paleomagnetism of small basalt exposures in the west puget sound area washington and speculations on the accretionary origin of the olympic mountains twenty two sites from two small exposures of basalt of probable eocene age immediately east of the olympic mountains have the following mean paleomagnetic direction declination 176 5 inclination 66 5 circle of 95 confidence 8 6 this is the only paleomagnetic study of pre miocene rocks anywhere in western washington or oregon that matches the polar wandering curve for north america however a marked fanning of paleomagnetic declinations around the olympic core suggests that concordance in this case is fortuitous and results from oroclinal bending during or immediately after olympic orogenesis creation of the olympic mountains involved accretion of thickened basaltic crust followed by underthrusting of large quantities of oceanic sedimentary debris as the farallon plate pressed northeastward into a recess formed by the thick crystalline blocks of vancouver island and the north cascades

the global mio pliocene climatic equability and coastal ostracod faunas of southeast australia this study on the mio pliocene ostracod successions of southeast australia outlines several faunal events indicative of climate warming and or increased rainfall events ostracod faunas associated with a late late miocene sea level rise event suggest that the climate of this time in southeast australia was similar to or slightly warmer than that of present day southeast australia however it was probably wetter and significantly warmer than immediately preceding mid late miocene palaeoclimatic conditions within the region evidence for a change to wetter and warmer conditions during the late late miocene is seen in the appearance of various extant euryhaline and semi thermophilic ostracod species in coastal ostracod faunas the appearance of euryhaline species which are mostly absent from older shallow marine cenozoic strata of the bass strait hinterland suggests a major influx of fresh water into coastal marine settings which contributed to the initial phase of development of the southeast australian late neogene barrier coastline and associated marginal marine palaeoenvironments during the time interval latest miocene to earliest pliocene and during the early late pliocene two subsequent global sea level rise events are also preserved in the southeast australian coastal plain many of the species present in ostracod faunas associated with these two events are the same as in older local late late miocene faunas in earliest pliocene faunas there is minor evidence for the reappearance of semi thermophilic ostracods faunas of early late pliocene age often exhibit a conspicuous faunal dominance by or large abundance of euryhaline species indicating the particularly strong influence of fresh water influxes into coastal marine palaeoenvironments this may reflect the presence of especially wet local temperate palaeoclimatic conditions during a time of equable global climates succeeding estuarine lagoonal and coastal embayment ostracod faunas of late late pliocene age are associated with marginal marine sediments that are interbedded with coastal dune aeolianites this suggests an overall seaward retreat of marginal marine environments that was initiated by a major global sea level fall linked to the onset of cooler late pliocene and quaternary global climates

biostratigraphic strontium isotopic and geologic constraints on the landward movement and fragmentation of terranes within the tofino basin british columbia significant advancements in understanding the complex evolution of the tofino basin at a convergent accretionary margin are enabled by combining contextual geologic information with new isotopic and paleontological data a high resolution cenozoic chronostratigraphy of the basin is constrained by strontium isotope ages 36 9 1 3 ma of late eocene to pleistocene foraminifers together with a revised biostratigraphy foraminifers and ichthyoliths from six offshore wells and outcrop samples new specimen thermal alteration values and existing well log data these data are integrated with archival multichannel seismic and magnetic data to interpret offshore well positions with relation to sub basins and structural highs of the pacific rim and crescent terranes and other accreted strata six regions of the tofino basin are defined based on structure and depositional differences during the eocene to holocene history of accretion and fragmentation of the crescent terrane and it underthrusting the pacific rim

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

on the causes of mid pliocene warmth and polar amplification the mid pliocene 3 to 3 3 ma ago is a period of sustained global warmth in comparison to the late quaternary 0 to 1 ma ago and has potential to inform predictions of long term future climate change however given that several processes potentially contributed relatively little is understood about the reasons for the observed warmth or the associated polar amplification here using a modelling approach and a novel factorisation method we assess the relative contributions to mid pliocene warmth from elevated co2 lowered orography and vegetation and ice sheet changes the results show that on a global scale the largest contributor to mid pliocene warmth is elevated co2 however in terms of polar amplification changes to ice sheets contribute significantly in the southern hemisphere and orographic changes contribute significantly in the northern hemisphere we also carry out an energy balance analysis which indicates that that on a global scale surface albedo and atmospheric emmissivity changes dominate over cloud changes we investigate the sensitivity of our results to uncertainties in the prescribed co2 and orographic changes to derive uncertainty ranges for the various contributing processes

subsidence in magma chamber and the development of magmatic foliation in oman ophiolite gabbros abstract in the oman ophiolite the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified starting in the root zone of the sheeted dike complex rzsdc located above this horizon 18 sections down to the upper gabbros unit have been mapped in great detail in selected areas of the southern massifs of this ophiolite they are complemented by 133 sites located throughout the entire ophiolite where the transition from the rzsdc to the uppermost foliated gabbros is well exposed altogether half the sites and 11 cross sections display within a few tens of meters beneath the rzsdc a magmatic foliation which is parallel to the overlying sheeted dikes in the other sites and cross sections the gabbro foliation is either flat lying or steep but not parallel to the sheeted dikes compared to the rzsdc isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths in the topmost steeply foliated gabbros clinopyroxene is idiomorphic becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende moving down from these uppermost gabbros and over one hundred meters the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized granular gabbros of the gabbro unit these repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying main magma chamber the topmost gabbros beneath rzsdc which were expelled from the melt lens by drifting very soon after settling on the melt lens floor display in plagioclase a spectacular zoning pointing to a fast cooling moving downwards stronger foliation increased compaction and recovery recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude 50 to 500 yr over this vertical distance this field based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

shallow intrusive directions of sheeted dikes in the troodos ophiolite anisotropy of magnetic susceptibility and structural data sheeted dikes play a central role in the formation of oceanic crust it is commonly assumed that sheeted dikes intrude vertically upward from elongated mid ocean ridge mor magma chambers but there are no direct observational data bearing on this hypothesis this assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes the authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust the troodos ophiolite structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow 20 degree to nearly vertical upward magma transport directions these data suggest that horizontal emplacement has to be considered for sheeted dikes at mors implying more centralized mor plumbing systems than previously thought such plumbing systems provide ample opportunity for complex mixing fractionation and contamination of mor lavas in magma chambers and tabular magma storage volumes whether the mor magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems

permanent el nino and the onset of northern hemisphere glaciations mechanism and comparison with other hypotheses 1 the tropical pacific transitioned from a permanent el nino state that is without the current equatorial zonal gradient of sea surface temperatures to one with an equatorial cold tongue in a process that started 4 ma during the early pliocene the latter stages of this transition around 2 75 ma marked the onset of northern hemisphere glaciations we explore the hypothesis that this onset of glaciation is causally linked to the development of the cold tongue guided by atmosphere general circulation model agcm simulations forced by various hypothesized climate influences a unique feature of our simulations is imposing permanent el nino conditions not by imposing sea surface temperatures anomalies directly but rather through changing the implied ocean heat transport in the slab ocean component of the agcm done in such a way as to maintain global ocean heat balance similar to previous studies we find increased near surface temperature conditions occur during simulated permanent el nino conditions at the locations of known ice sheet growth during glacial periods we establish that this tropical extratropical connection is mediated through atmospheric stationary waves a well known atmospheric pathway for today s el nino influence on north america the influence of the permanent el nino condition is compared through model simulations to other mechanisms proposed to date to explain the onset of glaciations at 2 75 ma changes in atmospheric carbon dioxide co2 concentrations closure of the panama seaway and changes in north pacific seasonality the exact comparison is done by calculating differences in the equilibrium line altitude at the known locations of ice sheet growth an approach that permits the translation of climate anomalies into a measurement of glaciation trends in terms of glacier mass balance we find that permanent el nino and atmospheric co2 are the leading influences in this comparison leading us to conjecture that the two influences combined determined the long term background conditions governing ice sheet growth within this long term background evolution however we additionally find that the exact timing of northern hemisphere glaciations is likely to have come from favorable orbital conditions our results regarding permanent el nino on northern hemisphere glaciations is however conditioned on a strong sensitivity of northern hemisphere climate changes to how the oceanic heat compensation to the permanent el nino is achieved

quaternary fluvial history of the delaware river new jersey and pennsylvania usa the effects of glaciation glacioisostasy and eustasy on a proglacial river system abstract fluvial glacial and estuarine deposits in the delaware valley record the response of the delaware river to glaciation sea level change and glacioisostasy during the quaternary incision following an early pleistocene glaciation created the present valley which is inset into a pliocene strath and fluvial plain middle and upper pleistocene and holocene deposits were laid down in this inset valley estuarine terraces in the lower valley and bayshore at 20 m probably marine isotope stage mis 11 8 m mis 5e and 3 m mis 5a or c and a fluvial deposit that correlates to offshore mis 3 marine deposits at 20 m are at elevations consistent with glacioisostatic models successive incisions during lowstands in the middle and late pleistocene lengthened deepened and narrowed the channel in the lower valley and shifted the channel westward in delaware bay during mis 2 glaciation from 25 to 18 ka the delaware was diverted to the hudson shelf valley by glacioisostatic tilting most glacial sediment was trapped in fluvial lacustrine valley fills north of the terminal moraine incision of the valley fill was accomplished during the early stage of rebound between 17 and 12 ka drainage to the delaware shelf was restored between 15 and 13 ka as the forebulge collapsed during incision multiple postglacial terraces formed where the valley was perpendicular to rebound contours and so was steepened and elevated northward and a single terrace formed where the valley paralleled the contours and there was no differential elevation or steepening about 65 of the original volume of mis 2 glacial sediment remains in the main valley and most of the eroded volume is in the channel in the lower valley beneath holocene estuarine fill little glacial sediment reached the delaware or hudson shelf overbank deposition on the lower postglacial terrace and modern floodplain spans the holocene the volume of holocene sediment in the estuary and bay yields a basinwide denudation rate of about 20 m my

formation of fast spreading lower oceanic crust as revealed by a new mg ree coupled geospeedometer abstract a new geospeedometer is developed based on the differential closures of mg and rare earth element ree bulk diffusion between coexisting plagioclase and clinopyroxene by coupling the two elements with distinct bulk closure temperatures this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples as the existing mg exchange thermometer was calibrated for a narrow temperature range and strongly relies on model dependent silica activities a new thermometer is developed using literature experimental data when the bulk closure temperatures of mg and ree are determined respectively using this new mg exchange thermometer and the existing ree exchange thermometer this speedometer can be implemented for a wide range of compositions mineral modes and grain sizes applications of this new geospeedometer to oceanic gabbros from the fast spreading east pacific rise at hess deep reveal that the lower oceanic crust crystallized at temperatures of 998 1353 c with cooling rates of 0 003 10 2 c yr stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies together with existing petrological geochemical and geophysical evidence results from this new speedometry suggest that the lower crust formation at fast spreading mid ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal scale hydrothermal circulations the replenished melts become chemically and thermally evolved accumulate as small magma bodies at various depths feed the shallow axial magma chamber and may also escape from the mush zone to generate off axial magma lenses

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

clockwise rotation of the entire oman ophiolite occurred in a suprasubduction zone setting the oman ophiolite provides a natural laboratory for understanding oceanic lithospheric processes previous paleomagnetic and structural investigations have been used to support a model involving rotation of the ophiolite during formation at a mid oceanic microplate however recent geochemical evidence indicates the ophiolite instead formed in a nascent forearc environment opening the potential for alternative rotation mechanisms central to the conundrum is the contrast between ese to se magnetizations and nnw magnetizations from the northern and southern ophiolitic massifs respectively attributed previously to either differential tectonic rotations during spreading or complete emplacement related remagnetization of the southern massifs here we report new paleomagnetic data from lower crustal rocks of the southern massifs that resolve this problem sampling of a continuous section in wadi abyad reveals ene magnetizations in the dike rooting zone at the top of the lower crust that change systematically downwards to nnw directions in underlying foliated and layered gabbros this is only consistent with remagnetization from the base upwards replacing early remanences in layered and foliated gabbros completely but preserving original ene magnetizations at higher levels comparison with new data from wadi khafifah provides a positive fold test that shows this event occurred before late campanian structural disruption of the regional orientation of the petrologic moho these data show that the entire ophiolite experienced large intraoceanic clockwise rotation prior to partial remagnetization leading to a new tectonic model in which formation rotation and emplacement of the ophiolite are all linked to late cretaceous motion of arabia and roll back of the oman subduction zone

evidence for iceberg armadas from east antarctica in the southern ocean during the late miocene and early pliocene abstract sediments from ocean drilling program site 1165 in the indian ocean sector of the southern ocean off prydz bay contain a series of layers that are rich in ice rafted debris ird here we present evidence that ird rich layers at site 1165 at 7 4 8 and 3 5 ma record short lived massive discharges of icebergs from wilkes land and adelie land more than 1500 kilometers to the east of the depositional site this distant source of icebergs is clearly defined by the presence of ird hornblende grains with 40ar 39ar ages of 1200 1100 ma and 1550 1500 ma ages that are not found on the east antarctic continent in locations closer to site 1165 this observation requires enormous amounts of detritus carrying drifting icebergs most likely in the form of large icebergs these events probably reflect destabilization surge and break up of ice streams on the wilkes land and adelie land margins of the east antarctic ice sheet in the vicinity of the low lying aurora and wilkes basins they occurred under warming conditions but each coast seems to have produced ice rafting events independently at different times the data presented here constitute the first evidence of far traveled icebergs from specific source areas around the east antarctic perimeter launch of these icebergs may have happened during quite dramatic events perhaps analogous to heinrich events in the north atlantic

nature of the antarctic peninsula ice sheet during the pliocene geological evidence and modelling results compared abstract in this paper we examine the nature of the pliocene antarctic peninsula ice sheet by comparing the terrestrial and marine geological records of the antarctic peninsula and surrounding sea floor with estimated net snow accumulation in the region derived from numerical palaeoclimate model experiments pliocene geological data and our new modelling results are consistent and mutually supportive in suggesting that an ice sheet was present even during the warmest episodes of the pliocene the combined results suggest that the ice sheet in the antarctic peninsula is more robust to globally warmer conditions than is generally assumed at least up to the climatic limits examined in our study

moist morb axial magmatism in the oman ophiolite the evidence against a mid ocean ridge origin the oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid oceanic ridge basalt morb its internal structure is regarded by many as being directly comparable to ocean lithosphere from the east pacific rise however there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn because the morb like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites to some this implies that the entire ophiolite formed above a subduction zone others maintain that it formed at a true open ocean mor and that the water required to generate the arc like magmas derived from descending near axis hydrothermal fluids or from ancient subduction a popular compromise posits that the axial suite formed at a true mor and the later magmatism documents the initiation of obduction we test these models by reexamining the morb like character of the early axial lavas and dikes we show that fractionation trends require the presence of water at concentrations significantly higher than any open ocean morb instead trends are identical to those of backarc basin and intraoceanic forearc volcanics by showing that the entire ophiolite formed in a hydrous system we rule out all models in which the oman ophiolite was generated at an open ocean mor instead it formed at a submarine spreading center above a probably newly initiated subduction zone

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

paleomagnetism and counterclockwise tectonic rotation of the upper oligocene sooke formation southern vancouver island british columbia the age of the sooke formation on the southern coast of vancouver island british columbia canada has long been controversial prior paleomagnetic studies have produced a puzzling counterclockwise tectonic rotation on the underlying eocene volcanic basement rocks and no conclusive results on the sooke formation itself we took 21 samples in four sites in the fossiliferous portion of the sooke formation west of sooke bay from the mouth of muir creek to the mouth of sandcut creek after both alternating field af and thermal demagnetization the sooke formation produces a single component remanence held largely in magnetite which is entirely reversed and rotated counterclockwise by 35 12 this is consistent with earlier results and shows that the rotation is real and not due to tectonic tilting since the sooke formation in this region has almost no dip this rotational signature is also consistent with counterclockwise rotations obtained from the northeast tip of the olympic peninsula in the port

stratigraphy and paleoceanography of pliocene deposits of karaginsky island eastern kamchatka u s s r abstract the lithology and paleontology of pliocene deposits from karaginsky island off eastern kamchatka peninsula u s s r are described in order to provide a paleoceanographic reconstruction of this region of the eastern north pacific ocean the stratigraphic sequence is divided into three suites the limimtevayamian the ust limimtevayamian and the tusatuvayamian which correspond to three standard pacific diatom zones the thalassiosira oestrupii neodenticula koizumi n kamtschatica and n koizumi the entire sequence is further subdivided into six members and 18 distinct beds on the basis of molluscan assemblages and three members and seven beds on the basis of benthic foraminiferal assemblages marine ostracodes are described from the limimtevayamian suite the marine invertebrate assemblages from karaginsky island provide evidence that 1 ocean water temperatures were warmer than today about 4 ma and again about 3 5 3 0 ma 2 several arctic atlantic species of molluscs and ostracodes migrated to the eastern north pacific between 4 2 3 0 ma presumably through the proto bering strait 3 many species found in the karaginsky island pliocene represent pacific taxa which migrated through the arctic into the north atlantic during the late pliocene

mid pliocene warmth stronger greenhouse and stronger conveyor abstract three million years ago prior to the onset of northern hemisphere glaciation global mean temperatures may have been as much as 3 5 c warmer than at present we present evidence based on the carbon isotopic composition of marine organic matter that atmospheric co2 levels at this time were on average only about 35 higher than the preindustrial value of 280 ppm we also present carbon isotopic evidence for stronger thermohaline circulation in the atlantic ocean during the warmest intervals and propose that the north atlantic conveyor belt may act as a positive feedback to global warming by enhancing sea ice retreat and decreasing high latitude albedo based on our results it seems unlikely that the mid pliocene warm period was a doubled co2 world

paleomagnetism of ocean layers 2 and 3 evidence from the metchosin complex vancouver island abstract the early eocene metchosin complex is an ophiolitic sequence and is the northernmost part of the crescent terrane it was underthrust beneath wrangellia later in the eocene directions of magnetization in sheeted diabase layer 2 and gabbroic intrusions layer 3 from the complex are internally consistent and both normal and reversed polarities are present mean directions irrespective of sign differ not only from the expected eocene palaeofield but also from one another these divergences have not therefore been caused by tectonic rotation of the metchosin complex as a whole but by variable tilting of bodies within the complex it is argued that these tilts probably were imposed during the emplacement of the metchosin complex adjacent to and beneath wrangellia the systematic clockwise rotations found further south in eocene rocks of the crescent terrane of washington and oregon do not occur the remanent magnetizations of the gabbros of layer 3 are commonly strong and stable these are the only rocks in the metchosin complex that had they been part of the present oceanic crust would be capable of contributing to marine magnetic anomalies the stable magnetization of diabase of layer 2 is too weak the dominant magnetization being a soft viscous remanence imposed by the present geomagnetic field the remanence of basalt lavas of layer 2 as previous work on the metchosin complex has shown is weak and scattered in direction

magma flow inferred from ams fabrics in a layered mafic sill insizwa south africa the insizwa sill is a 25 km diameter 1000 m thick layered mafic intrusion part of the karoo igneous province in south africa the peridotitic and gabbronoritic rocks are undeformed and mineral fabrics demonstrably result from magma flow a horizontal centimeter scale model layering is visible in numerous outcrops plagioclase crystals are both tabular and elongated their preferred orientation parallel to the layering forms a foliation and a nw se lineation respectively interpreted as the magma flow plane and flow direction throughout the 78 stations of this study 699 specimens magnetic susceptibilities km range from 750 to 10 000 10 6 si the magnetic anisotropy pj ranges from 1 03 to 1 08 magnetic ellipsoids are both prolate and oblate average tjc0 anisotropy of magnetic susceptibility ams fabrics are dominated by multidomain to pseudo single domain magnetite high field magnetic experiments indicate that the paramagnetic contribution from the mafic silicates is less than 50 for low susceptibility rock types the anisotropy results from magnetite grain shape solely as shown by no significant increase in pj with increasing km the magnetic lineation 305j 0 5j is consistent throughout the sill at various scales and coincides with the mineral lineation in average in contrast the magnetic foliation 125j ne 10j is generally perpendicular to the mineral foliation and to the layering several explanations for this odd configuration are discussed the variations of magnetic parameters across the layering and field observations point to a multiple injection the magnetic lineation is consistent with the presence of a single feeder dike situated to the se of the sill d 2002 elsevier science b v all rights reserved

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

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

micropaleontological evidence for increased meridional heat transport in the north atlantic ocean during the pliocene the middle pliocene approximately 3 million years ago has been identified as the last time the earth was significantly warmer than it was during the last interglacial and holocene a quantitative micropaleontological paleotemperature transect from equator to high latitudes in the north atlantic indicates that middle pliocene warmth involved increased meridional oceanic heat transport

pliocene pleistocene evolution of eastern tropical pacific surface water circulation and thermocline depth we evaluated the response of eastern equatorial pacific upper water column hydrography to the panama seaway closure and the initiation of large scale northern hemisphere glaciation the 18o gradient between globorotalia tumida and globigerinoides sacculifer indicates a general shoaling of the thermocline between 4 2 and 3 0 ma which may be related to a recorded decrease in southeast trade wind strength the 13c gradients suggest that surface water nutrient concentrations have increased during the last 5 myr and that upwelling intermediate waters began to change sources at 3 2 ma changes in the trans pacific east west temperature and nutrient gradients at 4 0 and 1 5 ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation changes in the sensitivity of surface circulation to milankovitch forcing 18o records linearly related to orbital variations only after 3 2 ma are associated with the seaway closure pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to milankovitch forcing is enhanced by air sea interactions which maintain the relatively steep pleistocene trans pacific gradients the 13c records are dominated by the 41 kyr period and are usually coherent with high latitude climate suggesting that associated changes in the global carbon reservoir dominate the tropical 13c signals

what do variable magnetic fabrics in gabbros of the oman ophiolite reveal about lower oceanic crustal magmatism at fast spreading ridges the magmatic processes responsible for accretion of the lower oceanic crust remain one of the least constrained components of the global seafloor spreading system samples of gabbroic rocks recovered by scientific ocean drilling are too limited to allow effective assessment of spatial variations in magmatic flow within in situ lower crust extensive exposures of gabbros in ophiolites on the other hand provide opportunities to study accretion processes in three dimensions across wide areas and at a resolution that allows variations in magmatic fabrics through the crust to be quantified here we show that magnetic anisotropy provides a reliable proxy for lower crustal magmatic fabrics in the world s largest ophiolite in oman important differences in magnetic fabrics are detected that reflect variations in magmatic processes on a range of scales fabrics in layered gabbros are aligned with modal layering and display a consistency in the orientation of maximum principal axes of anisotropy between localities at a regional scale these fabrics are compatible with subhorizontal preferred alignment of crystals orthogonal to the inferred orientation of the oman spreading axis resulting from magmatic flow or deformation of melt rich crystal mushes during spreading in contrast magnetic anisotropy in foliated gabbros at the top of the lower crust reveals for the first time distinctly different linear and anastomosing fabric styles between localities sampled at the same pseudostratigraphic level these differences reflect spatial variations in the style and trajectory of flow in the crystal mush beneath the axial melt lens during upward melt migration at the spreading axis

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

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

the role of co2 decline for the onset of northern hemisphere glaciation the pliocene pleistocene transition ppt from around 3 2 to 2 5 million years ago ma represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern eurasia and north america paleo evidence indicates that the ppt was accompanied and possibly caused by a decrease in atmospheric co2 but the temporal resolution of co2 reconstructions is low for this period of time and uncertainties remain large therefore instead of applying existent co2 reconstructions we solved an inverse problem by finding a schematic co2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records to this end we performed an ensemble of transient simulations with an earth system model of intermediate complexity from which we derived a best guess transient co2 scenario for the interval from 3 2 to 2 4 ma that gives the best fit between the simulated and reconstructed benthic 18o and global sea surface temperature evolution our data constrained co2 scenarios are consistent with recent co2 reconstructions and suggest a gradual co2 decline from 375 425 to 275 300 ppm between 3 2 and 2 4 ma in addition to a gradual decline the best fit to paleoclimate data requires the existence of pronounced co2 variability coherent with the 41 kyr 1 kyr 1000 years obliquity cycle in our simulations the long term co2 decline is accompanied by a relatively abrupt intensification of northern hemisphere glaciation at around 2 7 ma this is the result of a threshold behaviour of the ice sheets response to gradual co2 decrease and orbital forcing the simulated northern hemisphere ice sheets during the early pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m both ice volume and benthic 18o are dominated by 41 kyr cyclicity our simulations suggest that before 2 7 ma greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation a fully glaciated greenland comparable to its present day ice volume is modelled only during glacial maxima after 2 7 ma and more continuously after 2 5 ma

high eustatic sea level during the middle pliocene evidence from the southeastern u s atlantic coastal plain the middle pliocene approximately 3 5 2 5 ma was a period of global warmth preceding the growth of major northern hemisphere ice sheets the authors report on eustatic sea level for the middle pliocene based on microspaleontologic study of marine deposits of the duplin formation of south carolina and north carolina the duplin was deposited during a middle pliocene marine transgression that formed the orangeburg scarp a prominent wave cut geomorphic paleoshoreline of the southeastern u s atlantic coastal plain they concluded that 1 the scarp in south carolina was formed mostly during the middle pliocene 3 5 3 0 ma 2 eustatic sea level was about 35 plus minus 18 m higher than modern sea level the scarp has been uplifted about 50 65 m since the middle pliocene and 3 ocean water temperatures along eastern north america were warmer when the scarp was formed that they are at present

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

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

pliocene climate variability northern annular mode in models and tree ring data the northern annular mode nam and its regional expression the north atlantic oscillation nao are the primary interannual oscillatory systems in the northern hemisphere in the modern climate nam has been linked with a number of weather extremes including large terrestrial temperature increases and extreme snowfalls its connection to climate is more controversial although a change in its behaviour between 1970 and 2000 coincided with rapid temperature increases in the northern hemisphere the north atlantic and nordic seas are a key component in pliocene climate showing the largest increases from modern sea surface temperature to understand these changes and the ability of climate models to reproduce them we must consider simulations of the nam here we show that existing mid pliocene simulations exhibit large changes to the mean state and variability in the north atlantic and a significant dampening of the nao through sensitivity experiments this change is primarily attributed to the impact of the lowering of the rocky mountains as the timing of rocky mountains uplift is still disputed simulations of the north atlantic region contain significant uncertainty particularly relating to interannual variability and its climate feedbacks new high temporal resolution climate proxy data is required to test these model reconstructions here we report new annual resolution data and an analysis of climate variability from fossil tree rings these fossils from sites in the canadian arctic provide support for a strong north atlantic oscillation during the pliocene

constraints on the amplitude of mid pliocene 3 6 2 4 ma eustatic sea level fluctuations from the new zealand shallow marine sediment record ice volume calibrations of the deep ocean foraminiferal delta18o record imply orbitally influenced sea level fluctuations of up to 30m amplitude during the mid pliocene and up to 30 per cent loss of the present day mass of the east antarctic ice sheet eais assuming complete deglaciation of the west antarctic ice sheet wais and greenland these sea level oscillations have driven recurrent transgressions and regressions across the world s continental shelves wanganui basin new zealand contains the most complete shallow marine late neogene stratigraphic record in the form of a continuous cyclostratigraphy representing every 41 and 100ka sea level cycle since ca 3 6ma this paper presents a synthesis of faunally derived palaeobathymetric data for shallow marine sedimentary cycles corresponding to marine isotope stages m2 100 ca 3 4 2 4ma our approach estimates the eustatic sea level contribution to the palaeobathymetry curve by placing constraints on total subsidence and decompacted sediment accumulation the sea level estimates are consistent with those from delta18o curves and numerical ice sheet models and imply a significant sensitivity of the wais and the coastal margins of the eais to orbital oscillations in insolation during the mid pliocene period of relative global warmth sea level oscillations of 10 30m were paced by obliquity

ongoing oroclinal bending in the cascadia forearc and its relation to concave outboard plate margin geometry author s finley t morell k leonard l regalla c johnston st zhang w abstract 2019 geological society of america the concave inboard concave toward the overriding plate geometry of most convergent margins is considered a natural consequence of the depression of the edge of a thin spherical cap whereas concave outboard margin segments commonly form around indenters on the subducting plate at the cascadia subduction zone the apex of a g 500 km long concave outboard bend in the trench presently shows no obvious subduction of an indenter but does coincide with the axis of an outboard facing concavity in upper plate rocks arched around the olympic peninsula in northwestern washington state usa here we synthesize paleomagnetic and structural data together with new analyses of global navigation satellite system data to show that the upper plate at cascadia has been folded from the miocene to the present into an orocline with an axial trace that bisects the olympic peninsula the processes that accommodate bending which we suggest include 1 folding by flexural slip on the orocline limbs and 2 shortening uplift and escape within the core of the fold at the olympic mountains have the combined result of relative motion of the forearc towards the arc at the core of the orocline and sustained opposing rotations of the upper plate on the orocline limbs we propose that oroclinal bending is promoted and maintained by along strike variations in plate boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of concave outboard margins without the need for a subducting indenter

the eocene southern vancouver island orocline a response to seamount accretion and the cause of fold and thrust belt and extensional basin formation abstract a deflection of the fault controlled southwestern coastline of vancouver island suggests the presence of a minor orocline with a southern crustal block south of barkley sound alberni inlet rotated 20 counterclockwise relative to a northern fixed crustal block about a pole of rotation located northeast of port alberni in this paper two models of orocline development one of pure block rotation and one of pure bending are proposed the predictions of these models are tested against available geological maps structural orientation data identified regions of extension and contraction and paleomagnetic data structural orientation and paleomagnetic data are consistent with 18 of post late cretaceous counter clockwise rotation of the southern crustal block relative to the northern fixed crustal block a southward increase in the magnitude of rotation evident in the structural orientation data argues for a model of bending both bending and block rotation models predict the development of a zone of contraction along the northeast margin of the southern crustal block coincident with the location of the eocene cowichan fold and thrust belt that diminishes northward toward the pole of rotation as predicted the fold and thrust belt is characterized by a northerly decrease in the amount of shortening from 30 at the south end of the thrust belt to 0 shortening north of port alberni the northerly decrease in shortening is complemented by a north to south change in structural style from cylindrical to conical folds and finally to planar undeformed strata the model of block rotation predicts the presence of a zone of extension extending southwest from the zone of rotation coincident with the location of eocene extensional structures within barkley sound and with horst and graben structures in the offshore eocene to miocene tofino basin extension is less than predicted by a model of pure block rotation and suggests that much of the oroclinal rotation was accommodated by bending timing constraints indicate that orocline development was coeval with and resulted from the eocene accretion of seamounts of the crescent terrane these findings demonstrate that oroclinal orogeny the buckling of a linear crustal beam about vertical axes of rotation can significantly impact the geometry structure and character of an orogenic belt even where the buckles are minor

variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge plagioclase crystal size distributions in gabbros from the oman ophiolite 1 analysis of crystal size distributions csd of plagioclase in gabbros from the oman ophiolite indicates that cooling rates in the lower crust beneath a medium to fast spreading oceanic ridge did not vary smoothly with depth cooling rates in the upper half of the gabbro section were approximately 1 5 2 times faster than in the lower half of the gabbro section during igneous crystallization and or high temperature grain growth processes our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism instead the oman csd data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced near vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

evidence from gabbro of the troodos ophiolite for lateral magma transport along a slow spreading mid ocean ridge the lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid ocean ridge segmentation and variations in crustal thickness direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust but such studies can be made on ophiolites fragments of oceanic crust accreted to a continent in the oman ophiolite small scale radial patterns of flow have been mapped along what is thought to be the relict of a fast spreading mid ocean ridge here we present evidence for broad scale along axis flow that has been frozen into the gabbro of the troodos ophiolite in cyprus thought to be representative of a slow spreading ridge axis the gabbro suite of troodos spans nearly 20 km of a segment of a fossil spreading axis near a ridge transform intersection we mapped the pattern of magma flow by analysing the rocks magnetic fabric at 20 sites widely distributed in the gabbro suite and by examining the petrographic fabric at 9 sites we infer an along axis magma flow for much of the gabbro suite which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow spreading mid atlantic ridge