flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

fine scale segmentation of the crustal magma reservoir beneath the east pacific rise mid ocean ridges are composed of segmented faults and magma reservoirs seismic images from the east pacific rise show that the magma reservoirs are segmented on the same fine scale as the surface faults and that distinct lava eruptions are sourced from largely isolated magma lenses

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

magma flow directions in the sheeted dike complex at superfast spreading mid ocean ridges insights from iodp hole 1256d eastern pacific integrated ocean drilling program iodp hole 1256d successfully sampled a complete section of an intact oceanic crustal sheeted dike complex sdc from 1061 to 1320 meters below seafloor mbsf on a 15 ma old cocos plate a series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end member superfast spread 200 mm yr full rate oceanic crust results indicate that main ferromagnetic minerals are predominantly pseudo single domain titano magnetite crystals responsible for both anisotropy of magnetic susceptibility ams and magnetic remanence signals ams fabrics were reoriented into a geographic reference frame using magnetic remanence data and corrected for a counterclockwise rotation of the cocos plate relative to the east pacific rise epr ca 15 ma corrected ams fabrics were then compared with the orientations of chilled margins previously obtained from formation microscanner fms images of the sdc at hole 1256d for some samples taken from close to dike margins a dike normal orientation of the minimum ams axes kmin of prolate ams ellipsoids mean that the long axis kmax can be used to infer magma flow directions subvertical kmin orientations in the interior of the dikes however may have required settling or compaction of the magma shortly after intrusion thus rearranging the ams fabric despite this orientation of kmin axes orientation of kmax axes indicate a rather constant subhorizontal paleo flow direction suggesting that magmas most probably traveled to the surface considerable distances from source regions within the epr system

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

effects of plate boundary geometry and kinematics on mantle melting beneath the back arc spreading centers along the lau basin abstract the back arc spreading centers that extend along the lau basin exhibit trends in axial morphology crustal thickness and geochemistry which are opposite those typically observed at mid ocean spreading centers we develop 2d numerical models of mantle flow thermal structure and melting of the lau back arc tonga subduction system for each of three lau spreading centers valu fa ridge the eastern lau and the central lau our goal is to determine whether along strike variability in the circulation pattern or hydration within the mantle wedge could explain the trends observed we use present day plate and subducted slab geometries and velocities to explore a range of mantle potential temperature and water content scenarios and test whether predictions match observations of crustal thickness and water content of the magmas erupted at the spreading ridges within the range of mantle parameters tested we find that a potential temperature of 1300 c and source water contents greater than 0 22 wt are required to match observed crustal thickness and lava water contents at valu fa substantially less water in the sub arc mantle source is required to match the eastern lau and central lau observations at the same or higher mantle potential temperatures a small background hydration of 0 01 wt water in the mantle wedge is required to match the observations of water in the central lau magmas we predict that the arc and back arc melting regions are interconnected for all potential temperatures and sub arc water contents at the eastern lau spreading center and at the valu fa ridge while at the central lau they are only connected for cases when mantle potential temperature is 1400 c or greater we hypothesize that the longer lived eastern lau and central lau rifting and axial volcanism may have dehydrated the mantle wedge and slowed melt production beneath these spreading centers the valu fa ridge which is actively propagating into a more hydrated wedge associated with the tofua arc experiences enhanced melting relative to the other two spreading centers for the valu fa case we show fast subduction in combination with the proximity of the trench and spreading center results in enhanced upwelling and therefore increased crustal production slower subduction with a convergence rate of 45 mm yr a dry mantle and a 1350 c mantle potential temperature reduces the difference in crustal thickness between the central lau and valu fa to within 1 2 km in contrast for present day kinematics with a dry mantle and 1350 c mantle potential temperature our models predict the crustal thickness at valu fa to be 3 1 km thicker than central lau much closer to the observed values

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

paleomagnetism of tilted dikes in fast spread oceanic crust exposed in the hess deep rift implications for spreading and rift propagation tilted dikes occur within the pillow lava sheeted dike and gabbroic sections of 1 million year old fast spread oceanic crust exposed on the walls of the hess deep rift during recent alvin dives oriented samples from both tilted and vertical dikes were collected from several of the outcrops these are the first oriented paleomagnetic samples ever collected directly from outcrops on the ocean floor and were obtained with the help of a specially constructed geological compass remanent magnetic inclinations measured on the dike samples are significantly different from the near horizontal inclinations expected the present attitudes of the dikes and their remanent magnetizations indicate that the dikes have been technically rotated since their intrusion submersible observations from other dive programs that examined parts of nearby escarpments suggest that the structural complexity observed in the alvin study areas may be only a local phenomenon or at least not a pervasive characteristic of oceanic crust although tectonic rotations such as those observed may have taken place as a result of the opening of the hess deep rift crosscutting vertical dikes and other geologic considerations suggest that substantial tectonic rotations occurred at or very near the east pacific rise axis probably as a result of intermittent axial collapse the observed structures suggest that tectonic processes beneath fast spreading ridges and the internal structure of crust produced at them may be significantly more complex than previously assumed

paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the hess deep rift implications for axial processes at the east pacific rise 1 studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers we present paleomagnetic remanence data from 45 fully oriented samples from dikes gabbros and a small number of basaltic lavas from fast spread crust exposed along the hess deep rift over 25 km along this escarpment dikes and dike subparallel fault zones dip consistently away from the east pacific rise epr while lava flows dip toward the ridge underlying gabbro is less deformed but contains widely spaced low angle fractures tentatively interpreted as shear zones as expected from the crustal age 1 07 1 48 ma most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site overly steep and directionally scattered gabbro remanence and observed low angle shear structures within this unit are tentatively interpreted as the manifestation of three dimensional strain along anastomosing shear zones although some remanence directions are incompatible with any plausible deformation history and thus likely reflect orientation errors the overall data set is consistent with a model involving sequential rotations on 1 outward dipping epr parallel n s normal faults and 2 hess deep rift parallel e w normal faults average rotations for these sequential events are 22 to the east defined by the mean dike attitude and 10 to the south estimated by bathymetry respectively this model best explains the remanence data observed dikes and lava orientations presence of dike parallel fault zones and the observation of steep little deformed dikes cutting both east dipping dikes and faults the data support a structural model for spreading at the epr in which outcrop scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis because these faults form within the neovolcanic zone they are subject to burial and are expected to have subdued to little surface expression

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

primitive melt replenishment and crystal mush disaggregation in the weeks preceding the 2005 2006 eruption 9 50 n epr abstract the 2005 2006 eruption at 9 50 n east pacific rise provides an exceptional opportunity to investigate the magma plumbing system beneath a well studied ridge system the eruption was preceded by two years of increasingly intense seismicity and occurred from the same location as a previous and well characterized eruption in 1991 1992 here we use the crystal cargo of samples from this eruption to investigate magma reservoir processes in the lead up to the eruption as well as their temporal relationship to the seismicity that preceded it compositional zoning in some plagioclase crystals indicates primitive melt replenishment occurred roughly six weeks or less before the eruption this replenishing event is seen only in the crystals from the central region of the eruption 9 50 9 52 n this is also the area where the most primitive lava compositions are observed and together these observations support models of replenishment being spatially focused the short time between the input of a more primitive melt and eruption onto the seafloor suggests replenishment likely contributed to triggering the eruption rare resorbed plagioclase crystals and glomerocrysts of plagioclase and olivine some of which have rims far from equilibrium with their host melt suggest that disaggregation of a crystal mush occurred within a few days prior to eruption interstitial melt from within this mush zone must have been mixed back into the erupted lava a form of in situ crystallization thus the erupted magmas evolved in a replenished tapped magma reservoir in which at least a part of the crystallization occurred in situ

large rotation of the easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor one of the goals of the pito93 cruise along the perimeter of the easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study using a new orienting device the geocompass 14 oriented blocks were obtained during 6 dives conducted using the ifremer dsrv nautile paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and af demagnetizations and reveals a mean characteristic remanent magnetization chrm direction within each block with both normal and reverse polarities after elimination of data from 5 blocks situated outside the microplate and one block which bears an anomalous direction we have computed a mean chrm direction at dm 49 0 im 39 0 k 10 2 95 19 8 n 7 although the mean inclination is in agreement with the inclination of the recent dipole magnetic field d 0 i 43 the average declination is significantly different from this direction which we interpret as due to a bulk clockwise rotation of 48 5 11 after the bivariate average of the easter microplate over the past 2 3 ma this value is in excellent agreement with previous estimates based on remote geophysical measurements and models

