deformation and hydrothermal metamorphism of gabbroic rocks within the godzilla megamullion parece vela basin philippine sea abstract microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the godzilla megamullion site kh07 02 d18 located along the parece vela basin spreading ridge parece vela rift philippine sea reveal the development of a high temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust the deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene amphibole porphyroclasts in a fine grained matrix plagioclase crystallographic preferred orientations vary from 010 100 and 001 100 patterns in the mylonites to a weak 001 100 pattern in the some mylonites and ultramylonite suggesting a change in the deformation mechanism from dislocation creep to grain size sensitive creep with increasing intensity of deformation the chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts secondary amphibole is ubiquitous consisting mainly of pargasite and magnesiohornblende brown hornblende and actinolite green hornblende the mineral assemblage is consistent with the hydrothermal metamorphic reaction clinopyroxene calcic plagioclase fluid amphibole sodic plagioclase compared with deformed gabbroic rocks from the breakaway and termination areas of the godzilla megamullion the samples record ductile shearing under high temperature conditions possibly related to the development of the godzilla megamullion although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

geotectonic significance of gneissic amphibolites from the vema fracture zone equatorial mid atlantic ridge a large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the vema fracture zone serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites in the shallowest of the dredge hauls petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies on the other hand the associated meta gabbros display sequences of secondary minerals indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium we tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the vema fracture zone hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions k ar dating suggests within relatively large analytical uncertainties that the amphibolite metamorphism took place 10 m y ago i e at a time when the dredging sites were located in the vicinity of the spreading center

the rate of oceanic detachment faulting at atlantis bank sw indian ridge abstract the rates of slip on oceanic detachment faults and how those rates compare to sea floor spreading rates constitute fundamental data required to constrain how oceanic core complexes form and their role during crustal accretion we combine sea surface magnetic data with the magnetic polarity of shallow core samples and pb u shrimp ages of igneous zircon to determine the time averaged half spreading rate during oceanic detachment faulting at atlantis bank 100 km south of the ultraslow spreading southwest indian ridge swir the pb u zircon ages correlate well with the magnetic ages and so highlight that magmatic accretion and faulting were coeval for over 2 myr creating and exposing a 1 5 km thick layer of gabbro for 35 km parallel to spreading we use bivariate linear regression of distance age data and forward modeling of magnetic anomaly data to calculate a half spreading rate during detachment faulting of 14 1 1 8 1 5 km myr 95 confidence limits when integrated with regional constraints on spreading history we note that detachment faulting coincided with a short lived regional increase in the full spreading rate along the swir and for the ridge segment containing atlantis bank spreading was highly asymmetric with 80 of plate motion accommodated by detachment faulting consequently the detachment fault effectively formed the plate boundary at the surface in this spreading segment highly asymmetric spreading was confined to the spreading segment containing atlantis bank and to the duration of detachment faulting so the ridge segment containing atlantis bank migrated northward relative to its symmetrically spreading eastern neighbour such that the intervening non transform discontinuity shortened we suggest that the highly asymmetric spreading may be a characteristic feature of oceanic detachment faulting an inference supported by more poorly constrained half spreading rates determined at several other oceanic core complexes

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

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

a revised subduction inception model to explain the late cretaceous double vergent orogen in the precollisional western tethys evidence from the northern apennines the meso cenozoic alpine belts of the mediterranean area are characterized by complex architectures result of a complex subduction and collision evolution that preserve also a legacy of the rifting related configuration of the continental margins the northern apennines is a segment of these belts originated during closure of the ligure piemontese ocean and collision between the europe and adria plates the different configuration of the adria and europe margins inherited from asymmetric rifting is recorded in the ligurian units that preserve incorporation into the subduction factory of fragments of the oceanic domain internal ligurian units and portions of the ocean continent transition zone octz toward adria external ligurian units we provide here unpublished data on the stratigraphy and sedimentology of these units together with a review of what is already established in literature both datasets combined testify that at 80 m a an accretionary prism was growing between the deposition basins of the two groups of units and fed both basins with clasts from the ocean realm the continental crust and the subcontinental mantle we propose that closure of the ligure piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned adria margin and developed a double vergent prism by accreting oceanic material and continental extensional allochthons from the octz we believe the revised site of subduction initiation and the pre collisional architecture inherited from the rifting and spreading phases allow reconciling most of the discrepancies between the various interpretation proposed in literature for the pre collisional evolution of the apennines

