microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

a different pattern of ridge segmentation and mantle bouguer gravity anomalies along the ultra slow spreading southwest indian ridge 15 30 e to 25 e abstract the results of a recent bathymetric and geophysical investigation of a 650 km long portion of the very slowly opening 16 mm yr full rate southwest indian ridge swir between 15 30 e and 25 e are presented bathymetry and mantle bouguer gravity anomalies mba caused by variations in crustal thickness and or crustal and upper mantle densities show different characteristics from those observed at faster spreading centers like the mid atlantic ridge mar 20 30 mm yr full rate with the exception of the du toit transform none of the ridge axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off axis traces the mba patterns associated with individual segments are much more complex than the simple circular bull s eyes lows reported along the mar while the short wavelength ridge segment length is comparable to that of the mar there is little correlation with mba amplitude and segment length and axial relief furthermore an eastward propagating magma source and an 84 km long zone of oblique spreading appears to define a fundamental boundary along the swir between two 250 300 km long sections characterized by distinctly different axial morphology and gravity signatures we interpret these results to indicate a long wavelength segmentation pattern of the underlying upwelling mantle melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes fissure eruptions within the rift valley build linear ridges defining a short wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle our results and interpretation are quite different than that predicted by extending current models of the faster spreading mar to these ultra slow spreading rates

seamount volcanism at the super slow spreading southwest indian ridge between 57 and 70 using high resolution bathymetric data of the southwest indian ridge axial valley floor 523 seamounts were identified between the atlantis ii fracture zone 57 and the rodrigues triple junction 70 we find that the seamount population has a much larger spatial density 103 per 10 3 km 2 and shorter characteristic height 48 m to the west of 62 than to the east 8 per 10 3 km 2 and 105 m respectively these densities of seamounts are much lower than those found on the mid atlantic ridge suggesting that magma production is attenuated under the southwest indian ridge compared with faster spreading oceanic ridges we propose that cooler mantle temperatures which induce lower degrees of partial melting and less magmatic supply to the ridge axis result in construction of taller and fewer seamounts between 62 and 70

three dimensional gravity and bathymetry analysis of the mid atlantic ridge between 20 n and 24 n flow geometry and temporal evolution of the segmentation bathymetry and gravity data covering 10 m y on both flanks of the mid atlantic ridge south of the kane fracture zone reveal a complex segmentation pattern well correlated with short scale temporal variations in crustal production segment and paleosegment centers are associated with crustal thickening while their extremities appear as elongated bands of thin crust subsidence rates vary between centers and extremities of a segment with higher than world average values at segmert midpoints and closer to average values at the edges this is consistent with segment centers being the site of focused mantle upwellings with associated thin lithosphere and with thick crust a pronounced circular minimum in the long wavelength of the mantle bouguer anomaly associated to a maximum in the long wavelength of the bathymetry marks the site of a mantle upwelling associated with a particularly robust segment located north of a small fracture zone 21 20 n an offset is often observed between extreme values of topography and crustal thickness resulting in a lack of isostatic equilibrium for off axis short wavelength features an obliquity between maxima of crustal thickening associated with active segments and the topographic axial domain suggests that the shift between topography and crustal structure may arise partly from a decoupling between crust and topography formation processes

upper mantle heterogeneities in the indian ocean from waveform inversion a waveform inversion method is applied to 156 love and rayleigh wave seismograms to build up a 3 d model of the shear velocity in the upper mantle beneath the indian ocean the first step of the method consists in finding for each path a radially stratified upper mantle model compatible with the love and rayleigh wave seismograms relative to that path the fundamental mode and few higher modes are modelled in the waveform inversion in a second step the models related to the different paths are used in a tomographic inversion to map the 3 d upper mantle structure the 3 d velocity model has a lateral resolution of 1000 km no significant velocity anomalies are found below 300 km although the resolution is still good continental roots are confined to the upper 300 km and low velocities below mid oceanic ridges to the upper 250 km depending on their spreading rates at shallow depths 80 km we find a positive velocity anomaly beneath the west indian ridge near the rodriguez triple junction where gravimetric and bathymetric data indicate a reduced volcanic activity at greater depth around 200 km a low velocity signature is found beneath the hot spot of reunion mauritius islands and the central indian ridge it could reflect a real connection between the two structures

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

the southwest indian ridge between 49 15 e and 57 e focused accretion and magma redistribution abstract bathymetric gravity magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow spreading southwest indian ridge swir full rate 14 mm yr between 49 15 e and 57 e this analysis reveals that the segmentation defined by along axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along axis variations of backscattering strength mantle bouguer anomaly mba and amplitude of the central magnetic anomalies cma at axial discontinuities with offsets larger than 15 km thin crust and reduced volcanic production are suggested by the occurrence of mba highs almost non existent cma and 50 lower backscattering strength relative to the segment centres by contrast smaller non transform discontinuities with offsets smaller than 15 km correspond to very weak variations or to no variation of the mba the cma or the reflectivity of the seafloor suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities these small axial discontinuities bound low relief bathymetric segments 500 700 m corresponding to weak or no mba lows amplitude 1000 m corresponding to large mba lows amplitude 30 mgal we suggest that the magma supply to these low relief segments is controlled by near surface processes such as melt migration and or crustal plumbing from adjacent high relief segments pronounced mba lows at high relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle these spreading cells are spaced by about 100 km along the swir axis we suggest that the spacing of spreading cells on slow spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow spreading ridges than on slow spreading ridges

geophysical and geochemical constraints on crustal accretion at the very slow spreading mohns ridge the composition of upper mantle and lower crustal material at very slow spreading centers cannot be reliably determined by seismic studies alone since the range of p wave velocities for serpentinized peridotites and gabbros overlap additional information provided by the major and rare earth element ree content of the basalts is useful to constrain interpretations of seismic data refraction seismic data from the very slow spreading 16 mm a full rate mohns ridge in the norwegian greenland sea yields a highly variable thin crust of 4 0 0 5 km thickness analysis of s waves suggests that layer 3 is composed primarily of gabbro containing at most a small percentage 20 of mantle material the na8 content of mohns ridge basalts suggests a magmatic crustal thickness of 4 5 km inversion of the ree concentrations yields a melt thickness of 5 km this agreement between seismic and geochemical data suggests that neither large quantities of mantle material are found in the lower crust nor is a large volume of basaltic magma frozen in the upper mantle

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

evidence from gravity anomalies for interactions of the marion and bouvet hotspots with the southwest indian ridge effects of transform offsets abstract the ultra slow spreading southwest indian ridge swir presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry using recently available satellite free air gravity and shipboard bathymetry data we obtain mantle bouguer mba and residual mantle bouguer anomalies rmba by removing from free air gravity the attractions of seafloor topography sediment thickness variations a reference crust and theoretically predicted effects of lithospheric cooling the bouvet hotspot previously observed to be associated with anomalous bathymetry and geochemistry near the bouvet triple junction has an mba axial gravity low of 100 mgal implying pronounced localized crustal thickening off axis the rmba lows along previously calculated bouvet hotspot tracks are variable in amplitude suggesting the possibilities that bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center along axis geophysical anomalies suggest that the marion hotspot has a significant effect on accretionary processes in the central portion of the swir importantly the marion axial anomaly appears to be compartmentalized between the andrew bain and discovery ii fracture zones implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented ultra slow spreading system

gravity evidence of very thin crust at the gakkel ridge arctic ocean abstract gakkel ridge the active spreading center in the arctic ocean is the slowest spreading portion of the global mid ocean ridge system total spreading rates range from 0 6 cm yr in the east where the ridge disappears beneath the laptev shelf to 1 3 cm yr in the west near greenland bathymetry and gravity surveys of four sections of the gakkel ridge were carried out in 1996 by the u s navy nuclear submarine uss pogy as part of scicex 96 in order to sample variations in seafloor morphology and gravity anomalies as a function of spreading rate the ridge axis throughout the survey area is characterized by a continuous axial rift valley similar to that observed at other slow spreading ridges the continuous rift axis suggests that well organized seafloor spreading is occurring at total spreading rates of less than 1 cm yr in three faster spreading 1 13 1 24 cm yr western survey areas located between 7 e and 54 e the gakkel ridge is deep compared with other ridge axes axial depths range between 4600 and 5100 m and ridge flanks at about 3200 m the ridge flank morphology is very blocky and is characterized by large scarps and deep fault bounded troughs very large amplitude free water anomalies with peak to trough amplitudes of 85 150 mgal are observed centered on the axis of the gakkel ridge modeling of the free water anomalies by varying the crustal thickness and average crustal density including the gravity effect of the cooling of the mantle away from the axis implies that if the average crustal density is less than 2900 kg m3 the crustal thickness must be less than 4 km the axial rift valley at the fourth survey area near 98 e where the total spreading rate is 0 99 cm yr is buried by sediments the axis in this region is associated with a continuous 70 mgal gravity minimum implying the presence of a large buried rift valley the rift flanks at 95 e are at a depth of greater than 3800 m 600 m deeper than the average depth at the gakkel ridge axis west of 60 e simple isostatic calculations suggest that the crust in this region may be vanishingly thin beneath the sediment cover these observations indicate a relationship between melt production and seafloor spreading rate at very slow spreading rates suggesting that ultra slow spreading may suppress melt production or delivery at the gakkel ridge