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

diverse magma flow directions during construction of sheeted dike complexes at fast to superfast spreading centers abstract dike intrusion is a fundamental process during upper oceanic crustal accretion at fast to superfast spreading ridges based on the distribution of magma along fast spreading centers inferred from marine geophysical data models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends anisotropy of magnetic susceptibility ams fabrics from 48 fully oriented block samples of dikes from upper oceanic crust exposed at hess deep rift and pito deep rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models the ams is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape preferred orientation of opaque oxide and plagioclase crystals generally parallel to ams maximum eigenvectors most dike samples show normal ams fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal the distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern even after structural correction we use a kolmogorov smirnov test ks test to show that the distribution of bootstrapped flow lineation rakes from pito deep are not statistically distinct from hess deep and neither are distinguishable from oman and troodos ophiolite ams data magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted these results indicate distinct compositional sources feed melt that is injected into dikes at fast to superfast spreading ridges with no preference for subhorizontal or subvertical magma flow collectively results imply ephemeral melt lenses at different along axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge centered or off axis sources

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

paleomagnetic constraints on deformation of superfast spread oceanic crust exposed at pito deep rift 1 the uppermost oceanic crust produced at the superfast spreading 142 km ma 1 full spreading rate southern east pacific rise epr during the gauss chron is exposed in a tectonic window along the northeastern wall of the pito deep rift paleomagnetic analysis of fully oriented dike 62 and gabbro 5 samples from two adjacent study areas yield bootstrapped mean remanence directions of 38 9 8 1 16 7 15 6 n 23 area a and 30 4 8 0 25 1 12 9 n 44 area b both are significantly distinct from the geocentric axial dipole expected direction at 23 s regional tectonics and outcrop scale structural data combined with bootstrapped remanence directions constrain models that involve a sequence of three rotations that result in dikes restored to subvertical orientations related to 1 inward tilting of crustal blocks during spreading area a 11 area b 22 2 clockwise vertical axis rotation of the easter microplate a 46 b 44 and 3 block tilting at pito deep rift a 21 b 10 these data support a structural model for accretion at the southern epr in which outcrop scale faulting and block rotation accommodates spreading related subaxial subsidence that is generally less than that observed in crust generated at a fast spreading rate exposed at hess deep rift these data also support previous estimates for the clockwise rotation of crust adjacent to the easter microplate dike sample natural remanent magnetization nrm has an arithmetic mean of 5 96 a m 3 76 which suggests that they significantly contribute to observed magnetic anomalies from fast to superfast spread crust

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

seismological evidence for three dimensional melt migration beneath the east pacific rise the extent to which crustal processes along mid ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known models of mantle upwelling vary from two dimensional passive flow1 to three dimensional diapiric flow2 3 4 similarly beneath a ridge segment bounded by tectonic offsets crustal magma chambers may be replenished continuously along the ridge5 6 7 or at a central injection zone2 3 4 from which magma migrates towards the segment s ends here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the east pacific rise the anisotropy is consistent with two dimensional mantle flow diverging from the rise whereas the anomalous isotropic structure requires a three dimensional but continuous distribution of melt near the crust mantle interface our results indicate that crustal magma chambers are replenished at closely spaced intervals along axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle

magma flow directions inferred from field evidence and magnetic fabric studies of the streitishvarf composite dike in east iceland abstract anisotropy of magnetic susceptibility ams and rock magnetic studies have been made on three outcrops separated by 12 km along strike nne ssw on the streitishvarf composite dike in east iceland samples for this study have been collected from the inner quartz porphyry part of the dike which show clear field evidence of a lateral flow component from north to south at one of the sites this flow component is consistent with margin ams results from all three sites the quartz porphyry has a substantial bulk magnetic susceptibility 10 2 si mainly carried by magnetically soft titanium poor titanomagnetite mdf 15 mt the ferrimagnetic grains yield a characteristic remanent magnetization in all three sites which gives a virtual geomagnetic pole at latitude 52 6 s and longitude 319 6 e the degree of anisotropy is low p j 1 033 and the magnetic fabrics shifts from oblate to prolate shapes depending on dike margin and outcrop the magnetic fabric has been interpreted according to the imbrication model using the minor susceptibility axis as shear plane indicator the absolute directions given by the minor susceptibility are then quantified using vector algebra the magma flow is indicated as an upward directed flow flowing from north to south with an inclination between 30 and 64 with a 95 confidence ellipse of 3 9 a model for the intrusion of the streitishvarf dike has been constructed where a magma pocket with felsic magma is punctured by a mafic dike enabling the felsic magma to rise and extend to the south within the pathway created the results of this study confirm the applicability of ams in studies of magma flow directions in igneous dikes of felsic composition

tonga ridge and lau basin crustal structure from seismic refraction data 1 the crustal structure across the tonga lau arc back arc system from the lau ridge to the pacific plate 178 170 w 18 19 s is modeled using data from an 840 km long air gun refraction line over 19 ocean bottom seismometers and one land station the data reveal that the pacific plate crust is 5 5 km thick with a velocity structure similar to that found at the present day east pacific rise epr beneath tonga ridge an intermediate velocity layer 6 7 km s is up to 7 5 km thick and has a velocity depth distribution similar to andesitic rocks found in continental crust the crust is abnormally thin 4 km at the boundary between the tonga ridge and the lau basin at the east end of lau basin the crust is 5 5 6 5 km thick and resembles crust formed at the epr except for a thicker sheeted dike section 2 3 km and thinner lower crust 2 km the lau basin crust thickens to 7 8 km near the central lau spreading center clsc mostly through thickening of the lower crust the crust thickens again to 8 5 9 5 km at 50 km west of the clsc mostly through thickening of the midcrust in the thick westernmost section the crustal structure is uniform even though one part of this section formed through extension of arc type crust while the rest was created at an oceanic spreading center the relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center

geologic structure of the uppermost oceanic crust created at fast to intermediate rate spreading centers abstract geological investigations of major fault scarps tectonic windows and dsdp odp drill holes provide direct views of the uppermost oceanic crust generated at fast to intermediate rate spreading centers these areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing inward dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of outward dip widespread fracturing faulting and hydrothermal metamorphism accompanied magmatic construction these geological relationships can be interpreted in terms of dramatic asymmetrical subaxial subsidence of upper crustal rock units that diminishes across the very narrow few kilometers wide zone of lava accumulation and dike intrusion at the ridge axis this type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

geochemistry of dikes and lavas from the north wall of the hess deep rift insights into the four dimensional character of crustal construction at fast spreading mid ocean ridges 1 an investigation of 1 m y old dikes and lavas from the north wall of the hess deep rift 2 15 n 101 30 w collected during alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust the study area encompasses 25 km of an east west flow line representing 370 000 years of crustal accretion at the east pacific rise samples analyzed exhibit depleted incompatible trace element abundances and ratios la sm n 1 indices of fractionation mgo and incompatible element ratios la sm nb ti show no systematic trends along flow line rather over short 4 m and long 25 km distances significant variations are observed in major and trace element concentrations and ratios modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions these findings combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift dominated systems suggest a more realistic model of the magmatic system underlying the east pacific rise relative to the commonly assumed two dimensional model in this model melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir dikes emanating from these distinct reservoirs transport magma along axis resulting in interleaved dikes and host lavas with different evolutionary histories this model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin

direct evidence from anisotropy of magnetic susceptibility for lateral melt migration at superfast spreading centers 1 rare fault bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers we mapped the geologic structure of ocean crust produced at the east pacific rise epr and now exposed along steep cliffs of the pito deep rift near the northern edge of the easter microplate the upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking moderately to steeply southeast dipping dikes paleomagnetic remanence of oriented blocks of dikes collected with both alvin and jason ii indicate clockwise rotation of 61 related to rotation of the microplate indicating structural coupling between the microplate and crust of the nazca plate to the north the consistent southeast dip of dikes formed as the result of tilting at the epr shortly after their injection anisotropy of magnetic susceptibility of dikes provides well defined magmatic flow directions that are dominantly dike parallel and shallowly plunging corrected to their original epr orientation magma flow is interpreted as near horizontal and parallel to the ridge axis these data provide the first direct evidence from sheeted dikes in ocean crust for along axis magma transport these results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present

basaltic lava domes lava lakes and volcanic segmentation on the southern east pacific rise meter scale dsl 120 sonar mapping and coregistered argo ii photographic observations reveal changes in eruptive style that closely follow the third order structural segmentation of the ridge axis on the southern east pacific rise 17 11 18 37 s near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology the ubiquity of pillow lava suggests low effusion rate eruptions the abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices near segment centers we observe fewer but larger lava domes voluminous drained and collapsed lava lakes and smooth lobate and sheet lava flows with very little pillow lava the abundance of sheet flows suggests that high effusion rate eruptions are common fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources this pattern was observed on eight third order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale the third order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across axis width of the region of seismic layer 2a thickening along the one segment where sufficient cross axis seismic lines exist the geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries although rock sampling density is not adequate to constrain the variation across all the third order volcanic segments that we identify these observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading 140 km myr full rate mid ocean ridges

rapid mantle flow beneath the tonga volcanic arc abstract flow in the earth s asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates suggesting flow rates comparable to plate velocities however arc parallel shear wave splitting and geochemical observations imply along strike flow in the lau basin presenting a challenge to geodynamic models of plate driven subduction systems as plate driven flow models predict arc perpendicular fast direction with simple a type mineral texturing although b type mineral texturing would result in the fast directions developing 90 relative to a type and likely occurs in the forearc mantle such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail by tracking mineral texturing development within geodynamic flow models patterned after the tongan system in the lau basin that include along strike flow we find that the observations are best explained by models including rapid along arc flow within the subarc mantle the effect of melt on the anisotropy does not appear to be a primary control but needs to be investigated more fully the best fit to the observations is a model with along strike flow within a low viscosity channel beneath the arc at a rate of nearly 50 cm yr several times the plate motion velocities

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

determining paleo flow directions from groundmass fabrics in the lyttelton radial dykes new zealand abstract the poles to 010 of groundmass feldspar in radial dykes cutting the dissected miocene volcano lyttelton banks peninsula form generally oblique girdles about the paleo flow direction the obliquity is a response to velocity gradients of magma within the dykes and the sense of obliquity relative to each dyke wall provides a unique determination of the sense of flow the fabrics give support to the model of blanchard et al 1979 for the movement of rigid particles in flowing magmas this method of flow determination overcomes the problem inherent in most others which provide a flow line but no sense of movement along that line methods of sampling and examples of the feldspar fabrics are described together with the relationship of these fabrics to other dyke features such as secondary shears and stretched vesicles

using cosmogenic nuclides to contrast rates of erosion and sediment yield in a semi arid arroyo dominated landscape rio puerco basin new mexico analysis of in situ produced 10 be and 26 al in 52 fluvial sediment samples shows that millennialscale rates of erosion vary widely 7 to 366 m ma 1 through the lithologically and topographically complex rio puerco basin of northern new mexico using isotopic analysis of both headwater and downstream samples we determined that the semi arid rio puerco basin is eroding on average about 100 m ma 1 this rapid rate of erosion is consistent with estimates made using other techniques and is likely to result from a combination of easily eroded lithologies sparse vegetation and monsoon dominated rainfall data from 331 stream water samples collected by the us geological survey between 1960 and 1995 are consistent with basin wide average chemical denudation rates of only about 1 4 m ma 1 thus the erosion rates we calculate may be considered rates of sediment generation because physical weathering accounts for almost 99 per cent of mass loss the isotopic data reveal that sediment is generally well mixed downstream with the areaweighted average sediment generation rate for 16 headwater samples 234 ton km 2 a 1 for

axial crustal structure of the lau back arc basin from velocity modeling of multichannel seismic data located west of the tonga trench the lau back arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone within the basin lie two complementary intermediate rate spreading systems the central lau spreading center clsc and the eastern lau spreading center valu fa ridge elsc vfr that are positioned 170 to 40 km away from the active volcanic arc respectively multichannel seismic mcs images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc but also related to spreading rate morphology and petrology upper crustal refraction data selected from the along axis seismic lines collected during the 1999 mcs survey were modeled in both the time versus range t x and intercept time versus slowness p domains to provide validation and detail to the seismic reflection observations the results show that as both the proximity to the arc and the spreading rate decrease southward 1 seismic layer 2a thickens by 0 6 km between the clsc and vfr from 0 4 km to 1 0 km 2 the average depth of the axial magma chamber amc increases from 1 5 km at the clsc to 2 8 km at the southern vfr excluding the northern section of the elsc that shows no continuous amc reflector but does show an isolated melt sill 3 the upper crustal basement velocity decreases from 2 1 km s at the clsc to 1 8 km s at the vfr and 4 the velocities of both layer 2a and 2b decrease between the clsc and northern vfr from 3 2 to 2 5 km s and 5 0 to 3 9 km s respectively along with a broad axial high morphology these features of the clsc thinner layer 2a shallower amc and faster crustal velocities correlate best with the fast spreading east pacific rise and intermediate spreading juan de fuca ridge conversely the structural characteristics of the central elsc vfr have no known counterpart in the global mid ocean ridge system we attribute this primarily to the volatile concentration in the magmas coming from the tonga subduction zone

emplacement of magma in eastern iceland dikes insights from magnetic fabric and rock magnetic analyses abstract the anisotropy of magnetic susceptibility ams coupled with mineral magnetic identification has been investigated in about 640 samples from 27 basaltic dikes of tertiary age in eastern iceland the dikes are a few meters thick and they have been sampled all across three types of magnetic fabrics with well defined mean principal axes of anisotropy are recognized in the first one the maximum axis is vertical and the minimum axis coincides with the pole of the dike plan geometrically normal fabric the second fabric shows opposite orientation geometrically inverse fabric the third type is when these two fabrics coexist with the normal one at the margins and the inverse one in the center of the dike mixed fabric no relationship is observed between the type of fabric and the width of the dikes each fabric is carried by clear magnetic assemblages made of low x 0 15 ti content fine grained magnetites for the normal one and high ti content around tm60 coarse magnetite grains for the inverse fabric upon heating the magnetic properties and the orientation of the anisotropy ellipsoid characterizing the inverse fabric in the mixed dikes change and evolve toward the normal type these two geometrically normal and inverse fabrics are interpreted as primary ones representing different physico chemical conditions during the emplacement of the magma the ams results indicate that during the volcanic events occurring about 10 myr ago the extrusion of the magma was vertical at the depth of the studied sites in eastern iceland

early establishment of seafloor hydrothermal systems during structural extension paleomagnetic evidence from the troodos ophiolite cyprus abstract paleomagnetic data from the troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of large tonnage sulfide deposits at oceanic ridges we have sampled dikes from the troodos sheeted complex that have been subjected to variable hydrothermal alteration from greenschist alteration typical of the low water rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks epidosites altered within high water rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits these dikes are moderately to highly tilted from their initial near vertical orientations due to rotations in the hangingwalls of approximately dike parallel oceanic normal faults comparison of characteristic remanence directions from these dikes with the late cretaceous troodos reference direction therefore allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the late cretaceous troodos reference direction upon restoration of dikes to near vertical positions about a nnw trending horizontal axis these data along with field observations of focused alteration along normal faults suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation this sequence of events is similar to that observed for creation of large tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension we suggest that the dike parallel normal faults were initiated as extensional fractures during this early stage of crustal extension thus providing the necessary permeability for focused fluid flow and that later slip along these structures during rotational planar normal faulting caused reduction in permeability due to gouge formation

crustal faults exposed in the pito deep rift conduits for hydrothermal fluids on the southeast pacific rise 1 the escarpments that bound the pito deep rift northeastern easter microplate expose in situ upper oceanic crust that was accreted 3 ma ago at the superfast spreading 142 mm a full rate southeast pacific rise sepr samples and images of these escarpments were taken during transects utilizing the human occupied vehicle alvin and remotely operated vehicle jason ii the dive areas were mapped with a deformation intensity scale revealing that the sheeted dike complex and the base of the lavas contain approximately meter wide fault zones surrounded by fractured damage zones fault zones are spaced several hundred meters apart in places offset the base of the lavas separate areas with differently oriented dikes and are locally crosscut by younger dikes fault rocks are rich in interstitial amphibole matrix and vein chlorite prominent veins of quartz and accessory grains of sulfides oxides and sphene these phases form the fine grained matrix materials for cataclasites and cements for breccias where they completely surround angular to subangular clasts of variably altered and deformed basalt bulk rock geochemical compositions of the fault rocks are largely governed by the abundance of quartz veins when compositions are normalized to compensate for the excess silica the fault rocks exhibit evidence for additional geochemical changes via hydrothermal alteration including the loss of mobile elements and gain of some trace metals and magnesium microstructures and compositions suggest that the fault rocks developed over multiple increments of deformation and hydrothermal fluid flow in the subaxial environment of the sepr faults related to the opening of the pito deep rift can be distinguished by their orientation and fault rock microstructure some subaxial deformation increments were likely linked with violent discharge events associated with fluid pressure fluctuations and mineral sealing within the fault zones other increments were linked with the influx of relatively fresh seawater the spacing of the faults is consistent with fault localization occurring every 7000 to 14 000 years with long term slip rates of 3 mm a once spread from the ridge axis the faults were probably not active and damage zones likely played a more significant role in axial flank and off axis crustal permeability

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

magnetic polarity structure of the lower oceanic crust a newly developed rock drill was used to take the first fully oriented samples from in situ gabbros exposed on atlantis bank which formed 11 m y ago on the southwest indian ridge swir these paleomagnetic data locate the seabed outcrop of a normal polarity magnetic lineation within an outcrop of lower crust that correlates with overlying deep towed and sea surface magnetic field measurements a systematic lateral offset between the seabed boundary and measured anomalies implies that the reversal boundary dips 25 to the south away from the ridge axis paleomagnetic results suggest that following magnetization acquisition the platform tilted south by 20 range 33 south to 7 north implying an original reversal boundary dip of only 5