elementary theory of bed sediment entrainment by debris flows and avalanches 1 analyses of mass and momentum exchange between a debris flow or avalanche and an underlying sediment layer aid interpretations and predictions of bed sediment entrainment rates a preliminary analysis assesses the behavior of a coulomb slide block that entrains bed material as it descends a uniform slope the analysis demonstrates that the block s momentum can grow unstably even in the presence of limited entrainment efficiency a more detailed depth integrated continuum analysis of interacting deformable bodies identifies mechanical controls on entrainment efficiency and shows that entrainment rates satisfy a jump condition that involves shear traction and velocity discontinuities at the flow bed boundary explicit predictions of the entrainment rateeresult from making reasonable assumptions about flow velocity profiles and boundary shear tractions for coulomb friction tractions predicted entrainment rates are sensitive to pore fluid pressures that develop in bed sediment as it is overridden in the simplest scenario the bed sediment liquefies completely and the entrainment rate equation reduces toe 2 1gh1 cos 1 1 v 1 where is the slope angle 1 is the flow s coulomb friction coefficient h1 is its thickness 1 is its degree of liquefaction and v 1is its depth averaged velocity for values of 1ranging from 0 5 to 0 8 this equation predicts entrainment rates consistent with rates of 0 05 to 0 1 m s measured in large scale debris flow experiments in which wet sediment beds liquefied almost completely the propensity for bed liquefaction depends on several factors including sediment porosity permeability and thickness and rates of compression and shear deformation that occur when beds are overridden

submarine metamorphism of gabbros from the mid cayman rise petrographic and mineralogic constraints on hydrothermal processes at slow spreading ridges the coarse grained submersible collected rocks from the mid cayman rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro petrographic and mineralogic studies showed that 1 some deformation is present in all rocks 2 the distribution of deformation is not uniform within a specimen and 3 alteration has taken place preferentially in the deformed zones the abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth the compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied na k and h2o to and removed ca from the rock in all about 15 of the original rock has been transformed to amphibolite in the 700 m vertical section our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur if deformation continues to take place during the spreading of seafloor alteration may also continue to occur along newly formed fractures and cracks

collision induced rifting the grenville orogeny and the keweenawan rift of north america abstract we propose that the keweenawan rift formed as a result of convergence related to the synchronous grenville orogeny this proposal is based on 1 1 isotopic ages in the keweenawan rift and the grenville province are similar according to the best u pb and rb sr dates the grenville orogeny existed from 1150 to 1100 ma and keweenawan rifting occurred 1120 1100 ma 2 2 a similar modern orogenic belt and rift relationship in the middle east serves as an actualistic model the red sea gulf of aden rift system has formed as a continuous series of coalesced pull apart basins and is opening parallel to the orogenic front of the zagros and bitlis mountains rifting began in the early miocene while the collision began in the eocene suggesting that rifting is related to the collision 3 3 many other ancient rift collision pairs have a similar age relationship and geometrical configuration based on comparisons of these tectonic regimes we have derived a model whereby these rifts formed as a result of strike slip faults forming in either the foreland or the hinterland of orogenic zones

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

extremely asymmetric magmatic accretion of oceanic crust at the ends of slow spreading ridge segments we report the results of a deep towed magnetic survey of part of a mid atlantic ridge spreading segment analysis of the magnetic reversals indicates that for the past 0 7 m y magmatic accretion at the end of the segment has been effectively one sided with new crust being added only to the outside corner of the ridge offset eastern flank and not to the inside corner western flank spreading on the inside corner was accommodated by significant displacement on a single large fault the area between the fault and the axial volcanic ridge was effectively a thin static sliver at the plate boundary during this process in the short term asymmetric magmatic accretion was probably accomplished by progressively shifting the axial volcanic ridge to a new location at the inside corner western edge of the old one asymmetric spreading is unlikely to be sustainable as a steady state process the termination of a period of asymmetric spreading may be achieved either by establishing a new axial volcanic ridge to a position on the outside corner eastern plate thus isolating the old axial volcanic ridge on the inside corner plate or by simply arresting movement on the large fault and reverting to symmetric spreading at the axial volcanic ridge highly asymmetric accretion may be a common process at slow spreading segments particularly near discontinuities this asymmetry cannot be maintained for long periods and may be directly linked to intervals of spreading by tectonic extension

