a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

along axis gravity gradients at mid ocean ridges implications for mantle flow and axial morphology along axis gradients in mantle bouguer anomalies mbas were calculated for detailed gravity surveys at the southwest central and southeast indian ridges with half spreading rates of 0 9 2 3 and 3 1 cm yr respectively mba gradients at the very slow spreading southwest indian ridge are 0 4 0 7 mgal km similar to the gradients found at the slow spreading mid atlantic ridge whereas gradients at the central indian and southeast indian ridges are 0 2 0 35 mgal km the results of this study when combined with other published data show that axial mba gradients at ridge segments with an axial high are about 0 1 mgal km independent of spreading rate gradients at segments with a median valley are higher and generally decrease with increasing spreading rate the spreading rate independent mba gradient at ridge segments with an axial high represents the mantle gravity signal due to the focusing of upwelling within segments the larger gravity gradients at segments with an axial valley result from the superposition of intrasegment variations in crustal thickness on this mantle signal the consistent relation between along axis mba gradient and across axis axial morphology suggests that the axial morphology is related to the efficiency of along axis transport of melt

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

hydraulic fracture diagnostics from krauklis wave resonance and tube wave reflections abstractfluid filled fractures support guided waves known as krauklis waves the resonance of krauklis waves within fractures occurs at specific frequencies these frequencies and the associated attenuation of the resonant modes can be used to constrain the fracture geometry we use numerical simulations of wave propagation along fluid filled fractures to quantify fracture resonance the simulations involve solution of an approximation to the compressible navier stokes equation for the viscous fluid in the fracture coupled to the elastic wave equation in the surrounding solid variable fracture aperture narrow viscous boundary layers near the fracture walls and additional attenuation from seismic radiation are accounted for in the simulations we then determine how tube waves within a wellbore can be used to excite krauklis waves within fractures that are hydraulically connected to the wellbore the simulations provide the frequency dependent hydraulic impedance of the fracture which can then be used

causes of variation in fault facing direction on the ocean floor a marked variation in fault facing direction with spreading rate is observed on the flanks of mid ocean ridges on slow and intermediate rate spreading centers inward facing faults predominate whereas at fast spreading centers large numbers of both inward and outward facing faults are observed the distribution and character of inward and outward dipping faults derived from seamarc ii data from the flanks of the east pacific rise and galapagos spreading center are compared with observations from previously published data we suggest that the fault facing direction variation observed may reflect relative differences in the mean stress on potential fault planes through the oceanic lithosphere which might favor inward vs outward facing faults at different spreading rates for example due to the thermal structure of a slow spreading ridge a fault dipping toward the axis extends to shallower depths than one dipping away from the axis thus favoring the occurrence of inward dipping faults on a fast spreading ridge the difference in fault length is less so both inward and outward facing faults are likely to occur

causes and consequences of variations in faulting style at the mid atlantic ridge both volcanism and faulting contribute to the rugged topography that is created at the mid atlantic ridge mar and preserved off axis in atlantic abyssal hill terrain distinguishing volcanic from fault generated topography is essential to understanding the variations in these processes and how these variations are affected by the three dimensional pattern of mantle upwelling ridge segmentation and offsets here we describe a new quantitative method for identifying fault generated topography in swath bathymetry data by measuring topographic curvature the curvature method can distinguish large normal faults from volcanic features whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes the combination of curvature and slope information allows inward and outward facing fault faces to be mapped we apply the method to sea beam data collected along the mar between 28 and 29 30 n the fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework long linear small throw faults occur toward segment centers while shorter larger throw curved faults occur toward ends these variations reflect those of active faults within the axial valley we investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the mar in the first model only one fault is active at a time on each side of the rift valley each fault grows while migrating away from the volcanic center due to dike injection extension across the fault causes a flexural rotation of nearby inactive faults the amount of stress necessary to displace the fault increases as the fault grows when reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley this model yields a realistic terracelike morphology from the rift valley floor into the rift mountains the relief is caused by the net rotation accumulated in the lithosphere from the active faults e g 10 reached 20 km from the active fault fault spacing is controlled by lithospheric thickness fault angle and the ratio of amagmatic to magmatic extension we hypothesize that this mechanism may be dominant toward ridge segment offsets an alternative model considers multiple active faults each fault relieves stresses as it grows and inhibits the growth of nearby faults causing a characteristic fault spacing such fault interactions would occur in a region of necking instability involving deformation over an extended area this mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults we hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints causing the homogeneous abyssal hill fabric in these regions

transform zone migration implications of bookshelf faulting at oceanic and icelandic propagating ridges at a propagating ridge a migrating transform zone mtz offsets the propagating and retreating spreading axes we examine several kinematic models of mtz deformation to determine which models can generate observed seafloor fabric orientations vertical tectonics and faulting patterns within the 95 5 w galapagos propagating ridge pr system models that involve transform parallel simple shear within the mtz can fit observed seafloor fabric rotation patterns within the 95 5 w mtz however these models predict pervasive transform parallel strike slip faults within the mtz that are not observed and do not predict the dramatic shear concurrent deepening and post shear uplift that is seen in this propagating ridge system since we only measure the amount of structural rotation of the abyssal hill fabric within the mtz and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills this structural information can only determine the rotational component of the finite strain tensor thus this simple shear family of kinematic deformation models is actually only a small subset of mtz deformation patterns that can produce the same finite rotation patterns but with different amounts of shortening and or extension parallel or normal to abyssal hills there is strong seismic and surface fault break evidence in iceland that the south iceland seismic zone mtz propagates along an array of en echelon transform perpendicular strike slip faults a likely reason for this is that if deformation were accommodated through transform parallel strike slip faults then migration of the transform zone would require the creation of new transform parallel strike slip faults for each increment of transform migration instead if transform migration can be accommodated through slip on preexisting faults then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration we examine the kinematic consequences of finite mtz deformation within this generalized bookshelf faulting deformation scenario we find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95 5 w galapagos pr system vertical tectonics are an almost inevitable by product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation thus local extension within the abyssal hills of de steiguer deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated we conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a mtz since it can explain not only the structural rotations observed at the 95 5 w propagating ridge but can also relate these rotations to the uplift history recorded there

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

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

segmentation and morphotectonic variations of the central indian ridge 21 10 s 22 25 s bathymetric magnetic and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the central indian ridge cir between 21 10 s and 22 25 s volcanoes abyssal hills and steep slopes corresponding to fault scarps and to volcanic constructional slopes have been obtained by image processing methods applied to sea beam data in the survey area segmentation of the cir is similar to that of a slow spreading ridge the plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions hourglass shaped valleys are bounded by second and third order discontinuities located at local along axis depth maxima 400 700 m depth variations and marked by oblique extensional basins and en echelon jogs respectively large throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places along the axial valley the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs the morphology and structure of the rift valley reflect the three dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center abyssal hills are bounded by both inward and outward facing faults in 1 5 ma crust indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20 40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills reconstructions of the plate boundary geometry through time using abyssal hills as isochrons provide evidence to show that the nontransform discontinuity located at 21 47 s with an offset as small as 2 3 km is associated with v shaped off axis traces in the form of alignments of disrupted abyssal hill lineations these reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery three dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0 780 and 0 984 ma and suggests postaccretionary crustal block rotations within the discordant zones

robust description of statistically heterogeneous seafloor topography through its slope distribution statistical heterogeneity of abyssal hill properties is often evident in seafloor topography even under periods of relatively constant spreading direction and rate in this paper we relate the statistics of topographic slopes computed on finite spatial scales to the autocovariance function and investigate the practicality of using these functions in describing such heterogeneous abyssal hill terrains for a two dimensional homogeneous surface a direct relation exists between the sample autocovariance and the slope distributions at different spatial scales however for a heterogeneous field characterized by large transient signals the computed autocovariance estimate no longer has a clear statistical interpretation and becomes dominated by the transients in contrast the family of slope distributions can still be used to derive stable descriptors of the field slope statistics are thus useful in deriving a more robust estimate of the autocovariance than the usual sample autocovariance moreover slope statistics may also be used to derive stable estimates of quantities not measurable with the autocovariance function or power spectra such as the statistical asymmetry of features examples of the use of slope statistics and a comparison with autocovariance methods are presented we document and quantify evidence of statistical asymmetry in a region of abyssal hills in the northeast pacific and in a second example the presence of multiple lineations in a region where a fracture zone cuts through abyssal hill terrain

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

origin of overlapping spreading centers a finite element model overlapping spreading centers oscs appear to result from two ridges propagating head on toward each other the origin of the most elongated and the largest ones is almost perfectly accounted for by the stresses due to the relative elevation of the tips of the propagating ridges rather than to the elevation of the whole ridges ridge push on the other hand stresses due to the ridge push or to the forcible injection of magma at stationary centers produce wider oscs than those on the east pacific rise between 3 n and 13 n we conclude that cross sections of ridges more than a few kilometers away from spreading centers are essentially in a state of plane stress our modelling method uses the finite element method and dynamic fracture propagation the mesh is modified at every time step to minimize computational errors

seafloor slopes at mid ocean ridges from submersible observations and implications for interpreting geology from seafloor topography abstract observations from 145 submersible dives are used to create a database of mid ocean ridge scarp topography and lithology seafloor lithologies are classified into extrusives basaltic talus dykes gabbros and serpentinites and the dive locations are broadly classified according to whether they are close to transform valleys and as a function of ridge spreading rate the database is used to determine whether there is any difference in the maximum slope for each rock type which might relate to differences in rock jointing cohesion or friction properties there is a common perception that lower crustal rocks form steeper slopes than shallower crustal rocks but until now there has been little evidence to support or refute this idea from our analysis we find a tendency for gabbro and dykes to form steeper slopes than serpentinite the 90th percentile of each lithology slope distribution used as a measure of limiting slope is 43 39 and 32 for gabbro dykes and serpentinite respectively lithologic control on slope is weak however compared to overall slope variability in these mid ocean ridge settings so seafloor relief is likely to be a poor guide to underlying geology we speculate on the structure of eroding fault scarps outline the implications for attempts to infer active faults from talus ramp activity and discuss more generally factors affecting the geomorphology of mid ocean ridge slopes

effects of pleistocene environmental changes on the distribution and community structure of the mammalian fauna of mexico article i nfo biological communities in mexico experienced profound changes in species composition and structure as a consequence of the environmental fluctuations during the pleistocene based on the recent and fossil mexican mammal checklists we determine the distribution composition diversity and community structure of late pleistocene mammalian faunas and analyze extinction patterns and response of individual species to environmental changes we conclude that 1 differential extinctions occurred at family genus and species level with a major impact on species heavier than 100 kg including the extinction all proboscideans and several ruminants 2 pleistocene mammal communities in mexico were more diverse than recent ones and 3 the current assemblages of species are relatively young furthermore pleistocene relicts support the presence of biogeographic corridors important refugia existed as well as centers of speciation in isolated regions we identified seven corridors eastern usa sierra madre oriental corridor rocky mountains sierra madre occidental corridor central united states northern mexico corridor transvolcanic belt sierra madre del sur corridor western usa baja california corridor tamaulipas central america gulf lowlands corridor and sonora central america pacific lowlands corridor our study suggests that present mammalian assemblages are very different than the ones in the late pleistocene

petrology of basalts from the central indian ridge lat 25 23 s long 70 04 e estimates of frequencies and fractional volumes of magma injections in a two layered reservoir abstract mineralogy melt inclusions in crystals and reverse zoning in minerals of oceanic basalts can be used to get an insight into the functioning of a magma chamber beneath a spreading zone dredged basalts from the central indian ridge cir 21 km from the rodriguez triple junction rtj exhibit two groups of olivine and plagioclase of different composition the first group is composed of forsteritic olivine fo 90 and very anorthitic plagioclase an 89 90 resorbed megacrysts containing numerous melt inclusions those observed in the core of the plagioclase megacrysts are the most primitive their compositions have been considered as the parental magma composition mg number 0 77 low tio 2 0 31 wt and low na 2 o 1 2 1 4 wt these magmas have a density about 2 70 g cm 3 and viscosity close to 100 poises at 1250 c liquidus temperature in the diagramfeo mgo vs tio 2 mgo 100 the zigzag evolution of the melt inclusions from core to rim of these plagioclase megacrysts has been interpreted as the result of three cycles of injection of parental liquid mixing and olivine crystallization the second group of minerals is essentially composed of more evolved olivine fo 86 88 and plagioclase an 67 72 euhedral mesocrysts some olivines are reverse zoned 3 steps they crystallized from hybrid liquids resulting from three successive mixings between batches rising from the base of the magma chamber and an evolved magma at the top of the reservoir time between the mixings can be calculated from the diffusion profiles of ni in the reverse zoned olivines these three mixings have been achieved within 4 days a two layer model for the magmatic reservoir is proposed to account for the mineralogy the melt inclusions and glassy matrix compositions of those basalts an estimation of the total volume of the chamber has been made knowing the volume of extruded magmas at zero age the chamber would have a radius of 400 500 m relative volume of lower layer where the first group of minerals crystallized and the upper layer where the second group of minerals formed is around 1 3

lithospheric stresses associated with nontransform offsets of the mid atlantic ridge implications from a finite element analysis nontransform offsets are a fundamental aspect of the offset geometry exhibited along the mid oceanic ridge system independent of spreading rate along the slow intermediate opening 40 mm y full rate mid atlantic ridge these offsets of the ridge axis range in length from less than 10 km to approximately 30 km and vary in age offset from 0 5 to 2 0 m y the variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike slip tectonism and that the geometries are unstable in time in many cases there appears to be an evolutionary relationship between transform fault boundaries and nontransform offsets as the result of prolonged differential asymmetric spreading between adjoining ridge segments the finite element method is used to study the complex stress field associated with these small offset discontinuities of ridges with slow 30 mm y and fast 100 mm y total opening rates a plane stress plate model examines the variation in the horizontal tectonic stress field produced by offsets with different lengths and changes in the ratio of a ridge normal tensile stress resisting plate separation to a shear stress resisting relative plate motion along the discontinuity the predicted fault patterns based on the calculated stress field are compared with seafloor observations in terms of the morphotectonic patterns and evolution of nontransform offsets for a slow spreading rate the analysis shows that all structural geometries observed can be modeled by a range of offset lengths 5 10 20 30 and 40 km and by a ridge normal stress 3 to 5 times greater than the discontinuity shear stress these findings suggest that nontransform offsets are zones of mechanical weakness relative to the surrounding lithosphere an offset length between 10 and 20 km is predicted to be the threshold length for maintaining a transform fault geometry as inferred from ridge axis morphology there seems to be a strong link between the magnitude of the stress ratio and the time varying magmatic activity along and between ridge segments while our models are consistent with a weak discontinuity shear stress relative to the ridge normal stress to explain the geometries of nontransform offsets of slow spreading centers a weaker ridge normal stress to discontinuity shear stress most closely models the development of an overlapping spreading center geometry the distinctive geometry of nontransform offsets of spreading centers opening at fast rates this difference is attributed to magma supply along axis relatively continuous for fast spreading centers and intermittent for slow spreading centers and a preexisting zone of mechanical weakness linked to the evolution of nontransform offsets from transform faults on slow spreading centers

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

reconstruction of the flanks of the mid atlantic ridge 28 to 29 n implications for evolution of young oceanic lithosphere at slow spreading centers mar flank reconstruction

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

distributed extension at the indian ocean triple junction the indian ocean triple junction is a ridge ridge ridge type joining two medium spreading ridges with one slow spreading ridge gloria long range side scan sonar images show that while the axial valleys of the two medium spreading ridges are almost colinear apart from a small 5 km offset the valley of the slow spreading third axis does not meet the other two in a simple fashion the axis of this slow spreading southwest indian ridge swir beyond the rift valley walls of the other two ridges steadily deepens by over 1000 m away from the triple junction to reach 5000 m at 35 km the gloria images show large normal faults around the deep swir rift valley which increase in heave away from the triple junction and crosscut the abyssal hills formed at the faster spreading central indian ridge indicating that extension across the swir is distributed over a zone 10 km or more wide this zone also shows no evidence for the formation of new oceanic crust suggesting that the extension may be amagmatic near the triple junction the high relief of the swir rift flanks containing tilted seafloor of the other two ridges may be an isostatic response of the lithosphere to the deep valley produced by this rifting these observations which suggest the progressive development of a propagating swir rift by the extension of preexisting seafloor may have general implications for the dynamics of oceanic spreading centers in particular the deepening and widening of the valley away from the triple junction is attributed to the competing effects of tectonic thinning and lithospheric cooling thermal models of mid ocean ridges spreading at 30 mm yr e g lin and parmentier 1989 predict that the depth to the 700 c isotherm increases with distance from the spreading axis by a proportion similar to the observed doubling in the total width of the deformation with distance from 10 to 35 km away from the triple junction this rate of widening of the deformation zone may therefore support lithospheric necking models for oceanic rift valleys if the width and depth of the deforming lithosphere are related in a simple manner

abyssal hill characterization at the ultraslow spreading southwest indian ridge 1 the morphology of the flanks of the southwest indian ridge holds a record of seafloor formation and abyssal hill generation at an ultraslow spreading rate statistical analysis of compiled bathymetry and gravity data from the flanks of the southwest indian ridge from 54 e to 67 e provides estimates of abyssal hill morphologic character and inferred crustal thickness the extent of the compiled data encompasses a spreading rate change from slow to ultraslow at 24 ma a significant inferred variation in sub axis mantle temperature and a patchwork of volcanic and non volcanic seafloor making the southwest indian ridge an ideal and unique location to characterize abyssal hills generated by ultraslow spreading and to examine the effect of dramatic spreading rate change on seafloor morphology root mean square abyssal hill height in ultraslow spreading seafloor ranges from 280 m to 320 m and is on average 80 m greater than found for slow spreading seafloor ultraslow spreading abyssal hill width ranges from 4 km to 12 km averaging 8 km abyssal hill height and width increases west to east in both slow and ultraslow spreading seafloor corresponding to decreasing inferred mantle temperature abyssal hills persist in non volcanic seafloor and extend continuously from volcanic to non volcanic terrains we attribute the increase of abyssal hill height and width to strengthening of the mantle portion of the lithosphere as the result of cooler sub axial mantle temperature and conclude that abyssal hill height is primarily controlled by the strength of the mantle component of the lithosphere rather than spreading rate

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

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

periodicity in the accretion process on the southeast indian ridge at 27 40 s abstract image processing of a seabeam bathymetric grid on the southeast indian ridge seir at 27 40 s has been used to analyse the morphotectonics of the ridge flanks steep topographic slopes indicating inward facing or outward facing faults and those representing tilted blocks were automatically separated an autocorrelation process allows an estimation of the periodicity of the fault spacing the outward facing faults first appear at 16 18 km from the axial valley are more common on bathymetric highs and correspond to tilted block zones in adjacent deeper areas the tilting of the blocks is 4 the transformation of the rift valley into the smooth undulating relief of the flanks is accomplished by these outward facing faults and tilted blocks inward facing faults appear either as large faults with major throws or as a series of smaller faults with minor throws the minor faults are mainly localized on bathymetric highs and are densely organized the major faults continuous and symmetrically spaced on each side of the axis zone are organized with a periodicity of 8 2 km these faults seem to be old inner walls which have been moved outwards periodically during the spreading process they alternate with positive magnetic lineaments reflecting old neovolcanic zones linked to intense magmatic periods considering a mean half spreading rate of 31 km m y during the past million years this would mean the development of a major fault each 0 26 m y we propose a model for the evolution of the seir at 27 40 s controlled by a periodic succession of magmatic and tectonic episodes 1 1 faulting and volcanism are concentrated in the axial zone above the melt reservoir where the crust is weakened magmatic period outside this zone the external relief is frozen in supported by the finite strength of the plate 2 2 the axial valley is enlarged fills up with lavas and bulges due to thermal effects 3 3 at the end of the magmatic period this bulged zone collapses by thermal subsidence along lines of weakness such as the faults forming the inner walls 4 4 when the melt reservoir is empty tectonic period the processes are no longer concentrated and active faulting extends over tens of kilometres from the axis the lithosphere is thinned a new axial valley is created the flanks are uplifted and outward facing faults and block tilting are induced

along axis variations within the plate boundary zone of the southern segment of the endeavour ridge abstract the width of and along axis variations within the plate boundary zone of the southern segment of the endeavour ridge of the juan de fuca ridge system have been quantified using sea marc i sidelooking sonar imagery the sonar imagery was calibrated with video and photographic surveying to provide visual corroboration of the activity spacing and offset of faults and fissures this ridge segment contains an axial high bisected along its length by a summit depression whose width relief of its rims depth of its floor and spacing of faults and fissures vary systematically away from the midpoint of the ridge segment toward its tips the axial high and its summit depression are likened to an elongated shield volcano that is being disrupted by collapse of its crest into a widening linear caldera the observed distribution of faults and fissures could be produced by mechanisms which stretch young ocean crust which is characterized by rheological properties which vary with distance from the center of the elongated volcano reprints jhd

evidence for active normal faulting on 5 9 ma crust near hole 504b on the southern flank of the costa rica rift single channel and multichannel seismic reflection data show evidence for recent movement of faults on 5 9 ma crust near ocean drilling program odp hole 504b on the southern flank of the costa rica rift 200 km from the ridge axis these faults which are associated with n85 w trending ridge parallel basement escarpments can be traced upward into the thick overlying sedimentary section that blankets volcanic crust in this area the offset of sedimentary horizons indicates the style and history of faulting it consistently shows the downdropped side to the north signifying inward facing normal faults or grabenlike structures indicative of crustal extension perpendicular to the ridge axis although most of the movement on these faults appears to have occurred in young crust near the ridge axis many of these faults have a long history of activity with cumulative displacements of several tens of metres occurring over the past several million years these results are inconsistent with inferences from borehole stress measurements made in hole 504b that the crust in this area is in horizontal compression or with previous assumptions that crustal extension at mid ocean ridges is limited to within 10 20 km of the spreading axis although the broad zone of crustal extension on the south flank of the costa rica rift could reflect anomalous stresses within the nazca plate several independent lines of evidence suggest that the active tectonic zone of crustal extension and normal faulting at mid ocean ridges may be significantly wider than previously suspected

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

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

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

the spreading rate dependence of three dimensional mid ocean ridge gravity structure we analyze over 1300 km of high resolution along axis gravity profiles at ridges with half spreading rates ranging from 1 2 to 5 5 cm yr the results show consistently higher along axis gradients of mantle bouguer anomaly at the slow spreading mid atlantic ridge mar 0 3 1 2 mgal km than at the intermediate to fast spreading cocos nazca ridge and east pacific rise epr 0 1 0 2 mgal km the regional peak to trough amplitude of mantle bouguer anomaly is also greater along the mar 30 60 mgal than the cocos nazca ridge and the epr 10 20 mgal with increasing spreading rate the regional peak to trough amplitude of axial seafloor depth decreases from 1000 1700 m to 200 700 m 3 d numerical experiments suggest that mantle contributions to the gravity can be significant only near large offset transforms at the more commonly observed non transform offsets gravity anomalies will reflect crustal thickness variations the along axis gravity data thus indicate that the amplitude of along axis crustal thickness variation decreases with increasing spreading rate we propose that this spreading rate dependent crustal accretion style may originate in the mantle finite amplitude mantle upwelling is intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate small amplitude 3 d upwellings may occur at a fast spreading ridge but their effects on crustal thickness variations will be significantly reduced by along axis melt flows along a persistent low viscosity crustal magma chamber in contrast the large crustal thickness variations due to 3 d mantle upwellings will be maintained at a slow spreading ridge because less along axis melt flows can occur in the colder and more rigid crust there

generation of morphotectonic fabric on the mid atlantic ridge flanks 28 to 29 n implications for the limits of tectonic deformation and abyssal hill formation 1 high resolution multibeam bathymetry is used to analyze the morphotectonic fabric of a 35 000 km2 zone of the mid atlantic ridge crest between 28 and 29 n interpolated isochrons at 1 myr intervals derived from a kinematic reconstruction provide a temporal framework for the structural analysis the axial valley within the survey area is offset by three non transform discontinuities dividing the axis into en echelon segments the flanks display a predominantly ridge parallel fabric of elongate abyssal hills crosscut by several discordant zones throughout the zone faulting is the predominant factor in generating the morphotectonic fabric analysis of fault scarp distribution places the limit of active tectonic deformation between 2 and 3 ma 25 to 35 km on either side of the spreading axis fault scarp relief on the ridge flanks varies with a period of approximately 2 myr which corresponds to the wavelength of abyssal hills within the area we propose an interpretation in which abyssal hill formation occurs periodically 2 myr within the ridge mountains as the lithosphere transitions from the dynamic supported regime near the axis to the isostatic regime of the flanks at 2 3 ma 25 35 km from the axis

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

lithospheric stress near a ridge transform intersection oceanic lithosphere at a ridge transform intersection has distinct structural characteristics the spreading axis and normal fault scarps which parallel the ridge axis away from the intersection locally curve towards the transform fault this implies that the least compressive stress direction becomes oblique to the spreading direction near a ridge transform intersection this stress field may be the result of a tensile ridge axis stress resisting plate separation and a transform shear stress resisting relative plate motion a plane stress plate model with prescribed plate boundary stresses is formulated to explore this hypothesis the orientation of the least compressive horizontal stress near an intersection depends strongly on the ratio of the applied boundary stresses thus ridge axis curvature may place an important constraint on plate boundary forces

correction to reconstruction of the flanks of the mid atlantic ridge 28 to 29 n implications for evolution of young oceanic lithosphere at slow spreading centers 1 we reconstruct the flanks of the mid atlantic ridge mar between 28 and 29 n from 1 to 10 ma at intervals of 1 myr for the purpose of investigating evolution of young oceanic lithosphere morphology and its variation through time using an innovative method that combines seafloor subsidence correction with interpolated isochrons and rotation poles reconstruction results are consistent with formation of abyssal hills every 1 to 2 myr in 2 3 myr old lithosphere at the outer edge of the ridge mountains as a result of transition from dynamic regime near the axis to isostatic regime of the flanks the oblique passage of structures formed at the axial valley walls through this transition zone may play a role in the development of inside corner high bathymetry asymmetric juxtaposition of abyssal hill morphology in reconstruction is indicative of independent formation and evolution of the morphotectonic fabric on opposing flanks the two major factors affecting asymmetric ridge flank morphology are found to be sense of axial offset and fluctuation of magmatic activity at the segment scale sense of axial offset determines the relative distribution of inside and outside corner bathymetry on the flanks enhanced magma supply and associated segment propagation may contribute to half spreading rate asymmetry accretion of thicker crust and formation of larger abyssal hills on the faster spreading flank consistent alignment in reconstruction of the base of the steep walls bounding inside corner bathymetry confirms that they mark the boundaries between segments on the flanks and that the deep sediment filled basins typically identified as discordant zones are attributable to outside corner bathymetry

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process