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

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

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

formation and tectonic evolution of the cretaceous jurassic muslim bagh ophiolitic complex pakistan implications for the composite tectonic setting of ophiolites the muslim bagh ophiolitic complex balochistan pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the indian and eurasian plates these ophiolites were obducted onto the indian continental margin around the late cretaceous prior to the main collision between the indian and eurasian plates the upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes each pluton has a transitional dunite rich zone at its base and new geochemical data suggest a similar mantle source region for both the plutons and dykes in contrast the lower nappe consists of pillow basalts deep marine sediments and a me lange of ophiolitic rocks the rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid ocean ridge the basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the indian plate the two nappes of the muslim bagh ophiolitic complex are thus distinctively different in terms of their age lithology and tectonic setting the recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved

a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

ultramafic exposures and the gravity signature of the lithosphere near the fifteen twenty fracture zone mid atlantic ridge 14 16 5 n abstract we analyze geological and geophysical data near the fifteen twenty fracture zone to constrain the processes of magmatic accretion and tectonic extension in the area characterized by extensive outcrops of ultramafic rocks at the seafloor close to the fracture zone 60 km the crust is thicker associated with circular or elongated gravity lows bull s eyes axis parallel faults and abyssal hills and no peridotite outcrops consistent with a more homogeneous and magmatic crust simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry we explain these discrepancies primarily a by the entrapment of melt in the lithosphere at deep levels particularly near the transform b by along axis melt migration away from the transform and c by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite the along axis variations in melt supply lithospheric thickness and alteration of the lithosphere can result in a change of rheological structure and therefore in fault pattern as observed along the ridge axis identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor these detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere the origin and evolution of these structures is yet unconstrained but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere and that once formed they localize strain due to the presence of weak serpentinites along the fault planes

experimental study of the effects of solute transport on reaction paths during incipient serpentinization abstract this paper presents the results of 4 reactive percolation experiments set up for investigating the impact of flow rate on serpentinization reaction paths for conditions relevant of the oceanic peridotite sub seafloor during the initial stages of its hydrothermal alteration the experiments consisted in injecting artificial seawater into porous compressed olivine powder cores at constant flow rates q 0 24 0 48 1 14 and 5 21 ml h 1 the experiments were conducted at constant temperature 170 c and pressure 25 mpa and lasted 11 to 28 days at the end of the experiments the outlet fluids composition displayed similar compositions buffered by the formation of serpentine amg2 a h 2 9 7 10 asio2 3 9 to 5 2 ph in situ 6 1 these values were achieved in a few to up to 300 h for the high flow rate experiment suggesting that they corresponded to a steady state regime of mass transfer which depended on flow rate differences in the composition of fluid versus time and in the structure of reacted samples during and after the four reactive percolation experiments suggested also various incipient serpentinization reaction paths the low q experiments produced sio2 aq enriched outlet fluids and nodular aggregates were identified covering the reacted olivine surfaces during high q experiments fibrous filaments of proto serpentine were formed on the olivine surfaces and the fluids progressively achieved steady state compositions similar to the other experiments these results together with those of previously published reactive percolation experiments lead us to propose two end member reaction paths for incipient serpentinization of olivine dominated permeable rocks infiltrated by seawater derived hydrothermal fluids 1 a transport controlled reaction path occurring in diffusion dominated zones is characterized by transient brucite precipitation which produces mg trapping and si release in solution followed by serpentine precipitation and 2 a kinetics controlled reaction path occurring in advection dominated zones where transport conditions are favorable to mg leaching and where serpentine precipitates first the occurrence of these two end member reaction paths is determined locally by the composition of the fluid which varies along flow paths thus both reaction paths can coexist in the sample depending on the local pore geometry our study shows that the interplay between fluid transport and reaction kinetics controls the chemical fluxes between the mineral surface and the bulk solution and the incipient serpentinization reaction paths in natural systems the scale and distribution of these reaction domains will depend on the complex structure of the ultramafic basement our results suggest that the precipitation of serpentine and silica rich phases will be favored in fluid focusing zones such as faults and fractures whilst formation of brucite will preferentially occur as part of pervasive background serpentinization

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

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

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

different tdm ch4 hydrothermal plume signatures tag site at 26 n and serpentinized ultrabasic diapir at 15 05 n on the mid atlantic ridge as a part of the 1988 noaa vents program ch4 and mn tracers were used to identify and compare hydrothermal plumes found above the tag field 26 n and in the rift valley at 15 n close to the eastern intersection of the ridge axis with the 15 20 n fracture zone at the mid atlantic ridge mar active hydrothermal venting was confirmed at tag based on elevated concentrations of total dissolved mn tdm up to 30 nmol kg high ch4 concentrations up to 200 nl l and elevated nephelometry signals plumes of a different composition were identified at 15 n with high ch4 concentrations up to 400 nl l low total dissolved mn concentrations tdm 1 nmol kg and no significant nephelometry signal the different properties of these tracers and the different tracer ratios can be used to deduce vent fluid characteristics and compare one hydrothermal area to another tdm ch4 and nephel ch4 ratios at tag are of the same order of magnitude as those observed at other spreading axis hydrothermal fields at 15 n the low tdm ch4 ratio provides evidence of fluid circulation into ultrabasic rocks and offers a potentially useful and single method of exploring for hydrothermal activity associated with serpentinization mantle degassing through hydrothermal activity associated with serpentinization is an important process with respect to chemical and thermal exchanges between the upper mantle and the ocean different ratios of hydrothermal tracers i e tdm ch4 provide a useful framework for identifying subseafloor processes along mid oceanic ridges

direct geologic evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n comment and reply comment macleod et al 2002 1 made a strong case that subhorizontal corrugated surfaces adjacent to the slowly spreading mid atlantic ridge near 15 45 n are exhumed fault surfaces that have been uncovered by normal faulting associated with oceanic lithosphere divergence see also cann et al 1997

