structural control on sea floor hydrothermal activity at the tag active mound the tag active mound1 13 on the mid atlantic ridge near 26 n is one of the largest known actively forming volcanogenic massive sulphide deposits construction of such a deposit requires that the upflow of hydrothermal fluids be localized at this site over tens of thousands of years but the cause of this localization has been controversial two popular hypotheses propose that it results from young volcanism immediately adjacent to the mound6 or from the intersection of a transfer fault orthogonally crossing the rift valley floor with ridge parallel faults8 here we present high resolution sonar and photographic data which provide no evidence for young eruptions nearby or for transverse through going faults intersecting the tag mound instead our data suggest that the localization of hydrothermal activity at the tag mound is strongly controlled by permeable conduits at the intersection of an actively developing ridge parallel fissure fault complex with a newly recognized but older oblique fault system these observations may aid the interpretation of similar structures in ancient sea floor sulphide deposits where the tectonic disruption caused by emplacement on land has obscured the original geometry

hydrothermal activity at the trans atlantic geotraverse hydrothermal field mid atlantic ridge crest at 26 n the first submersible observations of the only known active submarine hydrothermal field on a slow spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity duration and varying intensity of hydrothermal activity at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge crest near latitude 26 n hydrothermal activity has persisted at this location for at least 1 106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age the hydrothermal activity has been cyclic multistage and episodic prior high temperature hydrothermal venting stages with a periodicity of the order of 1 104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in cu fe and zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the mid atlantic ridge the most recent episode of high temperature venting is inferred to have ceased in the recent past based on metal enrichment cu fe zn in the surficial sediment layer low temperature hydrothermal venting stages with a duration of the order of 1 104 years intervene between the short high temperature stages and produce stratiform deposits of layered and earthy manganese oxide iron oxide hydroxide and silicate bivalve like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs the fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits conductive heat flow anomalies in the sediment column and the record of temperature anomalies and excess 3he in the near bottom water column suggest that the low temperature hydrothermal discharge is intermittent at individual vents on a time scale of years

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

relict hydrothermal zones in the tag hydrothermal field mid atlantic ridge 26 n 45 w two relict hydrothermal zones were delineated between water depths of 3400 and 3500 m at the lower part of the east wall of the rift valley of the mid atlantic ridge in the tag hydrothermal field using a deep towed side scan sonar tow and a camera temperature tow along the northern 3 km of the wall and a submersible transect named the north and mir relict zones they are located about 4 km and 2 km northeast respectively of the known active high temperature sulfide mound between water depths of 3625 and 3670 m on the rift valley floor near the base of the east wall the north zone extends about 2 km along the northern end of the lower east wall the zone includes two moundlike features up to 30 m high by 200 m in diameter imaged by side scan sonar within a 2 km long line of discontinuous hydrothermal deposits comprising inactive toppled and standing chimneys layered material and patchy dark stains on sediment photographed by the camera temperature tow several other moundlike features were imaged with the side scan sonar outside of the photographic coverage the mir relict hydrothermal zone 2 km south of the north zone named after the mir submersible used to investigate it consists of three subzones 1 a 200 m wide area of diverse types of hydrothermal materials exposed by normal faulting at its western margin 2 a 400 m wide by 700 m long central area of discrete groups of toppled and standing inactive sulfide chimneys up to 25 m high on a substrate of red metalliferous sediment and carbonate lutite spires sampled on the highest chimneys are composed of coarse grained recrystallized sulfides dominated by pyrite and chalcopyrite which contain the first primary free gold grains 2 3 m diameter found at a hydrothermal site on a mid ocean ridge and 3 a 150 m wide hummocky area of layered hydrothermal material with the appearance of low temperature precipitates and carbonate lutite with patchy dark stains at its eastern margin the active sulfide mound the north zone and the mir zone are each located on the fractured western margins of gentle dome shaped areas of pillow flows typically 500 m in diameter interpreted as summits of volcanic centers that may have supplied heat to drive adjacent hydrothermal activity the distribution and size of the active and inactive hydrothermal zones of the tag field the chronology and the characteristics of relict samples recovered indicate a history of at least 100 103 years of high temperature hydrothermal episodes with multiple overprinting stages of mineralization accompanied by alteration

geological fate of seafloor massive sulphides at the tag hydrothermal field mid atlantic ridge deep sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply increasing demand for these metals many of which are

crustal magnetization reveals subsurface structure of juan de fuca ridge hydrothermal vent fields a near bottom geophysical survey on the endeavour segment of the northern juan de fuca ridge shows that regions of well defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites in particular at the main endeavour field we find discrete magnetization lows associated with each cluster of vents magnetization lows are directly centered beneath the vent clusters and have diameters of 100 m which implies a near vertical narrow pipe like source region located directly beneath the surface expression of the vent edifices lows are also separated from each other by only 200 m which further implies highly focused zones magnetization lows are also associated with inactive and extinct vent areas which indicates that alteration of the magnetic minerals in the crust rather than necessarily temporary thermal demagnetization is the primary process responsible for the low magnetization these narrow pipe like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up flow zones thus each magnetization low may define an individual upwelling zone with distinct subsurface plumbing and thermal structure the crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems at the main endeavour field magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of 200 m arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

comprehensive analysis of marine magnetic vector anomalies we present a comprehensive description of the analysis of marine magnetic vector anomalies the marine magnetic vector field can be measured with a shipboard three component magnetometer stcm the stcm has the advantage of measuring three components but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton precession magnetometer we have estimated the quality of stcm data constructed an optimal filter and evaluated the reliability of the filtered data then we have organized an analytical procedure composed of four steps 1 the discrimination between two and three dimensional magnetic sources 2 the determination of magnetic boundaries 3 the calculation of the boundary strikes and 4 the estimation of the boundary magnetization contrasts we show examples of these procedures using the data near the east pacific rise 28 s 31 s collected during leg 5 of the gloria expedition all the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

morphology mineralogy and chemistry of hydrothermal deposits from the tag area 26 n mid atlantic ridge abstract metalliferous hydrothermal deposits have been observed and sampled by submersible from the tag hydrothermal field on the mid atlantic ridge at 26 n the deposits include surficial metal rich staining on the surface of carbonate ooze deposits over considerable areas of the sea floor as well as discrete massive layered deposits the latter include hydrothermal precipitates of fe silicate nontronite mn oxide birnessite and fe oxide amorphous the deposits range in size from less than 1 m to 15 m 20 m the composition and morphology of individual deposits varies from thick laminated crystalline birnessite precipitates through fe rich tubular vents to loose earthy birnessite nontronite or amorphous fe oxide interlayered deposits fractionation and crystallinity are presumably controlled by local eh conditions and the hydrodynamics of circulation in the upper crust they are presumed to represent deposition from low temperature hydrothermal solutions which had originally reacted with basaltic oceanic crust at depth at high temperature and had precipitated metal rich sulfides within the crust occasional short duration high temperature black smoker type venting is inferred to have occurred based upon cu fe zn rich layers in the sediments and observations of clam shells in bottom photographs

extremely asymmetric magmatic accretion of oceanic crust at the ends of slow spreading ridge segments we report the results of a deep towed magnetic survey of part of a mid atlantic ridge spreading segment analysis of the magnetic reversals indicates that for the past 0 7 m y magmatic accretion at the end of the segment has been effectively one sided with new crust being added only to the outside corner of the ridge offset eastern flank and not to the inside corner western flank spreading on the inside corner was accommodated by significant displacement on a single large fault the area between the fault and the axial volcanic ridge was effectively a thin static sliver at the plate boundary during this process in the short term asymmetric magmatic accretion was probably accomplished by progressively shifting the axial volcanic ridge to a new location at the inside corner western edge of the old one asymmetric spreading is unlikely to be sustainable as a steady state process the termination of a period of asymmetric spreading may be achieved either by establishing a new axial volcanic ridge to a position on the outside corner eastern plate thus isolating the old axial volcanic ridge on the inside corner plate or by simply arresting movement on the large fault and reverting to symmetric spreading at the axial volcanic ridge highly asymmetric accretion may be a common process at slow spreading segments particularly near discontinuities this asymmetry cannot be maintained for long periods and may be directly linked to intervals of spreading by tectonic extension

comparing dune migration measured from remote sensing with sand flux prediction based on weather data and model a test case in qatar this study explores validating and calibrating the wind regime predicted by global circulation models gcm on earth and other planets using optical remote sensing of dune dynamics we use spot 5 images to track the migration of 64 barchan dunes in qatar using the cosi corr technique we estimate the volume of the dunes using a scaling law calibrated from one particular dune which was surveyed in the field using volume and migration rate we determine the sand flux from a single dune q dunes and scale this estimate to the whole dune field we compare the measured sand flux with those derived from wind velocity measurements at a local meteorological station as well as with those predicted from era interim a global circulation model the comparison revealed that the wind velocity predicted by era interim is inappropriate to calculate the sand flux this is due to the 6 h sampling rate and to systematic bias revealed by a comparison with the local wind data we describe a simple procedure to correct for these effects with the proposed correction similar sand flux are predicted using the local and era interim data independently of the value of the value of the shear velocity threshold u t the predicted sand flux is about 65 of q dunes the agreement is best assuming the value u t 0 244 m s which is only slightly larger than the value of u t 0 2612 m s estimated based in the sand granulometry measured from field samples the influence of the dune topography on the wind velocity field could explain the underestimation in any case the study demonstrates the possibility of validating gcm model and calibrating aeolian sand transport laws using remote sensing measurements of dune dynamics and highlights the caveats associated to such an approach

magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges magnetic properties natural remanent magnetization nrm susceptibility curie point temperature tc saturation isothermal remanent magnetization irms and koenigsberger ratio q and opaque mineralogy were determined for basalts diabases gabbros peridotites and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites 15 n 17 n 23 n the snake pit hydrothermal field 26 n the tag hydrothermal field on the mid atlantic ridge and 42 n the sea cliff hydrothermal field on the northern gorda ridge the magnetomineralogy is interpreted in terms of deuteric hydrothermal metasomatic and ambient seawater alteration evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases gabbros and ultramafic rocks this lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases gabbros and peridotites the studies reveal distinct correlations between magnetization intensity thermomagnetic behavior and magnetic mineralogy grain size style and intensity of alteration and rock type basalts have the highest nrm intensities and the lowest curie points 105 294 c basalts with nrm irms ratios less than 10 2 may have been remagnetized and have nrm intensities and q values much lower one third and one half respectively than basalts with nrm irms ratios of 10 2 or greater in the oceanic lithospheric suite examined remanent q 1 magnetization is present in basalts and induced magnetization q 1 occurs in metabasalts and the other rock types layer 2a basalts are the source of median valley magnetic anomalies the magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2b dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites this shift is influenced by fluctuations in the curie isotherm related to the duration of active magma chambers and convective hydrothermal cells the magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere magmatic cooling history and hydrothermal circulation

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

a near bottom magnetic survey of the mid atlantic ridge axis at 26 n implications for the tectonic evolution of the tag segment 1 an extensive deep tow magnetic survey of the tag ridge segment on the mid atlantic ridge reveals new information about the relationship between the magnetic anomaly field and the tag hydrothermal deposits results show the strongest magnetization is located over the neovolcanic axis and asymmetrically toward the western side of the central brunhes anomaly a well defined linear magnetization low is located over the eastern rift valley wall of the tag segment the near bottom data show no direct correlation between this crustal magnetization low and the hydrothermal deposits the magnetization low is explained by crustal thinning caused by 4 km of horizontal extension along a normal fault previous observations and sampling indicate exposures of gabbros and dikes in the eastern rift valley wall suggesting slip along a normal fault has revealed this crust modeling suggests the fault has been active since 0 35 0 1 ma at a horizontal slip rate of roughly half the spreading rate of 22 km myr the tag hydrothermal system is located on the hanging wall of this fault within 3 km of its termination over the past several hundred thousand years movement on the detachment fault may have episodically increased the permeability of the hanging wall reactivating the overlying hydrothermal systems significant vents like tag may be typically associated with hanging walls of long term detachment faults near seafloor spreading centers this would imply that it is the reactivation of permeability in the hanging wall related to repeated fault movement that controls the longevity of these hydrothermal systems

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

structural control evolution and accumulation rates of massive sulfides in the tag hydrothermal field the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge is one of the best studied hydrothermal systems to date however high resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle auv reveal new information about the distribution of active and inactive hydrothermal deposits and their relation to structural features the discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow spreading ridges the interpretation of ship based and high resolution auv based data sets allowed for the determination of the main tectonic stress regimes that have a first order control on the location and distribution of past and present hydrothermal activity the data reveal the importance of cross cutting lineament populations and temporal variations in the prevalent stress regime a dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material 29 mt the accumulation rate of 1 500 t yr is comparable to those of other modern seafloor vent fields however our observations suggest that the tag segment is different from many other slow spreading ridge segments in its tectonic complexity which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation the inactive and weakly active mounds contain almost 10 times the amount of material as the active high temperature mound providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

reduced crustal magnetization beneath relict hydrothermal mounds tag hydrothermal field mid atlantic ridge 26 n submersible magnetic profiles over the mir and alvin relict hydrothermal zones of the tag hydrothermal field on the mid atlantic ridge at 26 n show magnetic anomalies that are consistent with the presence of reduced crustal magnetization these magnetic anomalies are similar to the anomaly found associated with the tag active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

magnetization of 0 29 ma ocean crust on the mid atlantic ridge 25 30 to 27 10 n a sea surface magnetic survey over the west flank of the mid atlantic ridge from 0 to 29 ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust we find that magnetization decays rapidly within the first few million years although the filtering effect of water depth on the sea surface data and the slow spreading rate 13 km m y preclude us from resolving this decay rate a distinctly asymmetric along axis pattern of crustal magnetization is rapidly attenuated off axis suggesting that magnetization dominated by extrusive lavas on axis is reduced off axis to a background value off axis we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust indicative of induced magnetization in thin inside corner ic crust in general we find that off axis segment ends show an induced magnetization component regardless of polarity and that ic segment ends tend to have slightly more induced component compared with outside corner oc segment ends possibly due to serpentinized uppermost mantle at ic ends we find that remanent magnetization is also reduced at segment ends but there is no correlation with inside or outside corner crust even though they have very different crustal thicknesses this indicates that remanent magnetization off axis is independent of crustal thickness bulk composition and the presence or absence of extrusives remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in oc crust and a combination of crustal thinning and alteration in ic crust from all these observations we infer that the remanent magnetization of extrusive crust is strongly attenuated off axis and that magnetization of the lower crust may be the dominant source for off axis magnetic anomalies

tectonics of the mid atlantic rift valley between the tag and mark areas 26 24 n evidence for vertical tectonism abstract a segment of the mid atlantic ridge between the tag and mark areas between 26 and 24 n was studied during march april of 1988 from the r v akademik mstislav keldysh using mir submersibles the rift valley is depressed here to depths of 3200 4700 m and includes seven independent magmatic cells which are in different phases of their evolution some of them are now in the constructive phase whereas others are in the destructive phase and are strongly fractured slopes of two types flank the rift valley the normal slopes similar to the famous area are block faulted terrains with small elevations above the neovolcanic ridge crest in the center of the rift valley the abnormal slopes are elevated from 1 to 3 km above the neovolcanic ridge crest an almost undisturbed section of the upper oceanic crust from gabbro through sheeted dikes to pillow lavas outcrops on the abnormal slopes as the dikes are dipping nearly vertically and pillow lavas preserve their initial inclinations no block rotation or back tilting occurred during the development of rift mountains pure vertical uplift was responsible for rift mountain growth speculations are made that serpentinization of the uppermost mantle peridodite can be considered as the main process leading to oceanic crust uplift serpentinization takes place during the nonvolcanic destructive phases when seawater can easily penetrate into the deeper crust and uppermost mantle reaching the peridodites as a result serpentinites are added to the oceanic crust of slow spreading ridges

generalized dix equation and analytic treatment of normal moveout velocity for anisotropic media despite the complexity of wave propagation in anisotropic media reflection moveout on conventional common midpoint cmp spreads is usually well described by the normal moveout nmo velocity defined in the zero offset limit in their recent work grechka and tsvankin showed that the azimuthal variation of nmo velocity around a fixed cmp location generally has an elliptical form i e plotting the nmo velocity in each azimuthal direction produces an ellipse and is determined by the spatial derivatives of the slowness vector evaluated at the cmp location this formalism is used here to develop exact solutions for the nmo velocity in anisotropic media of arbitrary symmetry for the model of a single homogeneous layer above a dipping reflector we obtain an explicit nmo expression valid for all pure modes and any orientation of the cmp line with respect to the reflector strike the contribution of anisotropy to nmo velocity is contained in the slowness components of the zero offset ray along with the derivatives of the vertical slowness with respect to the horizontal slownesses quantities that can be found in a straightforward way from the christoffel equation if the medium above a dipping reflector is horizontally stratified the effective nmo velocity is determined through a dix type average of the matrices responsible for the interval nmo ellipses in the individual layers this generalized dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous arbitrarily anisotropic media for models with a throughgoing vertical symmetry plane i e if the dip plane of the reflector coincides with a symmetry plane of the overburden the semi axes of the nmo ellipse are found by the more conventional rms averaging of the interval nmo velocities in the dip and strike directions modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one zero offset ray remarkably the expressions for geometrical spreading along the zero offset ray contain all the components necessary to build the nmo ellipse this method is orders of magnitude faster than multi azimuth multi offset ray tracing and therefore can be used efficiently in traveltime inversion and in devising fast dip moveout dmo processing algorithms for anisotropic media this technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces the high accuracy of our nmo expressions is illustrated by comparison with ray traced reflection traveltimes in piecewise homogeneous azimuthally anisotropic models we also apply the generalized dix equation to field data collected over a fractured reservoir and show that p wave moveout can be used to find the depth dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy

experimental reassessment of the shaw paleointensity method using laboratory induced thermal remanent magnetization 1 artificial thermoremanent magnetization imposed on natural basalt samples was used to check the reliability of shaw s palaeointensity method shaw 1974 kono 1978 rolph and shaw 1985 and to give an indication of the accuracy of paleointensity determinations first thermal remanence was induced in all samples 50 in total in a known field 50 0 1 t this remanence was regarded as the original natural remanent magnetization nrm the samples were divided in to three subsets a b and c and intensity experiments were performed each subset of samples used different thermal remanent magnetization trm acquisition parameters rock magnetic properties on 14 representative subsamples were also measured both before and after the trm acquisition steps to monitor possible magnetochemical alteration it has been demonstrated that intensity estimates by the shaw method give values close to the expected value 50 5 t in all three sets of basalt samples the modified method with rolph and shaw 1985 correction i e using ratios of anhysteretic remanent magnetization given before the trm step arm1 and after arm2 is proved to be a powerful approach to correct the trm capacity changes that result from magnetochemical alteration this is especially so for sample set c where the samples were heated up to 700 c and then were held at this temperature for 40 min during trm acquisition after correction 45 of set c samples gave acceptable results however some samples with strong magnetochemical alteration sometimes invalidated the arm1 arm2 ratio correction and gave wrong intensity estimates monitoring rock magnetic properties is therefore very important when applying the shaw method

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

kinematics of the north american african plate boundary between 28 and 29 n during the last 10 ma evolution of the axial geometry and spreading rate and direction abstract a high resolution survey sara segmentation ancienne de la ride atlantique collected sea beam bathymetry magnetic gravity and seismic reflection data on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to complement a portion of an on axis study carried out between the atlantis and kane transform faults which revealed the configuration of the spreading axis the sara survey extends this coverage to an age of approximately 10 ma on either flank a sinuous isochron pattern was established from anomaly identification made using a two dimensional model the plate tectonic reconstruction based solely on this isochron pattern provides an accurate kinematic description of the relative motion between the african and north american plates within the zone our results confirmed by data from the famous area and a flowline comparison with the trend of the kane fracture zone show that a change in axial orientation spreading direction and spreading rate occurred at approximately anomaly 4 time 7 01 ma this was coincident with the appearance of a new axial segment all the axial segments observed have changed length continously during the last 10 ma by advancing or retreating at one or both ends in any combination so as to leave non parallel trails on the flanks that do not align with either the spreading direction or with the absolute plate motion factors contributing to the sinuous magnetic anomaly pattern are discussed

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

magnetic modeling near selected areas of hydrothermal activity on the mid atlantic and gorda ridges sea surface magnetic and bathymetric data from two areas of observed high temperature hydrothermal activity in the atlantic trans atlantic geotraverse or tag hydrothermal field and pacific sea cliff hydrothermal field and two areas of inferred hydrothermal activity in the atlantic mid atlantic ridge mar at 15 n and 17 n were studied to determine the effect of vent activity on magnetization of the oceanic crust all of the sites examined show a low in the magnetic anomaly in varying degrees magnetization lows within the axial marine magnetic anomaly have been associated with some areas of active and relict high temperature hydrothermal venting a three dimensional magnetic inversion was performed on magnetic and bathymetric data from the tag hydrothermal area on the mid atlantic ridge 26 n and two dimensional magnetic inversions were done on individual profiles from the other study sites the map from the tag area shows a negative in the crustal magnetization solution just to the east of the median valley axis which encompasses the active hydrothermal area a low in magnetization continues up the eastern wall which suggests alteration of the crust a distinct low in magnetization is present within the axial anomaly at the sea cliff hydrothermal field located on the eastern wall of the gorda ridge the magnetization low becomes less pronounced farther south of the known vent field profiles from 15 n and 17 n on the mid atlantic ridge show magnetization lows over areas of inferred hydrothermal activity these lows are present over the the eastern and western walls of the median valley respectively the lows observed in the axial marine magnetic anomaly in these four study sites all occur slightly off axis on crust that is at least 200 000 years old on the mid atlantic ridge and at least 100 000 years old on the gorda ridge on the basis of magnetic modeling and measurements of rock magnetic properties the most probable cause of the low in the magnetization is alteration of the oceanic crust with other subsidiary contributing factors which vary from site to site the magnetization low over tag is most probably due to high temperature hydrothermal alteration of the crust and a thinner layer 2 with possible contributions from reversal events and magma chamber curie isotherm effects the low near the sea cliff hydrothermal field on the gorda ridge is probably the product of tectonically controlled low temperature nonhydrothermal alteration high temperature hydrothermal alteration and magnetic reversal events at 15 n and 17 n on the mar the lows may be produced by alteration of the crust due to possible hydrothermal activity

evolution of an active sea floor massive sulphide deposit hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to 400 c before venting at spectacular black smokers these hydrothermal systems exert a strong influence on ocean chemistry1 yet their structure longevity and magnitude remain largely unresolved2 the active transatlantic geotraverse tag deposit at 26 n on the mid atlantic ridge is one of the largest oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black smoker fields here we report ages of sulphides and anhydrites from the recently drilled3 tag substrate structures determined from 234u 230th systematics analysed by thermal ionization mass spectrometry the new precise ages combined with existing data4 5 show that the oldest material 11 000 to 37 000 years old forms a layer across the centre of the deposit with younger material 2 300 7 800 years old both above and below this stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid6 the age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2 000 years

fissuring near the tag active hydrothermal mound 26 n on the mid atlantic ridge abstract analysis of 12 000 electronic still camera images collected with the argo ii vehicle near the trans atlantic geotraverse tag active hydrothermal mound 26 n on the mid atlantic ridge has made possible the first quantitative in situ assessment of both fissure orientation and width within the median valley of a slow spreading ridge fissures near the tag mound are partially rubble filled extensional fractures that cut lightly sedimented seafloor and in 1 of our observations host pillow lavas fissure widths range from 0 15 to 3 5 m with a mean of 0 7 m and do not vary systematically within the survey area first order estimates of crack depth based on these width measurements and reasonable elastic moduli indicate that fissures are restricted to depths 45 from the strike of the ridge axis the formation of obliquely oriented fissures requires that the local least compressive stress direction varies at least temporarily from that predicted by the regional tectonic stress field associated with plate separation this stress field reorientation may be facilitated by variations in the style of magma emplacement within the rift the close spatial association of long term hydrothermal activity fissure hosted lava flows and faults and fissures trending oblique to the spreading axis suggests a causal relationship between the impact of dike intrusion and the maintenance of localized hydrothermal flow