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

erosional effects on runout of fast landslides debris flows and avalanches a numerical investigation the mechanism of mass changes in debris transportation process is an important topic in the study of fast landslides debris flows and avalanches basal erosion is recognised as a dynamic interaction between the original moving material and the entrained basal topsoil shearing along their non slip contact surface in this paper we propose a new concept of yield rate and establish the erosional relationship to bridge these two systems a pertinent mathematical model and numerical implementation are formulated parametric numerical experiments are conducted to compare the erosional effects the simulated results are consistent with available experimental and field observations the influence of the involvement of the erosive material on runout behaviour and the global mobility of the moving material are elucidated the proposed method is then employed to analyse a recent debris flow event in northern italy the excellent match to the field data gives it a plausible potential application to the analysis of this type of gravity driven flow with significant erosion defined in a dimensionless form the proposed yield rate can be estimated conveniently in general geotechnical practices

evidence for the protracted construction of slow spread oceanic crust by small magmatic injections abstract gabbroic cumulates drilled south of the kane transform fault on the slow spread mid atlantic ridge preserve up to three discrete magnetization components here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust by comparing the paleomagnetic data with mineral compositions and based on thermal models of local reheating we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter scale sills some of these magmas were crystal laden and the subsequent expulsion of interstitial liquid from them produced cumulus sills these small scale magmatic injections took place over at least 210 000 years and at distances of 3 km from the ridge axis and may have formed much of the lower crust this model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow spread ridges

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

erosion and mobility in granular collapse over sloping beds 1 we describe laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed designed to mimic erosion processes of natural flows travelling over deposits built up by earlier events two controlling parameters are the inclination of the plane and the thickness of the erodible layer we show that erosion processes can increase the flow mobility i e runout over slopes with inclination close to the repose angle of the grains r by up to 40 even for very thin erodible beds erosion efficiency is shown to strongly depend on the slope of the topography entrainment begins to affect the flow at inclination angles exceeding a critical angle c r 2 runout distance increases almost linearly as a function of the thickness of the erodible bed suggesting that erosion is mainly supply dependent two regimes are observed during granular collapse a first spreading phase with high velocity followed by a slow thin flow provided either the slope or the thickness of the erodible bed is high enough surprisingly erosion affects the flow mostly during the deceleration phase and the slow regime the avalanche excavates the erodible layer immediately at the flow front waves are observed behind the front that help to remove grains from the erodible bed steep frontal surges are seen at high inclination angles over both rigid or erodible bed finally simple scaling laws are proposed making it possible to obtain a first estimate of the deposit and emplacement time of a granular collapse over a rigid or erodible inclined bed

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

fault rotation and core complex formation significant processes in seafloor formation at slow spreading mid ocean ridges mid atlantic ridge 13 15 n 1 the region of the mid atlantic ridge mar between the fifteen twenty and marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension normal faults show large amounts of rotation dome shaped corrugated detachment surfaces core complexes intersect the seafloor at the edge of the inner valley floor and extinct core complexes cover the seafloor off axis we have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments 13 and 15 n segments steep outward facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30 the rotation occurs very close to the ridge axis as much as 20 within 5 km of the volcanic axis and is complete by 1 my producing distinctive linear ridges with roughly symmetrical slopes this morphology is very different from linear abyssal hill faults formed at the 14 n magmatic segment which display a smaller amount of rotation typically 15 we suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults if faults are long lived a dome shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex a single ridge segment can have several active core complexes some less than 25 km apart that are separated by swales we present two models for multiple core complex formation a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales either model can explain the observed morphology

baddeleyite zro2 and zircon zrsio4 from anorthositic rocks of the laramie anorthosite complex wyoming petrologic consequences and u pb ages

faulting and fracturing in part of the duluth complex northeastern minnesota extensive faulting and fracturing have been documented by detailed field studies of a 48 km2 area in the upper precambrian duluth complex northeastern minnesota faults and fractures dip steeply

fossil oceanic core complexes recognized in the blueschist metaophiolites of western alps and corsica tethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks by contrast tothe complete mantle crustal sections of semail type ophiolite sheets tethyan ophiolites are characterized by asmaller amount of mafic rocks gabbros and basalts by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle derived peridotites and associatedgabbroic intrusions therefore they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean continent transition at distal passive margins in the very first models offormation of the tethyan ophiolites in the years 1980 the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present dayoceans in particular at that time little work had focused on the distribution origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics here we reconsider the geology of sometypical metaophiolites from the western alps and corsica and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the tethyan ophiolites in general revisitedexamples include a set of representative metaophiolites from the blueschists units of the western alps queyrasregion and from alpine corsica golo valley field relationships between the ophiolitic basement andthe metasedimentary metavolcanic oceanic cover are described outlining a typical character of the tethyanophiolite lithological associations jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle gabbo basement in a way strictly similar to what is observed in the non metamorphicappennine ophiolites or chenaillet massif this confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction this juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of oceanic core complexes occs at the axis of slow spreading ridges this fundamental plate tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries in addition fieldrelationships between the discontinuous basaltic formations and the ultramafic mafic basement indicate that thistectonic stage is followed or not by a volcanic stage we discuss this issue in the light of available field constraints

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

constraints on crustal structure of adjacent occs and segment boundaries at 13 n on the mid atlantic ridge the 13 n segment of the mid atlantic ridge is an example of a morphologically well studied slow spreading ridge segment populated with oceanic core complexes occs in this paper we present the results of a 200 km long 2d seismic and gravity transect through this segment the bounding fracture zones to the south and the ridge discontinuity to the north we use this transect to consider the two end member models of occ evolution in which one referred to as the segment scale model implies they are interconnected with their detachments being part of a single segment long feature and the other the localised model that each occ is structurally isolated we show using the 7 5 km s 1 velocity contour as the base of crust marker that the crust is consistently relatively thin ridge parallel at 5 km thick on average and that beneath the occs the moho marks the top of a velocity gradient transition into the mantle rather than a distinct velocity discontinuity although each occ is not traversed in an identical structural location they show a different crustal velocity density structure with depth with along axis variations in this structure mirrored by the bathymetric deeps between them older occs have a contrasting velocity depth signature to the currently active 13 20 n occ the 13 20 n occ is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours while the 13 30 n occ has an apparently thinner crust our combined p wave seismic travel time tomography and gravity forward modelling suggests that the occs of the 13 n segment are not interconnected at depth to the north of the 13 30 n occ our modelling also suggests that the crust is being magmatically refreshed or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge the profile also crosses the marathon and mercurius fracture zones that mark the southern limit of the 13 n segment and the southern ridge transform intersection outside corner along profile marathon fracture zone offsets younger 1 my vs 8 my oceanic crust than mercurius fracture zone 8 my vs 11 my when considered in combination both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of marathon fracture zone coupled with a region of low density that most likely reflects serpentinisation of the uppermost mantle in addition the crust captured between fracture zones appears relatively rotated about an e w axis and uplifted to the north with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre both the outside corner and the crust bounded by fracture zones have velocity depth characteristics similar to that of the 13 n segment occs rather than normally accreted oceanic crust particularly in the upper to middle crust overall our results support the localised model of occ evolution and suggest that fracture zones do not become locked immediately on transform to fracture transition as current models dictate

depth integrated equations for entraining granular flows in narrow channels flowing over erodible beds channelized granular avalanches can alter their volume by entraining or detraining basal grains in detail entrainment results from a gradual adjustment of stress and velocity profiles over depth bringing bed material past yield and vice versa for detrainment to capture this process we propose new depth integrated equations that balance kinetic energy in addition to mass and momentum the equations require a local granular rheology assumed viscoplastic but no extra erosion law entrainment rates are instead deduced from the depth integrated layer dynamics to check the approach we obtain solutions for non equilibrium heap flows and compare them with experiments conducted in a seesaw channel

fuji dome a large detachment fault near 64 e on the very slow spreading southwest indian ridge a continuous domed detachment surface fuji dome has been imaged on the very slow spreading southwest indian ridge using deep towed side scan sonar and has been investigated by manned submersible and sea surface geophysics the dome is morphologically similar to other oceanic detachments core complexes or mega mullions in addition to bathymetric mullions observed in ship borne bathymetry finer scale spreading parallel striations were imaged with the side scan on the detachment surface metabasalt crops out near the termination probably as part of a thin fault sliver gabbro and troctolite probably crop out near the summit of the dome the rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite most of the detachment surface dips toward the ridge axis at 10 20 but near the breakaway it is strongly rotated outward and dips away from the axis at up to 40 normal undeformed volcanic seafloor crops out adjacent to the detachment modeling of sea surface magnetic data suggest the detachment was active from 1 95 ma for about 1 ma during a period of reduced and asymmetric magmatic accretion modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the dome and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the dome

the magmatic hydrothermal transition in the lower oceanic crust clues from the ligurian ophiolites italy abstract the gabbroic bodies from the jurassic ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the mid atlantic ridge initial cooling of the ligurian gabbros is associated with local development of hornblende bearing felsic dykes and hornblende vein networks the hornblende veining is correlated with the widespread development of hornblende as coronas pseudomorphs after the clinopyroxene in the host gabbros in addition the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins the hornblendes from the felsic dykes and the hornblende rich rocks within the mantle sliver show a similar geochemical signature characterized by low mg cao and al2o3 negligible cl and high tio2 k2o ree y zr and nb concentrations the whole rock sm nd isotopic compositions of the felsic dykes and the hornblende rich rocks define a sm nd isochron corresponding to an age of 154 20 ma and an initial end of 9 2 0 5 the 18o of the hornblendes and coexisting zircons from these rocks about 4 5 and 5 8 respectively do not indicate the presence of a seawater component in these melts the formation of the felsic dykes and of the hornblende rich rocks within the mantle sliver involved sio2 rich silicate melts with negligible seawater component which presumably were derived from high degree fractional crystallization of mor type basalts the vein and the coronitic pseudomorphic hornblendes show high mg and cao significant cl 0 02 0 17 wt and low tio2 and k2o concentrations the coronitic pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and 18o values 1 0 to 0 7 close to seawater suggesting an origin by reaction between migrating seawater derived fluids and the host gabbros the vein hornblendes commonly show slight lree enrichment relatively high concentrations of nb up to 2 5 ppm and 18o ranging from 3 7 to 0 8 the crystallization of these hornblendes most likely involved both seawater and magmatic components

crustal structure of the trans atlantic geotraverse tag segment mid atlantic ridge 26 10 n implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges new seismic refraction data reveal that hydrothermal circulation at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 10 n is not driven by energy extracted from shallow or mid crustal magmatic intrusions our results show that the tag hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km and we find no evidence for low seismic velocities associated with mid crustal magma chambers our tomographic images support the hypothesis of tivey et al 2003 that the tag field is located on the hanging wall of a detachment fault and constrain the complex dome shaped subsurface geometry of the fault system modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation which may enhance footwall cooling however heat extracted from the footwall is insufficient for sustaining long term high temperature hydrothermal circulation at tag these constraints indicate that the primary heat source for the tag hydrothermal system must be a deep magma reservoir at or below the base of the crust

building of the deepest crust at a fossil slow spreading centre pineto gabbroic sequence alpine jurassic ophiolites this work presents new field and petrological data on a poorly known lower crustal section from the alpine jurassic ophiolites the pineto gabbroic sequence from corsica france the pineto gabbroic sequence is estimated to be 1 5 km thick and mainly consists of clinopyroxene rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gab bros in its deeper sector the sequence also encloses olivine rich troctolite and mantle peridotite bodies at dif ferent stratigraphic heights the composition and the lith ological variability of the pineto gabbroic sequence recall those of the lower crustal sections at slow and ultra slow spreading ridges the gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros which approximately constitute 2 3 of the section in particular the lower sector of the pineto gab broic sequence shows the existence of large scale frag ments of the deepest oceanic crust displaying a highly primitive bulk composition the mineral chemical varia tions document that the origin and the evolution of the pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation the clinopyroxenes from the olivine gabbros and the olivine rich troctolites also record the infiltration of olivine dissolving cr2o3 rich melts that presumably formed within the mantle into replacive dunite bodies cooling rates of the troctolites and the olivine gabbros were evaluated using the ca in olivine geospee dometer we obtained high and nearly constant values of 2 2 to 1 7 c year log units which were correlated with the building of the pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold litho spheric mantle

plate tectonics piecing together rifts earth s crust is formed where tectonic plates rift apart and upwelling magma solidifies disparate observations from rifts beneath the oceans and on land provide insights into the dynamics of rifting and opportunities for synthesis

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

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

phase relations and liquid lines of descent in hydrous ferrobasalt implications for the skaergaard intrusion and columbia river flood basalts crystallization experiments using a hydrous ferrobasalt as starting material conducted at 200mpa 940 12008c at a wide range of water activities 0 1 1 and redox conditions qfm 3 to qfm 4 where qfm is the quartz fayalite magnetite oxygen buffer show that h2o influences significantly the differentiation history of ferrobasaltic magmas a combination of our data with published experiments on dry ferrobasalt at 1atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical fe rich tholeiitic system under both dry and hydrous conditions the addition of h2o decreases liquidus temperatures and changes significantly the proportions temperature range and sequence of crystallizing mineral phases the dry liquidus is at about 11708c whereas the liquidus for h2o saturated melts is at 10608c the main phases crystallizing from h2o bearing ferrobasalt at the investigated conditions are olivine ol clinopyroxene cpx plagioclase pl magnetite mt hematite hm ilmenite ilm and amphibole am the phase assemblage is similar to that of the dry system except for the presence of hm at extremely oxidizing conditions and am at low temperatures 59508c and h2o saturated conditions the important observation made in this study is that the stability of fe ti oxides and in particular mt as well as the simultaneous coprecipitation of mtand ilm are almost independent of the activity of h2o ah2o in the system whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing ah2o the stabilities of oxides are controlled mainly by the redox conditions prevailing in the system the most pronounced effect of ah2o on the liquidus temperatures of silicates is observed for pl which shows a considerable delay in crystallization with progressive magma differentiation early crystallization of fe ti oxides in h2o bearing ferrobasaltic compositions precludes any significant fe enrichment during differentiation as fe enrichment is a characteristic feature of the skaergaard intrusion it implies that the skaergaard parental magma did not contain considerable amounts of water on the other hand our experiments indicate that the differentiation of some ferrobasaltic series from the columbia river flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles 0 5 3 wt h2o

evolution of the southwest indian ridge from 55 45 e to 62 e changes in plate boundary geometry since 26 ma from 55 45 e to 58 45 e and from 60 30 e to 62 00 e the ultraslow spreading southwest indian ridge swir consists of magmatic spreading segments separated by oblique amagmatic spreading segments transform faults and nontransform discontinuities off axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the swir individual magmatic segments show varying magnitudes and directions of asymmetric spreading which requires that the shape of the plate boundary has changed significantly over time in particular since 26 ma the atlantis ii transform fault grew by 90 km to reach 199 km while a 45 km long transform fault at 56 30 e shrank to become an 11 km offset nontransform discontinuity conversely an oblique amagmatic segment at the center of a first order spreading segment shows little change in orientation with time these changes are consistent with the clockwise rotation of two 450 km wide first order spreading segments between the gallieni and melville transform faults 52 60 e to become more orthogonal to spreading we suggest that suborthogonal first order spreading segments reflect a stable configuration for mid ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge axis that can more easily accommodate seafloor spreading

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

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

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

granular surface avalanching induced by drainage from a narrow silo using theory and experiments we investigate granular surface avalanching due to material outflow from a narrow silo the assumed silo geometry is a deep rectangular box of moderate spanwise width and small gap thickness between smooth front and back walls a small orifice deep below the free surface lets grains drain out at a constant rate the resulting granular flows can therefore be assumed quasi two dimensional and quasi steady over most of the surface descent history to model these flows we couple a kinematic model of deep granular flow with a dynamic model of shallow surface avalanching we then compare the calculated flow fields with detailed particle tracking measurements letting the silo ascend relative to the high speed camera to increase spatial resolution the results show that the avalanching surface shape and near surface flow are controlled by the spanwise gradient in subsidence velocity and how this gradient is in turn controlled by the height above orifice and the gap thickness whereas the deep flow pattern is rate independent shallow avalanching is paced by the granular rheology

diversity of melt conduits in the izu bonin mariana forearc mantle implications for the earliest stage of arc magmatism abstract magmatic processes during the earliest stage of subduction initiation are still not well understood we examined peridotites recovered from an exhumed crust mantle section exposed along the landward slopes of the northern izu bonin trench using the japan agency for marine earth science and technology9s remotely operated vehicle kaiko7000ii based on the cr cr cr al atomic ratio of spinel two distinctive groups 1 high cr 0 8 dunite and 2 medium cr 0 4 0 6 dunite occur close to each other and are associated with refractory harzburgite two distinctive melts were in equilibrium with these dunites a boninitic melt for the high cr dunite and a mid oceanic ridge basalt morb like melt for the medium cr dunite the tio 2 content of the latter melt is lower than typical morb compositions we suggest that the medium cr dunite was a melt conduit for a basalt recently reported from the mariana forearc that was erupted at the inception of subduction the wide range of variation in the cr s of spinels in dunites from the izu bonin mariana forearc probably reflects changing melt compositions from morb like melts to boninitic melts in the forearc setting due to an increase of slab derived hydrous fluids and or melts during subduction initiation