strain localization on an oceanic detachment fault system atlantis massif 30 n mid atlantic ridge 1 microstructural observations mineral chemistry and the spatial distribution of deformation fabrics recorded in outcrop samples collected from atlantis massif the active inside corner high at 30 n mid atlantic ridge suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault deformation textures in peridotite and gabbro indicate that high temperature 500 c strain occurred via crystal plastic flow and diffusive mass transfer low temperature 400 c shear zones containing brittle and semibrittle microboudinage textures in which tremolite chlorite and or talc replace fractured serpentine or hornblende cut earlier formed high temperature deformation fabrics in peridotite textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism gabbro is only weakly deformed below amphibolite facies 500 c indicating that strain was partitioned into altered peridotite at low temperature there is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the massif discontinuous high intensity crystal plastic deformation fabrics are found at all structural depths 0 520 m beneath the surface indicating that high temperature granulite and amphibolite grade deformation was not localized in a single shear zone in contrast semibrittle and brittle low temperature shear zones are concentrated less than 90 m structurally beneath the surface and the most intensely brittlely deformed samples concentrated in the upper 10 m localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault the structural evolution of atlantis massif is therefore analogous to a continental metamorphic core complex strain was localized onto the fault by reaction softening and fluid assisted fracturing during greenschist and subgreenschist grade hydrothermal alteration of olivine clinopyroxene serpentine and hornblende to tremolite chlorite and or talc

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

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

geochemical constraints on the modes of carbonate precipitation in peridotites from the logatchev hydrothermal vent field and gakkel ridge the ultramafic hosted logatchev hydrothermal field lhf at 15 n on the mid atlantic ridge and the arctic gakkel ridge gr feature carbonate precipitates aragonite calcite and dolomite in voids and fractures within different types of host rocks we present chemical and sr isotopic compositions of these different carbonates to examine the conditions that led to their formation our data reveal that different processes have led to the precipitation of carbonates in the various settings seawater like 87sr 86sr ratios for aragonite in serpentinites 0 70909 to 0 70917 from the lhf are similar to those of aragonite from the gr 0 70912 to 0 70917 and indicate aragonite precipitation from seawater at ambient conditions at both sites aragonite veins in sulfide breccias from lhf also have seawater like sr isotope compositions 0 70909 to 0 70915 however their rare earth element ree patterns show a clear positive europium eu anomaly indicative of a small 1 hydrothermal contribution in contrast to aragonite dolomite from the lhf has precipitated at much higher temperatures 100 c and yet its 87sr 86sr ratios 0 70896 to 0 70907 are only slightly lower than those of aragonite even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine talc fault schists form north of the lhf these calcites show unradiogenic 87sr 86sr ratios 0 70460 to 0 70499 indicative of precipitation from evolved hydrothermal fluids a simple mixing model based on sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation dolomite precipitated from a mixture of 97 seawater and 3 hydrothermal fluid that should have had a temperature of approximately 14 c assuming that no heat was transferred the much higher apparent precipitation temperatures based on oxygen isotopes 100 c may be indicative of conductive heating probably of seawater prior to mixing the hydrothermal calcite in the fault schist has precipitated from a mixture of 67 hydrothermal fluid and 33 seawater which should have had an isenthalpic mixing temperature of 250 c the significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults rare earth element patterns corroborate the results of the mixing model since the hydrothermal calcite which formed from waters with the greatest hydrothermal contribution has ree patterns that closely resemble those of vent fluids from the lhf our results demonstrate for the first time that 1 precipitation from pure seawater 2 conductive heating of seawater and 3 conductive cooling of hydrothermal fluids in the sub seafloor all can lead to carbonate precipitation within a single ultramafic hosted hydrothermal system

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

the geological setting of the ultramafic hosted logatchev hydrothermal field 14 45 n mid atlantic ridge and its influence on massive sulfide formation the logatchev hydrothermal field 14 degrees 45 n on the mar is one of a few submarine hydrothermal systems associated with ultramafic rocks it is situated on the eastern inner flank of the rift valley wall 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation geological mapping during various rov dives geological sampling and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks the mixed mafic ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor the logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems one of these are so called smoking craters seafloor depressions that are several meters wide characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material at these smoking craters hydrothermal venting occurs directly from holes within the craters and from small cm to dm high cu rich chimneys occurring at the crater rim based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic hosted off axis location with abundant landslides as well as off axis gabbroic intrusions providing the heat for the hydrothermal convection cell c 2009 elsevier b v all rights reserved

drilling constraints on lithospheric accretion and evolution at atlantis massif mid atlantic ridge 30 n expeditions 304 and 305 of the integrated ocean drilling program cored and logged a 1 4 km section of the domal core of atlantis massif postdrilling research results summarized here constrain the structure and lithology of the central dome of this oceanic core complex the dominantly gabbroic sequence recovered contrasts with predrilling predictions application of the ground truth in subsequent geophysical processing has produced self consistent models for the central dome the presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100 220 kyr and not as a single magma pulse isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth below 800 m alteration is restricted to narrow zones surrounding faults veins igneous contacts and to an interval of locally intense serpentinization in olivine rich troctolite hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies but was predominantly in the amphibolite and greenschist range deformation of the sequence was remarkably localized despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation presumably during unroofing via detachment faulting both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent southern ridge of the massif there the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70 of recovered samples we develop a working model of the evolution of atlantis massif over the past 2 myr outlining several stages that could explain the observed similarities and differences between the central dome and the southern ridge

