric evolves from elongated to planar these observations can be accounted for by considering the existence of both isotropic and anisotropic crack distributions and their evolution with applied pressure

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

propagation of dispersed compressional and rayleigh waves on the texas coastal plain during a refraction profile on the texas coastal plain a strong surface wave with predominant frequencies between 2 and 10 hz and group velocities near 1 9 km sec was observed to ranges as great as 64 km this dispersed wave with velocities near the compressional wave speed of near surface sediments corresponds to the leaky compressional or pl wave pl dispersion can be predicted from the theory of the normal modes of a layered liquid medium efficient propagation of the pl wave is related to the high poisson ratio of the unconsolidated sediments in the shallow subsurface and additional examples from the published literature show that the pl wave is commonly excited by shallow sources both on land and offshore in addition to the pl waves dispersed waves with group velocities between 0 3 and 0 7 km sec were observed at ranges less than 10 km these are identified as rayleigh waves lr smoothly varying p and s velocity structures for the upper 1 km are obtained by fitting theoretical dispersion cu

potential field evidence for a volcanic rifted margin along the texas gulf coast potential fi eld data along the texas portion of the gulf of mexico indicate a large amplitude coast parallel magnetic maximum and a smaller bouguer gravity high models constrained by seismic refraction data indicate that these maxima manifest a deeply buried volcanic rifted passive margin or other magnetic high in the outer transitional crust buried 12 15 km the source is 220 km wide similar to the voring plateau in norway and the u s east coast this margin which formed during the opening of the gulf of mexico differs in origin from the transform boundary of the northeast mexico margin tehuantepec transform and we infer a jurassic triple junction related to the borderland rift system which is traceable as far as southeast california

cyclone development in the south pacific convergence zone during fgge 10 17 january 1979 a diagnosis of the life cycles of three south pacific cyclones each tropical in origin is presented the study focuses on two of the cyclones which propagate south eastward along the western edge of the south pacific convergence zone spcz and reach middle latitudes before decaying based on enhanced ir satellite imagery from goes west both cyclones appear to go through stages of development similar to those described by troup and streten even though their classification system generally applies to higher latitude systems the primary data set used for surface and upper air analyses is based on ecmwf fgge level iii b analyses variables presented include mean sea level pressure m s l p horizontal divergence relative vorticity and winds a comparison between m s l p patterns obtained manually from station observations and those produced objectively by ecmwf shows good agreement except when the cyclonic disturbances are subgrid scale maximum low level convergence occurs in the spcz north and east of the cyclone tracks however maxima of low level cyclonic vorticity correspond to the individual cyclones cyclone tracks appear to be influenced by the upper level subtropical jet the two cyclones which propagate into middle latitudes are accompanied by lower tropospheric warm cold air advection ahead of behind the system both cyclones also exhibit upper tropospheric anticyclonic cyclonic vorticity ahead of behind the system

large offset seismic surveying using ocean bottom seismographs and air guns instrumentation and field technique repeatable closely spaced signal sources from large capacity air guns and detection and recording of signals using highly flexible microprocessor controlled digital ocean bottom seismographs allow us to acquire high quality large offset marine seismic refraction and reflection data the acquired data are readily adaptable to various processing techniques originally developed for seismic reflection data there are several requirements and problems specific to the technique for example bubbly signals from one or two large capacity air guns are often preferable to bubble suppressed signals from tuned arrays in identifying weak arrivals at large offset distances recorded water wave signals at near ranges provide precise locations of detectors relative to shots

the role of diabatic heating in maintaining the upper tropospheric baroclinic zone in the south pacific a modified set of ecmwf fgge level iii b analyses is used to examine the four dimensional structure of the region containing the south pacific convergence zone spcz during a period when two cyclones form in the tropics and propagate south eastward along the spcz into middle latitudes during this period 10 18 january 1979 the atmosphere is characterized by a highly baroclinic state which together with the existence of an upper tropospheric subtropical jet supports the growth and development of the observed cyclone activity in a previous study it was found that convective activity was prominent during the life cycle of each cyclone the goal of the present study is to examine the significance of kinematic and thermodynamic processes in maintaining the upper baroclinic region coincident with the subtropical jet this baroclinic zone lies just poleward of the warm core associated with the convectively active spcz a partitioned form of the fronto genetical function is used to diagnose the adiabatic and diabatic contributions to the maintenance of baroclinicity in the vicinity of the spcz a major balance is found between the frontogenetical contribution by differential diabatic heating and the opposing diabatic tilting processes in contrast to the results of most mid latitude cyclone studies the adiabatic contributions from the deformation and tilting terms appear to be of lesser importance also the contribution from the deformation associated with the horizontal branch of the diabatically forced circulation is found to be relatively unimportant the latter finding is in disagreement with some previously reported numerical results concerned with middle latitude moist frontogenesis possible reasons for these differences are discussed

crustal structure in gulf of mexico from obs refraction and multichannel reflection data results from 12 reversed refraction profiles each 110 km long have been combined with multichannel reflection data to produce a series of crustal structure sections across the gulf of mexico the refraction data were collected using ocean bottom seismographs obs and explosive charges ranging in size from 1 to 120 lb 0 45 to 54 42 kg these data show as many as three layers of sedimentary rocks with total thickness between 5 and 9 km and layer velocities between 1 7 and 3 5 km sec beneath most of the gulf this sedimentary section is underlain by a layer with velocity between 4 5 and 5 4 km sec the acoustic basement as defined by reflection data is confined within this layer this basement layer which may be the top of volcanic layer 2 is too thin to be detected in the refraction results beneath this layer in most of the deep gulf is an oceanic crustal layer layer 3 3 to 6 km thick which thickens to about 12 km under the mississippi fan and 10 km in the southeastern gulf where it is interpreted to be transitional crust oceanic layer 3 has a velocity between 6 4 and 7 0 km sec and overlies a mantle with v locity between 7 6 and 8 2 km sec the top of oceanic layer 3 was not observed on regional multichannel seismic profiles these data confirm earlier refraction interpretation that most of the deep gulf basin is underlain by an oceanic crustal layer flanked by transitional crust this layer may have been formed by a mantle thermal event accompanied by a period of rapid sea floor spreading

high heat flow anomalies over salt structures on the texas continental slope gulf of mexico we made 74 closely spaced 2 km apart heat flow measurements around and over two salt structures on the texas continental slope gulf of mexico the values outlined the shape of the heat flow anomalies over both structures based on a preceding high resolution seismic survey we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall shaped feeder the heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation the values over the salt plug are nearly all greater than 70 mw m the measurements over the salt tongue have a sharp heat flow peak of 90 mw m associated with the presumed feeder and rather uniform values around 60 mw m over the remainder the variation of heat flow over both structures is smooth and shows no apparent scatter heat flow values off these features are uniformly low around 30 mw m thermal effects from bottom water temperature fluctuation slope sedimentation diapiric movement of the salt body and pore fluid migration appear unable to provide a satisfactory explanation for the observations however thickness variations of a highly conductive salt body can easily account for the heat flow anomalies we suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt

transform margin of the northern levant eastern mediterranean from formation to reactivation 1 the levant continental margin eastern mediterranean is usually defined as a passive margin that developed between the late paleozoic early mid mesozoic previous studies suggest that the margin is composed of two segments with different structural characteristics the southern segment south of the carmel structure was formed by rifting whereas the origin of the northern segment is not known the present study focuses on the northern segment which is a key for understanding the tectonic history of the eastern mediterranean investigation is carried out offshore on two scales 1 deep crustal observations analysis of seismic refraction and magnetic data and their comparison with other continental margins worldwide and 2 shallow crustal scale observations of recent tectonic deformations based on multichannel and single channel seismic reflection profiles integration of the deep and shallow observations with reevaluation of geological data onland show strong structural resemblance between the northern levant segment and other transform margins a modified scenario is suggested for the formation and development of the levant basin and its margins combining all previous studies into one simple picture where a signature of older structures on more recent features is evident located in the midst of the progressive afro eurasian collision the northern levant presents a unique lifecycle for transform margins worldwide from formation to reactivation

p and s wave anisotropy and shear wave splitting at pressure and temperature in possible mantle rocks and their relation to the rock fabric the seismic properties and their directional dependence have been measured in three potentially mantle derived rocks peridotite ivrea zone italy lherzolitic xenolith massif central france serpentinite the experimental data include the measurement of elastic wave velocities and densities at confining pressures up to 600 mpa and temperatures up to 600 c 600 mpa and the determination of velocity anisotropy and shear wave splitting the petrophysical data can be related to mineral composition and to microstructural characteristics of the rocks macroscopic anisotropy of p and s wave velocities is strongly controlled in the peridodites by the olivine and in the serpentinite by the serpentine antigorite lattice preferred orientation lpo respectively shear wave splitting was found to be an important characteristic of the anisotropic rocks the study shows that for the interpretation of shear wave splitting the direction of wave propagation in relation to the lpos of the major minerals and to the symmetry of their lattice fabric is of great importance in the ivrea peridotite exhibiting rhombic symmetry there is practically no shear wave splitting to be observed for s waves propagating parallel to the olivine 100 axes and the maximum shear wave splitting is not coupled with the direction of highest p wave velocity in the transversely isotropic massif central xenolith shear waves do not split when propagating parallel to the olivine 010 maximum coinciding with the direction of the radial symmetry axis of the olivine fabric the experimental data are principally in accordance with velocity calculations based on the measured olivine lpo and the olivine single crystal stiffness coefficients implications for the interpretation of seismic data are discussed

deep crustal structure in the eastern gulf of mexico we use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along gulf of mexico basin opening line 4 a profile extending from the northwestern florida peninsula across the florida escarpment to the central gulf of mexico moderately thinned continental crust with a moho depth of 32 33 km average sediment thickness of 6 km and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore florida thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6 7 km mean seismic velocities of the crust in this region increase from 6 55 km s to 6 95 km s from northeast to southwest and are evidence for increased magmatic input as rifting developed oceanic crust with an average thickness of 5 6 5 7 km is observed over a distance of 175 km on the southwest end of the profile with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile anomalously high upper oceanic crust velocities of 6 0 6 7 km s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement integrating well seismic reflection and our seismic refraction data allow us to estimate a full spreading rate of 2 2 cm yr for seafloor spreading along the profile this indicates that oceanic crust was emplaced at a slow spreading center

continental rifting and sediment infill in the northwestern gulf of mexico the opening of the gulf of mexico was an important mesozoic tectonic event that provides new insight in the role of magmatism and lithospheric stresses in the initiation of continental rifting a new seismic velocity profile based on seismic refraction data in the northwestern gulf of mexico offshore texas where the basin started opening in the early jurassic shows a rifted margin with strong lateral heterogeneity beneath the shelf and slope the structure of the thinned crust is consistent with large scale extensional faulting and moderate amounts of synrift magmatism before continental breakup these new seismic constraints do not indicate the presence of a volcanic margin along the texas coast as has sometimes been proposed based on magnetic data the laurentian continental lithosphere of central texas may have been too thick at the onset of rifting 100 km to let magmatic diking control the extension in contrast the continental lithosphere of the northeastern gulf of mexico may have been thinner such that magma assisted rifting formed a volcanic margin there later in the jurassic

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

evidence from apatite fission track analysis for regional cretaceous cooling in the ouachita mountain fold belt and arkoma basin of arkansas geologic note 1 a reconnaissance study involving apatite fission track analysis of surface paleozoic sedimentary and cretaceous intrusive rocks was undertaken to evaluate the low temperature thermal history of the ouachita mountain fold belt and arkoma basin of arkansas paleozoic rocks have apparent apatite fission track ages ranging from 184 or 12 ma to 95 or 11 ma with mean track lengths in the range of 12 95 or 0 17 micrometers to 13 51 or 0 13 micrometers indicating that these samples have undergone significant postdepositional annealing cretaceous samples have apatite fission track ages between 104 or 4 and 88 or 6 ma that are concordant with rb sr and k ar ages and mean track lengths in the range 14 69 or 0 16 micrometers to 15 36 or 0 10 microme ers indicating that these rocks cooled rapidly to paleotemperatures below 50 60 degrees c following their emplacement and have subsequently remained cool paleozoic strata of the ouachita mountain fold belt and arkoma basin in arkansas are inferred to have undergone regional cooling from paleotemperatures greater than 110 degrees c beginning in the early cretaceous to paleotemperatures generally less than 80 90 degrees c prior to the intrusion of alkaline plutons along the western margin of the mississippi embayment during the middle cretaceous whether regional cooling during the cretaceous was caused by uplift and erosion or whether a dramatic reduction in the paleogeothermal gradient was involved remains uncertain although a significant component of uplift and erosion is inferred from independent geological observations the results of this study indicate that paleozoic rocks of the ouachita mountain fold belt and arkoma basin of ar ansas were exposed to maximum paleotemperatures during late paleozoic burial prior to the emplacement of cretaceous plutons in the region

comparison between permeability anisotropy and elasticity anisotropy of reservoir rocks we developed new experimental and theoretical tools for the measurement and the characterization of arbitrary elasticity tensors and permeability tensors in rocks they include an experimental technique for the 3 d visualization of hydraulic invasion fronts in rock samples by monitoring the injection of salt solutions by x ray tomography and a technique for inverting the complete set of the six coefficients of the permeability tensor from invasion front images in addition a technique for measuring the complete set of the 21 elastic coefficients a technique allowing the identification and the orientation in the 3 d space of the symmetry elements planes axes and a technique for approximating the considered elastic tensor by a tensor of simpler symmetry with the quantification of the error induced by such an approximation have been developed we apply these tools to various types of reservoir rocks and observed quite contrasted behaviors in some rocks the elastic anisotropy and the hydraulic anisotr

diabatic rossby waves in the southern hemisphere diabatic rossby waves drws are a special type of low level cyclone with a self maintaining mechanism given an environment of moderate or strong baroclinicity and abundant moisture under certain conditions these shallow cyclones associated with a cyclonic low level pv anomaly can serve as precursors for explosive cyclone intensification the present study is the first to investigate drws in the southern hemisphere sh two typical drw case studies are presented and a sophisticated algorithm is used to detect drws in operational analysis data from 2001 to 2012 drws occur in all ocean basins and seasons with a maximum in summer on average about four drws are found per month and 12 intensify explosively thereby they contribute 6 to the total set of explosively deepening extratropical cyclones in the sh drw tracks originate slightly closer to the equator than normal extratropical cyclones and they follow the convergence zones eastward and poleward this results in a climatological drw track density similar to a spiral from the western south pacific ending in the eastern south indian ocean broken only by the andes typical synoptic situations associated with drw genesis are identified the categories include i low level pv generation by upper level induced lifting ii low level jet induced diabatic pv generation and iii transformation of the low tropospheric pv anomaly from another system

circulation features over the south pacific during 10 18 january 1979 abstract a diagnostic study of the time averaged large scale circulation features over the equatorial and south pacific ocean during 10 18 january 1979 fgge sop 1 is presented results are based on ecmwf level iii b analyses for an area bounded by 10 n 105 w 42 5 s and 170 e during the nine day period the south pacific convergence zone spcz and its accompanying cloud band were quasi stationary features in the area of study analyses of surface pressure geopotential height horizontal winds temperature relative humidity vertical p velocity horizontal divergence and relative vorticity are presented and discussed particular attention is devoted to examining the structure of the spcz through the use of vertical cross sections along and across both the west east zonal and northwest southeast diagonal portions of the zone

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

implications of continental crust extension for plate reconstruction an example from the gulf of mexico the prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup continental extension is not expected to be pervasive throughout the plate but to be confined to a zone of up to several hundred kilometers in width the direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates the best fit between rifted continental margins is then described by the rotation angle about an euler pole which best closes both the oceanic crust and the extension within the continental crust along small circles about the euler pole as an example application of these concepts the pattern of late mesozoic crust extension within the gulf of mexico basin is used to constrain the location of the euler pole and angle of rotation of the yucatan block with respect to north america both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point by point basis from bathymetry and basement depth the root mean square rms misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening a contour map of misfit versus pole location reveals a global minimum rms misfit of 47 km for a 45 counterclockwise opening of the gulf of mexico basin about an euler pole at 25 n 79 w an elongate trend of poles for which the misfit is less than 100 km extends from the bahama islands eastward across the atlantic and through north africa the reconstruction of the gulf of mexico for the best fit pole places the palinspastically restored yucatan block in contact with north america along the texas and louisiana shelf the resulting restored southern margin of north america fits against the the northern margin of south america in its prerift position to within the uncertainty in the north america africa south america plate circuit we conclude that the prerift placement of the yucatan block between north and south america along the texas louisiana shelf is consistent with both the pattern of extension within the gulf of mexico basin and seafloor spreading history of the central atlantic basin

elastic anisotropy and pore space geometry of schlieren granite direct 3 d measurements at high confining pressure combined with microfabric analysis pore space geometry of granitic rocks and its evolution with depth are key factors in large scale seismics or in projects of enhanced geothermal systems or of deep hazardous waste repositories in this study we studied macroscopically anisotropic schlieren bearing granite by experimental p wave velocity v p measurements on spherical sample in 132 directions at seven different confining pressures in the range 0 1 400 mpa in order to discriminate the phenomena affecting the rock elastic properties we analysed the orientation of microcracks and of grain boundaries and we measured the anisotropy of magnetic susceptibility of the rock three sets of microcracks were defined with two of them linked to the massif exfoliation process and one to cooling contraction of the massif during pressurization the measured mean v p and v p anisotropy degree at ambient pressure and at highest confinement 400 mpa yielded 3 3 km s 1 and 24 per cent and 6 2 km s 1 and 3 per cent respectively the associated v p anisotropy pattern was transversely isotropic and governed by the schlieren with a minimum v p direction perpendicular to them and a girdle of high v p directions parallel to them the highest change in v p was observed between 0 1 and 10 mpa suggesting a significant closure of porosity below depths of 500 m change of the v p anisotropy pattern to orthorhombic together with increase of mean v p and v p anisotropy degree during depressurization was attributed to inelastic response of one of the sets of microcracks to the loading unloading cycle

the effect of rock fabric on p wave velocity distribution in amphibolites abstract this study presents contribution to the laboratory investigation of elastic properties and rock fabric of amphibolites p wave velocity was determined on four spherical samples prepared from a shallow borehole core the measurement was conducted in 132 directions under various conditions of hydrostatic pressure up to 400 mpa the rock fabric was investigated by image analysis of thin sections that enabled precise determination of grain size modal composition and shape parameters of rock forming minerals laboratory measurement of p waves revealed pseudoorthorhombic symmetry of rock fabric in amphibolites studied this symmetry reflects rocks macro and microfabric maximum p wave velocity corresponds to the macroscopically visible stretching lineation minimum p wave velocity is oriented perpendicular to the foliation plane the average grain size is the main microstructural factor controlling mean p wave velocity

florida a jurassic transform plate boundary magnetic gravity seismic and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern florida during the jurassic on the basis of this integrated suite of data the pre cretaceous florida bahamas region can be divided into the pre jurassic north american plate jurassic marginal rift basins and a broad jurassic transform zone including stranded blocks of pre mesozoic continental crust major tectonic units include the suwannee basin in northern florida containing paleozoic sedimentary rocks a central florida basement complex of paleozoic age crystalline rock the west florida platform composed of stranded blocks of continental crust the south georgia rift containing triassic sedimentary rocks which overlie block faulted suwannee basin sedimentary rocks the late triassic jurassic age apalachicola rift basin and the jurassic age south florida bahamas and blake plateau marginal rift basins the major tectonic units are bounded by basement hinge zones and fracture zones fz the basement hinge zone represents the block faulted edge of the north american plate separating paleozoic and older crustal rocks from jurassic rifted crust beneath the marginal basins fracture zones separate mesozoic marginal sedimentary basins and include the blake spur fz jacksonville fz bahamas fz and cuba fz bounding the blake plateau bahamas south florida and southeastern gulf of mexico basins the bahamas fz is the most important of all these features because its northwest extension coincides with the gulf basin marginal fault zone forming the southern edge of the north american plate during the jurassic the limited space between the north american and the south american african plates requires that the jurassic transform zone connecting the central atlantic and the gulf of mexico spreading systems was located between the bahamas and cuba fz s in the region of southern florida our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the gulf of mexico southern florida and the bahamas indicate that the gulf was sealed off from the atlantic waters until callovian time by an elevated florida bahamas region restricted influx of waters started in callovian as a plate reorganization and increased plate separation between north america and south america africa produced waterways into the gulf of mexico from the pacific and possibly from the atlantic

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

the evolution of middle america and the gulf of mexico caribbean sea region during mesozoic time a plate tectonic model for the evolution of middle america and the gulf of mexico caribbean sea region is presented the model which is based upon the existence of the mojave sonora megashear incorporates into the triassic pangea reconstruction three microplates between north and south america thus avoiding the overlap of the bullard fit these plates are the yaqui bounded on the north by the mojave sonora megashear the east and west maya plates bounded on the north by the mexican volcanic zone and on the south by a predecessor of the motagua fault zone and the chortis plate parts of guatemala and honduras during late jurassic time as north america split away from europe africa and south america shear with left lateral sense of displacement occurred along the transform faults that bounded the micro plates if 800 km of left lateral displacement along the mojave sonora megashear 300 km along the mexican volcanic belt and 1 300 km along a proto motagua megashear are restored and if yucatan and cuba are rotated to fit against northern south america then 1 a curvilinear belt of late paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in texas eastern mexico nuclear central america and colombia 2 a mediterranean like sea is delineated that was a precursor of most of the present gulf of mexico 3 correlation is implied between the distinctive quartzose san cayetano formation of cuba and the caracas and juan griego groups of venezuela geometric constraints suggest that probably shear initially occurred along the mexican volcanic zone near the end of the middle jurassic subsequently probably about 160 m y ago displacements that total 800 km began along the mojave sonora megashear contemporaneously yucatan and fragments of pre cretaceous rocks that compose parts of central and western cuba migrated northward toward their present positions rotation of yucatan was facilitated by considerable displacement along the proto motagua zone and along a zone that is probably coincident with the modern salina cruz fault accumulation of widespread major salt units of late jurassic callovian to early oxfordian age in the gulf basin probably occurred contemporaneously with the arrival of these blocks at their present positions clastic units that interfinger with some of the youngest salt units and rim the gulf of mexico have not recorded major recognized translations since their accumulation clockwise rotation of south america and the chortis plate occurred during early cretaceous time this movement which was manifested by subduction of jurassic ocean floor against the previously rifted precursor of the island of cuba and under parts of hispaniola and puerto rico is recorded by circum caribbean orogeny abrupt changes in the relative motions between north and south america during late cretaceous time may have resulted in extension and outpourings of basalt upon the jurassic rocks of the ocean floor of the venezuelan basin west of beata ridge sea floor spreading formed the colombian basin related subduction occurred as the chortis plate including part of central america the nicaraguan rise and southeastern cuba was sutured against the maya east plate along the present motagua fault and cayman trench our model is constrained by published geologic data the relative positions of north and south america from atlantic sea floor magnetic anomalies and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between north and south america paleomagnetic data from middle america are sparse but do not conflict with the predicted motions of some of the microplates especially chortis

sedimentary and crustal structure of the northwestern gulf of mexico we conducted a large offset seismic experiment in the northwestern gulf of mexico using largecapacity air guns and digital ocean bottom seismographs to determine the velocity structure of the sediments crust and upper mantle five multiple detector split profile lines were shot over an area extending from the shallow midshelf south of galveston to the continental rise just south of the sigsbee escarpment the data were of adequate spatial density and quality to allow combined use of near vertical reflections wide angle reflections and refractions for interpretation several techniques were used to obtain velocity depth functions 1 conventional constant velocity layer analysis 2 interval velocities from moveouts of the precritical reflections 3 analysis in the tau p domain to determine extremal depth bounds 4 estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions and 5 forward modeling using two dimensional 2 d ray tracing a sedimentary sequence of nearly constant thickness 13 to 15 km covers the study area various mobilized salt features exist within the sediments in the slope area ranging from deeply buried layers and diapirs under the inner slope to shallow thin allochthonous bodies under the outer slope in contrast to the sediments the crust shows considerable variation in thickness from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf the transition under the slope however is not monotonie but includes thinning to nearly oceanic thickness under the midslope possibly suggesting an incipient rift that failed to materialize during the opening of the gulf

nacr14 a 3d model for the crustal structure of the north american continent abstract based on the large number of crustal seismic experiments carried out in the last decades we create nacr14 a 3d crustal model of the north american continent at a resolution of 1 1 we present maps of thickness and average velocities of the main layers that comprise the north american crystalline crust obtained from the most recent seismic crustal models within the usgs crustal structure database however the crustal data are unevenly distributed and in some cases discrepancies exist between published models in order to construct a consistent 3d crustal model with three layers in the crystalline crust we refrained from a direct interpolation of the crustal seismic parameters in the database instead we implemented the following sequence of steps 1 definition of the geometry of the main tectonic provinces of north america 2 selection and evaluation of the reliability of seismic crustal models in the database 3 estimation of the p wave seismic velocity and thickness of the upper middle and lower crust for each tectonic province 4 estimation of the interpolated pn velocity distribution the resulting average velocity of the crystalline crust is mostly consistent with that of the seismic points the main variations of the structure of the crystalline crust of north america displayed in the model can be related to its tectonic evolution the model available in a digital form can be used in various geophysical applications such as the correction for the crustal effects in gravity and seismic tomography and models of dynamic topography in order to detect heterogeneities characterizing the underlying upper mantle

comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions abstract this article presents a systematic analysis of the anisotropic behaviours of the bentheim and rothbach sandstones using ultrasonic p wave velocity electrical conductivity and magnetic susceptibility measurements for each sandstone the data were obtained from three core samples drilled perpendicularly to each other and tested in dry and water saturated conditions for acoustic and magnetic investigations the same statistical analysis was applied in order to present the data on comparable stereoplots surprisingly the bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the rothbach sandstone in which cross laminations were clearly identified as confirmed by a sedimentary magnetic fabric a discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies first by comparing electrical and acoustic properties in the bentheim sandstone we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape the inclusion model developed by kachanov kachanov m 1993 elastic solids with many cracks and related problems advances in applied mechanics vol 30 academic press boston ma pp 259 445 accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio second acoustic electrical and magnetic properties indicate that the observed anisotropy in the rothbach sandstone can be attributed to the matrix and more specifically to cementation we modified the dvorkin and nur geophysics 61 5 1996 1363 model of cemented granular media by introducing a spatially variable contact length and the model suggests that a very small variability of cemented contact length is enough to account for the observed p wave velocity anisotropy we emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks

development of magnetic and elastic anisotropies in slates during progressive deformation abstract magnetic and elastic anisotropies were investigated in rocks of the nizk jesenik mountains northeast bohemian massif ranging in lithology from almost unmetamorphosed sediments through slate to phyllite and showing a range of structural styles from sedimentary through spaced and slaty cleavage to metamorphic schistosity in unmetamorphosed and undeformed sedimentary rocks both the anisotropies display close relationships to the sedimentary fabric during the development of the spaced and slaty cleavage they are gradually re oriented into the attitudes of the deformational fabrics and in the rocks with metamorphic schistosity they are fully related to the deformational fabric elements which can be oriented in a very different way from the original sedimentary structures the magnetic anisotropy is mostly due to the preferred orientation of phyllosilicates generated during very weak regional metamorphism and subordinately due to the preferred orientation of magnetite the elastic anisotropy is probably controlled by the preferred orientation of phyllosilicates and by the existence of oriented systems of microcracks

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity