a simple magma driven thermal balance model for the formation of volcanogenic massive sulphides abstract a simple model is used to examine the balance of heat water flow and temperature in a long lived single pass hydrothermal system heated by magmatic heat the geometry of the system is based on mid ocean ridge observations cold water percolates downward to the top of a shallow magma chamber is heated by conduction through a thin boundary layer and discharges through a permeable fault zone modelled as a bundle of narrow rough pipes as heat input is increased the model shows a catastrophic temperature transition at an exit water temperature of between 340 and 410 c depending on seafloor pressure which in nature would probably be represented by surging flow and sudden changes in the specific volume of the circulating fluids to generate a 3 million tonne sulphide deposit with 70 efficiency of deposition from water at 350 c within 4000 years would require a heat flux of 110 w m 2 from the magma chamber for a mass flow rate of 140 kg s 1 if the plan area of the reacting part of the system is 2 km 2 the total heat input would be 3 10 19 j under these conditions the thickness of the solid boundary layer between liquid magma and circulating water will be about 10 m and the integrated reactive water rock ratio over the lifetime of the system will be about 30 decreasing the water temperature to 250 c would require a heat flux of only 60 w m 2 but the time to form a 3 million tonne deposit is increased to 33 000 years and the overall heat requirement is more than quadrupled

seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

the process of formation of ocean crust ocean crust is the outermost layer of earth under the oceans it is separated from the underlying mantle by a seismic transition zone called the moho a widely held view is that the moho represents a petrologic change from basaltic type rocks to a mantle composed mostly of olivine and pyroxene according to this view crust is formed by a steady segregation of basaltic melt derived from partial melting of the mantle into a crustal magma chamber wherein cooling and crystallization bring about steady state accretion to the continuously spreading plates there is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process at least two other processes are more compatible with the geophysical data in one the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the moho is a primary petrologic boundary in the other the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking by comparison in the mantle thermal contraction is accommodated by flow the upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt the lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature

tectonics of the east pacific rise near 12 50 n a submersible study abstract using the diving saucer cyana a portion of the east pacific rise between 12 38 and 12 54 n has been studied and mapped four main zones are defined 1 an active volcanic zone located in the central graben the graben displays a variable relief along strike from less than 10 m up to 55 m and has an en echelon pattern in plan view 2 an active tectonic zone comprising the central graben and extending up to about 2 km from either side of the rise axis 3 an inactive tectonic zone starting 2 km away from the axis and formed by discontinuous horst and graben type of structures and 4 off axis volcanoes seamounts located less than 20 km on either side of the rise axis at 12 50 n the active tectonic zone is extremely narrow and the mode of deformation is through fissuring in strong contrast with that observed in slow or intermediate rate ridges the presence or absence of a magma reservoir at shallow depth explains the difference in the width of the active tectonic zone the state of isostatic equilibrium or lack of equilibrium explains the different modes of deformation

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

debris flows rainfall thresholds in the apennines of emilia romagna italy derived by the analysis of recent severe rainstorms events and regional meteorological data abstract recent severe rainstorms events in october 2014 and september 2015 triggered more than a hundred debris flows in the western part of the apennines of emilia romagna italy in this work we tested a novel method to define debris flows rainfall thresholds for the 2014 and 2015 rainstorms which have been used as reference events and to extend these results across the apennines of emilia romagna making use of long term rainfall data of the regional raingauges network results are compared for validation to rainfall rates recoded during other past rainstorm debris flows events which have been used as validation events at first the method involves a spatial discriminant analysis between the spatial distribution of debris flows and the high frequency weather radar rainfall data for the 2014 and 2015 reference events the analysis defines rainfall cutoff values over rainfall durations from 30 to 6 h related to verification indices in the roc curves which are used as debris flows rainfall thresholds exceedance ratios are calculated between the computed rainfall thresholds and the rainfall rates at 10 years return periods at corresponding rainfall durations computed for raingauges located in the areas affected by the 2014 and 2015 events the ratios are then used as multipliers of the rainfall rates at 10 years return periods over rainfall durations from 30 to 6 h calculated for all other raingauges in the regional study area to spatialize thresholds calculation to the regional scale the computed thresholds are interpolated across the apennines of emilia romagna the research resulted in the assessment of two levels debris flows rainfall thresholds curves which seem to be adequate to discriminate rainfall rates recorded during past debris flows events used for validation discussion evidences advantages and limits of our approach compares results to existing debris flows thresholds and highlights their possible use in a multi stage warning procedure at regional scale

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

geophysical evidence for the absence of a crustal magma chamber under the northern juan de fuca ridge a contrast with rose results deep tow reflection data seismic refraction data to ocean bottom seismometers and gravity interpretations of the northern juan de fuca ridge all suggest a very narrow or absent crustal magma chamber under this portion of the rise axis the seismic features that have been suggested as indicating crustal magma chambers on other parts of the east pacific rise such as travel time delays and reflections from the top of a shallow magma reservoir are not found on the northern part of this ridge these results indicate that at spreading rates of 3 cm yr crustal cooling probably by water penetration is sufficiently rapid to prevent the existence of a steady state crustal magma chamber and therefore crustal accretion may be episodic

microearthquake activity on the orozco fracture zone preliminary results from project rose we present preliminary hypocenter determinations for 52 earthquakes recorded by a large multi institutional network of ocean bottom seismometers and ocean bottom hydrophones in the orozco fracture zone in the eastern pacific during late february to mid march 1979 the network was deployed as part of the rivera ocean seismic experiment also known as project rose the orozco fracture zone is physiographically complex and the pattern of microearthquake hypocenters at least partly reflects this complexity all of the well located epicenters lie within the active transform fault segment of the fracture zone about half of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the cocos pacific relative plate motion these events appear to be characterized by strike slip faulting the second major group of activity occurred in the central portion of the transform fault the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading and several are not obviously associated with distinct topographic features hypocentral depth was well resolved for many of the earthquakes reported here nominal depths range from 0 to 17 km below the seafloor

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

a finite element computation of seismic diffraction about the rise axis magma chamber in a previous study models of the crust and magma chamber at 12 50 n on the east pacific rise were derived with a two dimensional ray tracing analysis of rise normal refraction lines the best model featured a 4 km wide axial magma chamber but the data contained diffraction effects that were not modeled by geometric ray theory this raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber we have tested the model with a finite element calculation of synthetic seismograms this method incorporates full wave effects including diffraction the synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data the previous travel time analysis was largely successful and was only slightly compromised by diffraction diffraction in rise normal refraction experiments is therefore not obscuring a large low velocity magma chamber

plate waves and the 1980 huajuapan de leon mexico earthquake abstract we propose two alternate structural models featuring a dipping oceanic plate in the subduction zone of southwestern mexico based on the relocation by the master event method of 77 aftershocks of the 1980 huajuapan mexico earthquake recorded by the mexican continental aperture seismic network resmac the aftershock locations roughly define the rupture surface of the main event as a normal fault dipping towards the continent the rupture involved a thickness of about 25 km in the top of the subducted plate these results are consistent with the fault plane solution both models feature a waveguide in the top of the subducted plate which may account for the large relative amplitudes of plate waves relative to direct waves at all stations and the large high frequency content observed at a station on the coast compared with stations inland

the generation and composition of partial melts in the earth s mantle abstract a set of equations is presented which combines the constraints of fluid dynamics and multicomponent phase equilibrium to provide a unified description of partial melting in the earth s mantle the equations are applied to a one dimensional model for pressure release melting of a simplified mantle material which contains only two chemical components exhibiting either a complete solid solution or b a binary eutectic in both cases melting occurs over a range of depths the unmelted crystalline residue matrix is modeled as a saturated porous medium through which the melt can migrate because of its differential buoyancy since melt interacts continuously with the matrix during ascent melting occurs by equilibrium rather than fractional fusion this equilibrium fusion is not the same as batch fusion however since material elements are quickly dispersed by migration of melt relative to the matrix to a first approximation the temperature profiles adiabats in the partially molten zone are independent of melt migration the slope of the adiabats varies in inverse proportion to the number of degrees of freedom which characterizes the melting melting of a complete solid solution occurs along a wet adiabat whose slope is controlled by absorption of latent heat melting of a eutectic system occurs along a steeper univariant adiabat until one solid phase is exhausted and subsequently along a wet adiabat the velocity of melt migration can exceed the mantle upwelling velocity by an order of magnitude or more the volume fraction of melt present is always less than the fraction of the material which has melted and is unlikely to exceed a few percent for a wide range of initial conditions melting of a eutectic system produces erupted melts having constant major element composition and widely varying trace element composition this result may provide a partial explanation for the characteristic major and lil element patterns observed in morb liquid compositional paths during pressure release melting cannot be determined from isobaric sections of phase diagrams and can only be calculated by solving the energy equation subject to the appropriate depth dependent phase equilibrium constraints

depth versus age new perspectives from the chemical compositions of ancient crust petrological data provide a new approach to an evaluation of the depth age problem for ancient seafloor the correlations among basalt chemical composition axial depth and mantle temperature at current ocean ridges allow the determination of initial depth and mantle temperature for any portion of ancient seafloor that was created at a spreading center provided the chemical composition of the ancient crust is determined it is then possible to calculate a petrologically constrained depth at any age which can be compared to observed depths and depths from the classical half space models we evaluate data from dsdp and odp drill holes on crust older than 80 ma considering chemical composition back tracked depth and crustal thickness the data are complex and interpretation of their chemical composition requires consideration of alteration absence of glass compositions data quality and the influence of off axis volcanism and near ridge hot spots to check and expand the data set we develop and use trace element proxies for major element compositions since many trace element ratios are less influenced by alteration and by variable proportions of phenocrysts the twenty drill holes for which reliable data can be obtained are well distributed around the globe and include multiple sites on old crust in the atlantic pacific and indian ocean basins comparison of the chemical and crustal distributions between ancient and current n morb show that the oceanic crust older than 80 ma has significantly lower na 8 0 z r y sm ybn and higher cao al2o3 f e 8 0 and crustal thickness quantitative modeling of these results suggests that the mantle was hotter in this time period by about 50oc that the crust was several hundred meters shallower and 1 2 km thicker these observations show that half to two thirds of the observed flattening relative to a half space model is due to the change in mantle temperature and crustal composition thus only a few hundred meters of flattening by plate reheating by hot spots or by other mechanisms is required these results are consistent with the existence of abundant oceanic plateaus even at fast spreading rates in the mesozoic and with the apparent thickening of ocean crust with time 1999 elsevier science b v all rights reserved

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

tomographic image of the axial low velocity zone at 12 50 n on the east pacific rise the 1982 magma seismic refraction experiment yielded a large set of accurate p wave travel times corrected for bathymetry and anisotropy which sample the structure of the east pacific rise epr at 12 50 n the arrivals were recorded using ocean bottom seismographs at three sites on axis and at 7 km and 16 km east of the axis we invert 2320 travel times for the twodimensional crustal seismic velocity structure across the epr axis assuming that the velocity structure is invariant along strike the travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to 10 faster velocities for propagation parallel to the axis than perpendicular to the axis the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy our preferred model contains only the structure clearly required by the data structure which is stable under excessive smoothing and achieves a variance reduction of 81 relative to the laterally homogeneous starting model we resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0 4 to 0 7 km s this low velocity zone lvz is about 7 km wide and extends from a depth of about 1 5 2 0 km down to moho at a depth of 5 5 km the lvz is slightly asymmetric extending 1 km further to the east than to the west of the axis in the shallow 1 0 km depth crust a pattern of velocity variations is imaged in which velocities are high at the spreading axis decrease between 3 km and 7 km east of the axis and then increase again between 10 and 15 km east of the axis we investigate the resolution of the inversion using an impulse response method the lvz and off axis upper crustal variations are well resolved in addition the travel time data indicate that an axial high velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion the small velocity reductions of the lvz are consistent with hot rock containing only a small quantity of melt these results combined with the multichannel seismic reflection lines and expanded spread profiles from the northern epr suggest that the zone of high melt fraction under the spreading center is confined to a narrow thin lens capping a broad zone of hot plutonic rocks the upper crustal velocity reduction within 1 km of the axis reflects near axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near axial tectonism the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration

constraints on the structure of the east pacific rise from gravity gravity data across the east pacific rise at 9 n and 12 n the rose area are characterized by a high of 12 20 mgal with a width of about 20 km it is shown that this axial gravity high is not adequately explained by uncompensated constant density topography or an isostatically compensated thermally expanded ridge crest it can be adequately explained by airy compensation with a depth of compensation of at least 20 km or by elastic plate compensation with a flexural rigidity between 1018 and 1020 n m the significance of the depth and flexural rigidity parameters is that in both cases they result in a gravity field from the compensation whose amplitude is below the data resolution for wavelengths less than about 50 km that is the gravity data do not require compensation of the rise axis for wavelengths less than about 50 km this suggests that these wavelengths are either supported mechanically or by other forces dynamic the mechanical interpretation is not consistent with partial melt under the axis indicating that dynamic forces may be important in supporting the rise axis the same conclusion is arrived at from the thermal model the thermal model can be made to fit the gravity data if we allow departures from isostasy and the addition of a dike like body about 2 km wide in the crust having a positive density contrast of about 0 25 mg m 3 this geometry is consistent with a seismically determined low velocity zone in the crust in the rose area in this model the nonhydrostatic pressures must be supported dynamically a general conclusion of this study for the portions of the east pacific rise studied here is that the axial topography is not buoyantly supported by low density material in the crust therefore either the crust under the rise axis mechanically supports the axial topography or a mass excess at the thermally expanded rise axis is supported by other forces such as plate driving forces the latter option is favored on the basis of heat flux and seismic data

the east pacific rise and the thermal model although the thermal mode of lithosphere cooling can be used to predict the east pacific rise topography reasonably well it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor the misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography generating a theoretical gravity anomaly smaller than that observed it is shown using data at 12 n as an example that improved agreement with the thermal model is obtained if to first order the crust is treated as an isothermal constant thickness constant density layer with variable depth this result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation it implies that convective cooling of the whole crust is very rapid even at the axis of fast spreading ridges the oceanic moho may represent the depth to which cracking and convective cooling extends

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 rose seismic data storage and exchange facility the rivera ocean seismic experiment was a multi institutional marine seismic experiment held off western mexico during early 1979 dense spatial sampling was provided by ocean bottom seismometers and ocean bottom hydrophones recording both natural events and explosions the rose experiment and a companion land experiment generated a data base of approximately 76 000 events source instrument pairs procedures for efficient data storage retrieval and exchange were designed and implemented at the hawaii institute of geophysics data capabilities of the exchange are demonstrated by using data from several participants to obtain qualitative estimates of the attenuation of seismic energy in the oceanic mantle near the east pacific rise and through the ocean continent margin near petatlan guerrero mexico the data base is now available for general use

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

amplitude constraints in linear inversions of seismic data linear programming inversions of seismic data may include constraints on the second derivative of the delay time d2 dp2 dx dp x geometric ray theory predicts that the second derivative of the delay time is proportional to the inverse of the complex seismic amplitude squared so that the use of this additional constraint constitutes the incorporation of amplitude information in linear inversions the realizability constraint that velocity be a single valued function of depth must be included explicitly for higher order derivatives of delay time than the second the realizability constraint is nonlinear and cannot be included in a linear scheme given that the data are adequate to allow identification of the major prograde and retrograde branches and the intervening caustics useful bounds on x can be established the function x is calibrated with respect to measured amplitudes by requiring that the t x curve obtained by successively integrating the x curve satisfy the observed travel times this calibration may be cast as a linear least squares problem applications to data from the rose rivera ocean seismic experiment area reveal a welldefined layer 2 layer 3 transition an apparently isotropic and nearly homogeneous layer 3 and a moho transition that is quite gradual this transition appears to be more rapid in the fast pn direction

a two and three dimensional analysis of gravity anomalies associated with the east pacific rise at 9 n and 13 n we have used a unique data set collected during the 1985 east pacific rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the east pacific rise epr the close spacing of track lines within the two main survey areas 8 45 n 9 55 n and 12 20 13 30 n allowed us to perform three dimensional analyses of high quality gravity data 1 mgal uncertainity obtained with the bell aerospace bgm 3 gravity meter our gravity modeling was enhanced by the availability of high resolution sea beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data to model the crustal and upper mantle density structure at the ridge axis we first calculated the gravity anomalies due to the density contrasts at the water crust and crust mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free air anomaly the residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the epr our results show that over 90 of the power in the observed free air anomaly can be modeled by these predictable components of the gravity signal the amplitude and wavelength of the small residual anomaly can be modeled by a broad region 20 km wide centered on the rise axis of slightly lower than normal crustal and or upper mantle densities 0 03 mg m 3 this region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high the anomalies are consistent with but do not require the presence of a low density crustal magma chamber at the rise axis if a largely molten magma chamber exists then the gravity data require that it be a narrow volumetrieally small body there are first order differences in the distribution of anomalous mass between the two survey areas with the 9 n region generally being characterized by lower densities in the axial region along axis variations in anomalous mass within each of the survey areas correlate with first order changes in the depth and cross sectional shape of the epr axial high we attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along strike changes in the size of the crustal magma chamber the width of the surrounding zone of cooling crustal rocks and densities in the uppermost mantle beneath the rise axis

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

the structure of young oceanic crust near a very fast spreading ridge we have modeled the structure of the young oceanic crust near the fastest spreading ridge ever to be profiled using seismic refraction techniques the data were originally collected during the capricorn expedition of the scripps institute of oceanography in 1953 one of the split leg profiles from that expedition c22 was conducted near the east pacific rise at 15 s where the half spreading rate is over 80 mm yr by digitizing the data and modeling the two splits of c22 using synthetic seismograms we have found that the crust is normal in thickness 5 to 7 km and in velocity structure an upper crustal transition zone is present on both legs and would appear to be a regular feature of young oceanic crust it is probably too deep 1 5 to 2 km beneath the seafloor to be correlated with the boundary between pillow basalts and sheeted dikes instead we attribute this transition zone to changes in crustal porosity and or hydrous minerals in the oceanic crust from the appearance of wide angle moho reflections near the rise axis we suggest that the very large crustal magma chamber that might be expected under this fast spreading ridge is not present this presumably results from the cooling of the crust by pervasive hydrothermal circulation

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

gloria survey of the east pacific rise near 3 5 s tectonic and volcanic characteristics of a fast spreading mid ocean rise abstract a long range sidescan sonar gloria survey of an approximately 100 km square area of the east pacific rise crest between 3 s and 4 s extends results obtained in the same area by lonsdale 1977 using deep tow the axial linear volcano was found to be continuous over a distance of 75 km the presence of major inward and outward facing fault scarps was confirmed but the gloria data show several distinct differences between the two fault sets the inward dipping faults are more numerous more closely spaced and longer than the outward dipping ones and their dip slopes backscatter sound more extensively than those of the outward dipping faults moreover most of them appear to be formed within 2 km of the axis whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis these differences suggest that the two sets of faults have different origins the horizontal pattern of inward dipping faults is similar to those observed on other mid ocean rises at all spreading rates though the lengths and separations of the scarps are slightly and their throws considerably less than on slower spreading rises this common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate horizontal tension is probably the dominant factor but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2 8 km from the axis the similarity of major fault spacings on the east pacific rise to those on slower spreading rises suggests that faulting is invariant in space rather than time and that the lithosphere where these faults are formed about 2 km from the spreading axis has a similar small thickness for all spreading rates this is attributed to the regulating effect of hydrothermal circulation and plate cooling

the back barrier and barrier depositional history of romney marsh walland marsh and dungeness kent england previous stratigraphical investigations of romney marsh have tended to be local in scale and this has hindered efforts to establish a unifying stratigraphical framework for this area this paper addresses this problem by describing the results of a 12 km transect across romney marsh linking previously studied back and fore marsh sites horsemarsh sewer and broomhill respectively and presenting additional pollen diatom and radiocarbon data from an intermediate mid marsh site brookland one main organic unit is recorded across much of romney marsh although its age altitude and composition varies microfossil and radiocarbon data from brookland and elsewhere on romney marsh show that this organic unit accumulated under a general removal and return of marine conditions that took place between ca 5100 and 2000 yr bp a recently proposed model of barrier development is used to investigate the history of back barrier sedimentation in romney and walland marshes this model suggests a three phase life history for gravel barriers which consists of initiation stability and breakdown although there are problems in relating back barrier deposits directly to barrier dynamics nevertheless the romney marsh data do for the most part agree with the expected number and sequence of sea level tendencies predicted by this model the back barrier stratigraphical data suggest that initiation stability and breakdown of the dungeness foreland occurred between ca 6000 and 5000 yr bp 5000 and 2000 yr bp and 2000 yr bp and present respectively

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

structure and ageing of oceanic crust at 14 s on the east pacific rise a wide angle reflection refraction survey was conducted along a flow line on the eastern flank of the east pacific rise at 14 s the data were acquired with a single 32 litre airgun and ocean bottom hydrophones obh and consist of six profiles on 0 5 8 3 myr old seafloor in total 17 obhs provided seismic records of excellent quality to generate 2 d velocity models traveltimes and amplitudes have been calculated using asymptotic ray theory a comparison of the traveltimes and amplitudes of p wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust the uppermost crust is composed of a 0 4 0 7 km thick low velocity 2 9 4 6 km s 1 surficial layer layer 2a and is bounded at its base by a sharp velocity increase to 5 5 km s 1 at the top of layer 2b overall layer 2 is 1 4 1 8 km thick small seamounts frequently superposed on fast spreading crust are characterized by velocities lower than within the adjacent upper crust velocities 6 0 km s 1 exist throughout the edifices suggesting that the seamounts are dominantly extrusive decreasing amplitudes beyond 8 10 km offset constrain a moderate gradient lower crust layer 3 with velocities of 6 8 7 2 km s 1 crustal thickness is found to be remarkably uniform at 6 11 0 14 km nevertheless the seismic data provide evidence for age dependent variations in both the low velocity surficial layer and the upper mantle we find that velocities in layer 2a increase from 2 9 to 4 3 km s 1 velocities in the middle and lower crust show uniform values while velocities within the uppermost mantle increase gradually from 7 5 km s 1 to 8 0 km s 1 the most viable explanation for increasing velocities in the upper crust is that off axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces thereby decreasing the porosity and increasing the velocity low mantle velocities beneath young crust are suggested to be a temperature controlled phenomenon and may indicate the presence of partial melt in the upper mantle

axial and off axial heterogeneity of basaltic rocks from the east pacific rise at 12 35 n 12 51 n and 11 26 n 11 30 n surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the epr at 12 35 n 12 51 n and at 11 26 n 11 30 n and from four adjacent seamounts centered at less than 18 km from the rise axis the basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition there is no correlation between lava flow morphology geological settings and compositions based on mineral and compatible element composition these basalts were classified into three types the least evolved olivine basalts and highly phyric plagioclase basalts type a have high fo87 89 high mg 66 70 and high ni 100 contents the most evolved plagioclase olivine basalts with or without pyroxene type b and c have relatively low fo78 86 low mg 55 65 and low ni 60 100 ppm contents simple crystal fractionation 3 18 accounts for some of the observed compositional range however based on their incompatible element contents the above basalts are divided into three groups the various types a to c are found in each group depleted basalts with low k ti 0 04 0 15 ratios low zr 100 ppm and low nb 0 4 ppm contents undepleted basalts with the highest k ti 0 25 0 46 zr 150 170 ppm and nb 8 16 ppm contents and the transitional basalts having compositions between these two extremes these three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3 4 km in width near 12 43 n 12 51 n on the epr similar volcanic diversity in a more limited area 3 km2 is observed on off axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle the transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting we suggest that a lherzolitic mantle with minerals having a variable behavior under partial melting constitutes this type of composite mantle indeed clinopyroxene enriched in incompatible elements will be consumed at an earlier stage of melting than the olivine plus orthopyroxene eruption of compositionally distinct magma batches in close proximity on axial and off axial structures implies periods of quiescence between successive magmatic stages and cycles

basement slivers within the new britain accretionary wedge implications for the emplacement of some ophiolitic slivers using multichannel seismic data from the western solomon sea we have imaged several 2 5 to 3 km thick oceanic basement slivers within the new britain accretionary wedge which may serve as a modern analogue for the detachment of some ophiolitic slivers the most recently accreted basement sliver is located just over 5 km from the thrust front an approximate area balance indicates the subjacent section of the downgoing plate from which this sliver originated is between 11 km and 15 km arcward of its present position though we have not imaged the basement ramp sedimentary strata beneath the basement sliver pinch out between 9 and 12 km from its seaward edge suggesting that detachment occurred in that vicinity normal faults reactivated in response to the flexural bending of the downgoing oceanic plate are a prominent feature of this region significantly almost all basement normal faults near the recently accreted basement sliver including those imaged beneath the toe of the wedge dip arcward based on these data we suggest that small 1 3 km thick basement slivers are decoupled along favorably oriented preexisting zones of weakness formed by normal fault detachments within the oceanic basement

finite difference seismic modeling of axial magma chambers we tested the feasibility of using finite difference methods to model seismic propagation at 10 hz through a two dimensional representation of an axial magma chamber with a thin liquid lid our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of p and s energy propagation the results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries we observe a high energy downward traveling shear phase observable only with borehole receivers that would be useful in studying the nature and shape of magma chambers the ability of finite difference methods to model high order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near axis drilling of the east pacific rise in order to optimize the benefits from shear wave imaging of sub axis structure

anisotropic gradients in the upper mantle pn amplitudes in some widely spaced sets of orthogonal marine refraction lines on young oceanic crust are greater in the fast direction than in the slow direction this is inconsistent with the predicted amplitude behavior for simple head waves but can be explained by an increase in anisotropy with depth it appears that these gradients are due to increasing olivine crystal orientation although changes in the relative abundance of two anisotropic minerals without variable tectonization could also account for the observations depth variation of tectonization most probably indicates very high temperature gradients at the moho this would imply a substantial amount of convective heat transport in the whole oceanic crust near mid ocean rises

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

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b