dissecting palimpsests in a late lower and middle paleolithic flint acquisition site on the madaba plateau jordan abstract wzm 2 a flint source on the edge of the madaba plateau jordan exemplifies many of the problems archaeologists confront in investigating open air sites this site has a complex history of alternating episodes of deposition erosion and colluvial movement of sediments as well as bioturbation and recent plowing that has altered the spatial relationships of artifacts creating a cumulative palimpsest on the surface but with limited stratigraphic integrity below surface techniques for investigating these types of sites are discussed including transect surface collections with finds recorded by hand held gps units systematic total collection of grids and the use of geological and archaeological test trenches the assemblages obtained by these methods were subject to statistical analysis of technological attributes combined with the identification of typological specimens and techniques of manufacture known to have chronological significance in order to identify the parts of the paleolithic sequence present potentially time sensitive types were also subject to spatial analysis with the exception of a spatially limited and un diagnostic holocene chipping area at the northeastern end of the site wzm 2 is primarily a middle paleolithic lithic acquisition and processing site probably dating to mis 5 with limited evidence of exploitation during the preceding late lower paleolithic acheulo yabrudian and also possibly the early middle paleolithic this site also extends the known geographical distribution of the acheulo yabrudian to the south and east although the disturbed nature of open air sites such as this limits the types of behavioral information that can be obtained by archaeologists their location on the paleo landscape as well as the aggregate characteristics of their assemblages can provide important clues to early hominin land use economies including provisioning strategies and settlement patterns

tectonic and magmatic control of hydrothermal activity along the slow spreading central indian ridge 8 s 17 s the complex geology and expansive axial valleys typical of slow spreading ridges makes evaluating their hydrothermal activity a challenge this challenge has gone largely unmet as the most undersampled mor type for hydrothermal activity is slow spreading 20 55 mm yr here we report the first systematic hydrothermal plume survey conducted on the central indian ridge cir 8 s 17 s the most extensive such survey yet conducted on a slow spreading ridge using a combined ctd miniature autonomous plume recorder mapr package we used 118 vertical casts along seven segments of the cir 700 km of ridge length to estimate the frequency of hydrothermal activity evidence for hydrothermal activity particle and methane plumes was found on each of the seven spreading segments with most plumes found between 3000 and 3500 m generally 1000 m above bottom we most commonly found plumes on asymmetric ridge sections where ultramafic massifs formed along one ridge flank near ridge transform intersections or nontransform offsets the estimated plume incidence ph for axial and wall casts ph 0 30 35 of 118 casts is consistent with the existing global trend indicating that the long term magmatic budget on the cir is the primary control on the spatial frequency of hydrothermal venting our results show that the tectonic fabric of the cir strongly determines where hydrothermal venting is expressed and that using only near axial sampling might underestimate hydrothermal activity along slow spreading and ultraslow spreading ridges serpentinization is a minor contributor to the plume inventory based on 15 profiles with methane anomalies only predominantly at depths above the local valley walls

horst and graben structures on the flanks of the mid atlantic ridge in the mark area 23 22 n submersible observations abstract south of the kane fracture zone 23 40 n the mid atlantic ridge is segmented into 20 100 km long spreading cells in which morphology seismic and gravity signatures change along and across the ridge axis the rift valley considered as a typical structure along slow spreading ridges can be symmetric or asymmetric and segment centers experience strong magmatic activity whereas low magma supply results in thinning of the crust at segment tips along the segment centered on the snake pit neovolcanic ridge 23 22 n the rift valley is clearly asymmetric and mantle and lower crustal rocks are commonly exposed on the steepest and highest ridge flanking wall in this area we analyzed submersible observations gravinaute cruise collected up to 50 km on both sides of the axis to discuss the proceses responsible for the axial valley shape its possible relationship with deep rock exposures and to determine the origin of off axis structures our data reveal that the structure and the geology vary on both walls of the axial valley and is different from the volcanic and tectonic organization observed on the neovolcanic ridge itself furthermore the structure of the off axis domain in different from the axial valley and rift mountains are organized in a set of regularly spaced relief about 10 km on both flanks of the ridge scarps which built these topographic highs face either toward the axis or away thus the off axis morphology appears as a set of horsts and grabens which could partly result from off axis tectonic processes and or from a shifting of the volcano tectonic activity out of the axial valley during the last 3 ma off axis data show that serpentinized peridotites more commonly observed on the western rift valley wall crop out on the western flank up to 30 away from the axis models proposed to account for such rock exposure are usually related to the formation of the deep axial valley along slow spreading ridges here we propose that mantle rocks are exposed only in a very particular structural context which is directly controled by the ridge segmentation and is independent of the shape of the axial valley symmetric or not serpentinites crop out within a v shaped structure corresponding with the boundary between two adjacent segments where seismic and gravity data reveal that the crust is thin and the lithosphere is thick here the crust is fed by a lower magma supply than at the segment center snake pit area thus we interpret this segment end to be a region of extremely thin crust where modest normal fault can bring mantle rocks to the surface

southeastern lewis hills bay of islands ophiolite geology of a deeply eroded inside corner ridge transform intersection the lewis hills massif bay of islands ophiolite newfoundland preserves a deeply eroded transform fault a major low angle extensional mylonite zone is next to the transform assemblage and probably merges laterally into it the mylonite developed in a high temperature environment and occurs at the top of relatively rigid lithospheric mantle the lithospheric mantle represents exhumed older arc type basement of the little port complex that is also exposed across the transform in the western lewis hills it differs dramatically from an asthenospheric mantle unit of the bay of islands complex exposed in the eastern lewis hills that formed in a spreading ridge environment the fossil ridge transform segment and associated low angle normal fault in the lewis hills formed at an inside corner structural setting similar to recently discovered core complexes at oceanic ridge transform intersections however the lewis hills segment developed in a setting where a spreading center propagated across a transform margin and rifted older arc type lithosphere factors that contributed to formation of the low angle detachment fault are the lithospheric nature of the mantle basement rheological weakening and strain focusing by intrusive sills and weak lateral coupling realized by the extensional transform assemblage

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

quasi 3 d seismic reflection imaging and wide angle velocity structure of nearly amagmatic oceanic lithosphere at the ultraslow spreading southwest indian ridge we present results from 3 d processing of 2 d seismic data shot along 100 m spaced profiles in a 1 8 km wide by 24 km long box during the sismosmooth 2014 cruise the study is aimed at understanding the oceanic crust formed at an end member mid ocean ridge environment of nearly zero melt supply three distinct packages of reflectors are imaged 1 south facing reflectors which we propose correspond to the damage zone induced by the active axial detachment fault reflectors in the damage zone have dips up to 60 and are visible down to 5 km below the seafloor 2 series of north dipping reflectors in the hanging wall of the detachment fault these reflectors may correspond to damage zone inherited from a previous north dipping detachment fault or small offset recent faults conjugate from the active detachment fault that served as conduits for isolated magmatic dykes and 3 discontinuous but coherent flat lying reflectors at shallow depths 1 5 km below the seafloor and at depths between 4 and 5 km below the seafloor comparing these deeper flat lying reflectors with the wide angle velocity model obtained from ocean bottom seismometers data next to the 3 d box shows that they correspond to parts of the model with p wave velocity of 6 5 8 km s suggesting that they occur in the transition between lower crust and upper mantle the 4 5 km layer with crustal p wave velocities is interpreted as primarily due to serpentinization and fracturation of the exhumed mantle derived peridotites in the footwall of active and past detachment faults

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 changing vision of marine minerals abstract non fuel marine minerals are reviewed from the perspective of resources and their value as active analogs that can advance understanding of types of ancient ore deposits that formed in marine settings the theory of plate tectonics is the largest influence in expanding our vision of marine minerals and in developing our understanding of geologic controls of mineralization in space and time prior to the advent of plate tectonics we viewed the ocean basins as passive sinks that served as containers for particulate and dissolved material eroded from land this view adequately explained marine placer deposits heavy minerals and gems aggregates sand and gravel and precipitates phosphorites and manganese nodules although numerous sites of placer mineral deposits are known on continental shelves worldwide current activity pertains to diamond mining off southwestern africa tin mining off southeastern asia and intermittent gold mining off northwestern north america which are all surpassed economically by worldwide recovery of marine sand and gravel in turn dwarfed by offshore oil and gas with the advent of plate tectonics plate boundaries in ocean basins are recognized as active sources of mineralization in the form of hydrothermal massive sulfide deposits and proximal lower temperature deposits hosted in oceanic crust mafic at ocean ridges and felsic at volcanic island arcs and of magmatic ni cu sulfide chromite and pge deposits inferred to be present in the oceanic upper mantle lower crust based on their occurrence in ophiolites some 300 sites of hydrothermal active and relict mineralization most of them minor are known at this early stage of seafloor exploration on ocean ridges in fore arc volcanoes at back arc spreading axes and in arc rifts deposits formed at spreading axes and transported off axis by spreading are present in oceanic lithosphere but are virtually unknown the tag trans atlantic geotraverse hydrothermal field in the axial valley of the mid atlantic ridge latitude 26 n is considered to exemplify a major volcanogenic massive sulfide vms deposit forming at a spreading axis the most prospective of these occurrences lie within the 200 nautical mile 370 km wide exclusive economic zone eez of the nations of the volcanic island arcs of the western pacific where metal content of massive sulfides ag au ba cu pb sb zn exceeds that at ocean ridges plate tectonics early provided a framework for mineralization on the scale of global plate boundaries and is providing guidance to gradually converge on sites of mineralization through regional scales of plate reorganization with the potential to elucidate the occurrence of individual deposits e g eocene carlin type gold deposits investigation of the spectrum of marine minerals as active analogs of types of ancient mineral deposits is contributing to this convergence consideration of questions posed by brian skinner 1997 of what we do and do not know about ancient hydrothermal mineral deposits demonstrates the ongoing advances in understanding driven by investigation of marine minerals

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

serpentinized ultramafics and hydrothermal activity at the mid atlantic ridge crest near 15 n the initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the fifteen twenty fracture zone the ultramafics crop out in block faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m they comprise cumulate harzburgites pyroxenites iherzolites and wehrlites as well as gabbronorites olivine gabbronorites gabbropegmatites and alteration products including serpentinites bastite serpentinites and asbestos the ti zr y relations and relatively constant zr ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust ongoing hydrothermal activity is indicated by chemical anomalies 3he mn in the near bottom water at the ultramafic outcrop the upwelling hydrothermal circulation apparently follows crust penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and or mantle the observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long offset 100 km ridge ridge transform faults where they are serpentinized by hydrothermal processes within the initial 1 106 years of generation of lithosphere at a slow spreading axis the corners formed by the intersections of the walls of a rift valley with both the transform rt corner and the nontransform rn corner portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics

vertical crustal movements at the vema fracture zone in the atlantic evidence from dredged limestones abstract a crustal block shallower by over 3 km than the level predicted by the thermal contraction model for the ocean crust is located on the southern side of the vema transform in the atlantic lithology magnetic and gravity data suggest the anomalous block is an uplifted piece of crust and upper mantle and not a constructional volcanic feature the crest and upper slopes of the anomalous block are capped by shallow water reef limestones determination of paleofacies and ages of samples of these limestones suggest that 1 a the crest of the anomalous crustal block reached sea level and was exposed subaerially in middle pliocene time about 3 m y ago and subsequently sank to its present depth at an average rate of about 0 3 mm year that is a rate one order of magnitude faster than that predicted by the thermal contraction model for crust of the same age 2 b the anomalous block reached at or above sea level either continuously or intermittently throughout the late miocene from roughly 10 m y ago 3 c paleocene sediments dated at about 55 58 m y have been recovered from the uplifted block where assuming a spreading rate of 1 1 cm year the crust should not be more than about 35 m y old vertical crustal movements at the vema transform provide an example of vertical tectonism in the large oceanic transform fracture zones

the architecture of an incipient oceanic basin a tentative reconstruction of the jurassic liguria piemonte basin along the northern apennines alpine corsica transect in this paper a scenario for the early evolution of the jurassic oceanic liguria piemonte basin is sketched for this purpose four selected examples of ophiolite sequences from the northern apennines and corsica are described and analyzed in the external ligurian units northern apennines the ocean continent transition of the adria plate was characterized by a basement made up of subcontinental mantle and lower continental crust covered by extensional allochthons of upper crust both the basement rocks and the extensional allochthons are cut by basaltic dikes and covered by basalts and pelagic deposits the conjugate ocean continent transition of the corsica margin represented by the balagne nappe corsica was composed of mantle peridotites and gabbros covered by basaltic flows and minor breccias that in addition include continent derived clasts by contrast the innermost i e closest to the ocean preserved area observed in the internal ligurian northern apennines and inzecca corsica units consists of former morphological highs of mantle peridotites and gabbros bordered by small basins where the basement is covered by a volcano sedimentary complex characterized by ophiolitic breccias and cherts interlayered with basaltic flows the overall picture resulting from our reconstructions suggests an asymmetric architecture for the liguria piemonte basin with a central area bounded by two different transition zones toward the continental margins this architecture can be interpreted as the result of a rifting process whose development includes a final stage characterized by passive asymmetric extension of the lithosphere along an east dipping detachment fault system

mantle deformation during slow seafloor spreading constrained by observations of seismic anisotropy in the western atlantic during mantle flow at mid ocean ridges viscous shear deformation imparts a structural fabric that remains in the lithosphere as it cools preserving a record of ridge processes over time this fabric can be detected by seismic imaging as it produces an azimuthal anisotropy in the velocity of seismic waves that propagate in the shallow mantle using data from a novel seismic refraction survey in the western atlantic we found that the maximum p wave velocity in the upper 10 km of mantle lithosphere formed at the slow spreading mid atlantic ridge is parallel to the paleo spreading direction consistent with viscous shear deformation dominated by corner flow the magnitude of the p wave anisotropy is 3 4f0 3 approximately one half that found in lithosphere formed at faster spreading rates in the pacific weaker anisotropy in the atlantic suggests that more pervasive conductive cooling at slow spreading ridges increases the proportion of localized brittle deformation in the mantle lithosphere thereby limiting the degree of viscous deformation by scaling our field observations to laboratory experiments we estimate that total viscous strain during slow seafloor spreading is of order 0 5 as opposed to 1 2 for the fast spreading case finally our travel time observations display an azimuthal asymmetry that can be interpreted in one of two ways either the average elastic tensor in this region is not orthorhombic as is commonly assumed or the underlying shear fabric is rotated 158 relative to fossil spreading d 2004 elsevier b v all rights reserved

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

evidence from the oman ophiolite for active mantle upwelling beneath a fast spreading ridge it is commonly accepted that as oceanic lithosphere cools it slides away from the spreading centre under its own weight1 it has been suggested2 5 that even at the spreading axis this process controls the dynamics causing asthenospheric mantle material to rise passively before being dragged away from the ridge by the overlying lithosphere alternatively the upwelling of the asthenosphere at the ridge might be active caused by the buoyancy of partial melt6 10 or of the less dense peridotite left behind by such melting11 12 at slow spreading ridges bull s eye gravity anomalies13 14 have been interpreted as suggesting focused accre tion and hence active mantle diapirs at fast spreading ridges however these anomalies are weaker and less clearly three dimensional15 which may reflect passive mantle upwelling16 here we show that structural data from the oman ophiolite which is thought to have been created at a fast spreading ridge17 18 are consistent with active mantle upwelling beneath the ridge and not with models of passive flow the absence of well marked bull s eye anomalies at fast spreading ridges may be explained by their relatively constant crustal thickness and near isostatic equilibrium

direct geologic evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n comment and reply reply we thank spencer for his comment and welcome the opportunity to debate the results of our study and its implications we are glad that he supports our principal finding subhorizontal corrugated surfaces adjacent to the mid atlantic ridge at 15 45 n are exhumed oceanic detachment faults macleod

fe ni co o s phase relations in peridotite seawater interactions serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen h2 aq release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by h2o we have compiled and evaluated thermodynamic data for fe ni co o s phases and computed phase relations in fo2 g fs2 g and ah2 aq ah2s aq diagrams for temperatures between 150 and 4008c at 50 mpa we use the relations and compositions of fe ni co o s phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the mid atlantic ridge in the 15820 0 n fracture zone area ocean drilling program leg 209 petrographic observations suggest a systematic change from awaruite magnetite pentlandite and heazlewoodite magnetite pentlandite assemblages forming in the early stages of serpentinization to millerite pyrite polydymite dominated assemblages in steatized rocks awaruite is observed in all brucite bearing partly serpentinized rocks apparently buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen which leads to the formation of awaruite associated with the prominent desulfurization of pentlandite sulfide is removed from the rock during the initial stage of serpentinization in contrast steatitization indicates increased silica activities and that highsulfur fugacity sulfides such as polydymite and pyrite vaesite solid solution form as the reducing capacity of the peridotite is exhausted and h2 activities drop under these conditions sulfides will not desulfurize but precipitate and the sulfur content of the rock increases the co evolution of fo2 g fs2 g in the system follows an isopotential of h2s aq indicating that h2s in vent fluids is buffered in contrast h2 in vent fluids is not buffered by fe ni co o s phases which merely monitor the evolution of h2 activities in the fluids in the course of progressive rock alteration the co occurrence of pentlandite awaruite magnetite indicates h2 aq activities in the interacting fluids near the stability limit of water the presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds

in situ measurements of hydrogen sulfide oxygen and temperature in diffuse fluids of an ultramafic hosted hydrothermal vent field logatchev 14 45 n mid atlantic ridge implications for chemosymbiotic bathymodiolin mussels the logatchev hydrothermal vent field 14 45 n mid atlantic ridge is located in a ridge segment characterized by mantle derived ultramafic outcrops compared to basalt hosted vents logatchev high temperature fluids are relatively low in sulfide indicating that the diffuse low temperature fluids of this vent field may not contain sufficient sulfide concentrations to support a chemosymbiotic invertebrate community however the high abundances of bathymodiolin mussels with bacterial symbionts related to free living sulfur oxidizing bacteria suggested that bioavailable sulfide is present at logatchev to clarify if diffuse fluids above mussel beds of bathymodiolus puteoserpentis provide the reductants and oxidants needed by their symbionts for aerobic sulfide oxidation in situ microsensor measurements of dissolved hydrogen sulfide and oxygen were combined with simultaneous temperature measurements high temporal fluctuations of all three parameters were measured above the mussel beds h2s and o2 coexisted with mean concentrations between 9 and 31 m h2s and 216 and 228 m o2 temperature maxima 7 4 c were generally concurrent with h2s maxima 156 m and o2 minima 142 m long term measurements for 250 days using temperature as a proxy for oxygen and sulfide concentrations indicated that the mussels were neither oxygen limited nor sulfide limited our in situ measurements at logatchev indicate that sulfide may also be bioavailable in diffuse fluids from other ultramafic hosted vents along slow and ultraslow spreading ridges