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

three dimensional gravity study of the mid atlantic ridge evolution of the segmentation between 28 and 29 n during the last 10 m y during the high resolution survey sara segmentation ancienne de la ride atlantique sea beam bathymetry magnetic gravity and seismic reflection data were collected on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to provide off axis information up to approximately 10 m y and to complement a detailed on axis survey carried out in 1988 and 1989 sempere et al 1990 between the atlantis and kane fracture zones a previous gravity study had revealed the existence of bulls eye shaped gravity lows centered on the axial segments and gravity highs centered on the non transform discontinuities lin et al 1990 we carried out a three dimensional calculation of the mantle bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center the off axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities after correcting for the gravity effect of lithospheric cooling away from the ridge the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry off axis bathymetry is characterized by southward pointing v shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously however the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows northward on the western flank and southward on the eastern flank the sense of offset between the gravity and bathymetric traces appears to be related to the right stepping nature of the axial discontinuities

morphology and tectonics of the mid atlantic ridge 25 27 30 s from sea beam and magnetic data high resolution sea beam bathymetry and sea surface magnetic data have been collected between 25 s and 27 30 s on the mid atlantic ridge analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three dimensional inversion of sea surface magnetic data are presented this 227 km long portion of the mid atlantic ridge is offset 42 km by the right lateral rio grande transform at 25 40 s and 9 5 km by the left lateral nontransform moore discontinuity at 26 30 s a third nontransform discontinuity the midway discontinuity exists near 26 12 s and is defined by a small displacement of the axial magnetization distribution and the disruption in the along strike continuity of ridge parallel terrain elements although the detailed survey presented in this paper extends out to only 5 to 7 m y old crust a regional compilation of magnetic data from this area by cande et al 1988 indicates that the relative positions and dimensions of the spreading cells have characterized this part of the mid atlantic ridge for at least the past 55 m y the offsets dimensions and geometries however have changed markedly because of prolonged differential asymmetric spreading between segments between and within the accretionary units that are spreading at uniform rates 35 mm yr total opening rate the morphology of the ridge axis varies dramatically from a well defined rift valley 20 25 km wide 1 1 5 km deep to an axial swell that rises 1500 m above the adjacent valley floor the axial magnetic structure is also highly variable along strike the inversion solution shows a series of generally linear short wavelength 10 km magnetization highs within the central brunhes anomaly over each of the four segments in the survey area these anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world s mid ocean spreading centers the distinct axial magnetization highs can extend out to 2 m y old crust at ridge discontinuity intersections suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks e g highly fractionated and strongly magnetized basalts enriched in iron and titanium the magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed in the case of an axial swell centered at 26 s attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and or a zone of substantial alteration taken together the bathymetric data and magnetic modeling document strong along axis gradients in crustal and upper mantle properties

timescales of beach cusp evolution on a steep megatidal mixed sand gravel beach abstract field observations are presented of the morphological evolution of beach cusps on a 1 10 slope megatidal 8 12 m range mixed sand gravel beach at the head of the bay of fundy nova scotia cusps had mean wavelengths of 3 6 m and displayed pronounced horn bay sediment size segregation with sand sized material in the bays and gravel sized material in the horns cusp occurrence was limited to the upper third of the beach face shoreline position during the tidal cycle was estimated at three minute intervals from time averaged video imagery three cusp events are examined in detail two exhibiting pronounced topographic relief and the third demonstrating sensitivity of the rate of cusp evolution to the beach surface grain size distribution forcing conditions were weak with significant offshore wave heights of 10 20 cm and peak periods of 4 7 s relict cusp morphology was inundated with the rising tide and destroyed or reworked during high tide new cusps formed during the falling tide through a combination of accretion at the horns and erosion in the embayments timescales of growth and decay were short ranging from 10 to 30 min the location and dynamics of cusp horns appeared to depend on the high water line and its location relative to any pre existing cusp morphology the apparent sensitivity of cusp formation timescales to the local grain size distribution suggests that size segregation is intrinsic to the process of mixed sand gravel cusp evolution

do layer 3 rocks make a significant contribution to marine magnetic anomalies in situ magnetization of gabbros at ocean drilling program hole 735b we estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies results from ocean drilling program hole 735b show that the drilled crustal section has a mean effective remanent magnetization of 2 5 a m olivine gabbros which make up 60 of the section have an average effective magnetization between 1 and 2 a m we used two dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface these results indicate that if polarity boundaries within oceanic layer 3 are near vertical crustal sections similar to 735b would contribute from 25 to 75 of the overlying marine magnetic anomalies at the sea surface the shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

from slow to ultraslow a previously undetected event at the southwest indian ridge at ca 24 ma changes in plate motion are thought to be recorded in the trend of fracture zones even though fracture zones provide no information about the spreading rate using newly compiled published and unpublished magnetic data from the southwest indian ridge we calculated fi nite rotation poles for a13 a8 and a6 from which we determined a 50 decrease in spreading rate from slow to ultraslow at ca 24 ma not accompanied by a signifi cant change in spreading direction this spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries finally we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca 24 ma at the global scale

a possible causal relationship between extinction of dinosaurs and k t iridium enrichment in the nanxiong basin south china evidence from dinosaur eggshells multiple distinct iridium anomalies have been observed in dinosaur eggshells in the k t boundary sections of the nanxiong basin guangdong province south china the eggshells collected at and near the fossil pollen defined k t boundary interval show iridium increases of about 19 and 28 times respectively above the background level the enrichment of other trace elements in the eggshells occurs mostly at and near the interval the distribution of ir bearing levels proves the existence of at least six ir delivering events from the latest cretaceous into the earliest paleocene the enrichment of ir and other trace elements in eggshells may have been caused by the assimilation of these elements into the dinosaur body through food and then into the eggs laid by them two types of pathological development i e variation in eggshell thickness and eggshell microstructure have been observed from the basin it seems that they occur frequently during the k t transition the physiological mechanisms producing pathologic dinosaur eggshells are evaluated in the light of homologous phenomena occurring in living birds on this basis it is concluded that the formation of pathologic dinosaur eggshells was caused by anomalous trace element concentrations the extinction of the dinosaurs in the nanxiong basin did not occur instantaneously but spread out within 250 ka with major extinction beginning at the boundary interval the cause may have been environmental poisoning and adverse changes in climatic conditions as indicated by the geochemical analyses of the dinosaur eggshells a repeating short and long term geochemically induced environmental stress adversely affected the reproductive process and contributed to the extinction of the dinosaurs c 2002 elsevier science b v all rights reserved

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

development of en echelon magmatic segments along oblique spreading ridges en echelon magmatic segments commonly develop along obliquely spreading oceanic ridges to clarify some of the dynamic aspects of this plate boundary we performed a series of thermo mechanical numerical tests when extension of oceanic lithosphere becomes oblique deformation within the axial region localizes into distinct upwelling centers temperatures are elevated in the upwelling cells which are shallow mantle features that form the new plate boundary the predicted features are similar to the axial volcanic ridges documented at mohns and reykjanes ridges and we conclude that they become the new loci of extensional deforma tion upwelling and magmatic activity these ridges suborthogonal to the plate spreading direction only develop when the axis rift zone is weak the subsegment length and spacing depend primarily on obliquity and axial width predicted crustal thickness along the subsegmented axis varies discernibly this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges

effusive and explosive volcanism on the ultraslow spreading gakkel ridge 85 e 1 we use high definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 arctic gakkel vents expedition agave to evaluate the volcanic characteristics of the 85 e segment of the ultraslow spreading gakkel ridge 9 mm yr 1full rate our seafloor imagery reveals that the axial valley is covered by numerous small volume order 1000 m3 lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm the valley floor contains two prominent volcanic lineaments made up of axis parallel ridges and small cratered volcanic cones the lava flows appear to have erupted from a number of distinct source vents within the 12 15 km wide axial valley only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site and these are associated with the oden and loke volcanic cones we model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of co2 that accumulated in possibly deep crustal melt reservoirs the energy released during explosive discharge combined with the buoyant rise of hot fluid lofted fragmented clasts of rapidly cooling magma into the water column and they subsequently settled onto the seafloor as fall deposits surrounding the source vent

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

variations of backscatter strength along the super slow spreading southwest indian ridge between 57 e and 70 e abstract simrad em12 backscatter strength data of the southwest indian ridge swir between 57 e and 70 e are used to reveal the along axis segmentation of this super slow spreading ridge the backscatter properties of different geologic domains like bathymetric highs and oblique basins within the rift valley are characterized using 66 small test sites we show that backscatter strength is higher on bathymetric swells corresponding to segment centres and lower in deep oblique basins corresponding to axial non transform discontinuities and fracture zones this contrast between segment centres and discontinuities is produced by both a thicker sediment cover and less frequent volcanic eruptions at segment ends using the model of mitchell 1993 sediments have been estimated to be 2 to 5 m thicker in these areas than at segment centres the distribution of the seamounts within the rift valley is controlling the long wavelength variations of the mean backscatter strength calculated along the axis lower densities of seamounts and thicker sediments are producing lower and heterogeneous reflectivity levels in the deepest part of the axial valley floor between 61 45 e and 63 45 e we propose that cooler mantle temperatures inducing construction of fewer volcanoes occur beneath this part of the ridge the mean backscatter strength along the swir axis decreases dramatically toward the rodrigues triple junction suggesting that volcanic production is reduced between 68 20 e and 69 20 e and that the transition from amagmatic tectonic deformation at the triple junction to new seafloor spreading occurs between 69 20 e and 70 e

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

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

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

magmatic segmentation of the northern knipovich ridge evidence for high pressure fractionation at an ultraslow spreading ridge 1 the ultraslow spreading knipovich ridge shows axial topographic highs that are associated with off axis linear arrays of seamounts that parallel the spreading direction these linear arrays suggest that the segmentation has been stable for at least 7 8 m y axial topographic highs are marked with distinct gravity anomalies suggesting that they represent thicker crust and hence volcanic segment centers dredging on the northern knipovich ridge between 76 30 and 77 50 n shows that the axial and off axial seamounts and the deeper axial rift valley between the segment centers are composed of volcanic rocks the basalts are subdivided into high intermediate and low feo groups low feo basalts with generally more enriched radiogenic isotope ratios and ree patterns were only dredged midway between segment centers whereas dredges close to the segment centers comprised high and intermediate feo basalts the variations in isotopes and ree patterns suggest that the mantle below the northern knipovich ridge is heterogeneous and the along axis variations can be explained by higher degrees of melting of this heterogeneous source close to segment centers the high and intermediate feo basalts define different mgo feo liquid lines of descents that may reflect differences in the crystallization pressures or the water contents of the magmas similar water contents of the high and intermediate feo basalts suggest that the differences in feo primarily are a pressure effect and that the intermediate feo group fractionated at higher pressure than the high feo group the high feo basalts show both low and high pressure phenocryst assemblages suggesting crystallization at high pressures 6 7 kbar followed by shallow low pressure fractionation on the basis of the new data we propose that magmas at this ultraslow spreading ridge fractionate at mantle depth except at segment centers where magma productivity occasionally is high enough to sustain shallow level magma reservoirs

tobi sidescan sonar imagery of the very slow spreading southwest indian ridge evidence for along axis magma distribution new deep tow sidescan sonar data from the southwest indian ridge reveal complex volcanic tectonic interrelationships in the axial zone of this ultra slow spreading ridge while some constructional volcanic features resemble examples documented at the slow spreading mid atlantic ridge such as axial volcanic ridges hummocky and smooth lava flows their distribution and dimensions differ markedly the largest axial volcanic ridges occur at segment centres but fresh looking volcanic constructions also occur at segment ends and in the deep basins marking the non transform discontinuities the orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes this observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes with associated fractures opening in response to magma pressure may control local emplacements of axial volcanic ridges at obliquely spreading ridges this non systematic pattern of ridge characteristics suggests an along axis variation between focused and distributed magmatic supply a model which is supported by our interpretation of low amplitude mantle bouguer anomalies calculated for the area we propose that a change of the axial segmentation pattern from two segments to the present day three segments may have introduced additional instability into the crustal accretion process

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data