the evolution of early oceanic lithosphere in the southern red sea abstract it seems generally agreed that the red sea has formed by the rotation of arabia from africa about a pole of rotation at 36 5 n 18 0 e but the exact way in which the oceanic lithosphere has evolved between the separating plates remains unclear this paper is concerned with the evolution of the earliest oceanic lithosphere in the southern red sea a combined study is made of gravity magnetic and seismic refraction data for a region on the southeast margin of the red sea between 16 3 n 17 3 n and 41 6 e 43 1 e a model is evolved which satisfies all the geological and geophysical data a feature of this model is that the ocean continent boundary might be 35 km inland from the coast and only 8 km from the exposed precambrian shield in this region the coastal plain being underlain by early oceanic lithosphere this implies that the break in the continental lithosphere was fairly clean

mechanical and electromagnetic properties of northern gulf of mexico sediments with and without thf hydrates using an oedometer cell instrumented to measure the evolution of electromagnetic properties small strain stiffness and temperature we conducted consolidation tests on sediments recovered during drilling in the northern gulf of mexico at the atwater valley and keathley canyon sites as part of the 2005 chevron joint industry project on methane hydrates the tested specimens include both unremolded specimens as recovered from the original core liner and remolded sediments both without gas hydrate and with pore fluid exchanged to attain 100 synthetic tetrahydrofuran hydrate saturation at any stage of loading test results demonstrate the extent to which the electromagnetic and mechanical properties of hydrate bearing marine sediments are governed by the vertical effective stress stress history porosity hydrate saturation fabric ionic concentration of the pore fluid and temperature we also show how permittivity and electrical conductivity data can be used to estimate the evolution of hydrate volume fraction during formation the gradual evolution of geophysical properties during hydrate formation probably reflects the slow increase in ionic concentration in the pore fluid due to ion exclusion in closed systems and the gradual decrease in average pore size in which the hydrate forms during hydrate formation the increase in s wave velocity is delayed with respect to the decrease in permittivity consistent with hydrate formation on mineral surfaces and subsequent crystal growth toward the pore space no significant decementation debonding occurred in 100 thf hydrate saturated sediments during unloading hence the probability of sampling hydrate bearing sediments without disturbing the original sediment fabric is greatest for samples in which the gas hydrate is primarily responsible for maintaining the sediment fabric and for which the time between core retrieval and restoration of in situ effective stress in the laboratory is minimized in evaluating the impact of core retrieval on specimen properties it is also important to consider how far removed hydrate bearing samples are from hydrate stability conditions

a methodology for 7d warping and deformation monitoring using time lapse seismic data a methodology is presented for vector analysis of the image displacements warping between successive 3d seismic image volumes that provides 7d analysis including lateral and vertical displacements of in situ subsurface deformation around hydrocarbon reservoirs the key challenges are 1 assessment of just vertical shifts is insufficient and vector displacements should be determined 2 robust vertical displacements can usually be derived but lateral shifts are less well defined because of the generally smooth data character in a horizontal horizon plane 3 subvoxel resolution is necessary for correct matching and deformation analysis 4 velocity and strain effects are intrinsically combined in time lapse seismic images 5 separation of accumulated and local effects is necessary 6 apparently coherent and smooth displacement fields do not necessarily provide good strain analysis 7 warping is easily degraded by noise and good cross matching is a prerequisite to address these challenges a full 3d local warp vector derivation methodology is proposed which involves 1 constraint using prior estimates 2 local refinement with subvoxel resolution and 3 3d and vectorial conditioning using a deformable mesh with sensitivity to image match quality the warping approach is extended to separate accumulated from local effects and to analyze in situ deformation based on the displacement vector volume this is achieved by a finite element approach to determine an elemental pseudostrain tensor field and an iterative procedure to separate the pseudostrain into velocity and strain components the approach up to the strain analysis is demonstrated using a real data example which indicates the potential of the methodology accumulated overburden effects are separated to reveal a local compaction signature in the reservoir but realistic quantitative values of strain have not yet been realized

attenuation measurements from sonic waveform logs in methane hydrate bearing sediments at the nankai trough exploratory well off tokai central japan 1 i have used full waveform logs from the nankai trough exploratory well off central japan to estimate both compressional and shear attenuation in sediments containing methane hydrate mh the attenuation estimates are based on a median frequency shift to the amplitude spectrum of the recorded waveforms this paper is concerned with attenuation at sonic frequencies of 10 20 khz for compressional waves and 500 1000 hz for shear waves i observed that the presence of mh increases the host sediments seismic attenuation and shear attenuation may be more helpful than compressional attenuation in detecting or characterizing mh bearing sediments as compressional attenuation is affected by the presence of gas moreover the ratio of compressional to shear attenuation is found to be a more sensitive indicator of the presence of low saturation gas than the corresponding velocity ratio

acoustic response of gas hydrate formation in sediments from south china sea abstract gas hydrate has been recognized as a potential energy resource in south china sea scs understanding the acoustic response of gas hydrate formation in the scs sediments is essential for regional gas hydrate investigation and quantification the sediments were obtained from gravity core sampling at e 115 12 52363 n 19 48 40299 gas hydrate was formed within a gas water saturated scs sediments system combination of a new bender element technique and coated time domain reflectometry tdr was carried out to study the acoustic response of hydrate occurrence in scs sediments the results show the acoustic signal becomes weak when hydrate saturation sh is lower than 14 the acoustic velocities vp vs of the sediments increase with sh during hydrate formation and vs increases relatively faster when sh is higher than 14 these results indicate that tiny hydrate particles may firstly float in the pore fluid which causes a significant acoustic attenuation but has little influence on shear modulus as time lapses and sh approaches 14 numerous particles coalesce together and contact with sediment particles as a result vs has a sharp increase when hydrate saturation exceeds 14 several velocity models were validated with the experimental data which suggests a combination of the bgtl biot gassmann theory modified by lee model and the weighted equation is suitable to estimate sh in scs

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

crustal and uppermost mantle structure beneath the afro arabian rift system abstract this contribution presents data and results from long range seismic refraction wide angle reflection lines in the afro arabian rift system from the derived crustal structure an evolutionary model is proposed in terms of the amount of crustal thinning in this model the gulf of aden and the axial trough of the southern red sea where the continental crust was thinned so much as to allow thin oceanic crust to form are representative of one end of the spectrum progressing through the northern part of the red sea main trough on the egyptian side with a crustal thickness of 11 14 km to the northern and southern parts of the red sea main trough on the arabian side and the northern afar depression with 15 20 km thick crusts followed by the southern afar depression with a crustal thickness of 22 26 km the members of the other end of the spectrum are reached these include the jordan dead sea and east african rifts which each with a crustal thickness of 30 36 km represent the least thinned closest to normal thickness continental crust in determining the amount and nature of opening of the red sea there remain however many problems argely due to the non unique correspondence of seismic velocity to rock type this may lead to uncertainty in moho depths especially in areas close to the mid ocean ridge system e g the southeastern part of the afar depression in the region around the gulf of tadjura where velocities as low as 6 8 6 9 km s have been interpreted as representirg hot anomalous mantle it also causes uncertainty as to what percentage of the present crust is stretched and thinned original continental crust and what percentage is new crust whether it be truly oceanic or basic intrusiors into continental crust thus it is felt that there is still an urgent need for geoscientific experiments in the afro arabian rift to resolve the details of continental rifting and subsequent oceanic crust emplacement

tectonic evolution of the northwestern red sea margin as related to basement structure abstract at the northwestern egyptian margin of the red sea a crystalline basement of pan african late proterozoic age is exposed beneath a phanerozoic sedimentary cover this tectonic situation provided an opportunity to study the younger red sea tectonic evolution in relation to features that are inherited from or reactivated in an underlying basement two structural maps compiled from published sources and our data covering the eastern egyptian basement from hurghada to the south show the most important structural elements the major central and southern parts of the basement in eastern egypt ced and sed are dominated by low angle thrusts other fractures n s e w nw se are distinctly less frequent and of local importance nw se oriented strike slip faults najd system overprinted the low angle thrusts of the northern ced these fault are related to ne sw trending normal faults which characterize the northern part of the basement ned before discussing general relationships between basement structure and red sea evolution a detailed example of the post pan african phanerozoic evolution of the low angle thrust dominated domains of the ced and the sed from a traverse south of 25 n is presented there detailed structural mapping showed phanerozoic normal faults close to the red sea trending nnw sse and at the margin of the phanerozoic cover east of the nile trending nnw sse and nw se leading to a horst like position of the basement within this horst deformation is restricted to a local sinistral strike slip fault trending n s the normal faults frequently used and reactivated fractures parallel or normal to the axial surface of earlier pan african folds igneous activity is confined to the western part of the basement where mafic dykes mafic to trachytic volcanics the alkaline gabal abu khrug ring complex and trachyte plugs in that time sequence from 104 to 80 ma are exposed mafic dykes are dependent on host rock properties distributed mainly in relatively brittle granitoid rocks trachyte intrusions are generally not related to particular host rocks but may follow pre existing structures crosscutting relationships with the igneous rocks imply a late cretaceous age for the faults in the west of the traverse however in the east at the red sea coast faults cut middle miocene sediments and are thus younger tectonic activity in the traverse area is therefore confined to two distinct episodes a late cretaceous event of alkaline magmatism and normal faulting at the western margin of the basement and a miocene or younger event of normal faulting at the present red sea coast in contrast to earlier views neither transform faults nor large scale riedel shears related to red sea tectonics have been met in the area this holds also true for the ced and sed as a whole where only local faults remain for potential reactivation during red sea tectonics except perhaps the duwi shear zone at quseir the prevailing n s e w or nw se directions of these faults preclude an interpretation as possible transform faults normal to the spreading axis of the red sea therefore it is no longer possible to assume a continuation of red sea transform faults into the continental crust

the afro arabian rift system an overview abstract the afro arabian rift system is reviewed beginning with the dead sea transform and gulf of suez in the north followed by the gulf of aden and red sea and ending with a brief mention of rifting through eastern and southern africa a consistent interpretation is obtained for geophysical data from the gulf of aden and red sea and geological data for the dead sea transform suez graben and red sea and gulf of aden margins geophysical data are used to estimate the locations of the ocean continent boundaries in the northern red sea these are found to coincide when the total 107 km shear along the dead sea is restored the red sea is therefore reconstructed in stages beginning with the 45 km post miocene shear and 62 km early miocene shear along the dead sea transform followed by estimates for extension in the early red sea suez graben when this is done there is a remarkable alignment of various features revealed on landsat tm imagery for both sides of the northern red sea north south differences in the red sea and west east differences hi the gulf of aden are discussed and found to be best explained by a propagating rift model

anisotropic models to account for large borehole washouts to estimate gas hydrate saturations in the gulf of mexico gas hydrate joint industry project leg ii alaminos canyon 21 b well abstract through the use of 3 d seismic amplitude mapping several gas hydrate prospects were identified in the alaminos canyon ac area of the gulf of mexico two locations were drilled as part of the gulf of mexico gas hydrate joint industry project leg ii jip leg ii in may of 2009 and a comprehensive set of logging while drilling lwd logs were acquired at each well site lwd logs indicated that resistivity in the range of 2 ohm m and p wave velocity in the range of 1 9 km s were measured in the target sand interval between 515 and 645 feet below sea floor these values were slightly elevated relative to those measured in the sediment above and below the target sand however the initial well log analysis was inconclusive regarding the presence of gas hydrate in the logged sand interval mainly because large washouts caused by drilling in the target interval degraded confidence in the well log measurements to assess gas hydrate saturations in the sedimentary section drilled in the alaminos canyon 21 b ac21 b well a method of compensating for the effect of washouts on the resistivity and acoustic velocities was developed the proposed method models the washed out portion of the borehole as a vertical layer filled with sea water drilling fluid and the apparent anisotropic resistivity and velocities caused by a vertical layer are used to correct the measured log values by incorporating the conventional marine seismic data into the well log analysis the average gas hydrate saturation in the target sand section in the ac21 b well can be constrained to the range of 8 28 with 20 being our best estimate

structure of the red sea at 20 n from gravity data and its implication for continental margins structure of the red sea at 20 n from gravity data and its implication for continental margins

non equilibrium compaction and abnormal pore fluid pressures effects on rock properties1 knowledge of pore pressure using seismic data will help in planning the drilling process to control potentially dangerous abnormal pressures various physical processes cause anomalous pressures on an underground fluid non equilibrium compaction is a significant process of overpressure generation this occurs when the sedimentation rate is so rapid that the pore fluids do not have a chance to escape from the pore space the model assumes a closed system and that the pore space is filled with water and hydrocarbon in a liquid state balancing mass and volume fractions yields the fluid pressure versus time of deposition and depth of burial thermal effects are taken into account the pore pressure together with the confining pressure determines the effective pressure which in turn determines the bulk moduli of the rock matrix we assume a sandstone saturated with hydrocarbons and water for which calibration of the model with experimental data is possible the seismic velocities and attenuation factors are computed by using biot s theory of dynamic poroelasticity and the generalized linear solid the example shows that the formation can be overpressured or underpressured depending on the properties of the saturating fluid wave velocities and quality factors decrease with decreasing differential pressure the effect is important below approximately 20 mpa the model is in good agreement with experimental data for berea sandstone and provides a tool for predicting pore pressure from seismic attributes

deep structure of a fault discontinuity relocated aftershocks at a fault discontinuity that ruptured in the 1992 landers earthquake show that there is no simple connection between the homestead valley and emerson faults at depth rather faulting within the discontinuity is as complex as observed at the surface both aftershocks and mainshock slip on the far side of the discontinuity that is on the emerson fault were confined primarily to depths above 10 km whereas slip and aftershock activity on the near side of the discontinuity extended to 17 km similar behavior was also observed at a fault discontinuity in the 1979 coyote lake earthquake and may be attributable to the greater effectiveness of dynamic stress in inducing fault rupture at shallow depths

focussed fluid flow on the hikurangi margin new zealand evidence from possible local upwarping of the base of gas hydrate stability the southern hikurangi subduction margin is characterized by significant accretion with predicted high rates of fluid expulsion bottom simulating reflections bsrs are widespread on this margin predominantly occurring beneath thrust ridges we present seismic data across the porangahau ridge on the outer accretionary wedge the data show high amplitude reflections above the regional bsr level based on polarity and reflection strength we interpret these reflections as being caused by free gas we propose that the presence of gas above the regional level of bsrs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids although we cannot entirely rule out alternative processes simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low permeability slope basins into the porangahau ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability focussing of fluid flow is predicted to take place deep in the accretionary wedge and or the underthrust sediments above the high amplitude reflections sediment reflectivity is low a lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas hydrate related amplitude blanking structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge a gap in the bsr in the southern part of the study area may be caused by a loss of gas during fluid expulsion we speculate that gaps in otherwise continuous bsrs that are observed beneath some thrusts on the hikurangi margin may be characteristic of other locations experiencing focussed fluid expulsion

an irregular feather edge and potential outcrop of marine gas hydrate along the mauritanian margin the dissociation of marine hydrate that surrounds continental margins is thought to be an agent for past and future climate change as the water depth decreases landwards the base of the hydrate stability zone progressively shallows until hydrate can occur at or immediately below the seabed where an increase in bottom water temperature can cause dissociation but the true extent of these most vulnerable hydrate deposits is unknown here we use exceptional quality three dimensional 3 d seismic reflection imagery offshore of mauritania that reveals a rare example of a bottom simulating reflection bsr that intersects the seabed and delineates the feather edge of hydrate the bsr intersects the seabed at the 636 m isobath but along the 32 km of the margin analysed the intersection is highly irregular intersections and seismic evidence for hydrate less than 4 3 m below the seabed occur in seven small localised areas that are 0 02 0 45 km2 in extent we propose gas flux below the dipping base of the hydrate to these places has been particularly effective the intersections are separated by recessions in the bsr where it terminates below the seabed seawards of the 636 m isobath recessions are areas where the concentration of hydrate is very low or hydrate is absent they are regions that have been bypassed by gas that has migrated landwards along the base of the hydrate or via hydraulic fractures that pass vertically through the hydrate stability zone and terminate at pockmarks at the seabed an irregular feather edge of marine hydrate may be typical of other margins

paleomagnetism of the samail ophiolite oman 1 the sheeted dike complex at ibra we have studied the rock magnetism and paleomagnetism of 200 oriented samples from the middle cretaceous sheeted dike sequence of the samail ophiolite oman the samples are from 28 sites spanning a cross strike distance of 30 km most of the dikes have undergone green schist metamorphism the natural remanent magnetization nrm of the samples is about 1 a m and qn is also near 1 thermal and alternating field demagnetization treatment resolves both normal north and reversed south polarities about one third of the samples have low median destructive fields mdf less than 100 oe 7 96 103 a turn m these low mdf samples have scattered directions and are classified as undetermined polarities u hysteresis measurements show pseudo single domain carriers predominate in all samples although the low mdf samples are suggested to have the largest grains in the samples studied thermomagnetic curves show both magnetite and minor maghemite thermal demagnetization and microscopic study reveals that hematite is abundant and is the carrier of the r remanence in addition several dikes exhibit both n and r polarities these facts coupled with the degree of metamorphism indicate that part of the remanence is secondary and could be due to hydrothermal metamorphism below the seafloor we suggest that the n polarity is primary and resides in exsolved magnetite while the r polarity is secondary and resides primarily in metamorphic origin hematite several short r events observed in the albian through cenomanian magnetostratigraphic record are candidates for the r event in the ophiolite if the r polarity originated near the spreading center alternately nonreproducible self reversal is another possible origin for the r polarity regardless of whether the reversal originated at the spreading center secondary magnetization in the dikes considerably increases the directional dispersion another major factor affecting dispersion is viscous remanent magnetization vrm in the larger magnetite grains this extreme dispersion disallows the samail ophiolite dike layer as a significant source for marine magnetic anomalies regardless of the strength of the nrm or the value of q the stable n and r directions yield a paleomagnetic pole which is far sided suggesting northward movement of the ophiolite however the paleomagnetic pole is not as far sided as the contemporaneous africa pole which suggests a component of north south closure between the ophiolite and africa prior to northward movement

a method for determining gas hydrate or free gas saturation of porous media from seismic measurements the occurrence of gas hydrate or free gas in a porous medium changes the medium s elastic properties explicit formulas for gas hydrate or free gas saturation of pore space on the basis of the frenkel gassmann equations describe the elastic moduli and seismic velocities of a porous medium for low frequencies a key assumption of the model is that either gas hydrate or free gas is present in the pore space in addition to water under this assumption the method uses measured p and s wave velocities and bulk density along with estimates of the moduli and densities of the solid and fluid phases present to determine whether gas or hydrate is present the method then determines the saturation level of either the gas or the hydrate i apply the method to published velocity and density data from seismic studies at the antarctic shetland margin and at the storegga slide offshore norway and to borehole log and core data from ocean drilling program odp leg 164 at blake ridge offshore south carolina a sensitivity analysis reveals that the standard deviations of the gas hydrate and free gas saturations reach 30 70 of the saturations if the standard deviations of the p and s wave velocities and of the bulk density are 50m s

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

miocene magmatism of sinai related to the opening of the red sea extension associated with the initial early miocene stages of opening of the red sea resulted in intrusion of a widespread system of dikes and smaller bodies primarily along the northeastern margin of the red sea gulf of suez axis dikes some up to more than 800 km in length were emplaced dominantly parallel to the red sea gulf of suez from the sinai peninsula to yemen intrusive rocks are mainly basaltic in composition but minor granophyres are also present in the sinai this magmatic event is about 20 m y old dikes of the southern and southeastern sinai crop out in precambrian terrane in the central and northern sinai they occur in phanerozoic terrane where they may be associated with linear grabens and cryptovolcanic features basaltic dikes of the sinai underwent variable degrees of alteration alteration phases include seriate quartz smectite chlorite and serpentine x ray diffraction and petrographic analysis indicate that in spite of extensive alteration primary minerals still control the major element compositions of the rocks basalts are tholeiitic in composition with sio2 48 5 to 50 and na2o k2o 3 to 3 5 rb and sr concentrations are probably dominated by alteration and by introduction of secondary material primarily carbonate zr tio2 which is relatively unchanged during alteration substantiates that the basaltic rocks are subalkaline initial 87sr 86sr determined after leaching of samples is approximately 0 7043 0 7063 much higher than red sea axial trough basalts 0 703 we infer that these sinai basalts equilibrated at depths of 35 50 km possibly within an enriched mantle whether the main trough of the red sea is underlain by extended and modified continental crust or by oceanic crust is controversial since miocene dikes were intruded along both coasts but primarily the saudi arabian coast of the red sea gulf of suez axis in a zone up to 150 km wide they may also underlie the main trough of the red sea and result at least partially in the discontinuous magnetic and gravity anomalies observed there possibly the abundance of dikes beneath the main trough is much greater than adjacent to the red sea giving rise to properties intermediate between those of continental and oceanic crust downwarping and widespread intrusion of dikes along the red sea 18 26 m y ago probably signified crustal dilation and the initiation of a continental rift because tholeiitic basalts of the sinai are higher in 87sr 86sr than red sea axial trough basalts we infer that the conditions of their origin must also be very different we suggest that the sinai basalts may have been generated within but near the base of subcrustal continental lithosphere perhaps indicating thermal thinning of lithospheric mantle above upwelled asthenosphere associated with continental rifting this inference if correct would indicate that the lithosphere was at least 35 50 km thick during the early miocene phase of rifting but possibly not much thicker since dikes farthest from the red sea are apparently identical in composition to those closest asthenospheric upwarping must have been broad enough that all magmas were derived from approximately the same depth in the yemen ethiopia areas volcanism began earlier about 30 m y ago and was much more voluminous than elsewhere along the red sea gulf of suez axis suggesting the presence of underlying mantle upwelling accompanying rifting of the red sea in spite of differences in age and volume the presence of large volumes of tholeiites among the miocene basalts of the yemen ethiopia areas suggests that the lithosphere was similarly thinned along the entire length of the red sea

a laboratory investigation into the seismic velocities of methane gas hydrate bearing sand remote seismic methods which measure the compressional wave p wave velocity vp and shear wave s wave velocity vs can be used to assess the distribution and concentration of marine gas hydrates in situ however interpreting seismic data requires an understanding of the seismic properties of hydrate bearing sediments which has proved problematic because of difficulties in recovering intact hydrate bearing sediment samples and in performing valid laboratory tests therefore a dedicated gas hydrate resonant column ghrc was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both vp and vs made at frequencies and strains relevant to marine seismic investigations thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested the results show a bipartite relationship between velocities and hydrate pore saturation with a marked transition between 3 and 5 hydrate pore saturation for both vp and vs this suggests that methane hydrate initially cements sand grain contacts then infills the pore space these results show in detail for the first time using a resonant column how hydrate cementation affects elastic wave properties in quartz sand this information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation

final laurentide ice sheet deglaciation and holocene climate sea level change abstract despite elevated summer insolation forcing during the early holocene global ice sheets retained nearly half of their volume from the last glacial maximum as indicated by deglacial records of global mean sea level gmsl partitioning the gmsl rise among potential sources requires accurate dating of ice sheet extent to estimate ice sheet volume here we date the final retreat of the laurentide ice sheet with 10 be surface exposure ages for the labrador dome the largest of the remnant laurentide ice domes during the holocene we show that the labrador dome deposited moraines during north atlantic cold events at 10 3 ka 9 3 ka and 8 2 ka suggesting that these regional climate events helped stabilize the retreating labrador dome in the early holocene after hudson bay became seasonally ice free at 8 2 ka the majority of laurentide ice sheet melted abruptly within a few centuries we demonstrate through high resolution regional climate model simulations that the thermal properties of a seasonally ice free hudson bay would have increased laurentide ice sheet ablation and thus contributed to the subsequent rapid labrador dome retreat finally our new 10 be chronology indicates full laurentide ice sheet had completely deglaciated by 6 7 0 4 ka which re quires that antarctic ice sheets contributed 3 6 6 5 m to gmsl rise since 6 3 7 1 ka

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

seismic wave attenuation in methane hydrate bearing sediments vertical seismic profiling data from the nankai trough exploratory well offshore tokai central japan 1 data from two vertical seismic profiles vsps from the nankai trough exploratory well offshore tokai central japan are used to estimate compressional attenuation in methane hydrate mh bearing sediments at seismic frequencies of 30 110 hz we compare spectral ratio and centroid frequency shift methods for measuring attenuation to isolate intrinsic attenuation from total attenuation attenuation is computed from multiples using one dimensional synthetic vsp data from sonic and density logs the use of two different measurement methods and two vsps recorded at just 100 m separation provides an opportunity to validate the attenuation measurements no significant compressional attenuation was observed in mh bearing sediments at seismic frequencies macroscopically the peaks of highest attenuation in the seismic frequency range correspond to low saturation gas zones in contrast high compressional attenuation zones in the sonic frequency range 10 20 khz are affected by the presence of methane hydrates in the same well locations thus this study demonstrated the frequency dependence of attenuation in mh bearing sediments mh bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range

the characteristics of gas hydrates recovered from shenhu area in the south china sea abstract for the first time gas hydrate samples were recovered in the south china sea during a drilling campaign implemented in shenhu area in june 2007 laboratory based studies for the characteristics of gas hydrate were carried out on two hydrate bearing sediment samples observations found that gas hydrates occurred in the pores of fine grained sediments the hydrates were structure i based on the raman spectroscopic results the guest gas molecules in hydrate cages were predominantly methane 99 3 and trace amount of ethane 0 4 and propane 0 04 as determined with the method of gas chromatography as estimated from the intensities of raman peaks the cage occupancy rates were more than 99 in large cage and 86 in small cage respectively corresponding to a hydration number of 5 9

paleomagnetism of the samail ophiolite oman 2 the wadi kadir gabbro section four stratigraphic vertical transects were obtained from the gabbroic and ultramafic section of the samail ophiolite middle cretaceous age the transect through wadi kadir was the most detailed 65 independent oriented cores were obtained from 22 sites on a 14 km traverse from field mapping we infer that this traverse corresponds to a vertical section of 6 0 km which includes sheeted dikes plagiogranites isotropic high level gabbro transitional gabbro cumulate gabbros cumulate ultramafics and ultramafic rocks there are some noticeable variations in magnetic mineralogy with depth 1 primary exsolved magnetite is found mainly in the late stage high level gabbro and 2 secondary magnetite is found mainly in the upper 3 5 km of the cumulate gabbros and in the cumulate ultramafics most of the gabbros have a stable magnetization direction that is almost horizontal and slightly west of north the median natural remanent magnetization nrm intensity for the cumulate gabbros is 0 93 a m the median qn value is 3 0 with most values falling in the range of 1 to 5 the ultramafic rocks below the gabbroic layer are more strongly magnetized due to higher concentrations of secondary magnetite and their magnetization is more stable to af demagnetization than that of the overlying gabbros it is not clear what fraction of their magnetization was acquired beneath the seafloor because some of the serpentinization is thought to be of subaerial origin hysteresis measurements on the gabbros suggest that the magnetic carriers in the gabbros are a few tenths of microns in diameter pseudo single domain the magnetic properties of the cumulate gabbro section divide it into an upper intensely magnetized layer 3 5 km thick and a lower weak layer 2 km thick this was also found for the point sal ophiolite california the low intensities below 2 km are in agreement with petrological observations of a minimum in the concentration of secondary magnetite the upper intense layer can contribute over one half of the amplitude of any marine magnetic anomaly which might have existed when the ophiolite was submarine the serpentinized rocks also are possible contributors to anomaly amplitude consideration of the probable variations in the magmatic and hydrothermal history of the ocean crust and comparison of our results to other studies indicate that it may not be possible to specify a generalized magnetic source model for the plutonic section of the ocean crust

problems concerning the evolution of oceanic lithosphere in the northern red sea abstract the pole of rotation for the opening of the red sea at 36 5 n 18 0 e arabia from africa implies the fastest spreading rates are in the south and are about 50 less in the north various studies show the spreading rate in the south to be about 1 cm yr over the last 4 5 ma problems arise as to the nature of the lithosphere beneath the northernmost red sea the problem is further complicated by the fact that the red sea may have evolved in three phases the first is the gulf of suez phase mainly oligocene the second corresponds to the first 62 km movement along the aqaba dead sea shear latest oligocene early miocene and the third to the second 45 km movement along the aqaba dead sea shear plio pleistocene these motions and their implications for the northernmost red sea are assessed the northern red sea differs from the central and southern red sea in having no clear magnetic lineations only a few isolated circular anomalies are observed there are two possible explanations 1 1 there is extremely thinned continental lithosphere occasionally pierced by strongly magnetised volcanic centres or 2 2 the oceanic lithosphere is present but without magnetic anomalies it is suggested that the first explanation applies to the first gulf of suez phase and the second explanation to the two later aqaba dead sea phases it is further suggested that the lack of magnetic anomalies may be due to a combination of large thicknesses of unstable sediments salt high temperatures and low sea floor spreading rates these lead to the evolution of oceanic lithosphere with slow cooling inhibiting the acquisition of strong magnetisation and giving subdued magnetic anomalies

possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine grained sediments results from the hikurangi margin new zealand abstract small amounts of free gas in interstitial sediment pores are known to significantly lower compressional p wave velocity vp this effect combined with moderately elevated vp from the presence of gas hydrates is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections bsrs at the base of gas hydrate stability bghs at several locations however weak bsrs have been observed which are difficult to reconcile with a presence of gas in sediment pores we here present a rock physics model for weak bsrs on the hikurangi margin east of new zealand thin sections of a fine grained mudstone sample from a submarine outcrop in the vicinity of a weak bsr show macroscopic porosity in the form of fractures and intrafossil macropores we apply the kuster toksoz theory to predict seismic velocities for a rock with water saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores we match field observations of a weak bsr with a reflection coefficient of 0 016 with two end member models 1 rocks with gas hydrate filled voids with a concentration of

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

seismic velocities for hydrate bearing sediments using weighted equation a weighted equation based on the three phase time average and wood equations is applied to derive a relationship between the compressional wave p wave velocity and the amount of hydrates filling the pore space the proposed theory predicts accurate p wave velocities of marine sediments in the porosity range of 40 80 and provides a practical means of estimating the amount of in situ hydrate using seismic velocity the shear s wave velocity is derived under the assumption that the p to s wave velocity ratio of the hydrated sediments is proportional to the weighted average of the p to s wave velocity ratios of the constituent components of the sediment in the case that all constituent components are known a weighted equation using multiphase time average and wood equations is possible however this study showed that a three phase equation with modified matrix velocity compensated for the clay content is sufficient to accurately predict the compressional wave velocities for the marine sediments this theory was applied to the laboratory measurements of the p and s wave velocities in permafrost samples to infer the amount of ice in the unconsolidated sediment the results are comparable to the results obtained by repeatedly applying the two phase wave scattering theory the theory predicts that the poisson s ratio of the hydrated sediments decreases as the hydrate concentration increases and the porosity decreases in consequence the amplitude versus offset avo data for the bottom simulating reflections may reveal positive negative or no avo anomalies depending on the concentration of hydrates in the sediments

the effect of hydrate content on seismic attenuation a case study for mallik 2l 38 well data mackenzie delta canada 1 observations of velocities in sediments containing gas hydrates show that the strength of sediments increases with hydrate saturation hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation however sonic log measurements in the mallik 2l 38 well and cross hole tomography measurements in the mallik field have shown that attenuation increases with hydrate saturation we studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation we found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect we modelled attenuation in terms of biot and squirt flow mechanisms in composite media we have used our model to predict observed attenuations in the mallik 2l 38 well mackenzie delta canada

physical property changes in hydrate bearing sediment due to depressurization and subsequent repressurization physical property changes in hydrate bearing sediment due to depressurization and subsequent repressurization w f waite 1 t j kneafsey 2 w j winters 1 d h mason 1 u s geological survey 384 woods hole road woods hole ma 02543 usa lawrence berkeley national laboratory 1 cyclotron rd ms 90r1116 berkeley ca 94720 usa