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

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

dynamic accretion beneath a slow spreading ridge segment iodp hole 1473a and the atlantis bank oceanic core complex 809 deep iodp hole u1473a at atlantis bank swir is 2 2 km from 1 508 m hole 735b and 1 4 from 158 m hole 1105a with mapping it provides the first 3 d view of the upper levels of a 660 km 2 lower crustal batholith it is laterally and vertically zoned representing a complex interplay of cyclic intrusion and ongoing deformation with kilometer scale upward and lateral migration of interstial melt transform wall dives over the gabbro peridotite contact found only evolved gabbro intruded directly into the mantle near the transform there was no high level melt lens rather the gabbros crystallized at depth and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal plastic deformation and faulting the residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif likely near the crust mantle boundary thus basalts erupted to the seafloor from 1 550 mbsf by contrast the mid atlantic ridge lower crust drilled at 23 n and at atlantis massif experienced little high temperature deformation and limited late stage melt transport they contain primitive cumulates and represent direct intrusion storage and crystallization of parental morb in thinner crust below the dike gabbro transition the strong asymmetric spreading of the swir to the south was due to fault capture with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion this caused rapid migration of the plate boundary to the north while the large majority of the lower crust to spread south unroofing atlantis bank and uplifting it into the rift mountains

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

geological investigation of a lineated massif at the kane transform fault implications for oceanic core complexes recently available new perspectives on the morphology of the sea floor near the mid atlantic ridge reveal the widespread development of dome like massifs with lineated upper surfaces in form and dimensions these features resemble continental metamorphic core complexes developed in regions of large magnitude extension and thus may be oceanic core complexes excellent examples of these features occur south of the kane transform fault where high resolution geological data from side looking sonar deeply towed cameras and manned submersibles are available the geological data from two different massifs representing somewhat different types of crustal structure provide important constraints on the development of these oceanic core complexes inferences from these direct geological investigations of oceanic core complexes are a first step toward understanding the processes by which these features form in slow spread crust specific relationships in these massifs and nearby areas suggest that these structures have formed over significant lengths of ridge axis tens of kilometres and are not necessarily confined to any specific setting with respect to morphologic segmentation patterns in some cases they are developed in thick gabbroic crust but elsewhere have formed in crust composed serpentinite riddled with gabbroic intrusions ductile stretching and thinning of these sections is recorded in extensive mylonites and shear zones syntectonic gabbroic intrusions are widespread and contradict the notion that the massifs develop during intervals of amagmatic spreading cataclastic fault zones in gabbros and serpentinites developed during uplift cooling and hydration of the stretched crust and may have been localized as major detachment faults sparse diabase dikes and basaltic lavas are the latest magmatic additions to these areas numerous joints and steeply dipping small offset faults are the youngest observed structures across part of the top surface of one massif small scale faults appear to bound elongated ridges of fractured basement rocks creating the lineated terrain thus the extensive lineated surfaces nearby need not reflect prolonged displacement along a continuous detachment fault surface geological relationships developed in the dome like massifs along the kane transform fault hint at a complex array of processes involved in the evolution of these features and raise new questions regarding their origin and analogies with continental core complexes

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

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

paleomagnetic evidence of large footwall rotations associated with low angle faults at the mid atlantic ridge exposures of gabbros and mantle derived peridotites at slow spreading oceanic ridges have been attributed to extension on long lived low angle detachment faults similar to those described in continental metamorphic core complexes in continental settings such detachments have been interpreted as having originated and remained active at shallow dips alternatively currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift while the latter interpretation would be more con sistent with andersonian faulting theory it predicts large footwall tilts that have not been observed in continental detachment faults here we use the magnetization of oceanic gabbro and peridotite samples exposed near the fifteen twenty fracture zone on the mid atlantic ridge to demonstrate that substantial footwall rotations have occurred widespread rotations ranging from 50 to 80 indicate that original fault orientations dipped steeply toward the spreading axis

separation of 3he and ch4 signals on the mid atlantic ridge at 5 n and 51 n abiogenic methane may be produced in submarine hydrothermal systems by degassing of basalts or serpentinization of ultramafic outcrops the latter process presumably releases little primordial helium and is therefore implicated by high ch4 3he ratios in vent fluids from the ultramafic hosted rainbow field and in methane plumes near ultramafic outcrops in two segments of the mid atlantic ridge at 5 4 n and 51 n we have observed depth separated ch4 and 3he plumes in both cases the helium plume was deeper near the valley floor it may be that the plumes issue from separate vents where the helium is discharged near the volcanic axis and the methane is generated by serpentinization on the valley wall however at the present time the locations of the vents that produce these plumes are not known using a one pass model we investigated whether separate venting could arise from heat conduction from a primary helium carrying hydrothermal circulation to a second shallower fracture loop intersecting ultramafic rock the model results indicate that the flow rate through the secondary loop would have to be relatively low in order for it to stay warm enough for serpentinization to proceed in this case some of the exothermic heat production is lost by conduction and the temperature increase in the circulating fluid is only a fraction of that expected from a water rock ratio of 1 1

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

a note on the fluxes of abiogenic methane and hydrogen from mid ocean ridges the concentrations of methane and hydrogen in hydrothermal vent fluids issuing from mid ocean ridges tend to fall into two groups one with high concentrations of these gases in ultramafic hosted vent fields and a second group with relatively lower concentrations in basalt hosted vent fluids ultramafic hosted systems however appear to be restricted to slow spreading ridges and constitute only a fraction of the hydrothermal systems found there in this note the hydrothermal fluxes of methane and hydrogen have been calculated by estimating the percentages of the total subsurface hydrothermal circulation that circulate through each type of host rock even though the percentage of the total subsurface flow that is affected by serpentinization appears to be rather small 8 it still appears that this process produces about 70 of the total mid ocean flux of these gases the total production of methane and hydrogen is calculated to be about 20 x 10 9 mol yr 1 and 190 x 10 9 mol yr 1 respectively the hydrogen flux is comparable to that most recently calculated on the basis of the rate of hydration of mantle rock in newly formed crust and the stoichiometry of the serpentinization reaction this suggests that except for the production of methane a major portion of the hydrogen produced in the subsurface is not consumed before venting

fluid circulation and carbonate vein precipitation in the footwall of an oceanic core complex ocean drilling program site 175 mid atlantic ridge carbonate veins recovered from the mafic ultramafic footwall of an oceanic detachment fault on the mid atlantic ridge record multiple episodes of fluid movement through the detachment and secondary faults high temperature 75 175 c calcite veins with elevated ree contents and strong positive eu anomalies record the mixing of up welling hydrothermal fluids with infiltrating seawater carbonate precipitation is most prominent in olivine rich troctolite which also display a much higher degree of greenschist and sub greenschist alteration relative to gabbro and diabase low temperature calcite and aragonite veins likely precipitated from oxidizing seawater that infiltrated the detachment fault and or within secondary faults late or post footwall denudation oxygen and carbon isotopes lie on a mixing line between seawater and logatchev like hydrothermal fluids but precipitation temperatures are cooler than would be expected for isenthalpic mixing suggesting conductive cooling during upward flow there is no depth dependence of vein precipitation temperature indicating effective cooling of the footwall via seawater infiltration through fault zones one sample contains textural evidence of low temperature seawater signature veins being cut by high temperature hydrothermal signature veins this indicates temporal variability in the fluid mixing possibly caused by deformation induced porosity changes or dike intrusion the strong correlation between carbonate precipitation and olivine rich troctolites suggests that the presence of unaltered olivine is a key requirement for carbonate precipitation from seawater and hydrothermal fluids our results also suggest that calcite talc alteration of troctolites may be a more efficient co2 trap than serpentinized peridotite

the continent ocean transition at the mid northern margin of the south china sea the northern margin of the south china sea scs has particular structural and stratigraphic characteristics that are somewhat different from those described in typical passive margin models the differences are attributable to poly phase tectonic movements and magmatic activity resulting from the interaction among the eurasian philippine sea and indo australian plates based on several crustal scale multi channel seismic reflection profiles and satellite gravity data across the northetn scs margin this paper analyzes the structures volcanoes and deep crust of the continent ocean transition zone cot at the mid northern margin of the scs to study the patterns and model of extension there

wide angle seismic imaging of two modes of crustal accretion in mature atlantic ocean crust we present a high resolution 2 d p wave velocity model from a 225 km long active seismic profile collected over 60 75 ma central atlantic crust the profile crosses five ridge segments separated by a transform and three nontransform offsets all ridge discontinuities share similar primary characteristics independent of the offset we identify two types of crustal segment the first displays a classic two layer velocity structure with a high gradient layer 2 0 9 s 1 above a lower gradient layer 3 0 2 s 1 here pmp coincides with the 7 5 km s 1 contour and velocity increases to 7 8 km s 1 within 1 km below we interpret these segments as magmatically robust with pmp representing a petrological boundary between crust and mantle the second has a reduced contrast in velocity gradient between the upper and lower crust and pmp shallower than the 7 5 km s 1 contour we interpret these segments as tectonically dominated with pmp representing a serpentinized alteration front while velocity depth profiles fit within previous envelopes for slow spreading crust our results suggest that such generalizations give a misleading impression of uniformity we estimate that the two crustal styles are present in equal proportions on the floor of the atlantic within two tectonically dominated segments we make the first wide angle seismic identifications of buried oceanic core complexes in mature 20 ma atlantic ocean crust they have a 20 km wide domal morphology with shallow basement and increased upper crustal velocities we interpret their midcrustal seismic velocity inversions as alteration and rock type assemblage contrasts across crustal scale detachment faults

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

morphotectonics of the carlsberg ridge between 62 20 and 66 20 e northwest indian ocean abstract the segmentation pattern of 440 km long section of the carlsberg ridge between 62 20 e and 66 20 e in the northwest indian ocean has been studied using multibeam bathymetry and magnetic data a well defined transform fault is the only first order discontinuity documented in this section of the ridge four segments have been identified based on the topographic fabric along axis depth variations and the presence of off axis traces of transform and non transform discontinuities these segments constitute four distinct domains representing magmatic and sparsely magmatic crustal accretion magnetic model studies show this section of the ridge is a slow spreading ridge with spreading half rates varying from 11 to 16 mm yr along axis variations in the magnetic anomalies and crustal magnetization the presence of axial volcanic ridges on the inner valley floor variations in the depth and geometry of the rift valley and recovery of mantle derived peridotites from the inner valley floor suggest distinct variations in the accretionary processes along the ridge it is suggested that the linear section in the east with ridge parallel topographic fabric continuous magnetic lineations and the segment with a shallow valley floor with along axis ridge propagation fabric represent a magmatic phase of crustal accretion the wide rift valley recovery of mantle rocks deformed topographic fabric along axis depth maxima and discontinuous magnetic lineations documented along the middle part of the ridge and the existence of short ridges east of the transform fault represent sparsely magmatic sections with dominance of tectonic extension

on the occurrence trace element geochemistry and crystallization history of zircon from in situ ocean lithosphere we characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite evolved gabbro and metamorphosed ultramafic host rocks collected along present day slow and ultraslow spreading mid ocean ridges mors the geochemistry of 267 zircon grains was measured by sensitive high resolution ion microprobe reverse geometry at the usgs stanford ion microprobe facility three types of zircon are recognized based on texture and geochemistry most ocean crustal zircons resemble young magmatic zircon from other crustal settings occurring as pristine colorless euhedral type 1 or subhedral to anhedral type 2 grains in these grains hf and most trace elements vary systematically with ti typically becoming enriched with falling ti in zircon temperature ti in zircon temperatures range from 1 040 to 660 c corrected for a tio2 0 7 a sio2 1 0 pressure 2 kbar intra sample variation is typically 60 150 c decreasing ti correlates with enrichment in hf to 2 wt while additional hf enrichment occurs at relatively constant temperature trends between ti and u y ree and eu eu exhibit a similar inflection which may denote the onset of eutectic crystallization the inflection is well defined by zircons from plagiogranite and implies solidus temperatures of 680 740 c a third type of zircon is defined as being porous and colored with chaotic cl zoning and occurs in 25 of rock samples studied these features along with high measured la cl s ca and fe and low sm la n ratios are suggestive of interaction with aqueous fluids non porous luminescent cl overgrowth rims on porous grains record uniform temperatures averaging 615 26 c 2sd n 7 implying zircon formation below the wet granite solidus and under water saturated conditions zircon geochemistry reflects in part source region elevated hree coupled with low u concentrations allow effective discrimination of 80 of zircon formed at modern mors from zircon in continental crust the geochemistry and textural observations reported here serve as an important database for comparison with detrital xenocrystic and metamorphosed mafic rock hosted zircon populations to evaluate provenance

quantitative constraint on footwall rotations at the 15 45 n oceanic core complex mid atlantic ridge implications for oceanic detachment fault processes the subsurface geometry of detachment faults at slow spreading mid ocean ridges is debated are they planar features that form and slip at low angles as often inferred for their continental equivalents or do they initiate at steep angles and then flatten in response to flexural unloading as displacement proceeds as predicted in rolling hinge conceptual models an essential difference is that significant rotation of the footwall should occur in the rolling hinge but not the planar fault model this can be tested using paleomagnetism previous attempts to address this question have relied upon data from azimuthally unoriented drill cores although results are consistent with large rotations having occurred these interpretations are very nonunique and other solutions that require minimal rotations are equally permissible we here present a rigorous analysis of paleomagnetic and structural data from a unique set of azimuthally oriented cores collected using a seabed rock drill from the 15 45 n oceanic core complex on the mid atlantic ridge by considering the full paleomagnetic remanence vector in combination with kinematic data from the detachment fault shear zone we are able to quantitatively constrain the geometrically permissible axes and magnitudes of rotation of the detachment fault footwall for the first time without having to make a priori assumptions about the orientation of the axis we show that significant rotations 64 16 have indeed occurred about a gently plunging near ridge parallel axis robustly supporting the rolling hinge models we further discuss the geological implications of this result for oceanic detachment fault processes

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

redox changes in a seafloor hydrothermal system recorded in hematite chalcopyrite chimneys abstract hydrothermal chimneys composed of hematite and chalcopyrite grow at the irina ii vent site logatchev hydrothermal field mid atlantic ridge hematite and chalcopyrite form layers that repeatedly alternate within the chimney wall the repeating alternation of hematite and chalcopyrite implies for repeating changes in the physico chemical parameters of the vent fluid the modeled stability diagrams of hematite and chalcopyrite at the physico chemical conditions of the irina ii hydrothermal site showed that the precipitation of these two fe minerals is primarily governed by the activity of h2s and ph fe isotope studies suggest that the successive deposition of chalcopyrite 56 54fe 0 20 and hematite 56 54fe 0 60 layers in the irina ii chimneys is a result of redox changes in the hydrothermal fluid the replacement of hematite by either magnetite or chalcopyrite at some places also suggests for a redox inversion in the hydrothermal system from oxic to reduced chondrite normalized rare earth element distribution patterns of the hematite layers show a strong negative eu anomaly which is interpreted to be a result of crystal chemical control on the rare earth element fractionation during hematite precipitation sr isotope systematics indicate significant and variable incorporation of seawater from 40 to 80 in the hydrothermal system nd pb isotope studies show that nd and pb in chalcopyrite and hematite layers are derived mainly from the underlying mafic and ultramafic rocks the chimneys are young 5 13 1 28 kyrs b p and their wall thickened inward the duration of redox cycles oxic and reducing within the scale of a single chimney is about several hundred years the hematite layers are enriched in w in sn and u indicating that primary hematite hydrothermal deposits at the seafloor may be a potential source for these strategic trace elements high concentrations of these elements in the studied hematite layers can be explained by their easy ion substitution for fe3 in the hematite crystal lattice whereas the most plausible source of the elevated w in and sn contents are the hydrothermal fluids the seawater is the possible major source of u

stacked gabbro units and intervening mantle a detailed look at a section of iodp leg 305 hole u1309d hole u1309d integrated ocean drilling program iodp legs 304 305 penetrated 1415 m into the seafloor of the atlantis massif an oceanic core complex at 30 n mid atlantic ridge more than 96 of all recovered rocks are gabbroic on the basis of a mineral chemical overview we suggest that between 800 and 1100 m below sea floor mbsf a magmatic unit occurs ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part below 1235 mbsf massive gabbronorites oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit the focus here is on the zone where both units interact and screens totaling 50 m of a microstructurally distinct olivine rich troctolite occur we argue that the olivine rich troctolite is a former mantle rock which was converted to a crust mantle transition zone dunite at the base of the upper magmatic unit later as melts derived from the lower magmatic unit percolated through it it was equilibrated to a more evolved chemistry and transformed to a fine grained olivine rich troctolite our main arguments against a possible cumulate nature of the olivine rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine rich troctolite its distinctly fine grained microstructure the high cr content of cpx and its ni rich olivine composition the high nio for a given mg mg fe in the olivine rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt evidence for the percolation of evolved melts through the olivine rich troctolites are ti rich interstitial pyroxenes and as inclusions in cr spinel highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter scale noritic veins the percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust mantle transition zone

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples