seismic and magnetic anisotropy of serpentinized ophiolite implications for oceanic spreading rate dependent anisotropy compressional and shear wave anisotropy shear wave birefringence and the anisotropy of magnetic susceptibility were measured on a series of dunites sampled from the dinardic hellenic ophiolites the densities of these materials ranged from 3330 kg m 3 to 2620 kg m 3 and indicate degrees of serpentinization from 2 3 to 87 9 respectively magnetic susceptibility increases and the compressional and shear wave velocities decrease in proportion to the degree of serpentinization as has been observed by other workers in all cases the magnetic susceptibility tensor is described by an oblate spheroid whose minor axis is closely aligned to the pole of the foliation but the magnetic anisotropy is not related to the degree of serpentinization the compressional wave anisotropy e monotonically decays from 12 for nearly pure olivine dunite to less than 2 for the most serpentinized sample this observation strongly suggests that serpentinization progressively destroys the original anisotropy by consuming the preferentially aligned olivines and replacing them with randomly oriented serpentines typical serpentine mesh textures seen in microscopic thin section examinations support this suggestion this loss of anisotropy with serpentinization may partly explain the apparent relationship between seismic compressional wave anisotropy of the oceanic lithosphere the more complex geological structure of slow spreading ridges may admit more sea water via faults for deep circulation which increases serpentinization and consequence of which is decreased seismic anisotropy 2007 elsevier b v all rights reserved

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

cryptic variation within the cumulate sequence of the oman ophiolite magma chamber depth and petrological implications the cumulate sequence of the oman ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic long lived crustal magma chamber that underlay a late cretaceous spreading ridge regional variations in cumulate sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions microprobe data are presented from a 600 m section of modally layered olivine gabbros coherent cryptic cyclic variation is shown with both normal and reversed geochemical gradients present these are explained in terms of fractionation mixing and eruption in an open system magma chamber the height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m the implications of this result are discussed a contrast between the fo an co variation between olivine and plagioclase in the oman cumulate sequence and that observed for morb votcanics is noted this may reflect compositional stratification in spreading ridge magma chambers or signifcant differences between the petrogenesis of the oman ophiolite and morb

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

metamorphism and deformation along the emplacement thrust of the samail ophiolite oman abstract the base of the samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement the classic exposure of this metamorphic sole at wadi tayin consists of a few meters of garnet clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite overlying 150 m of lower amphibolite facies mafic and sedimentary schist the sole is bounded by faults but is internally coherent peak metamorphic temperatures decrease exponentially from 825 c at the top of the sole to 500 c at its base as documented by changes in grain size hornblende composition the disappearance of chlorite and epidote garnet clinopyroxene element partitioning and garnet hornblende partitioning the inverted peak temperature gradient decreases from 4000 c km in the upper 50 m to 100 c km in the lower 150 m contrary to earlier reports greenschist facies tectonites are not present at wadi tayin although the entire sole underwent greenschist facies retrogression driven by the influx of fluids from the overlying peridotite or underlying carbonate calculations based on an amphibolite constitutive relation reveal that although the section represents a lithologic continuum the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column

seismic attenuation structure beneath the juan de fuca ridge from tomographic inversion of amplitudes tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the juan de fuca ridge to further constrain a velocity structure previously obtained by travel time velocity tomography and to obtain a model for attenuation variations within the shallow crust amplitude analysis has identified an additional boundary defined by a decrease in vertical velocity gradient at 2 0 km subbottom which was not required by the travel time data this transition zone likely represents the oceanic layer 2 layer 3 boundary the analysis also identified zones of relatively high attenuation qe 20 100 at subbottom depths of 0 3 2 0 km beneath the ridge axis which correlate well with low velocity zones velocities reduced by 0 4 km s in the velocity model the subaxial zone is interpreted as a region of increased fracture porosity and or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

azimuthally anisotropic model of the oceanic upper mantle abstract a mixed form of forward and backward modelings of azimuthally anisotropic oceanic upper mantle is attempted for a data set of surface wave velocities of periods 30 100 s one of the basic configurations of the anisotropy postulated in the lid and lvz is axial symmetry of elastic properties with a horizontal axis parallel to the spreading direction as found in ultramafic rocks from ophiolite complexes of the bay of islands and oman basic features of the resulting model are a thin lithosphere of 45 km a decrease of s wave velocities and an increase of p wave velocities and of partial melting dislocation relaxation with depth in the lvz this single model explains the overall features of anisotropy observations in the pacific ocean p n velocity anisotropy sh sv anisotropy and azimuthal anisotropy of surface waves and scs polarization anisotropy this fact leads us to a conclusion that the ultramafic rocks from the ophiolite complexes can explain the seismic properties of the oceanic lid and lvz

olivine fabrics in the bay of islands ophiolite implications for oceanic mantle structure and anisotropy petrofabric analysis of oriented ultramafic and mafic rock samples from six traverses representing all four massifs of the bay of islands ophiolite complex newfoundland indicate that the ultramaf

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

evolution of seismic layer 2b across the juan de fuca ridge from hydrophone streamer 2 d traveltime tomography how oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the earth s interior and the oceans although considerable work has been done characterizing the evolution of seismic layer 2a there has been little analysis of the processes that affect layer 2b after formation here we present high resolution 2 d tomographic models of seismic layer 2b along 300 km long multichannel seismic transects crossing the endeavour northern symmetric and cleft segments of the juan de fuca ridge these models show that seismic layer 2b evolves rapidly following a different course than layer 2a the upper layer 2b velocities increase on average by 0 8 km s and reach a generally constant velocity of 5 2 0 3 km s within the first 0 5 myr after crustal formation this suggests that the strongest impact on layer 2b evolution may be that of mineral precipitation due to active hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber variations in upper layer 2b velocity with age at time scales 0 5 ma show correlation about the ridge axis indicating that in the long term crustal accretion processes affect both sides of the ridge axis in a similar way below the 0 5 ma threshold differences in 2b velocity are likely imprinted during crustal formation or early crustal evolution layer 2b velocities at propagator wakes 5 0 0 2 km s where enhanced faulting and cracking are expected and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2b analysis of the layer 2b velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2b velocities respectively additionally the distance between present up flow and down flow zones is related to the amount of sediment cover because as sediment cover increases and basement outcrops become covered direct pathways from the igneous basement through the seafloor are cut off forcing convective cells to find alternate paths

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

a relic proterozoic subduction zone in western canada new evidence from seismic reflection and receiver function data lithoprobe seismic reflection profiles in the western canada sedimentary basin reveal a prominent band of dipping reflections that appear to extend into the upper mantle the nne strike of these reflections determined from intersecting profiles is parallel to the trend of the ca 1 8 ga rimbey domain an overlying magmatic belt receiver function analysis of teleseismic data recorded at a nearby site shows evidence for a low velocity zone lvz beneath the rimbey domain between 13 and 16 km depth the unusually low s wave velocity of the lvz 2 9 0 2 km s suggests a composition similar to serpentinized peridotites that occur in ophiolites taken together our data support a model of proterozoic subduction beneath the hearne province we interpret the dipping reflections observed on the seismic profiles as a shear zone that developed during subduction of a se dipping slab and the lvz as a fragment of oceanic lithosphere that was obducted to the overriding plate

neutral buoyancy and the structure of mid ocean ridge magma reservoirs the mechanical structure of the regions for shallow storage and lateral intrusion of mid ocean ridge basalt morb is explored in part by examining the density range appropriate for picritic to tholeiitic liquids and liquid crystal mixtures and its relationship to the nonlinear density depth structure of the ridge magma density spans the range 2 6 to at least 2 82 mg m 3 and incorporates the effects of fractional crystallization on the density evolution the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions the melt and melt crystal density band crosses the in situ rock density curve over the 1 to 3 km depth interval beneath the volcanic surface within that averaged interval picritic to tholeiitic melt and melt crystal mixtures are in approximate mechanical equilibrium with their surroundings and the interval is referred to as the morb horizon of neutral buoyancy hnb the detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity in the east pacific rise the hnb has a 1 1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region in ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros above the hnb is the region of negative buoyancy seafloor to an averaged 1 km depth within which negative buoyancy forces may induce the descent of nonvesiculated melt this region corresponds to the pillow basalt sediment sequence the region of positive buoyancy extends from 3 km to the depth of melt generation and picritic melt ascends in dikes and veins of finite height 500 m to 11 km and relatively narrow width 20 mm to 100 cm the finite strengths of high temperature fluid weakened rocks mandate finite height melt parcels along the ascent pathway the magmatic source region is therefore never hydraulically connected with the surface nor with the near surface reservoir thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy during the final stages of magma ascent hydrothermal fluid circulation enshrouds the hnb in a three dimensional envelope as suggested by the depth extent of alteration mineralogy in ophiolite complexes while hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading rate environments gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading rate regions such as the east pacific rise elastic crack penetration of rheological transition zones near the hnb identify modes of crack tip morphologic changes as a function of young s modulus ratios e2 e1 above and beneath the transition and affect the style of intrusion while melt buoyancy determines the ultimate equilibrium resting place elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic pt and picritic pp magmas to the vertically varying gradient in the horizontal stress component h and suggest further subdivision into two horizons of neutral buoyancy one for tholeiitic magma hnbt and one for the picritic liquid olivine mixtures hnbp these intervals are very approximate but are estimated as 600 m to 1400 m and 1400 to 3000 m depth respectively beneath the east pacific rise axis as oceanic lithosphere is created at the rise axis and spreads laterally the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off axis volcanism

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

seismic refraction study of upper crustal structure in the lassen peak area northern california in september 1982 the u s geological survey recorded a 125 km long refraction profile through the lassen peak area of the cascade range this profile extended northwestward to the klamath mountains and southeastward to the sierra nevada foothills interpretation of the refraction data determined the upper crustal structure in this volcanic region of northern california near surface velocities 1 5 5 55 km s are consistent with a thickness of 2 4 km for the cascade volcanic rocks between 0 5 km above and 1 8 km below sea level velocities are about 4 7 5 55 km s the highest velocities are modeled below the devils kitchen terminal geyser area possibly indicating hydrothermal alteration of the volcanic rocks the absence of low velocities less than about 3 4 5 km s deeper than 1 km above sea level suggests that the cretaceous sedimentary rocks exposed west of lassen peak do not underlie the volcano southeast of lassen peak at 1 8 km below sea level a 0 3 km thick layer with a velocity of 6 1 km s overlies a lower velocity 5 7 5 8 km s 2 9 km thick layer because these modeled velocities do not change across the boundary between the sierra nevada foothills and the cascade range sierran basement could continue under the cascade volcanic rocks as far northwest as lassen peak the velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust

seismic velocity structure at deep sea drilling project site 504b panama basin evidence for thin oceanic crust we present an analysis of wide angle reflection refraction data collected in the immediate vicinity of deep sea drilling project hole 504b in the panama basin currently the deepest drill hole 1 288 km into oceanic crust the data were acquired with a 1785 inch3 air gun array and fixed gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths near normal incidence multichannel seismic mcs reflection data were acquired simultaneously observed p and s wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness comparison of the travel times and amplitudes of the p and s wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method forward modeling shows that in contrast to standard oceanic velocity models a velocity depth profile that better explains the observed data is characterized by high velocity gradients up to 0 6 km s km in the middle crust a 1 8 km thick low velocity zone vp 7 1 6 7 km s immediately above moho and a total crustal thickness of only 5 km interpretation of the high velocity gradients in the middle crust is constrained by the observation of p wave amplitude focusing at ranges of 16 19 km although not as well developed in comparison to the p wave arrivals s wave arrivals show similar focusing total crustal thickness is constrained by the combined interpretation of a p wave wide angle reflection event observed at a range of 16 28 km and an mcs reflection event with a crustal travel time of 1 4 1 5 s although these events cannot be directly correlated their travel times are consistent with the assumption that both have a common origin amplitude modeling of the wide angle event demonstrates that these events are generated at the moho

variations in crustal structure related to intraplate deformation evidence from seismic refraction and gravity profiles in the central indian basin summary in this paper an analysis of seismic refraction and gravity profiles is presented to define the crustal structure across a ridge trough deformational feature in the central indian basin the purpose is to investigate what effects the deformation has had on the crust during the incipient rupture of oceanic lithosphere the wide angle seismic reflection refraction profiles which are modelled in this paper come from two previously reported data sets three r v conrad profiles and two r v mendeleev profiles located south of the odp leg 116 drilling sites where n s reflection profiles clearly delineate a symmetrically upwarped and faulted basement ridge bounded by two undeformed troughs previous analyses of the refraction data gave conflicting accounts of velocity and thickness variations between trough and ridge structures in this paper the refraction profiles are displayed in similar manner and both vs and vp structures are determined by forward modelling using 1 d reflectivity synthetics results indicate a typically mature oceanic upper crust for all profiles vp 4 5 6 3 km s 1 depth 0 1 2 km 0 35 0 27 above a transitional crust vp 6 3 6 8 km s 1 depth 1 2 2 6 km 0 25 in the middle and lower crust there are major changes between trough and ridge profiles i within the basement troughs there is a sharp transition between layer 3a vp 6 7 km s 1 thickness 1 7 km 0 26 and layer 3b vp 7 1 7 4 km s 1 thickness 2 0 2 5 km 0 25 ii on the ridge crest vp is increased to 7 2 km s 1 in layer 3a 0 26 but reduced to 6 6 m 0 2 km s 1 in layer 3b 0 24 0 30 forming a distinct low velocity zone in both vp and vs the crustal thickness is reduced progressively at its base from 6 2 m 0 3 km to 5 4 m 0 2 km a 2 d gravity model across the ridge trough structure based on the 1 d seismic velocity depth models is consistent with an observed anomaly of 40 45 mgal these results demonstrate that the crust does not thicken beneath the basement ridge crest as has previously been suggested the crustal structure is therefore more consistent with lithospheric deformation by buckling than by boudinage the crust beneath the ridge is altered primarily by the development of a low velocity and low density zone within the high velocity lower crust of seismic layer 3b a possible primary origin for the lvz is by serpentinization at low temperatures of olivine clasts within the mafic gabbro of layer 3b asymmetric folding of the lower crust or lateral flow of the more viscous serpentine might explain the possible crustal thinning under compression penetration of water to deep crustal levels within this region is consistent with recent deep multichannel reflection images of reverse faults which penetrate into or through the lower crust it is also consistent with fluid flow along faults within the sediment which have perturbed the surface geothermal gradients this process could explain the discrepancy between the high regionally averaged heat flow and normal basement depths when compared to standard lithospheric thermal models

the iceland research drilling project synthesis of results and implications for the nature of icelandic and oceanic crust study of a 3 km thick section of tertiary basalts in eastern iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the mid atlantic ridge deep drilling at reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3 5 km three major stratigraphic units composed largely of basalts are recognized each characterized by distinctive major and trace element patterns each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano extensive fractionation in shallow magma chambers led to intermediate and silicic lavas which mark the cores of the central volcanic complexes dike density does not increase downward below about 1 5 km crustal depth and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base in general the temperature of alteration increases downward but the distribution of individual minerals is irregular locally earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins the upper surface of greenschist facies metamorphism crops out at the surface in thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the reydarfjordur borehole located only 3 km east of the edge of thingmuli thus the metamorphic zones clearly crosscut the lava stratigraphy physical property profiles correlate well with variations in secondary mineralogy and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth magnetic susceptibility decreases with depth and is extrapolated to zero at about 4 3 km crustal depth close to the estimated upper limit of greenschist facies metabasalts a much greater crustal thickness leading to the emplacement of high level magma chambers within the extrusive part of the crust is thought to account for many of the differences in crustal structure and evolution between iceland and the rest of the mid atlantic ridge

effect of trench outer rise bending related faulting on seismic poisson s ratio and mantle anisotropy a case study offshore of southern central chile summary several trench outer rise settings in subduction zones worldwide are characterized by a high degree of alteration fracturing and hydration these processes are induced by bending related faulting in the upper part of the oceanic plate prior to its subduction mapping of p and s wave velocity structures in this complex tectonic setting provides crucial information for understanding the evolution of the incoming oceanic lithosphere and serves as a baseline for comparison with seismic measurements elsewhere active source seismic investigations at the outer rise off southern central chile 43 s were carried out in order to study the seismic structure of the oceanic nazca plate seismic wide angle data were used to derive 2 d velocity models of two seismic profiles located seaward of the trench axis on 14 5 ma old crust p01a approximately parallel to the direction of spreading and p03 approximately parallel to the spreading ridge and trench axes we determined p and s velocity models using 2 d traveltime tomography we found that the poisson s ratio in the upper crust layer 2 ranges between 0 33 at the top of the crust to 0 28 at the layer 2 3 interface while in the lowermost crust and uppermost mantle it reaches values of 0 26 and 0 29 respectively these features can be explained by an oceanic crust significantly weathered altered and fractured relative high poisson s ratios in the uppermost mantle may be likely related to partially hydrated mantle and hence serpentinization thus the seismic structure of the oceanic lithosphere at the southern central chile outer rise exhibits notable differences from the classic ophiolite seismic model normal oceanic crust these differences are primarily attributed to fracturing and hydration of the entire ocean crust which are direct consequences of strong bending related faulting at the outer rise on the other hand the comparison of the uppermost mantle p wave velocities at the crossing point between the perpendicular profiles 90 km oceanward from the trench axis reveals a low degree of pn anisotropy 2 per cent

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

permeability of the oceanic crust based on experimental studies of basalt permeability at elevated pressures christensen n i and ramananantoandro r 1988 permeability of the oceanic crust based on experimental studies of basalt permeability at elevated pressures tectonophysics 149 181 186 permeabilities have been measured as a function of confining pressure at constant temperature for basalt dredge samples from the juan de fuca and tonga kermadec regions permeabilities at 40 mpa are 1 1 x 10 19 m2 for the juan de fuca basalt and 7 4 x 10 19 m2 for the higher porosity tonga kermadec basalt permeabilities decrease significantly with increasing confining pressure the measured values of permeability are considerably lower than the estimated 10 14 to 10 15 m2 permeabilities of the upper few hundred meters of basement rocks of the oceanic crust thereby supporting oceanic crustal convection models in which seawater convection occurs largely through macrocracks at relatively shallow depths based on our laboratory measurements it is estimated that permeabilities of layer 2c and layer 3 are generally less than 1 0 x 10 19 m2 permeabilities of layers 2a and 2b in young oceanic crust are expected to be anisotropic due to the preferred orientation of fractures an anisotropic upper crustal model is presented in which layer 2 permeabilities are low parallel to spreading directions

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

velocity depth ambiguity and the seismic structure of large igneous provinces a case study from the ontong java plateau summary the seismic structure of large igneous provinces provides unique constraints on the nature of their parental mantle allowing us to investigate past mantle dynamics from present crustal structure to exploit this crust mantle connection however it is prerequisite to quantify the uncertainty of a crustal velocity model as it could suffer from considerable velocity depth ambiguity in this contribution a practical strategy is suggested to estimate the model uncertainty by explicitly exploring the degree of velocity depth ambiguity in the model space in addition wide angle seismic data collected over the ontong java plateau are revisited to provide a worked example of the new approach my analysis indicates that the crustal structure of this gigantic plateau is difficult to reconcile with the melting of a pyrolitic mantle pointing to the possibility of large scale compositional heterogeneity in the convecting mantle

magnetic signatures of serpentinization at ophiolite complexes we compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes pindos greece oman chenaillet france and the mid atlantic ridge near the kane fracture zone mark the pindos and oman sites show low susceptibility and remanence k 250 3008c occurring at the axis i e chenaillet similar to serpentinites from magmatically poor mid ocean ridges from lower temperature serpentinization 200 2508c likely occurring off axis and possibly during obduction i e pindos and oman at both settings serpentinization can result in significant hydrogen release

tonga ridge and lau basin crustal structure from seismic refraction data 1 the crustal structure across the tonga lau arc back arc system from the lau ridge to the pacific plate 178 170 w 18 19 s is modeled using data from an 840 km long air gun refraction line over 19 ocean bottom seismometers and one land station the data reveal that the pacific plate crust is 5 5 km thick with a velocity structure similar to that found at the present day east pacific rise epr beneath tonga ridge an intermediate velocity layer 6 7 km s is up to 7 5 km thick and has a velocity depth distribution similar to andesitic rocks found in continental crust the crust is abnormally thin 4 km at the boundary between the tonga ridge and the lau basin at the east end of lau basin the crust is 5 5 6 5 km thick and resembles crust formed at the epr except for a thicker sheeted dike section 2 3 km and thinner lower crust 2 km the lau basin crust thickens to 7 8 km near the central lau spreading center clsc mostly through thickening of the lower crust the crust thickens again to 8 5 9 5 km at 50 km west of the clsc mostly through thickening of the midcrust in the thick westernmost section the crustal structure is uniform even though one part of this section formed through extension of arc type crust while the rest was created at an oceanic spreading center the relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center

pore pressure and oceanic crustal seismic structure summary marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust in many localities compressional and shear wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth it is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability this is supported by compressional and shear wave velocity measurements as functions of confining pressure and pore pressure for basalt from the juan de fuca ridge and dolerite from the samail ophiolite oman within the oceanic crust regions of overpressure and underpressure will possess anomalous velocities the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic the influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low of significance poisson s ratio is shown to be dependent on pore pressure as well as confining pressure at constant confining pressure poisson s ratio increases with increasing pore pressure thus overpressured regions within the upper oceanic crust are likely to have relatively high poisson s ratios as well as low compressional and shear wave velocities

unsteady state of glaciers chhota shigri and hamtah and climate in lahaul and spiti region western himalayas a review of recent mass loss available published literatures on glacier mass loss and climate studies like temperature and precipitation are reviewed for lahaul and spiti region in northern india which is a part of the western himalayas chhota shigri and hamtah glaciers are both located in the lahaul and spiti region and have the longest record of in situ glaciological mass balance mb measurements we have compiled and compared all the dataset different methods related to glacier change with climate in the past few decades both the glaciers have experienced a significant mass loss during the study period different methods show diverse results for the glaciers studied for the same year however all results depicts overall unsteady state of both the glaciers data of indian meteorological department imd shows a significant increase in average temperature for the entire country and huge variability in precipitation particularly in the state of himachal pradesh temperature and precipitation are found to be the two main governing factors controlling the glacier health in this region thus it can be said that the glaciers of lahaul and spiti region are losing mass due to changing weather conditions especially the increasing air temperature however long term mb and climate data will give a better insight into understand and predict the future scenario of glacier health in this region

seismic evidence for hydrothermally altered mantle beneath old crust in the tydeman fracture zone abstract to study the structure of old atlantic fracture zones seismic refraction profiles were obtained using ocean bottom seismometers and two ship multichannel expanding spread experiments on two inactive sections of the tydeman fracture zone the results indicate that a crustal seismic velocities are low compared with normal oceanic crust b there is a marked absence of an identifiable layer 3 c the depth to the moho is shallow though not exceptionally so and d anomalously low seismic velocities of 7 2 7 5 km s 1 are observed over ranges at which upper mantle velocities are normally detected s wave arrivals corresponding to both crustal and mantle refractions are also observed which enable the first estimates of poisson s ratio within a fracture zone to be made using these results and those from other surveys we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age these findings suggest that hydrothermal circulation which can lower the upper mantle velocity by serpentinization persists under fracture zones for at least 70 ma and occurs at greater depths than for normal oceanic crust the serpentinization process is not sufficiently rapid to obscure the location of the original crust mantle boundary

azimuthal seismic anisotropy in a zone of exhumed continental mantle west iberia margin summary p wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension both recent and fossil in oceanic and continental settings we have determined the p wave anisotropy of a zone of exhumed continental mantle beneath the southern iberia abyssal plain as a possible indication of the initial direction of separation of iberia from north america in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction over 10 000 airgun shots were recorded by 11 ocean bottom seismometers hydrophones from which 24 900 arrivals that had traversed unaltered weakly serpentinized upper mantle were picked complete 360 azimuthal coverage was attained between the shots and receivers for ranges up to 55 km p wave raypaths through the zone of exhumed continental mantle were ray traced through a velocity model that incorporated the 3 d relief of the seabed and acoustic basement time residuals with respect to this model were fitted using an azimuthally dependent function small anisotropy 7 per cent with a fast direction of 143 was determined to exist between 3 1 and 6 7 km below the top of the basement although the fast direction suggests that the post breakup mantle stretching between iberia and north america was northwest southeast this hypothesis is hard to reconcile with other observations instead we conclude that motion along concave downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east west margin normal fast direction to the 143 azimuth observed today the degree of serpentinization was estimated to decrease from 20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle the degree of upper mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean p wave velocities

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

did the amalgamation of continents drive the end ordovician mass extinctions abstract global biodiversity has been punctuated throughout the phanerozoic by extinction events that vary in their degree of intensity and devastation the mass extinction event that occurred at the end of the ordovician period rapidly removed a wide range of species because taxonomic loss occurred during an ice age this is believed to have initiated the extinctions and thus these extinctions have often been viewed as a deep time analogue to the loss in species diversity during the present day glacial interval the current study however indicates that temperature though arguably being a trigger was not the sole reason for the crisis based on a large bibliographic database of rhynchonelliform brachiopods that specifically operates within very narrow time slices where every locality has been precisely georeferenced for the upper ordovician lower silurian interval we show that the extinctions were not uniformly distributed nor was the succeeding recovery here we argue that changing plate tectonic configurations during the ordovician silurian interval may have exerted a primary control on biotic extinction and recovery in particular the proximity and ultimate loss of microcontinents and associated smaller terranes around laurentia may have restricted shelf and slope habitats during the latest ordovician but nevertheless in a contracted iapetus ocean provided migration routes to help drive a diachronous early silurian recovery the conclusion that plate tectonics was a primary factor controlling the extent of the extinctions and the subsequent diversity rebound demonstrates that a reduction in diversity was perhaps the most important manifestation of the end ordovician crisis and further raises the question whether this could be applied to other large phanerozoic perturbations in biodiversity levels

shear wave velocity structure of the oceanic lithosphere from ocean bottom seismometer studies summary a comprehensive interpretation of two shear wave refraction profiles recorded on ocean bottom seismometers provides velocity depth models for the oceanic crust and upper mantle the data were obtained as part of an investigation of lithospheric structure carried out in the nootka fault zone area off the west coast of canada one dimensional s wave velocity depth models are derived from the data by trial and error comparisons with wkbj synthetic seismograms the results of this study confirm crustal characteristics interpreted previously from the compressional wave data layers 3a and 2b in the upper crust are characterized by rapid increases in seismic velocities shear wave velocity increases from 2 2 to 3 6 km s l over an interval of 3 5 km although the exact nature of the shearwave velocity gradient is uncertain in contrast the shear velocity in layer 3a remains fairly constant with depth at values near 3 6km s no velocity discontinuity is observed at the moho instead the transition from crust to upper mantle is marked by the presence of a velocity gradient zone layer 3b the shear wave velocities in the upper mantle are 4 5 and 4 6 km s l in directions approximately parallel to the spreading ridge and parallel to the direction of spreading respectively this low degree of velocity anisotropy contrasts sharply with the 10 per cent anisotropic effect observed for the pn velocities combining the p wave interpretation with this s wave study enables evaluation of poisson s ratio as a function of depth except for low values of poisson s ratio found in the upper crust the seismic properties of the oceanic crust and upper mantle interpreted in this study agree remarkably well with those of laboratory measurements on ophiolite samples

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges