a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

faulting pattern along slow spreading ridge segments a consequence of along axis variation in lithospheric rheology abstract we present here results from an analogue model designed to test the relative influence of along axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow spreading ridge rheologically calculated layered models are employed using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere respectively two parameters have been analysed in detail 1 the thickness of the brittle layer and 2 the viscosity of the ductile layer this study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults a decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer these results are consistent with observations along mid atlantic ridge segments for segment ends the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero offset discontinuity and on outside corners for segments bordered by a lateral offset discontinuity our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer the influence of segment length offset length and temporal variation in thermal input could explain the more or less important along axis variation in the deformation pattern observed along segments of slow spreading ridges

fast and slow spreading ridges structure and hydrothermal activity ultramafic topographic highs and ch4 output different parts of the world ridge system have quite different morphologies which reflect different constructional processes it appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust this hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes the tag 26 n and snake pit 23 n hydrothermal sites on the mid atlantic ridge mar display mn ch4 ratios in overlying seawater similar to those of nonsedimented east pacific rise sites i e about 0 2 mol l in contrast large methane anomalies with very low mn ch4 ratios of 0 005 mol l are associated with ultramafic topographic highs near 15 n close to the axis and on ultramafic walls of the rift valley the association of ultramafic bodies and ch4 anomalies in seawater indicates active serpentinization processes ch4 is produced during serpentinization according to the fisher tropsch reaction charlou et al 1991 many ultramafic rock assemblages have been sampled on the mar away from fracture zones in particular by the deep sea drilling project changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges

effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

sedimentation in the northern gulf of california after cessation of the colorado river discharge abstract the almost total elimination of sediment supply from the colorado river has produced a condition of sediment starvation in the entire northern gulf of california ngc textural and mineralogical analyses of 114 surface sediment samples collected in the area show that this situation has promoted alternate sediment sources to become more important in supplying and controlling the compositional characteristics of the sediments in the region by applying multivariate data analyses using q f ratios as well as heavy mineral and clay mineral content of samples four sedimentary provinces are identified colorado river delta province crdp concepcion river province ccrp transitional province tp and baja sonora province bsp the crdp dominates the regional distribution of sediments being mainly confined to the western side of the ngc the sediment source for this province is the deltaic structure of the colorado river sediment recycling through resuspension and reworking of the colorado delta and the shallow shelf of the upper gulf of california ugc is an important agent for supplying sediments to the ngc the ccrp reflects the mineralogical association of concepcion river while the tp contains a mineral association that is similar to crdp and ccrp but with intermediate values to these provinces this situation suggests that tp is a mixing zone between these two neighboring provinces the higher abundance of zircon and garnet in tp also suggests a clear contribution of sediment sources from the sonora desert in nw mexico clay mineral composition of these sediments indicates a greater abundance of smectite and chlorite relative to previous studies being mainly derived from eolian dust carried from the desert region of nw mexico and sw usa although eolian supply of fine grained sediments is important in the ngc colorado river delta sediments are still the largest single source of sediments to the region eolian supply of fine grained sediments is however more dominant in the southern half and along the eastern margin of the ngc the dispersion of sediments in the ngc is consistently cross basinal with a series of transport cells that follow a cyclonic circulation four depocenters were identified located all on the western side of the ngc and mainly associated to the deep basins the major effect that damming of the colorado river has had on the sedimentology of the area is that sediment dispersion is now cross basinal instead of long basinal this situation indicates that sediment dispersion at present is mainly controlled by oceanic forcing instead of fluvial processes as it was in the past

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

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

slow spreading ridge axis tectonics evidence from the lizard complex uk abstract the east coast section of the lizard ophiolite complex cornwall u k preserves a series of interrelated extensional faults and dyke sets which indicate that the formation of this section of obducted oceanic lithosphere resulted from a combination of contemporaneous structural and magmatic events three dolerite dyke sets are evident the first two sets 1 and 2 are cut by and rotated on a series of ridge parallel extensional faults and shear zones later dykes set 3 cross cut the extensional faults these data indicate that both rotation and significant thinning in the region of 40 of this part of the oceanic lithosphere occurred prior to the final period of dyke injection the thinning of the lithosphere was the result of extension along the normal faults all of the dolerite dykes show morb characteristics although the late dykes set 3 are more primitive tending to be aphyric to sparse olivine phyric and show higher cr ni and lower zr values than the typically plagioclase phyric dolerite dykes sets 1 and 2 intruded prior to the ductile shearing event the more primitive dolerites post date the tectonism and indicate the lack of a significant magma chamber following a period of amagmatic extension the data are consistent with observations at present day slow spreading ridge axes and some mafic ultramafic ophiolite sequences and provide insight into the nature of ridge related structures and the interrelationships between tectonism and geochemical evolution of the dykes and record a mechanism listric faulting to account for the presence of peridotites and gabbros commonly dredged at slow spreading centres

hydration and uplift of the oceanic crust on the mid atlantic ridge associated with hydrothermal activity evidence from boron isotopes boron isotope systematics of vent fluids from the mid atlantic ridge mar and the faster spreading east pacific rise epr reveal that at the mar 50 of seawater boron is removed from circulating fluids on the downwelling limb of the hydrothermal convection cell whereas little boron is removed at epr sites geophysical and petrographic studies suggest that downwelling fluids on the mar penetrate deep in the oceanic crust and upper mantle where boron is removed during serpentinization of peridotite the associated hydration of the oceanic crust reduces its density and modelling of the data suggests that this results in uplift leading to significant elevation of the median valley walls in the vicinity of active hydrothermal systems

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

intense ch4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15 20 n fracture zone and the mid atlantic ridge abstract as part of the fara french us program designed to study the mid atlantic ridge mar between 15 n and the azores twenty three dives with the submersible nautile were conducted during the french us faranaut 15n cruise on the eastern and western parts of the 15 20 n fracture zone ridge axis intersection south of the eastern ridge transform fault intersection nine nautile dives were made within the rift valley and along the western rift valley wall ch4 concentrations in the bottom waters reach 53 2 nmol kg along faulted zones on top and on the east flank of the ultramafic inner corner high 15 05 n 44 59 w where serpentinized rocks outcrop no 3he anomaly is associated with methane ruling out any primary mantle component fourteen dives were also made in the rift valley to the north close to the western intersection of the 15 20 n transform high ch4 anomalies up to 22 nmol kg are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop seven vertical hydrocasts carried out in the axial valley 4500 m deep show an intense ch4 anomaly with a maximum 35 8 nmol kg at 3200 m depth this ch4 concentration is among the highest found along the mid atlantic ridge charlou and donval 1993 charlou et al 1997 ch4 concentrations of 9 9 14 9 nmol kg are also present on the western wall along the 3200 m isobath the high ch4 concentrations correspond to only weak 3he anomalies this ch4 rich plume is also associated with active fault zones that expose peridotite rocks ch4 output from ultramafic outcrops on the western and eastern intersections of the 15 20 n fracture zone with the mar is believed to reflect ongoing serpentinization these results associated with many other ch4 anomalies measured in the water column above ultramafic outcrops found between 12 n and the azores most likely reflect serpentinization processes extending along the whole slow spreading mid atlantic ridge

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

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

spatial and temporal distribution of seismicity along the northern mid atlantic ridge 15 35 n 1 a detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern mid atlantic ridge between 15 and 35 n is presented two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones the seismically active zone extends 20 km to either side of the ridge axis consistent with earlier results from studies of fault morphology along the axis hydrophone recorded activity shows important variations areas with intense and persistent seismic activity stripes stand in sharp contrast to areas that lack seismicity gaps the regions of persistent activity are a new observation at mid ocean ridges in general the patterns of seismically active inactive regions are also recognized in the 28 year teleseismic record implying that these patterns are maintained at timescales between a few years and a few decades we find no simple relationship between individual segment variables e g length or trend of the segment maximum offset of discontinuities or along axis change in mantle bouguer anomaly mba and water depths and number of hydrophone recorded events there does appear to be a correlation between axial thermal structure and seismicity regions of low and high numbers of events would thus correspond to thinner hotter and thicker colder lithosphere respectively seismicity may reflect thermal structure at short timescales decadal or longer while relief and inferred crustal thickness may integrate this structure over longer periods of time order of 1 myr

a view of the lower crustal component of hydrothermal systems at the mid atlantic ridge an extensive suite of hydrothermally altered gabbros was recovered by alvin and dredging from the mid atlantic ridge in the vicinity of the eastern ridge transform intersection rti of the kane fracture zone mark where tectonic extension has provided a window into the lower crustal component of hydrothermal cells four alteration types are distinguished on the basis of metamorphic assemblage mineral composition and deformation textures a conceptual model is presented that places temperature fluid rock ratio relationships in conjunction with styles of deformation into a spatial and temporal framework exsolution of late stage magmatic fluids in the vicinity of the magma hydrothermal interface at temperatures 700 c marks the onset of fluid rock interaction in the mark gabbros kelley et al this issue with subsequent alteration involving seawater derived fluids being strongly influenced by deformation mechanisms in zones of ductile shear hydration was initiated at temperatures between 550 and 700 c and moderate fluid rock ratios elsewhere in the lower crust the onset of seawater penetration occurred at temperatures of 400 550 c and was facilitated by brittle fracturing early vein networks indicate very low initial fluid rock ratios and are interpreted as being related to the solidification of plutons later fractures indicate higher water rock ratios and may be related to the downward propagation of shallow fault systems throughout the crust fluid rock interaction ceased at temperatures 180 300 c subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the rti massif produced moderate fluid rock ratios within localized zones hydrothermal alteration in the crustal section exposed at mark was initiated at lower temperatures and water rock interaction proceeded to lower temperatures than many other plutonic suites these differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust

continuous exhumation of mantle derived rocks at the southwest indian ridge for 11 million years the global mid ocean ridge system where tectonic plates diverge is traditionally thought of as the largest single volcanic feature on the earth yet wide expanses of smooth sea floor in the easternmost part of the southwest indian ridge in the indian ocean lacks the hummocky morphology that is typical for submarine volcanism at other slow spreading ridges the sea floor can extend by faulting the existing lithosphere along only one side of the ridge axis however the smooth sea floor in the easternmost southwest indian ridge also lacks the corrugated texture created by such faulting instead the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle derived rocks here we use side scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the southwest indian ridge between 62 050 e and 64 400 e where the sea floor formed over the past 11 million years we show that the smooth floor is almost entirely composed of seawater altered mantle derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis faulting accommodates almost 100 of plate divergence and the detachment faults have repeatedly flipped polarity we suggest that this tectonic process could also explain the exhumation of mantle derived rocks at the magma poor margins of rifted continents

heterogeneous seismic velocity structure of the upper lithosphere at kane oceanic core complex mid atlantic ridge the kane oceanic core complex occ is a large corrugated megamullion that was formed by a long lived detachment fault at the axis of the mid atlantic ridge adjacent to kane fracture zone between 2 1 and 3 3 ma we use refracted arrivals recorded along a 6 km long hydrophone streamer during a multichannel seismic survey to constrain the shallow seismic velocity structure of the occ results are presented in high resolution traveltime seismic tomographic models along six lines that cover all of the main morphological features of the megamullion the models show large lateral variability in p wave velocity within the upper 0 5 2 0 km of the lithosphere and these variations correlate to first order with observed variations in lithology documented by in situ basement samples and seafloor morphology lithological interpretation of the velocity models indicates that there is marked lateral variability in distribution of gabbroic intrusions serpentinized peridotites and basalts at scales of a few kilometers to 10 km serpentinized peridotites appear to dominate the central and older parts of the occ high velocity gabbros are consistently but not exclusively present closer to the termination of the kane detachment fault and toward the ends of the occ the structure of the lithosphere exhumed by the kane detachment fault is far from the standard ophiolite based penrose model and it does not show segment centered magmatism that is commonly interpreted at slow spreading ridges if the gabbros exhumed toward the termination of the occ were emplaced deep 10 km beneath the spreading axis they may have constituted a weak zone that focused initiation of the kane detachment fault alternately as the occ footwall was being exhumed the gabbros may have been emplaced because of dynamic changes in melt supply changes in mantle fertility or decompression melting late stage volcanism is clearly associated with a major high angle normal fault that cuts the detachment surface this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall the shape of the large scale corrugated morphology of the occ is nearly invariant in the dip direction across major changes in basement lithology indicating that once established the form of the kane detachment fault was highly resistant to modification

strain localization along the atlantis bank oceanic detachment fault system southwest indian ridge microstructural observations and mineral thermometry from in situ samples collected from the atlantis bank oceanic core complex sw indian ridge indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile semibrittle and brittle regimes as strain localized along the exposed now subhorizontal fault surface ductile semibrittle and brittle fabrics are developed within dominantly gabbroic rocks footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault respectively sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910 c and 650 c by amphibole accommodated dissolution precipitation creep at temperatures 750 c 450 c by chlorite accommodated reaction softening at temperatures 450 c 300 c and by brittle fracturing and cataclasis at temperatures 300 c

non transform offsets along the mid atlantic ridge south of the azores 38 n 34 n ultramafic exposures and hosting of hydrothermal vents ten contiguous non transform offsets ntos along the mid atlantic ridge mar south of the azores between 38 n and 35 40 n have been studied in detail using swath bathymetric acoustic backscatter and deep tow high resolution sidescan sonar tobi data in contrast with discontinuities studied elsewhere at slow spreading ridges these left lateral ntos are consistently broader and larger with complex structural fabrics accommodating the offset they are characterized by a range of elevated and faulted massifs detached from their segment flanks with an irregular acoustic backscatter pattern some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks and sometimes associated with high temperature hydrothermal venting water column surveys adjacent to these massifs show high ch4 and low tdm total dissolvable manganese concentrations possibly resulting from the process of serpentinization of ultramafic rocks the correlation between the shallow dome like shaped massifs and the high concentrations of ch4 associated with low levels of mn is of particular interest to predict the outcrop of ultramafic rocks within the ntos where no geological data are available the exposure of the ultramafic massifs within the ntos is favored by low magmatic supply and low angle detachment faulting occurring at segment ends the pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high temperature vent sites

backscattered coherent noise and seismic reflection imaging of the oceanic crust an example from the rift valley of the mid atlantic ridge at 23 n seismic reflection surveys shot over oceanic crust are contaminated by source generated coherent noise that limits the identification and interpretation of subbasement reflections coherent noise arrivals are generated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface both types of arrival may be enhanced by conventional common midpoint cmp stacking some turning arrivals also exhibit negative moveout in a cmp gather due to the increase of crustal velocity with depth in unsedimented areas coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout with or without a subsequent zero offset migration prestack and suppressed but turning arrivals will usually remain turning arrivals generated by linear features in the seafloor such as fault scarps appear on a stack section as nearly linear coherent arrivals the apparent dip of which is related to the features angle to the seismic line after migration these turning wave arrivals can extend close to the topography that produced them and continue to late times leading to the possibility that such arrivals might be incorrectly interpreted as intrusions that crosscut but do not offset the moho careful identification of crustal reflections on a seismic profile along the mark area rift valley indicates that the snake pit hydrothermal field marks the northern limit of identifiable crustal reflections at least some of which appear to be related to magma injection the recent extensive volcanism of the northern mark area may well be a result of faulting caused by the stong northward growth of the southern spreading center permitting melt to rise more easily to shallow crustal levels

deformation associated with the denudation of mantle derived rocks at the mid atlantic ridge 13 15 n the role of magmatic injections and hydrothermal alteration 1 outcrops of deeply derived ultramafic rocks and gabbros are widespread along slow spreading ridges where they are exposed in the footwall of detachment faults we report on the microstructural and petrological characteristics of a large number of samples from ultramafic exposures in the walls of the mid atlantic ridge mar axial valley at three distinct locations at lat 13 n and 14 45 n one of these locations corresponds to the footwall beneath a corrugated paleo fault surface bearing in mind that dredging and rov sampling may not preserve the most fragile lithologies fault gouges this study allows us to document a sequence of deformation and the magmatic and hydrothermal history recorded in the footwall within a few hundred meters of the axial detachment fault at the three sampled locations we find that tremolitic amphiboles have localized deformation in the ultramafic rocks prior to the onset of serpentinization we interpret these tremolites as hydrothermal alteration products after evolved gabbroic rocks intruded into the peridotites we also document two types of brittle deformation in the ultramafic rocks which we infer could produce the sustained low magnitude seismicity recorded at ridge axis detachment faults the first type of brittle deformation affects fresh peridotite and is associated with the injection of the evolved gabbroic melts and the second type affects serpentinized peridotites and is associated with the injection of si rich hydrothermal fluids that promote talc crystallization leading to strain localization in thin talc shear zones we also observed chlorite serpentine shear zones but did not identify samples with serpentine only shear zones although the proportion of magmatic injections in the ultramafic rocks is variable these characteristics are found at each investigated location and are therefore proposed as fundamental components of the deformation in the footwall of the detachment faults associated with denudation of mantle derived rocks at the mar

physical modeling of slow seafloor spreading a properly scaled thermo mechanical experimental model of slow seafloor spreading is developed to investigate the mechanism of lithosphere accretion the melt of a specially fabricated hydrocarbon compositional system is cooled from above in a tank the crystallizing upper layer the lithosphere possesses semiplastic semibrittle properties horizontal tension of the layer is produced by a paddle drawn at a constant rate spreading in the model is very unstable asymmetric and involves regular jumps of the spreading center scaling up the experimental results to match seafloor spreading gives the length of the jumps of the order of 10 km and the period of the order of 105 106 years the jumps normally occur in the wall of a rift valley resulting in quasiplastic stretching of the lithosphere at this place and then in the formation of two intersecting major normal fault zones crossing the plate from some shallow depth to the base the wedge separated by these zones rises up symmetrically prior to the cessation of movement along one zone with motion only continuing along the other from this moment the deformation becomes asymmetrical the material crystallized at the base of the lithosphere is dragged along the active zone to the upper horizons of the plate and to the surface this process leads to both the nonisostatic uplifting of the central part of the pulled block and subsidence near the inlet on the surface of the fault detachment zone which forms the center of this asymmetric lithospheric accretion at a specific stage under the action of both horizontal tension and vertical nonisostatic forces the strength of the pulling block is exceeded within the rift valley the result is the next jump of the spreading center to the point of breakage and a repetition of the whole cycle the jump can also occur at other places weakened during previous cycles normally the asymmetry of the spreading alternates at a new center the dimensions of the valley as well as the overall seafloor relief are strongly dependent on the spreading rate v the lower the v value the rougher the relief the jumps of the spreading centers are not synchronous along the plate boundary moreover they can be of different types with varying distances and in opposite directions on adjacent segments of the spreading axis leading to the occurrence of transfer or accommodation zones

paleontological evidence for early exposure of deep oceanic crust on the vema fracture zone southern wall atlantic ocean 10 45 n abstract we present the results of a paleontological study of samples collected during the vemanaute cruise on the vema fracture zone southern wall where an almost complete section of oceanic crust is exposed the three studied samples are poorly lithified yellowish calcareous sediments lying horizontally over gabbros they contain abundant generally well preserved and relatively diverse tropical late neogene calcareous nannofloras and planktonic foraminiferal faunas and yield a few poorly preserved radiolarians the estimated ages are 8 8 10 ma 5 6 6 4 ma late miocene 4 5 4 2 ma early pliocene derived from deepsea correlations between microfossil zonations and magnetic stratigraphy our results are compared to theoretical ages proposed for the crust in the surveyed area this leads us to discuss the timing and location of mechanisms responsible for the exposure of the observed section and allows us to emphasize that early morphologies and structures may be preserved during the transform walls migration

tectonic evolution of 200 km of mid atlantic ridge over 10 million years interplay of volcanism and faulting we reconstruct the history of the mode of accretion of an area of the mid atlantic ridge south of the kane fracture zone using bathymetric morphology the area includes 200 km of the spreading axis and reaches to 10 ma on either side we distinguish three tectonic styles 1 volcanic construction with eruption and intrusion of magma coupled with minor faulting 2 extended terrain with abundant large offset faults 3 detachment faulting marked by extension on single long lived faults over 40 of the seafloor is made of extended terrain and detachment faults the area includes products of seven spreading segments the spreading axis has had detachment faulting or extended terrain on one or both sides for 70 of the last 10 ma in some parts of the area regions of detachment faulting and extended terrain lie close to segment boundaries regions of detachment faulting initiated at 10 ma close to the adjacent fracture zones to the north and south and then expanded away from them we discuss the complex evidence from gravity seismic surveys and bathymetry for the role of magma supply in generating tectonic style overall we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting but the relationship is not strong other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension perhaps through the lubrication of active detachment faults by the formation of low friction materials talc serpentine on detachment fault surfaces

cyclic volcanic stratigraphy in a late ordovician marginal basin west norwegian caledonides the well preserved extrusive sequence of the solund stavfjord ophiolite complex ssoc in the west norwegian caledonides enables reconstruction of the uppermost oceanic crust that developed in a marginal basin basaltic sheet flows pillow lavas and volcanic breccias are the main components of the extrusive sequence and show stratigraphic and structural evidence for a cyclic development the first stage in a volcanic cycle is characterized by high extrusion rates yielding sheet flows commonly with the thickest flow units at the base sequences of sheet flows can be correlated laterally for at least 6 5 km pillow lavas succeed the sheet flows later in a volcanic cycle with progressively smaller pillows forming at decreasing extrusion rates volcanic breccias occur towards the end of a volcanic cycle but may also occur at lower stratigraphie levels they are made generally of pillow breccias and hyaloclastites the extrusive sequence of the ssoc oceanic crust was constructed through seven volcanic cycles that resulted in stratigraphic units with thicknesses ranging from 40 to 225 m this architecture is comparable to sequences in in situ oceanic crust developed along slow to intermediate spreading ridges

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

dyke complex of the east pacific rise exposed in the walls of hess deep and the structure of the upper oceanic crust abstract detailed observations have been made of the extrusive and intrusive units of the uppermost crust emplaced at the axis of the east pacific rise during dives conducted with the submersible nautile on the walls of hess deep at four widely separated locations the same sequence was found an upper thin 100 200 m unit of extrusives pillow lavas underlain by a mixed zone of variable thickness 50 500 m averaging 200 m where extrusives are found and intrusives and massive subhorizontal layers which may represent sills the mixed zone itself is underlain by a sheeted dyke unit that may be up to 1200 m thick where dykes are subvertical i e untilted and parallel to the east pacific rise fabric this sequence may be a general feature of medium to fast spreading crust because it is similar to that observed in hole 504b of the costa rica rift and is consistent with images of the upper oceanic crust derived from seismic experiments on the axis of the east pacific rise dyke thickness is a good measure of the depth to the roof of the axial magma chamber the observed sections provide much needed ground truth for east pacific rise crust the thickness of the lava sequence and the overall thickness of layer 2 at most 1400 m appear to have been overestimated in previous studies of the oceanic crust

the oceanic crustal structure at the extinct slow to ultraslow labrador sea spreading center two seismic refraction lines were acquired along and across the extinct labrador sea spreading center during the seismic investigations off greenland newfoundland and labrador 2009 cruise we derived two p wave velocity models using both forward modeling rayinvr and traveltime tomography inversion tomo2d with good ray coverage down to the mantle slow spreading paleocene oceanic crust has a thickness of 5 km while the eocene crust created by ultraslow spreading is as thin as 3 5 km the upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading with a velocity drop of 0 5 to 1 km s when compared to paleocene upper crustal velocities 5 2 6 0 km s the overall crustal structure is similar to active ultraslow spreading centers like the mohns ridge or the south west indian ridge with lower crustal velocities of 6 0 7 0 km s an oceanic core complex is imaged on a 50 km long segment of the ridge perpendicular line with serpentinized peridotites 7 3 7 9 km s found 1 5 km below the basement the second ridge parallel line also shows extremely thin crust in the extinct axial valley where 8 km s mantle velocity is imaged just 1 5 km below the basement this thin crust is interpreted as crust formed by ultraslow spreading which was thinned by tectonic extension

a geophysical study of oceanic core complexes and surrounding terrain mid atlantic ridge 13 n 14 n 1 we describe a geophysical study of oceanic core complexes occ and surrounding seafloor on the mid atlantic ridge at 13 n 14 n and off axis to 1 9 myr data include a detailed deep towed side scan sonar magnetic field and bathymetry survey supplemented by concurrent sea surface bathymetry magnetic field and gravity measurements using side scan and bathymetry we infer areas and relative ages of seafloor volcanism revealing a complex pattern of melt accretion across the median valley including close to its walls we estimate tectonic and magmatic extension throughout the area and find that average tectonic extension since chron 2 on plates containing occs is up to three times that on their conjugates deep towed magnetic data reveal asymmetric spreading faster on occ containing plates and crustal magnetization that is highly heterogeneous on a scale of 5 km suggesting that exhumed domes of occs have highly variable lithologies perhaps comprising both serpentinized peridotite and gabbro improved fits to magnetic data are provided by models incorporating 45 of occ footwall rotation an axial zone of normal magnetization of presumed brunhes epoch has highly variable width and amplitude with parts of the ridge axis displaying very low or apparently reversed magnetization gravity requires that occs have dense cores capped by lower density zones several kilometers thick gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of occ formation and show that waxing magmatism may terminate occs

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

wavelet analysis of deep tow magnetic profiles modeling the magnetic layer thickness over oceanic ridges 1 the interpretation of marine magnetic anomalies usually consists either in determining the magnetization distribution assuming the source geometry and magnetization direction or in determining the magnetic layer thickness assuming the magnetization direction and intensity in this paper we introduce a new technique that allows modeling of the thickness of the magnetic source layer with very few a priori assumptions about the magnetization the magnetic layer is assumed to be made of a series of bodies each having a constant unknown magnetization and an unknown size this technique is based upon the application of the continuous wavelet transform recently introduced for the interpretation of potential field data as a multipole decomposition we present applications to synthetic data to one deep tow magnetic profile recorded across the juan de fuca ridge jdf and to three deep tow magnetic profiles recorded across the central indian ridge cir our results confirm that despite significant source thickness variations 100 1200 m across the cir measured magnetic anomalies mostly reflect past geomagnetic field intensity fluctuations however we show that within the axial region of high magnetization thickness variations have a significant contribution to short wavelength variations of deep tow magnetic signals 100 nt

3 d p wave velocity structure of oceanic core complexes at 13 n on the mid atlantic ridge the mid atlantic ridge at 13 n is regarded as a type locality for oceanic core complexes occs as it contains within 70 km along the spreading axis four that are at different stages of their life cycle the wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of occ development here we describe the results of p wave seismic tomographic modelling within a 60 60 km footprint containing several occs the ridge axis and both flanks which determines occ crustal structure detachment geometry and occ interconnectivity along axis a grid of wide angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean bottom seismographs was deployed approximately 130 000 first arrival travel times together with sparse moho reflections have been modelled constraining the crust and uppermost mantle to a depth of 10 km below sea level depth slices through this 3 d model reveal several independent structures each with a higher p wave velocity vp than its surrounds at the seafloor these features correspond to the occs adjacent to the axial valley walls at 13 20 n and 13 30 n and off axis at 13 25 n these high vp features display dipping trends into the deeper crust consistent with the surface expression of each occ s detachment implying that rocks of the mid to lower crust and uppermost mantle within the footwall are juxtaposed against lower vp material in the hanging wall the neovolcanic zone of the ridge axis has systematically lower vp than the surrounding crust at all depths and is wider between occs on average throughout the 13 n region the crust is 6 km thick however beneath a deep lava floored basin between axial occs the crust is thinner and is more characteristically oceanic in layering and velocity depth structure thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation while modelling of the sparse moho reflections suggests the crust mantle boundary is a transition zone throughout most of the 13 n segment our results support a model in which occs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each occ suggesting a geometry and mechanism of faulting that is more complicated than previously thought the steepness of the northern flank of the 13 20 n detachment suggests that it represents a transfer zone between different faulting regimes to the south and north we propose that individual detachments may not be linked along axis and that occs act as transfer zones linking areas of normal spreading and detachment faulting along ridge variation in magma supply influences the nature of this detachment faulting consequently not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive but also how when and where new occs are created

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

hydrothermal upflow zones in the oceanic crust abstract breccias recovered from the mid atlantic ridge south of the kane fracture zone mark and the east pacific rise at hess deep record distinct alteration signatures associated with the upwelling of hydrothermal fluids in the shallow oceanic crust two breccia types have been defined based on the mineralogy of the matrix type i breccias have a matrix characterized by fe rich chlorite quartz pyrite and anatase type ii breccias exhibit a matrix consisting of mg rich chlorite epidote quartz pyrite and titanite fluid inclusion analyses indicate a similar range in homogenization temperatures for the two breccia types 150 375 c but distinct ranges in salinity type i breccias have salinities greater than that of seawater 3 3 10 wt nacl whereas type ii breccias display much lower salinities mineral stability relationships coupled with the fluid inclusion data suggest that the type i breccias were produced by hydrothermal fluids relatively enriched in cl and depleted in h2s whereas the type ii breccias were associated with hydrothermal fluids relatively depleted in cl and enriched in h2s these chemical trends are consistent with those displayed by hydrothermal fluids actively venting on the seafloor thus we propose that the observed variability in the chemistry of modern submarine vent fluids is paralleled by distinct alteration assemblages in the upflow zones below the seafloor as defined by the type i and type ii breccia suites based on these results we predict that the relatively h2s depleted vent systems at tag southern juan de fuca and 11 13 n along the east pacific rise should be characterized by upflow zones with a mineralogy similar to the type i breccias conversely the relatively h2s enriched vent systems at 21 n and 11 13 n along the east pacific rise are likely associated with upflow zones that have a mineralogy similar to the type ii breccias as more fossil upflow zones in the oceanic crust are identified these results could be used to define temporal variability of upwelling fluid chemistry at specific ridge sites on a geologic time scale

comparison of the shapes and sizes of seafloor volcanoes on earth and pancake domes on venus the fact that flat topped pancake domes observed on venus resemble flat topped volcanoes on earth s seafloor suggests that volcanic processes on the seafloor and on venus may operate to construct volcanic forms with similar characteristics to test this the shapes and size distributions of terrestrial submarine volcanoes seamounts from several magmatic and tectonic provinces are quantitatively compared with those of venusian pancake domes seamount heights and diameters are highly correlated suggesting that there may be a universal mechanism for their growth the 2014 point sample mean of the height to diameter ratio is d 0 10 0 04 in contrast heights and diameters of 52 pancake domes exhibit no obvious trend and values of d are as much as three orders of magnitude smaller with a 52 point sample mean of d 0 02 0 03 seamounts with basal diameters db 10 km have flatness values ratio of top to bottom diameter that span the entire range between 0 pointed cone and 1 truncated cone larger sized seamounts tend to have small values of flatness and more pointed tops this is not true for pancake domes which show large flatnesses at all sizes additionally seamounts have distinctive profiles with their flank slopes increasing in gradient toward the summit flank slopes of pancake domes however commonly increase in gradient toward the base with respect to sizes the diameter distributions of the different seamount populations are consistent with a model in which the number of seamounts decreases exponentially with increasing diameter the diameter distribution of 126 pancake domes by contrast is more uniform its mean is db 22 13 km this size is at least an order of magnitude larger than the estimated characteristic diameters of the seamount populations based on the above results i conclude that seamounts have significantly different shapes and size distributions than venusian pancake domes that have been identified and measured to date this suggests that one or more of the variables e g magma composition crystallinity magma volume effusion rate cooling rate pre existing topography governing their formation differs in terms of shapes and size distributions the small db 20 km volcanic edifices first identified from venera data are good morphologic matches to seamounts and should be the focus of future work investigating the formation of comparable volcanic features in these two different planetary environments

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

uranium series age constraints on lavas from the axial valley of the mid atlantic ridge mark area abstract mass spectrometric measurements of 230 th 226 ra 235 u 231 pa and 238 u 230 th disequilibria are used to determine eruption ages for four mid ocean ridge basalts from the median valley of the mid atlantic ridge south of the kane fracture zone mark area three samples were collected across axis on the axial volcanic ridge i e the neovolcanic ridge near the snake pit hydrothermal mound and one sample was collected near the crest of serocki volcano 50 km south of snake pit 226 ra 230 th and 230 th 238 u activity ratios are low and uniform for all four samples while 231 pa 235 u activity ratios are elevated and somewhat more variable age constraints suggest that these lavas from the most robust volcanic edifices in the mark area are 10 000 20 000 yr old the age data are used to evaluate the efficacy of commonly used age estimate scales based on qualitative indicators e g sediment cover glass quality and to begin to quantify the temporal and spatial dependence of volcanic tectonic and hydrothermal processes at slow spreading oceanic ridges

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

three dimensional geometry of axial magma chamber roof and faults at lucky strike volcano on the mid atlantic ridge we present results from three dimensional 3 d processing of seismic reflection data acquired in june 2005 over the lucky strike volcano on the mid atlantic ridge as a part of the seismic study for monitoring of the mid atlantic ridge survey we use a 3 d tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time migrated seismic volume and provide 3 d geometry of the axial magma chamber roof fault reflectors and layer 2a gradient marker we also generate a high resolution bathymetric map using the seismic reflection data the magma chamber roof is imaged at 3 4 0 4 km depth beneath the volcano and major faults are imaged with dips ranging between 33 and 50 the magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10 of the total plate separation shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces

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

three dimensional seismic structure of the dragon flag oceanic core complex at the ultraslow spreading southwest indian ridge 49 39 e the southwest indian ridge swir is an ultraslow spreading end member of mid ocean ridge system we use air gun shooting data recorded by ocean bottom seismometers obs and multibeam bathymetry to obtain a detailed three dimensional 3 d p wave tomographic model centered at 49 degrees 39 e near the active hydrothermal dragon flag vent results are presented in the form of a 3 d seismic traveltime inversion over the center and both ends of a ridge segment we show that the crustal thickness defined as the depth to the 7 km s isovelocity contour decreases systematically from the center similar to 7 0 8 0 km toward the segment ends similar to 3 0 4 0 km this variation is dominantly controlled by thickness changes in the lower crustal layer we interpret this variation as due to focusing of the magmatic activity at the segment center the across axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting a locally corrugated oceanic core complex dragon flag occ on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient we infer that this occ may be predominantly made of gabbros we suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections hydrothermal activity at the dragon flag vents is located next to the detachment fault termination we infer that the detachment fault system provides a pathway for hydrothermal convection

detachment shear zone of the atlantis massif core complex mid atlantic ridge 30 n near bottom investigations of the cross section of the atlantis massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex integrated direct observations sampling photogeology and imaging define a mylonitic low angle detachment shear zone dsz along the crest of the massif the shear zone may project beneath the nearby corrugated upper surface of the massif the dsz and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme localized tectonic extension during seafloor spreading over the past 2 m y the dsz is characterized by strongly foliated to mylonitic serpentinites and talc amphibole schists it is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction the dsz foliation arches over the top of the massif in a convex upward trajectory mimicking the morphology of the top of the massif kinematic indicators show consistent top to east toward the mar axis tectonic transport directions foliated dsz rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop scale foliation the dsz and underlying basement rocks are cut by discrete anastomosing normal slip shear zones widely spaced steeply dipping normal faults cut all the older structures and localize serpentinization driven hydrothermal outflow at the lost city hydrothermal field a thin few meters sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the dsz and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present gently dipping orientation during lateral spreading and uplift

time series studies of vent fluids from the tag and mark sites 1986 1990 mid atlantic ridge a new solution chemistry model and a mechanism for cu zn zonation in massive sulphide orebodies

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

spreading induced faulting and fracturing of oceanic crust examples from the sheeted dyke complex of the troodos ophiolite cyprus

a possible triple junction at 14 50 s on the north fiji basin ridge southwest pacific auzende j m grficia mont e bendel v huchon p lafoy y lagabrielle y de alteriis g and tanahashi m 1994 a possible triple junction at 14 50 s on the north fiji basin ridge southwest pacific in j m auzende and t urabe editors north fiji basin sxarner french japanese program mar geol 116 25 35 recent full coverage bathymetric and geophysical surveys yokosuka 90 and 91 carried out in the framework of the starmer japanese french joint project reveal details of the structure of the n160 segment of the north fiji basin ridge despite its intermediate spreading rate this segment shows a slow spreading type morphology with steep 1000 m high walls reaching an axial depth of 4000 m its northern tip at 14 50 s is a complex area involving several plates boundaries this domain can be interpreted either as a complex boundary between two plates or as a new triple junction symmetrical with the 16 50 s triple junction characterizing its southern tip in either case the complexity of the spreading ridge geometry in this area illustrates the instability of the accretionary geometry due to the deformation of the whole north fiji basin between the much larger indo australian and pacific plates

crustal transect from the north atlantic knipovich ridge to the svalbard margin west of hornsund the crustal structure along a 312 km transect stretching from the axial mountains of the north atlantic knipovich ridge to the continental shelf of svalbard has been obtained using seismic reflection data and wide angle obs data the resulting seismic vp and vs models are further constrained by a 2 d gravity model the principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow spreading counterpart of the north atlantic ridge systems vp is estimated to be 3 50 6 05 km s for the upper oceanic crust oceanic layer 2 with a marked increase away from the ridge the measured vp of 6 55 6 95 km s for oceanic layer 3a and 7 10 7 25 km s for layer 3b both with a vp vs ratio of 1 81 except for slightly higher values at the ridge axis does not allow a clear distinction between gabbro and mantle derived peridotite 10 40 serpentized the thickness of the oceanic crust varies a lot along the transect from the minimum of 5 6 km to a maximum of 8 1 km the mean thickness of 6 7 km for the oceanic crust is well above the average thickness for slow spreading ridges 10 mm year half spreading rate the areas of increased thickness could be explained by large magma production rates found in the zones of axial highs at the ridge axis which also have generated the off axial highs adjacent the ridge we suggest that these axial and off axial highs along the ridge control the lithological composition of the oceanic crust this approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments the axial and off axial highs and mantle derived peridotite outside these zone

plagioclase wehrlites and peridotites on the east pacific rise hess deep and the mid atlantic ridge dsdp site 334 evidence for magma percolation in the oceanic upper mantle abstract textural and petrological data are presented that record the formation of plagioclase bearing wehrlites and peridotites at slow spreading and fast spreading ridges that are comparable to the wehrlites and peridotites formed in ophiolitic complexes evidence is provided for locally pervasive magma percolation in the oceanic residual upper mantle the samples studied come from the east pacific rise epr hess deep and the mid atlantic ridge mar dsdp site 334 at both sites parts of the peridotites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine and subidiomorphic spinel crystals petrofabric data for olivine suggest a magmatic origin for the hess deep wehrlite its phase chemistry is comparable to intrusive plagioclase wehrlites and lherzolite from ophiolites that are thought to have crystallized from a crystal mush it differs drastically from the associated hess deep diopside bearing harzburgites and dunites which are quite similar to oceanic and ophiolitic residual peridotites the epr wehrlite is therefore considered as an intrusive rock the leg 37 plagioclase peridotites display a weak lattice fabric of olivive which may have resulted from high temperature plastic flow these rocks have a phase chemistry that is typical of ophiolitic and oceanic residual rocks these rocks are hence interpreted as residual dunite or harzburgite which has been pervasively impregnated by a melt from which the clinopyroxene and plagioclase crystallized these peridotites could have an intrusive origin or may have formed in situ at the crust mantle transition in association with intrusions of large gabbro bodies

magnetic signatures of serpentinization at ophiolite complexes we compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes pindos greece oman chenaillet france and the mid atlantic ridge near the kane fracture zone mark the pindos and oman sites show low susceptibility and remanence k 250 3008c occurring at the axis i e chenaillet similar to serpentinites from magmatically poor mid ocean ridges from lower temperature serpentinization 200 2508c likely occurring off axis and possibly during obduction i e pindos and oman at both settings serpentinization can result in significant hydrogen release

emplacement of deep crustal and mantle rocks on the west median valley wall of the mark area mar 23 n abstract exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow spreading mid atlantic ridge nine dives of the submersible nautile explored the western axial valley wall in the northern cell of the mark area mid atlantic ridge kane fracture zone where peridotite and gabbro outcrops had been previously reported in order to constrain the structure and determine emplacement mechanisms the ridge transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl locally overlain by metabasalts and metadolerites capped by slightly weathered basalts the morphology is controlled by ridge parallel faults dipping moderately 40 65 to the east transform parallel scarps present in the northernmost dives become rare toward the south brittle deformation along moderately dipping fault scarps produced a dense microcrack network filled with greenschist facies minerals chlorite actinolite epidote quartz locally overprinting high temperature ductile deformation fabrics the small hill located at 23 20 in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl above 3100 mbsl the summit of the hill is composed of pillow basalts and sediments the peridotite outerops display a strong schistosity dipping 20 40 to the east parallel to striated normal fault planes some steeper east facing fault scarps truncate the lower dipping fault surfaces the serpentinites deriving from a harzburgitic protolith are cut by rare dikelets of highly differentiated gabbro these new data combined with previous results of alvin dives are used to draw a generalized geological map of the western axial valley wall this map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the mark area the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets low angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south

impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust one of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust layer 2a appears to thin and eventually disappear as its seismic velocity doubles from approximately 2 2 to 4 5 km s 1 in the first 40 m y the most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals a by product of off axis hydrothermal circulation and alteration of basalts new insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases a qualitative model of layer 2a crustal evolution is outlined that satisfies many geophysical and geochemical observations some seemingly unrelated somewhat surprisingly the crustal age at which layer 2a seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment which in turn controls the hydrologic regime necessary for secondary mineral precipitation speculative predictions can be used to verify refine or invalidate this dynamic model of crustal evolution major scientific and technical obstacles must be overcome before a definitive model can be quantified with such a rigorous model of crustal upper crustal evolution in hand remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state composition and past history of oceanic crust

ultramafic and gabbroic exposures at the mid atlantic ridge geological mapping in the 15 n region abstract the outcrops of mantle derived ultramafic rocks in the 15 n region are the most extensive yet reported for the mid atlantic ridge north of the fifteen twenty fracture zone these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall ultramafic rocks also crop out extensively south of the fifteen twenty fracture zone based on dredging and on a morphological analysis of the bathymetric map we propose that ultramafic outcrops may be common in the crust formed between 14 30 n and 15 50 n during at least the past 2 4 m y moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops diving observations also show that the ultramafic rocks are capped in stratigraphic contact by a thin layer of basalt this suggests that these rocks were tectonically emplaced at the axial seafloor or very close to it then uplifted in the footwall of the faults that bound the axial valley the occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region instead of one master shear zone there would be a complex array of cross cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds

fault structure and detailed evolution of a slow spreading ridge segment the mid atlantic ridge at 29 n we present preliminary results of a detailed near bottom study of the morphology and tectonics of the 29 n broken spur segment on the slow spreading mid atlantic ridge using principally the tobi deep towed instrument the survey covered two thirds of the segment length including all of its southern non transform boundary and extended off axis of 40 km 3 3 ma on either side we obtained nearly complete near bottom sidescan sonar coverage and deep towed three component magnetic observations along 2 km spaced e w tracks sidescan data reveal new details of fault structure and evolution faults grow by along axis linkage in the inside corner they also link in the axis normal direction by curving to meet the next outer older fault this leads to wider spaced faults compared to segment centre or outside corner outward facing faults exist but are rare the non transform offset is characterised by faults that are highly oblique not parallel to the spreading direction and show cross cutting relations with ridge parallel faults to the north suggesting along axis migration of the offset almost all volcanic activity occurs within 5 km of the axis most fault growth is complete within 15 km of the axis 1 2 ma though large scarps continue to be degraded by mass wasting beyond there crustal magnetisation is strongly three dimensional the current neovolcanic zone is slightly oblique to earlier reversal boundaries and its magnetisation rises to a maximum of 30 a m 1 near its southern tip the central magnetisation high tapers southwards and is asymmetric with a sharp western but gradual eastern boundary we infer a highly asymmetric accretion of layer 2 near the segment end older magnetic anomalies are kinked and sometimes missing we interpret these observations as evidence of a rapid 18 km southward migration of the segment boundary during the past 1 8 ma and present a series of reconstructions illustrating this tectonic history

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

geochronology of tag and snakepit hydrothermal fields mid atlantic ridge witness to a long and complex hydrothermal history geochronological studies of a large number of precipitates from the tag hydrothermal field and of few samples from snakepit hydrothermal field of the mid atlantic ridge show intermittent repeated hydrothermal events at both sites 210pb pb and 230th 234u measurements of sulfides iron and manganese oxides and 14c measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields the tag field shows intermittent activity over the past 120 000 years as evidenced by ages of low temperature mn oxides the presently active black smoker mound first formed about 40 000 50 000 years ago with precipitation of massive sulfides it has had intermittent pulsed high temperature activity every 5000 6000 years over the past 20 000 years which may reflect renewed magmatic activity at the ridge axis fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of n s ridge parallel lystric normal faults and an e w transform fault periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high temperature event the snakepit field had an initial event about 4000 years ago probably shortly after the eruption of the volcanic ridge on which it sits a recent renewal still presently active was probably initiated by recent fissuring of the volcanic pile

abyssal peridotite mylonites implications for grain size sensitive flow and strain localization in the oceanic lithosphere abstract microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation fabrics associated with deformation processes at slow spreading mid ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations the most abundant class are medium to coarse grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere these tectonites give geothermometric temperatures of 755 c or higher interpreted to represent lower temperature limits for diffusive exchange in coarse grained abyssal peridotites during cooling this conclusion is consistent with flow laws for olivine at these temperatures the second class of abyssal peridotites previously largely undescribed for the mid ocean ridge environment include fine grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle plastic transition extends into mantle rocks these mylonites give temperatures of 600 c which we suggest represent a lower temperature limit for plastic deformation reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse grained tectonites based on flow laws for olivine we suggest that deformation in some fine grained mylonites occurred by diffusion creep down to 600 c rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature we conclude that a reduction in grain size by cataclasis or dynamic recrystallization resulted in a transition in deformation mechanisms from dislocation to diffusion creep during uplift and or cooling the observation of these fine grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep weak faults may promote the development of long lived detachments in the upper mantle this inference supports mid ocean ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge axis discontinuities are exhumed along long lived detachment faults

ephemeral magma chambers in the trinity peridotite northern california abstract the trinity massif comprises the major lithologies of an ophiolite as defined at the 1972 penrose conference previous studies have shown however that it differs from the semail oman or table mountain newfoundland ophiolitic massifs particularly because its crustal section is thin and because its mantle section has vertical plastic flow planes these features have led to an interpretation of the trinity massif as a fragment of slow spreading oceanic lithosphere le sueur et al 1984 boudier and nicolas 1985 in this paper we show that the trinity gabbros occur in discontinuous kilometre sized pockets intrusive into the mantle peridotites the internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short lived magma chambers these ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle when it had cooled down to lithospheric temperatures we discuss the possibility that these small and ephemeral magma chambers formed at a slow spreading oceanic ridge

high resolution bathymetry reveals contrasting landslide activity shaping the walls of the mid atlantic ridge axial valley 1 axial valleys are found along most slow spreading mid ocean ridges and are one of the most prominent topographic features on earth in this paper we present the first deep tow swath bathymetry for the axial valley walls of the mid atlantic ridge these data allow us to analyze axial valley wall morphology with a very high resolution 0 5 to 1 m compared to 50 m for shipboard multibeam bathymetry revealing the role played by landslides slow spreading ridge axial valleys also commonly expose mantle derived serpentinized peridotites in the footwalls of large offset normal faults detachments in our map of the ashadze area lat 13 n ultramafic outcrops have an average slope of 18 and behave as sliding deformable rock masses with little fragmentation by contrast the basaltic seafloor in the krasnov area lat 16 38 n has an average slope of 32 and the erosion of the steep basaltic rock faces leads to extensive fragmentation forming debris with morphologies consistent with noncohesive granular flow comparison with laboratory experiments suggests that the repose angle for this basaltic debris is 25 we discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes we propose that along axial valley walls where serpentinized peridotites are exposed by detachment faults mass wasting results in average slopes 20 even in places where the emergence angle of the detachment is larger

the density structure associated with oceanic crustal rifting at the hess deep a seafloor and sea surface gravity study abstract seafloor and sea surface gravity measurements provide new constraints on the lithospheric structure of the hess deep at the triple junction between the pacific cocos and nazca plates as the cocos nazca ridge propagates westward breaking into young oceanic crust created at the east pacific rise rocks attributed to the base of the crust or to the upper mantle are exposed on the seafloor at hess deep the tip of the propagator seafloor gravity data were collected along two north south profiles the western profile runs through the deepest part of the hess deep valley and the eastern profile runs over the tip of the cocos nazca neovolcanic ridge shipboard gravity track lines provide coverage of the rift valley from the northern to the southern rim bouguer gravity anomalies are negative over the deepest part of the basin and positive north of it at the intra rift ridge where gabbros and hartzburgites have been drilled seafloor and sea surface gravity data are modeled together in a 2d stochastic inversion and a 3d ideal body inversion when interpreted in the light of geologic data gravity results favor an asymmetric emplacement mode for the deep origin rocks the alteration of the mantle material playing a key role in the gravity signal in addition new magma is emplaced from the east forming a neovolcanic ridge over strongly serpentinized peridotite the density distribution shows that positive buoyancy cannot be the driving force for magma ascent through the upper crust

lithospheric strength and rheological stratification at mid ocean ridges lithosphere at mid ocean ridges is rheologically stratified the extensional failure of rheologically stratified lithosphere can be modelled by integrating the appropriate failure criteria over the appropriate depths to the base of the lithosphere this integrated lithospheric strength is the force per unit length required for whole lithospheric failure to occur oceanic spreading centres are in a nearly constant state of whole lithospheric failure lithosphere at a given ridge axis has a characteristic integrated strength which is met for spreading to continue variations in integrated strength have important implications for tectonic phenomena such as jumps at spreading centres

accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading the magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes this load bends down the lithosphere and additional accretion forms a plate with concave upward curvature the topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate model predictions are consistent with the shape of the axial highs at fast spreading centers typically 400m high and 20 km wide as well as related gravity anomalies this model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis seismic data appear consistent with this lithospheric structure the magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis according to this model abandoned fast spreading centers should retain an elastically supported axial high while for previous models abandoned highs should collapse

observation of sections of oceanic crust and mantle cropping out on the southern wall of kane fz n atlantic the objective of the 20 nautile dives of the recent kanaut cruise was to study the southern wall of the kane fracture zone from its eastern intersection with the mid atlantic ridge mar to 5 myr in age the geological mapping shows four successive massifs wrench faulted and slightly tilted the transform facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites gabbros and basalts the entire crustal exposure is cataclased and metamorphosed to greenschist facies

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

seismic structure of an oceanic core complex at the mid atlantic ridge 22 19 n we present results from a seismic refraction and wide angle experiment surveying an oceanic core complex on the mid atlantic ridge at 22 19 n oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections tomographic inversion of our seismic data yielded lateral variations of p wave velocity within the upper 3 to 4 km of the lithosphere across the median valley a joint modeling procedure of seismic p wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle a gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks caused by un roofing lower crust rocks under the core complex crust is approximately 40 thinner than in the median valley and under the conjugated eastern flank clear pmp reflections turning under the western ridge flank suggest the creation of a moho boundary and hence continuous magmatic accretion during core complex formation

crustal structure of the mid atlantic ridge south of the kane fracture zone from seafloor and sea surface gravity data seafloor and sea surface gravity data are inverted together to construct a model for the near axis crustal structure of a slow spreading ridge the seafloor data set offers two main advantages it allows us to recover shorter wavelengths signal and to constrain the value of a potential field at two different levels the model we propose here would not have been derived from sea surface data alone it is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the mid atlantic ridge at kane mark area during the hydrosnake 1988 and gravinaute 1993 cruises the primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge first a forward two dimensional 2 d model of the crustal structure across the axis is fit to observed gravity anomalies using constraints from geological and structural observations bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling this forward model is the starting point of a 2 d monte carlo inversion of seafloor and sea surface data in addition to the crustal thickness variations along axis our data document the amplitude variations of the crustal thickness and or its density in the across axis direction the model resulting from our inversion exhibits several features of the crustal structure in the mark area 1 the presence of a low density ap 300 50 kg m 3 body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber or more probably to a highly hydrothermally fissured zone 2 both long and short wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain the mean value and the amplitude of bouguer anomalies are higher on the western part this difference suggests that axial processes in this area are very asymmetric 3 abyssal hills are not associated with a single gravity signature for instance on the west side of the axis one of the explored hills has no bouguer anomaly and is interpreted as a neovolcanic ridge whereas the others are associated with a shifted bouguer anomaly high and are interpreted as having thinner magmatic crust 4 the last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops in the mark area we find that pink hill a topographic high where serpentinized peridotites are outcropping is much more serpentinized on its east flank toward the axial valley than on its west flank alteration occurring mainly by fluid circulation through faulted zones the asymmetric serpentinization suggests that deep origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism

hess deep rift valley structure from seismic tomography we present results from a seismic refraction experiment conducted across the hess deep rift valley in the equatorial east pacific p wave travel times between seafloor explosions and ocean bottom seismographs are analyzed using an iterative stochastic inverse method to produce a velocity model of the subsurface structure the resulting velocity model differs from typical young fast spreading east pacific rise crust by approximately 1 km s with slow velocities beneath the valley of the deep and a fast region forming the intrarift ridge we interpret these velocity contrasts as lithologies originating at different depths and or alteration of the preexisting rock units we use our seismic model along with petrologic and bathymetric data from previous studies to produce a structural model the model supports low angle detachment faulting with serpentinization of peridotite as the preferred mechanism for creating the distribution and exposure of lower crustal and upper mantle rocks within hess deep

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

detailed geological mapping of two contrasting second order segments of the mid atlantic ridge between oceanographer and hayes fracture zones 33 30 n 35 n we present detailed geological mapping of the axial valley floor and ridge flanks of two neighboring but contrasting spreading segments oh1 and oh3 of the mid atlantic ridge between oceanographer and hayes fracture zones new in situ observations from the submersible nautile correlated with swath bathymetry and acoustic backscattering data from these segments reveal that the along and across axis volcanic tectonic variability within second order segments is larger than commonly acknowledged segment oh1 is a long and robust segment with a narrow and shallow axial valley the most intense magmatic activity is found at the segment center where the thickest crust has been imaged suggesting focused magma supply away from this part of the segment faulting and sedimentary cover predominate in contrast the center of segment oh3 is dominated by extensive sedimentary cover fissuring and faulting furthermore the most recent constructional volcanism is located away from the segment center in a region of greater depth and thinner crust this relocation of volcanism suggests either a recent shift in the magmatic source or the last vestige of a centrally located source fed by lateral dike injection segment tip magmatic oscillations are suggested by the distribution of rock types at both segment ends serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersection with nontransform offsets ntos whereas only basaltic rocks crop out on the nodal basin floors we suggest that the combination of low magmatic budget and extension taking place at the ntos during a segment retreat favors the uplift and exposure of ultramafic massifs

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

three dimensional analysis of gravity anomalies in the mark area mid atlantlc ridge 23 n we have carried out a three dimensional analysis of the gravity field over the mid atlantic ridge at kane mark area a well studied portion of the mid atlantic ridge south of the kane fracture zone between 23 40 n and 22 30 n the mid atlantic ridge in this area is composed of two distinct spreading segments separated by a small nontransform discontinuity at 23 15 n the mantle bouguer anomalies in the mark area are dominated by v shaped bands of anomalous mass and small amplitude gravity lows and highs located both on and off axis which indicate that the anomaly distribution is more complex than the simple bulls eye pattern described along some other parts of the mid atlantic ridge within the kane fracture zone and in the ridge transform intersection area mantle bouguer anomalies do not accurately reflect the extent of crustal thinning that has been documented seismically this is attributed to lower average crustal and mantle densities due to fracturing and alteration within the fracture zone and the inability of gravity data to resolve the abrupt change to thinner fracture zone crust which is confined to a narrow zone along the fracture zone trough and beneath the nodal basin at the ridge transform intersection outside of the kane fracture zone the magnitude of the crustal thickness variations predicted solely from the residual gravity anomalies compare reasonably well with the crustal structure derived from seismic refraction studies the largest residual gravity anomalies implying significant reductions in crustal thickness are observed over the small nontransform offset at 23 15 n the anomalous mass associated with this feature forms a v shape that can be traced well off axis suggesting that these discontinuities are fundamental ridge segmentation boundaries that can persist for millions of years the distinctive v shaped pattern of gravity anomalies in the mark area reflects systematic crustal thickness variations related to the northward migration of this nontransform offset over the past 3 4 m y