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

downward continued multichannel seismic refraction analysis of atlantis massif oceanic core complex 30 n mid atlantic ridge 1 detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the atlantis massif oceanic core complex occ contrasting notably with its conjugate ridge flank multichannel seismic mcs data are downward continued using the synthetic on bottom experiment sobe method providing unprecedented detail in tomographic models of the p wave velocity structure to subseafloor depths of up to 1 5 km velocities can vary up to 3 km s over several hundred meters and unusually high velocities 5 km s are found immediately beneath the seafloor in key regions correlation within situand dredged rock samples video and records from submersible dives and a 1 415 km drill core allow us to infer dominant lithologies a high velocity body ies found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the occ indicating a propagating magmatic source as the origin for this pluton s the western two thirds of the southern ridge is capped in serpentinite that may extend nearly to the base of our ray coverage the distribution of inferred serpentinite indicates that the gabbroic pluton s was emplaced into a dominantly peridotitic host rock presumably the mantle host rock was later altered via seawater penetration along the detachment zone which controlled development of the occ the asymmetric distribution of seismic velocities and morphology of atlantis massif are consistent with a detachment fault with a component of dip to the southeast the lowest velocities observed atop the eastern central dome and conjugate crust are most likely volcanics here an updated model of the magmatic and extensional faulting processes at atlantis massif is deduced from the seismic results contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow rate spreading centers

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

petrology and sr nd and pb isotope geochemistry of mid ocean ridge basalt glasses from the 11 45 n to 15 00 n segment of the east pacific rise 1 basaltic glasses from the geophysically well studied section of the east pacific rise epr between 11 45 n to 15 00 n range from normal mid ocean ridge basalts morb to transitional morb and their major element variations correlate with isotopic and trace element indices of enrichment to first order basalts enriched in na8 0 incompatible elements 87sr 86sr and 206pb 204pb but low in fe8 0 and 143nd 144nd are more prevalent along the shallow portions of the ridge axis in detail the samples can be divided into two chemical and geographical groups the southern bathymetric dome extending from the 11 45 n overlapping spreading center to 14 10 n and the northern group extending from 14 10 n to the orozco transform the boundary between these two groups is apparent in a change in isotopic composition results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11 45 n to 15 00 n segment of the epr a depleted mantle component a seamount type enriched mantle component and an indian morb like mantle component south of 14 10 n the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near ridge seamounts north of 14 10 n the low 206pb 204pb high 207pb 204pb indian morb like component exerts a major influence on the geochemical variability of the axial lavas regional averages of major element composition e g na8 0 and fe8 0 show relatively limited variability consistent with the restricted range in depth for this region and plot within the pacific field of the previously defined global trends major element variations among individual samples however parallel the global array and their correlation with indices of mantle enrichment supports the idea that the pacific type local trend results from small scale heterogeneities in the mantle beneath the epr our results also indicate that tectonic segmentation and magmatic boundaries are probably not causally related in this study area and that the sizes of the present magma chambers are not a dominant factor in determining the compositional variability of erupted lavas

geologic implications of seafloor character and carbonate lithification imaged on the domal core of atlantis massif abstract we document the seafloor character on atlantis massif an ocean core complex located at 30 n on the mid atlantic ridge with an emphasis on the distribution of carbonate features seafloor imagery near bottom backscatter and bathymetry were analyzed on the central dome and the western shoulder of the exposed footwall to the detachment and on the eastern block a hanging wall to the fault we merged argo ii still images to produce photo mosaics and evaluated these together with video imagery acoustic reflectivity and basic rock composition the seafloor was classified as unconsolidated sediment lithified carbonate crust consolidated carbonate cap exposed basement or rubble and the spatial distribution of each type was assessed unconsolidated sediment exposed basement and rubble were documented in all three regions studied lithified carbonate crust was also present on the western shoulder and eastern central dome consolidated carbonate cap was found on the eastern block the formation of the carbonate rock is interpreted to reflect precipitation and or sediment cementation via fluids derived from serpentinization both processes occur at the nearby lost city hydrothermal field the newly documented locations of seafloor carbonate lithification therefore mark pathways of past possibly recent fluid flux from subsurface water rock reaction zones and represent an additional constituent of the carbon cycling hosted by oceanic lithosphere

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

reactive flow as dominant evolution process in the lowermost oceanic crust evidence from olivine of the pineto ophiolite corsica the jurassic pineto ophiolite from corsica exposes a 1 km thick troctolite olivine gabbro sequence interpreted to represent a lowermost sector of the gabbroic oceanic crust from a ultra slow spreading system to constrain the petrogenesis of the olivine gabbros minor and trace element analyses of olivine forsterite 84 82 mol were carried out olivine from the olivine gabbros is depleted in incompatible trace elements sc v ti y zr and heavy rare earth elements with respect to olivines from associated troctolites depleted incompatible element compositions are also shown by olivine forsterite 86 mol from a clinopyroxene rich troctolite the incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization we propose that melts migrating through an olivine plagioclase crystal mush chemically evolved by reaction with the existing minerals changing in composition as it flowed upward the melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene generating olivine gabbro bodies within a troctolite matrix reactive flow was the major evolution process active in the 1 km crustal transect exposed at the pineto ophiolite producing lithological variations classically attributed to fractional crystallization processes

formation of levees troughs and elevated channels by avalanches on erodible slopes snow avalanches are typically initiated on marginally stable slopes with a surface layer of fresh snow that may easily be incorporated into them the erosion of snow at the front is fundamental to the dynamics and growth of snow avalanches and they may rapidly bulk up making them much more destructive than the initial release snow may also deposit at the rear base and sides of the flow and the net balance of erosion and deposition determines whether an avalanche grows or decays in this paper small scale analogue experiments are performed on a rough inclined plane with a static erodible layer of carborundum grains the static layer is prepared by slowly closing down a flow from a hopper at the top of the slope this leaves behind a uniform depth layer of thickness h stop at a given slope inclination due to the hysteresis of the rough bed friction law this layer can then be inclined to higher angles provided that the thickness does not exceed h start which is the maximum depth that can be held static on a rough bed an avalanche is then initiated on top of the static layer by releasing a fixed volume of carborundum grains dependent on the slope inclination and the depth of the static layer three different behaviours are observed for initial deposit depths above h stop the avalanche rapidly grows in size by progressively entraining more and more grains at the front and sides and depositing relatively few particles at the base and tail this leaves behind a trough eroded to a depth below the initial deposit surface and whose maximal areal extent has a triangular shape conversely a release on a shallower slope with a deposit of thickness h stop leads to net deposition this time the avalanche leaves behind a levee flanked channel the floor of which lies above the level of the initial deposit and narrows downstream it is also possible to generate avalanches that have a perfect balance between net erosion and deposition these avalanches propagate perfectly steadily downslope leaving a constant width trail with levees flanking a shallow trough cut slightly lower than the initial deposit surface the cross section of the trail therefore represents an exact redistribution of the mass reworked from the initial static layer granular flow problems involving erosion and deposition are notoriously difficult because there is no accepted method of modelling the phase transition between static and moving particles remarkably it is shown in this paper that by combining pouliquen forterre s j fluid mech vol 453 2002 pp 133 151 extended friction law with the depth averaged unicode stix x1d707 i rheology of gray edwards j fluid mech vol 755 2014 pp 503 544 it is possible to develop a two dimensional shallow water like avalanche model that qualitatively captures all of the experimentally observed behaviour furthermore the computed wavespeed wave peak height and stationary layer thickness as well as the distance travelled by decaying avalanches are all in good quantitative agreement with the experiments this model is therefore likely to have important practical implications for modelling the initiation growth and decay of snow avalanches for hazard assessment and risk mitigation

near solidus evolution of oceanic gabbros insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slow spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the mid atlantic ridge these new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin in turn this allows the gabbro solidus to be constrained to 860 30 c using amphibole plagioclase thermometry this is consistent with temperatures from associated secondary clinopyroxene magmatic amphibole which can be identified in almost all samples formed during metasomatism of a low porosity crystal mush by an evolved hydrous silicate melt these amphiboles are characterised by high f nb and f cl and low cl contents the amphibole forming reaction involved melt plagioclase and clinopyroxene amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals there is no evidence for a seawater component in the magmatic amphibole as would be expected if high temperature seawater ingress leads to flux melting as has recently been suggested however the ingress of seawater derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases these amphiboles are characterised by high cl b and cl f and low nb f and nb la the fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids