vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

potential field inversion for a layer with uneven thickness the tyrrhenian sea density model abstract inversion of large scale potential field anomalies aimed at determining density or magnetization is usually made in the fourier domain the commonly adopted geometry is based on a layer of constant thickness characterized by a bottom surface at a fixed distance from the top surface we propose a new method to overcome this limiting geometry by inverting in the usual iterating scheme using top and bottom surfaces of differing but known shapes randomly generated synthetic models will be analyzed and finally performance of this method will be tested on real gravity data describing the isostatic residual anomaly of the southern tyrrhenian sea in italy the final result is a density model that shows the distribution of the oceanic crust in this region which is delimited by known structural elements and appears strongly correlated with the oceanized abyssal basins of vavilov and marsili

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

origin of the absence of magnetic lineations in the yamato basin of the japan sea magnetic properties of mafic rocks from ocean drilling program hole 794 d we investigated magnetic properties of mafic rocks recovered from ocean drilling program hole 794 d in the yamato basin of the japan sea in contrast to other back arc basins in the western pacific lineated magnetic anomaly patterns have not been recognized in the yamato basin our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks the dominant magnetic mineral of hole 794 d is titanomaghemite as commonly observed for typical oceanic basalts but the grain size is much larger greater than several tens of microns remanence properties i e low natural remanent magnetization intensities 1 5 a m low koenigsberger ratios 1 0 high viscous remanent magnetization and mixed magnetic polarities indicate that the mafic rocks cannot be a source of lineated magnetic anomalies while curie temperature saturation magnetization and susceptibility values are not significantly different from those of typical oceanic basalts magnetic properties related to stability are greatly reduced the large grain size of the mafic rocks from hole 794 d plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts and is caused by the formation of sill sediment complexes beneath the yamato basin sill sediment complexes rather than extrusive basalts in typical oceanic crust are responsible for the absence of lineated magnetic anomalies in the yamato basin

the natural magnetization of a 3 kilometer section of icelandic crust the magnetization and magnetic properties of samples from a 3 km vertical section of icelandic crust the iceland research drilling project section are described the section is located at the head of reydarfjordur eastern iceland at 65 01 n 14 21 w and extends vertically from about 0 5 km to 3 5 km depth beneath the original surface the section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2 km drill core component consisting of 60 flows and minor volcaniclastics and 40 dikes the section is largely normally magnetized corresponding in age and polarity to seafloor spreading anomaly 5 geomagnetic polarity epoch 9 flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000 m thick equivalent sections which are exposed updip this great increase in downdip thickening is consistent with observations elsewhere in eastern iceland and models for icelandic crustal formation involving the combination of volcanic activity subsidence and spreading a combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section this overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2 8 km crustal original depth where moderate secondary magnetite development has occurred flows frequently show both normal and reverse stable polarities with the latter being the original secondary magnetite development is high where dike density in the epidote bearing flows is also high here very strong well defined pervasive normal magnetization characterizes the flows this also being the dominant polarity of the dikes magnetic overprinting of this type imposed shortly after crustal formation could occur at 800 to 1200 m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins the initial susceptibility of the flows shows the best defined depth trends over the section susceptibility increases with depth to about 2 km crustal depth and then decreases systematically to the bottom of the section the reason for the initial increase is not known but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates the decrease is sufficiently linear to warrant extrapolation to a zero value at 4 3 km crustal depth or just below the estimated level of onset of greenschist facies metamorphic conditions in typical oceanic crust the equivalent level is estimated to be at 1 2 km depth the net magnetization of the section the combined remanent and induced contributions when expressed as average values for 500 m depth intervals is everywhere positive that is is in the sense of the present field variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

investigation of a vine matthews magnetic lineation from a submersible the source and character of marine magnetic anomalies we report here the first attempt to map directly the boundary of a vine matthews magnetic stripe on the seafloor our objectives are to study the processes of oceanic crustal accretion as recorded in the reversal transition zone and to investigate the formation of the magnetic source of vine matthews magnetic anomalies our deep tow and alvin based magnetic studies focus on the matuyama brunhes reversal transition on the flanks of the east pacific rise near 21 n while the sea level magnetic anomalies are less than average in clarity for the east pacific rise a three dimensional inversion of deep tow data reveals a sharp strike linear polarity transition less than 1 8 km wide macdonald et al 1980a these measurements have been augmented by mounting a vertical magnetic gradiometer on alvin and making 280 reliable polarity determinations along and across the polarity transition zone even on long traverses across both sides of the boundary we find that nearly every magnetic target has the correct polarity i e the same polarity as the regional magnetic lineation this homogeneity in polarity of the magnetic lineations is surprising the magnetic polarity transition in the outcropping volcanic section is sharp and linear along strike delineated in some cases by a clear geologic contact of opposing flow fronts of different ages several weakly magnetized outcrops mapped within the transition zone may have erupted during the time in which the geomagnetic field was reversing the reversal boundary mapped on the seafloor from alvin is displaced 250 m to 500 m nw away from the spreading axis relative to the position of the average boundary as derived from inversion of the deep tow and sea level magnetic data this offset provides a means for estimating the spillover of lava flows away from the spreading axis during the time the crust was formed the combination of deep tow and alvin measurements suggests that circa 0 7 m y ago the crustal accretion zone magnetized volcanic intrusive and plutonic rocks was 2000 2800 m wide while the zone of recent volcanism alone was only 1000 2000 m wide the determination for the most recent reversal agrees well with submersible observations at the present spreading center where the zone of recent volcanism neovolcanic zone varies between 600 and 2000 m in width this very orderly picture for the formation of magnetic lineations and crustal accretion processes appears to conflict with complex deep sea drilling project dsdp magnetic results from the atlantic we suggest that the crustal generating processes and resulting magnetic structure vary significantly with spreading rate on the slow spreading mid atlantic ridge major episodes of volcanism are likely to be infrequent 104 years the magma chamber may be non steady state and the neovolcanic zone shifts or varies in width considerably this sporadic start and stop spreading process will contribute to a highly heterogeneous and complex crustal and magnetic structure as seen in dsdp holes in addition significant faulting and tilting may disrupt slow spreading crust for intermediate to fast spreading centers more frequent volcanism 50 600 years a nearly steady state magma chamber and a narrow stable neovolcanic zone will create a less complex magnetic and crustal structure both as seen from alvin and as inferred from clear sea level magnetic anomalies in the pacific

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

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

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

a study of mg and k isotopes in allende cais implications to the time scale for the multiple heating processes the measurements of magnesium and potassium isotopic compositions of refractory minerals in allende calcium aluminum rich inclusions cais 7r 19 1 hn3 1 and egg3 were taken by secondary ion mass spectrometry sims the 7r 19 1 contains 16 o rich and 16 o poor melilite grains and define a single isochron corresponding to an initial 26 al 27 al ratio of 6 6 1 3 10 5 the al mg isochron o isotope measurements and petrography of melilite in 7r 19 1 indicate that 16 o poor melilite crystallized within 0 4 myr after crystallization of 16 o rich melilite suggesting that oxygen isotopic composition of the cai forming region changed from 16 o rich to 16 o poor within this time interval the 16 o poor melilite is highly depleted in k compared to the adjacent 16 o rich melilite indicating evaporation during remelting of 7r 19 1 we determined the isochron for 41 ca 41 k isotopic systematics in egg3 pyroxene with 4 1 2 0 10 9 2 as an initial ratio of 41 ca 40 ca which is at least two times smaller than the previous result sahijipal et al 2000 the ratio of 41 ca 40 ca in the egg3 pyroxene grain agrees within error with the value obtained by hutcheon et al 1984 no evidence for the presence of 41 k excess decay product of a short lived radionuclide 41 ca was found in 7r 19 1 and hn3 1 we infer that the cai had at least an order of magnitude lower than canonical 41 ca 40 ca ratio at the time of the cai formation

fluctuations of the paleomagnetic field during chron c5 as recorded in near bottom marine magnetic anomaly data near bottom magnetic data contain information on paleomagnetic field fluctuations during chron c5 as observed in both the north and south pacific the north pacific data include 12 survey lines collected with a spatial separation of up to 120 km and the south pacific data consist of a single long line collected on the west flank of the east pacific rise epr at 19 s the north pacific magnetic profiles reveal a pattern of linear short wavelength 2 to 5 km anomalies tiny wiggles that are highly correlated over the shortest 3 8 km to longest 120 km separations in the survey magnetic inversions incorporating basement topography show that these anomalies are not caused by the small topographic relief the character of the near bottom magnetic profile from anomaly 5 on the west flank of the epr formed at a spreading rate more than twice that of the north pacific displays a remarkable similarity to the individual and stacked lines from the north pacific survey area over distances corresponding to 1 m y 19 lows in the magnetic anomaly profile can be correlated between the north and south pacific lines modeling the lows as due to short polarity events suggests that they may be caused by rapid swings of the magnetic field between normal and reversed polarities with little or no time in the reversed state owing to the implausibly high number of reversals required to account for these anomalies and the lack of any time in the reversed state we conclude that the near bottom signal is primarily a record of paleointensity fluctuations during chron c5 spectral analysis of the north pacific near bottom lines shows that the signal is equivalent to a paleointensity curve with a temporal resolution of 40 to 60 kyr while measurements of the smallest separations of correctable dips in the field suggest a temporal resolution of 36 kyr

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

east pacific rise at 21 n the volcanic tectonic and hydrothermal processes of the central axis abstract photographs obtained by the angus survey system at 21 n reveal many similarities to the geological processes delineated at other spreading centers and in particular those observed in the galapagos rift at 86 w the region of recent volcanism is restricted to a narrow zone zone 1 approximately 1 km wide this suggests that the width of the magma chamber feeding these flows is also narrow at the top variations in sediment cover are used to subdivide the flows within zone 1 into three sheet flow pillow flow pairs the youngest pair appears to have erupted form a linear fissure 8 km long running parallel to the valley axis this fissure is part of a larger en echelon pattern of eruptive fissures active hydrothermal vents associated with the youngest flows are situated directly above the apparent eruptive fissure the high 350 c temperature for some of the fluids exiting from the vents suggests a highly restricted circulation system involving at times little to no mixing with cold seawater the lava terrain in the remainder of zone 1 and bordering regions is characterized by extensive fracturing further suggesting a narrow upper dimension to the magma chamber and also that subsurface hydrothermal deposition of massive sulfides may lead to a reduction in the mixing processes

central anomaly magnetization high constraints on the volcanic construction and architecture of seismic layer 2a at a fast spreading mid ocean ridge the epr at 9 30 50 n abstract the central anomaly magnetization high camh is a zone of high crustal magnetization centered on the axis of the east pacific rise epr and many other segments of the global mid ocean ridge mor the camh is thought to reflect the presence of recently emplaced and highly magnetic lavas forward models show that the complicated character of the near bottom camh can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age this lava magnetization function is the product of geomagnetic paleofield intensity which has increased by a factor of 2 over the last 40 kyr and low temperature alteration which decreases the remanence of lava with exposure to seawater the success of the forward modeling justifies the inverse approach deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness this approach is tested on two near bottom magnetic profiles al2767 and al2771 collected using alvin across the epr axis at 9 31 n and 9 50 n our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer which is remarkably consistent with existing multichannel estimates of layer 2a thickness from lines cdp31 and cdp27 the similarity between magnetic layer and seismic layer 2a at the 9 10 n segment of the epr crest provides independent support to the notion that seismic layer 2a in young oceanic crust represents the highly magnetic lava layer and that the velocity gradient at the base of layer 2a is related to the increasing number of higher velocity dikes with depth in the lava dike transition zone the near bottom magnetic anomaly character of the camh is a powerful indicator of the emplacement history of upper crust at mors which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints

identification and implications of off axis lava flows around the east pacific rise 1 off axis eruptions at ocean ridges provide critical information with respect to underlying crustal plumbing and mantle melting systems a detailed study of basaltic glass samples around the east pacific rise from 12 00 to 12 30 n provides geological evidence for the existence of off axis eruptions with a distinctive chemical composition this composition has not been found along the axis of the epr from 8 to 14 n except at a ridge transform intersection but has been recovered in numerous locations that were farther than 1 km off axis these off axis normal mid ocean ridge basalts or oa nmorb are distinguished by low na2o sr and al2o3 and are unusually depleted in incompatible elements moderately enriched off axis transitional mid ocean ridge basalts oa tmorb with the same compositional tendencies can also be identified comparison of epr axis lavas and the oa type suggests that they come from the same range of unmelted mantle source compositions but that the source of the oa magmas was depleted in incompatible trace elements by removal of a small degree partial melt this would be consistent with the oa type as an epr pooled melt that is missing the low degree melt fraction from deep in the melting regime which provides a reasonable physical model for their formation in this case off axis magmas do not represent the same range of chemical variation as magmas delivered to the axial magma system the oa nmorb are similar to depleted lavas from near epr seamounts other seamount lavas with depleted trace elements have tmorb like major elements and may be classified as oa tmorb the similarity between seamount lavas and the lavas erupted off axis close to the epr suggests that the two are manifestations of the same phenomenon we suggest that seamount type volcanism effectively starts within 1 2 km of the axis this is within the range where lavas derived from the axial plumbing system may also erupt therefore there is a narrow zone where young lava flows from the axial plumbing system and from the off axis systems may overlap lavas erupted off axis may ultimately cover 20 of the seafloor around the epr which is substantially more than previous estimates that were based primarily on morphological studies

steady and non steady state magma chambers below the east pacific rise volcanic rocks collected on flowline traverses on the flanks of the east pacific rise epr document changes in axial magma chemistry with time providing a record of the thermal history of axial magma chambers amc we present data for a sample set of closely spaced 1 2 km samples along epr flowlines at three localities out to 800 ka showing both steady constant average temperature and non steady state behavior of magma chambers on time scales of 200 500 ka though based on only three symmetrical traverses so far it appears that magmatically robust ridge locations 11 20 n and 9 30 n have steady state chambers whereas a magmatically starved axis 10 30 n shows large temperature changes with time these observations provide a new petrologic perspective to the ongoing debate regarding the significance and causes of morphologic variations along the axis of the east pacific rise

magnetic polarity structure of the lower oceanic crust a newly developed rock drill was used to take the first fully oriented samples from in situ gabbros exposed on atlantis bank which formed 11 m y ago on the southwest indian ridge swir these paleomagnetic data locate the seabed outcrop of a normal polarity magnetic lineation within an outcrop of lower crust that correlates with overlying deep towed and sea surface magnetic field measurements a systematic lateral offset between the seabed boundary and measured anomalies implies that the reversal boundary dips 25 to the south away from the ridge axis paleomagnetic results suggest that following magnetization acquisition the platform tilted south by 20 range 33 south to 7 north implying an original reversal boundary dip of only 5

magnetic properties of three segments of the mid atlantic ridge at 37 n famous narrowgate and amar amar 2 during the amar 1978 expedition over 200 separate basalt samples were collected by alvin and dredge from the narrowgate and amar segments of the mid atlantic ridge near 37 n to determine the origin of reversely magnetized rocks previously recognized in the famous rift area to the north and to study the variations in rock magnetic properties that occur along the strike of the mid atlantic ridge magnetic intensity determinations of rocks collected in the famous area indicate that intensity decays with time by a factor of 5 in less than 106 years rocks with low intensities have high curie temperatures tc indicating that low temperature oxidation of the magnetic minerals takes place soon after the basalts are extruded and is pervasive in crust older than 106 years magnetic intensity jnrm of the uppermost oceanic crust can be expressed as a function of age t by the expression j 5 82t 1 3 by measuring jnrm of the surface rocks from the three valley segments the relative ages of the surface rocks from the famous amar and narrowgate areas can be estimated to be 1 104 years 3 104 years and 4 104 years respectively one exception is mt mars located in the center of the narrowgate area that has high jnrm values 84 10 3 emu cm3 and low tc values 165 c suggesting that this is an area where volcanic activity is becoming reactivated following a period of quiescence of the 55 oriented samples collected eight are reversely magnetized although three of the reversed samples have curie temperatures above 300 c and may have undergone self reversal the other five reversed samples have magnetic properties similar to normally magnetized rocks and may have been misoriented or extruded during a short reversed event within the brunhes epoch because most of the surficial rock samples are normally magnetized it is suggested that the previously recognized reversed zones which appear in a deep tow magnetometer profile that crossed the narrowgate area are located at depth and are the result of the spreading axis moving into older matuyama crust that is now covered by more recent volcanism

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

variations in layer 2a thickness and the origin of the central anomaly magnetic high the seismically determined off axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t ypic ally associated with the ridge crest the positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales 2 3 km comparable to that of the proposed layer 2a thickening we present magnetic rema nence data from basalts dredged on and near the east pacific rise axis at 12on which are compatible with a rapid magnetization reduction 20 k y to decay to l e together with near bottom magnetic profiles from the ultra fast spreading east pacific rise at 19 5os these data suggest that previous estimates of the time constant of alteration inferred from slow spreading ridges 0 5 m y may be more than an order of magnitude too high seismic studies of the fast spreading east pacific rise epr indicate a rapid thickening of seismic layer 2a within a few kilometers of the ridge axis vera et al 1990 toomey et ai 1990 detrick et al 1993 these seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness from 100 200 m to 300 500 m through addition of flows whi ch may extend 1 2 km off axis christeson et al 992 vera and diebold in press such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions e g macdonald et al 1983 however this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest klitgord 1976 sea surface magnetic profiles from the ultra fast spreading 77 mrn yr half rate epr near 19 5os figure 1 illustrate the short wavelength axial anomaly high within the 120 km wide central anomaly which has been termed the central anomaly magnetic high camh klitgord 1976 the camh typically has a wavelength of 8 10 km in sea surface profiles at in termediate spreading ridges 15 45 mm yr the camh occupies a larger proportion of the central anomaly and at slow spreading ridges it is not readily distinguishable in sea surface profiles from the central anomaly because the wavelengths become similar near bottom magnetic profiles figure 1 typically have higher amplitude and shorter wavelength 2 4 kin axial anomalies as expected from the closer proximity to the source klitg6rd et al 1975 miller 1977 perram et al 1990 and even allow distinction of the camh from the central anomaly at some slow spreading ridges macdonald 1977 the higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the camh thickness variations in the extraslye layer at fast spreading fi dges have a profound effect on the axial magnetic anomaly since the presence of an axial magma chamber and the low re rnanent intensity of intrusive rocks e g smith and banerice 1986 pariso et al 1991 minimize deeper source contributions in the near ridge environment based on seismic

magnetic structure of a slow spreading ridge segment insights from near bottom magnetic measurements on board a submersible 1 near bottom magnetic measurements on board submersible nautile were carried out on the mid atlantic ridge 21 40 n segment and deep sea geomagnetic vector anomalies along 19 dive tracks were obtained by applying the processing method for shipboard three component magnetometer data a forward modeling technique using short wavelength components of the anomalies arising from local topography and vertical motion of the submersible was designed to estimate the absolute magnetization intensity of the seafloor in the vicinity of the spreading axis a considerable number of magnetization estimations are reliably confirmed by the high correlation between observed and modeled anomalies whereas less reliable estimations are obtained off axis probably because the sediment buries the basement topography the natural remanent magnetization nrm measured on basalt samples collected during these dives is compared with the magnetization from anomalies though both results give a similar range of magnetization intensity no correlation is confirmed between them possibly because the magnetization from anomalies represents laterally averaged seafloor magnetization whereas the nrm has variations at the scale of individual pillow or lava pile equivalent magnetization inverted from the sea surface magnetic anomalies shows axial magnetization increases significantly from the segment center to the segment ends however the results of eight dives conducted near the spreading axis at different locations along the segment show much less variation in magnetization intensity along the axis we ascribe the high equivalent magnetization at segment ends to preferential serpentinization of peridotite near the segment ends and the associated formation of magnetite the results of three across axis transects composed of 15 dives running in the spreading direction can be consistently interpreted as recording geomagnetic paleointensity variations during the brunhes epoch although magnetization lows are generally correspondent to periods of low paleointensity they show deeper drop than predicted from the paleointensity variation

volcanic mound fields on the east pacific rise 16 19 s low effusion rate eruptions at overlapping spreading centers for the past 1 myr 1 volcanic mound fields identified on seamarc ii and hmr1 12 khz side scan data from the southern east pacific rise sepr occur near overlapping spreading centers oscs and migration traces of oscs the volcanic mound fields appear as a distinctive hummocky seafloor fabric due to side scan backscatter reflections from clusters of mound shaped reflectors the lack of growth of the mound fields away from the ridge axis and their occurrence in association with osc traces suggests that mound fields form along the ridge crest near oscs volcanic mound fields are found where 120 khz side scan and visual observations find fields of pillow mounds since pillow mounds are constructed by low effusion rate eruptions the volcanic mound fields found near the oscs and in their migration traces indicate that volcanic effusion rates tend to be lower near ridge discontinuities than midsegment regions this tendency for low effusion rate eruptions at oscs is documented for the past 1 myr three independent measurements of ridge segmentation 1 volcanic segment boundaries marked by the low effusion rate eruptions 2 tectonic segments defined by oscs and 3 magmatic segment boundaries based on continuity of parental magma composition all coincide in the study area high backscatter off axis lava fields not associated with seamounts are found on seafloor younger than 0 2 ma the 0 2 ma corridor corroborates previous results from the distribution of small isolated volcanoes that indicates randomly distributed off axis eruptions mainly occur on crust younger than 0 2 ma

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

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

nonuniform magnetization of seamounts a least squares approach residual total magnetic field anomalies relative to the best fitting uniform magnetization models are large and spatially coherent over three small tertiary seamounts on the cocos plate furthermore the paleomagnetic parameters determined from the uniform magnetization models in some cases disagree significantly with those predicted from models of absolute motion for the cocos plate inversion of the sea surface magnetic anomaly using a least squares procedure which allows magnetization to vary among different sections of each seamount shows clear polarity reversals in two of the seamounts studied the reversed zones constitute about 10 of the seamount volume for the third seamount the nonuniform magnetization solution more correctly determines the paleomagnetic inclination and declination for the only section of the dipolar anomaly not obscured by a magnetic lineation traversing the area the polarities indicated in the least squares solutions were confirmed by magnetic gradiometer measurements from the submersible alvin for all three seamounts the paleomagnetic latitudes and or rotation angles in the nonuniform magnetization solutions changed appreciably by as much as 20 to agree better with plate motion models the nonuniform magnetization models reveal that growth rates for such volcanoes approach 1000 km3 ma with the final phase of volcanism consisting of summit as opposed to flank eruptions the largest of the three volcanoes formed off ridge on lithosphere at least 0 5 ma old and may have remained active for 1 ma hydrothermal activity apparently persists at least 0 7 ma after the last major phase of volcanism

central anomaly magnetization high documentation of crustal accretion along the east pacific rise 9 55 9 25 n near bottom magnetic data collected along the crest of the east pacific rise between 9 55 and 9 25 n identify the central anomaly magnetization high camh a geomagnetic anomaly modulated by crustal accretionary processes over timescales of 104 years a significant decrease in camh amplitude is observed along axis from north to south with the steepest gradient between 9 42 and 9 36 n the source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption instead magnetic moment models show that it can be accounted for by an observed 50 decrease in seismic layer 2a thickness along axis layer 2a is assumed to be the extrusive volcanic layer and we propose that this composes most of the magnetic source layer along the ridge axis the 9 37 n overlapping spreading center osc is located at the southern end of the steep camh gradient and the 9 42 9 36 n ridge segment is interpreted to be a transition zone in crustal accretion processes with robust magmatism north of 9 42 n and relatively low magmatism at present south of 9 36 n the 9 37 n osc is also the only bathymetric discontinuity associated with a shift in the camh peak which deviates 0 7 km to the west of the axial summit trough indicating southward migration of the osc camh boundaries defined from the maximum gradients lie within or overlie the neovolcanic zone nvz boundaries throughout our survey area implying a systematic relationship between recent volcanic activity and camh source maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the nvz and camh boundaries lava dip angles average 14 toward the ridge axis which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed

the rise flank trails left by migrating offsets of the equatorial east pacific rise axis sea beam and magnetic surveys of the young 1 ma rise flank around nontransform rise crest offsets at 5 5 n 4 9 n 3 4 n 2 0 n and 2 8 s mapped distinctive trails of obliquely lineated highly magnetic crust that spreads from the characteristically curving overlapping rift zones a deep tow survey around the small 1 5 km 3 9 n offset found only normal rise flank terrain near bottom deep tow observations at the medium size 9 km 5 5 n offset and the large 27 km 2 0 n offset provided more detailed information on the formation of crust in the offset trails and of the structure of tectonically modified crust at interrift rise crest sites patches of interrift crust which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets are periodically shed from the axial zone together with abandoned portions of overlapped rift zone to become important elements of the rise flank trails the oblique azimuths of the paired fracture zone trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25 125 mm yr net movement of the 27 km 2 8 s offset has been slower with alternating periods of rapid northward and southward migration information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial east pacific rise epr whose trails were less thoroughly mapped most offsets of the southern pacific cocos boundary have migrated south and most on the northern pacific nazca boundary have migrated north the patterns are not readily explained by the inferred distribution of mantle upwellings along the epr axis

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

hydrothermal circulation anhydrite precipitation and thermal structure at ridge axes the thermal structure of the axial region of ridges spreading at 60 mm yr 1 was studied by comparing observations with numerical thermal models data from deep sea drilling project hole 504b constrain the temperature in the pillows and the upper part of the dike complex seismic reflection data constrain the depth to the magma chamber magnetic anomalies constrain the accumulation rate of pillows the thermal models assumed steady state and included the kinematics hydrothermal flow and the accumulation of pillows basalts in the successful thermal models hydrothermal fluid entered the dike intrusion zone at the ridge axis over a depth range from the base of the pillows to about 0 8 km above the top of the magma chamber the computed temperature at the axis in this depth range was nearly isothermal and suitable for the source regions of black smoker vents the computed temperature grid and assumed flow grid were used to model anhydrite precipitation and sulfate transport and thus more fully examine the implications of the flow pattern anhydrite was precipitated as seawater approached the hot axial zone the anhydrite was the carried outward with the spreading seafloor and redissolved when it reacted with pristine downwelling seawater the net effect was to deposit anhydrite near the base of vigorous hydrothermal circulation where it could not easily be leached by lower temperature off axis circulation and returned to the ocean this is a general feature of flow patterns where cold seawater vigorously enters the axial zone in broad downwellings that are not strongly heated by conduction at shallow depths alternatively the cold recharge is concentrated in the axial region and exchanges heat with nearby upwelling flow in the transition zone beneath the pillows anhydrite is then precipitated at shallow depths where it can later be leached back into the ocean the latter possibility is favored by studies of geochemical mass balances as well as the lack of evidence for a deep anhydrite layer in the oceanic crust

chemical remanent magnetization in oceanic sheeted dikes marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non vertical boundaries between normally and reversely magnetized crust the acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness a petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust odp ocean drilling program hole 504b demonstrates that these rocks carry a secondary chemical remanent magnetization crm and not a thermoremanent magnetization trm primary titanomagnetites have mostly been altered to non magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its curie temperature 600 c at ambient pressure mainly by the alteration of olivine the natural remanent magnetization nrm has been compared with laboratory anhysteretic remanent magnetization arm and trm the intensities relate nrm arm trm each by a factor of two because previous studies have shown that crms are much lower in intensity than trms and because of the common presence of secondary magnetite we conclude that the nrms of the lower hole 504b dikes are crms the dikes exhibit the same reversed magnetization as the overlying extrusive basalts and thus the timing of crm acquisition is most likely confined to the time span between the crustal age of hole 504b and the next younger field reversal i e 0 25 m y after emplacement

constraints on indian plate motion since 20 ma from dense russian magnetic data implications for indian plate dynamics we use more than 230 000 km of russian marine magnetic and bathymetric data from the carlsberg and northern central indian ridges comprising one of the most geographically extensive dense shipboard surveys anywhere in the ocean basins to describe in detail seafloor spreading since 20 ma along the trailing edge of the indian plate india somalia plate rotations for 1 myr intervals over the past 20 myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and 1400 crossings of fracture zones that offset these two ridges statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings contrary to the often made assumption that random gaussian distributed noise dominates the error budget seafloor spreading rates slowed gradually by 30 from 20 ma to 10 1 ma about a relatively stationary pole of rotation from 11 ma to 9 ma the rotation axis migrated several angular degrees toward the plate boundary modestly increasing the spreading gradient along the plate boundary india somalia kinematic data for times since 9 ma are consistent with remarkably steady motion with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data the timing and nature of changes in india somalia motion since 20 ma closely resemble those for the capricorn somalia plate pair indicating that india and capricorn plate motions are strongly coupled we speculate that the slowdown in seafloor spreading at the trailing edges of the indo capricorn composite plate from 20 ma to 10 1 ma resulted from the increasing amount of work that was needed to build topography in the himalayan collisional zone the transition to stable india somalia and capricorn somalia seafloor spreading at 10 9 ma corresponds well with the onset at 8 ma of folding and faulting across an equatorial plate boundary separating the indian and capricorn plates suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the indian and capricorn plates

small off axis volcanoes on the east pacific rise abstract a study of sea beam bathymetry and seamarc ii side scan sonar data allows a quantitative measure of the contribution of off axis volcanism to the creation of abyssal hill topography on the east pacific rise epr 9 15 n 9 50 n in order to assess the role of off axis volcanism the distribution of volcanic edifices within 35 km of the ridge was determined we measure the size and location of 55 edifices defined as local highs greater than or equal to 40 m with aspect ratios less than 2 the distribution of small volcanic edifices is notably different from that of larger volcanic constructions seamounts in that the former do not appear to be organized into discrete chains the volcanic edifices form 5 10 km off axis and are typically 40 70 m high in contrast the seamounts 200 m high measured by scheirer and macdonald 1 first form between 5 and 15 km off axis and then continue to grow in volume and size over a distance 2 4 times larger the smaller edifices make a minute contribution to crustal volume 0 02 0 03 but can cover 7 11 of the mature seafloor seamounts contribute 0 3 1 to the volume of oceanic crust and cover only 6 of the seafloor 1 2 we propose that within 10 km of the ridge seamounts and smaller volcanic edifices compete for a limited magma supply the mechanism of melt delivery to seamounts at these distances is not yet fully mature and some magma may leak in a random fashion onto the seafloor to form volcanic edifices typically less than 70 m high beyond 10 km the mechanism of melt delivery to seamounts is mature and no further formation or growth of small volcanic edifices occurs

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

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

kinematics of mantle flow beneath a fossil overlapping spreading center the wuqbah massif case oman ophiolite the wuqbah massif central oman ophiolite comprises a well preserved 2 km thick crustal unit over a 6 8 km thick mantle sequence the mantle comprises harzburgites and few dunites cut by various types of dykes with the exception of basal and late fault zones peridotites display granoblastic and porphyroclastic textures acquired by high temperature plastic deformation they locally suffered important recrystallization as shown by neoblastic shapes and arrangements macroscopically the peridotites have well defined foliations and lineations whose features allow the massif to divide into three structural domains it comprises a n110 trending central zone that suffered an anticlockwise rotation with an upwelling zone in its southwestern part bound between two ns areas sheared in right lateral conditions this peculiar geometry is thought to reflect an overlapping spreading center osc where the east and west bordering zones would define the trace in the mantle of two ridge segment terminations and the oblique n110 central zone the axial part of the overlap area it is proposed that the northward migration of the southwest branch and its concomitant ew spreading caused the deformation and rotation of the overlap zone in our osc model flow in the mantle would be continuous from one spreading center branch to the other being curved in the overlap area to accommodate the offset the mantle flow would therefore be more and more decoupled with the brittle crust when approaching the overlap zone where extensional tectonics can occur we consider that some mantle upwelling is possible due to its specific location in between two ridge segments and over the main mantle flow the distance between the two ridge axes is 15 km which means that this paleo osc can be ranked in the class of intermediate oscs illustrating a third order ridge segmentation at the scale of the ophiolite the wuqbah osc is located northwest of a major domain marked by the presence of a nw se trending propagating ridge responsible for the formation of a new oceanic crust it is suggested that the wuqbah osc formed earlier and underlines the trace of a ridge that had later migrated to the east

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

off axis volcanism at the east pacific rise detected by uranium series dating of basalts recent detailed surveys of the east pacific rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast spreading ocean ridge crests1 7 in parallel with geological and geochemical investigations it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium series disequilibria methods8 11 here we use 238u 230th and235u 231pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9 31 n on the east pacific rise most of the ages are younger than would be expected based on off axis distance and spreading rate we infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0 5 2 km outside the axial summit caldera with some volcanism occurring as far as 4 km off axis melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas

a baseline for upper crustal velocity variations along the east pacific rise at 13 n a wide aperture profile of the east pacific rise at 13 n provides data necessary to make a high resolution seismic velocity profile of the uppermost crust along a 52 km segment of ridge crest automated and objective processing steps including p analysis and waveform inversion allow the construction of models in a consistent way so that comparisons are meaningful a continuous profile is synthesized from 70 independent one dimensional models spaced at 750 m intervals along the ridge the resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability the main features are a thin low velocity layer 2 a at the top with a steep gradient to layer 2b the seafloor velocity is nearly constant at 2 45 km s 3 along the entire ridge the velocity at the top of layer 2b is 5 0 km s 10 the depth to the 4 km s isovelocity contour within layer 2a is 130 20 m from 13 to 13 20 n north of which it increases to 180 m the increase in thickness is coincident with a deviation from axial linearity deval noted by both a slight change in axis depth and orientation and in geochemistry the waveform inversion providing more details plus velocity gradient information shows a layer 2a with about 80 m of constant velocity material underlain by 150 m of high velocity gradient material putting the base of layer 2a at approximately 230 m depth south of 13 20 n and about 50 m thicker north of the deval the overall lack of variability combined with other recent measurements of layer 2a thickness along and near the axis indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity but the dynamics of emplacement the homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10 against which other observed variations in similar regimes can be compared

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge

three dimensional magnetic modeling of a propagating rift galapagos 95 30 w a combined deep tow and seabeam investigation at 95 30 w on the cocos nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems the observed 27 km offset between the axes creates an ideal situation for the application of magnetic methods the normally magnetized crust of the propagating rift tip penetrates into older crust which was created when the earth s main field was reversed as the v shaped structure of the rift clearly violates the conventional two dimensional assumptions a full three dimensional analysis is required in addition the distorting influence of over 1000 m bathymetric relief must be removed from the observed field inversions have been performed on the gridded representations of the observed magnetic field and bathymetry working in the fourier domain the result is a gridded rock magnetization distribution the inversion of the surface data covers a large area 6000 km2 and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites in general the propagating rift process appears to be much more orderly than the dying rift process the magnetic polarity transition widths are narrower and the boundaries have fewer undulations than the dying rift which appears to be quite episodic in behavior the average propagation rate is 52 mm yr compared to the average spreading half rate of 29 mm yr the locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagating rift requires about 250 000 years the inversion of the deep tow data near the seafloor provides a high resolution definition of the location of the tip of the propagating rift at 2 38 1 n 95 30 0 w

constructing the upper crust of the mid atlantic ridge a reinterpretation based on the puna ridge kilauea volcano the volcanic morphology of a number of segments of the slow spreading mid atlantic ridge mar have been reinterpreted based on our understanding of dike emplacement dike propagation and eruption at the east rift zone of kilauea volcano hawaii and its submarine extension the puna ridge the styles of volcanic eruption at the submarine puna ridge are remarkably similar to those of the axial volcanic ridges avrs constructed on the median valley floor of the mar we use this observation to relate volcanic processes occurring at kilauea volcano to the mar we now consider that volcanic features e g seamounts and lava terraces built on the flanks of the avrs are secondary features that are fed from lava tubes or channels not primary features fed directly from an underlying dike we examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there in addition deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate the volumes of individual volcanic terraces and seamounts on the puna ridge and at the mar are large 0 1 1 km3 and similar to the volumes of lava flows that are broadly distributed at the subaerial east rift zone of kilauea this striking difference in the volcanic morphology on a scale of 1 2 km producing terraces and seamounts underwater and low relief flows on land must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment we suggest that at the mar a crustal magma reservoir most likely located beneath shallow flat sections of the segment provides magma to the rift axis through dikes that propagate laterally tens of kilometers the zone of dike intrusion at least in the neighborhood of the magma body is likely narrower than the width resurfaced by flows yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes at segment ends the zone of dike intrusion is likely to be wider giving a resulting structure with a more gradual transition from lavas to dikes

relative motions of the pacific rivera north american and cocos plates since 0 78 ma we use magnetic anomaly and fracture zone crossings from 3 n 24 n along the east pacific rise to define the boundaries and relative velocities of the rigid pacific rivera north american and cocos plates since 0 78 ma and to test several hypotheses regarding the nature of deformation between these plates crossings of the 0 78 ma isochron from the pacific rivera rise north of 22 0 n are significantly better fit by the 0 0 78 ma pacific north america rotation than by a pacific rivera rotation thereby supporting a recently proposed model for accretion of the northern part of the rivera plate to north america a significant misfit of the pacific cocos rotation to crossings of anomaly ln from the east pacific rise north of 16 4 n suggests that seafloor north of 16 n moves relative to the rigid pacific or cocos plates we also find that nine crossings of the eastern rivera fracture zone and eight crossings of anomaly ln flanking the manzanillo spreading segment are fit well by a pacific rivera rotation the rivera cocos angular velocity derived from the best fitting pacific cocos and pacific rivera angular velocities predicts 19 3 mm yr 1 95 confidence limit of n s convergence near the center of the rivera cocos boundary nearby left lateral strike slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone the unambiguous kinematic and seismologic evidence for significant rivera cocos motion indicates that the absence of a well defined rivera cocos plate boundary cannot be interpreted as evidence that rivera cocos motion is slow or zero as is postulated in several previous studies the rivera cocos angular velocity predicts oblique rivera cocos convergence at the el gordo graben and beneath the manzanillo trough and southern colima graben any extension across these features since 0 78 ma thus cannot have resulted from divergence between the subducting rivera and cococs plates

the structure and paleomagnetism of north atlantic ocean layer 2 segregation vesicle determinations segregation vesicles contain solidified residual melt the segregation vesicle technique of seafloor basalt orientation is based on the assumptions that 1 the orientation of this residue is mainly gravity controlled and forms a geopetal feature and 2 rocks remain relatively stationary during the time of segregation solidification and magnetization by application of this technique it is possible to determine the horizontal cooling of certain seafloor basalts recovered in drill cores and then correct the magnetic direction for the dip of the rocks determine the true thickness of lithologic units measured perpendicular to the dip and estimate the dip direction with some degree of confidence this technique was applied to dsdp holes 332a 332b 334 395 395a and 396b which penetrated 117 to 583 m into basaltic basement crust of the north atlantic ocean segregation vesicles occur in 26 to 50 of the samples collected from each hole primarily upright dips of 4 to 41 were measured in holes 3

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies