reverse slip structures at oceanic diverging plate boundaries and their kinematic origin data from tertiary crust of west and south iceland abstract normal faulting predominates within rift zones but reverse fault plane solutions are occasionally found for earthquakes along the mid atlantic ridge furthermore field observations of striae and in situ measurements in places show off rift compression and reverse faulting we analyse eight cases of reverse slip motion observed in the eroded tertiary crust of the borgarfjor ur and hreppar rift jump blocks respectively in west and south iceland these structures were the only ones observed where both striae and marker horizons could be used to deduce the sense of reverse slip motion the vertical displacements are less than 10 m and the motions are of local origin we believe these structures are formed independently from regional compression as they have variable kinematic origins some occur in association with dykes sills or cone sheet without evidence of horizontal shortening across intrusions others are due to local bends of regional steeply dipping normal faults or secondary fractures riedel or secondary fractures reverse slip striations are found at the irregularities of steeply dipping normal faults or at the tips of steeply dipping normal faults uplifted by an underlying propagating dyke striations even conspicuous ones do not necessarily reflect significant amounts of displacement if horizontal compression acts in iceland its magnitude is not sufficient to shorten the rocks and to close the fractures of various tectonic settings the observed examples are explained as local kinematic adjustments to companion structures without regional compressional stress field or shortening and can lead to misinterpretation if not put into the correct context

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

a different pattern of ridge segmentation and mantle bouguer gravity anomalies along the ultra slow spreading southwest indian ridge 15 30 e to 25 e abstract the results of a recent bathymetric and geophysical investigation of a 650 km long portion of the very slowly opening 16 mm yr full rate southwest indian ridge swir between 15 30 e and 25 e are presented bathymetry and mantle bouguer gravity anomalies mba caused by variations in crustal thickness and or crustal and upper mantle densities show different characteristics from those observed at faster spreading centers like the mid atlantic ridge mar 20 30 mm yr full rate with the exception of the du toit transform none of the ridge axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off axis traces the mba patterns associated with individual segments are much more complex than the simple circular bull s eyes lows reported along the mar while the short wavelength ridge segment length is comparable to that of the mar there is little correlation with mba amplitude and segment length and axial relief furthermore an eastward propagating magma source and an 84 km long zone of oblique spreading appears to define a fundamental boundary along the swir between two 250 300 km long sections characterized by distinctly different axial morphology and gravity signatures we interpret these results to indicate a long wavelength segmentation pattern of the underlying upwelling mantle melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes fissure eruptions within the rift valley build linear ridges defining a short wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle our results and interpretation are quite different than that predicted by extending current models of the faster spreading mar to these ultra slow spreading rates

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

seismic gap beneath logachev seamount indicator for melt focusing at an ultraslow mid ocean ridge ultraslow spreadingmid ocean ridges have a low magma budget and melt is distributed unevenly along the ridge axis there is little or no basaltic crust between isolated magmatic centers the processes that focus melts to segments of robust magmatism are not yet understood during a seismic survey of the ultraslow spreading knipovich ridge in the norwegian greenland sea with ocean bottom seismometers we discovered a seismic gap in the upper mantle beneath logachev seamount where micro earthquakes clearly delineate a shallowing of the maximum depth of faulting a topography of the lithosphere that allows melts to travel laterally along its base and rise in areas of thin lithosphere has been proposed as a possible mechanism to explain the focusing of melts at volcanic centers but has never been confirmed observationally our results are the first geophysical evidence for an along axis variation of the lithospheric thickness at an ultraslow spreading ridge

microearthquake activity lithospheric structure and deformation modes at an amagmatic ultraslow spreading southwest indian ridge segment while nascent oceanic lithosphere at slow to fast spreading mid ocean ridges mor is relatively well studied much less is known about the lithospheric structure and properties at ultraslow mors here we present microearthquake data from a 1 year ocean bottom seismometer deployment at the amagmatic oblique supersegment of the ultraslow spreading southwest indian ridge a refraction seismic experiment was performed to constrain upper lithosphere p velocities and results were used to construct a 1d velocity model for earthquake location earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures frequent earthquake activity extends to 31 km beneath the seafloor indicating an exceptionally thick brittle lithosphere and an undulating brittle ductile transition that implies significant variations in the along axis thermal structure of the lithosphere we observe a strong relation between petrology microseismicity distribution and topography along the ridge axis peridotite dominated areas associate with deepest hypocenters vast volumes of lithosphere that deforms aseismically as a consequence of alteration and the deepest axial rift valley areas of basalt exposure correspond to shallower hypocenters shallower and more rugged axial seafloor focal mechanisms deviate from pure extension and are spatially variable earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges instead the seismicity band sharply terminates to the south immediately beneath the rift boundary considering the deep alteration large steep boundary faults might be present but are entirely aseismic

dinosaurs on the north slope alaska high latitude latest cretaceous environments abundant skeletal remains demonstrate that lambeosaurine hadrosaurid tyrannosaurid and troodontid dinosaurs lived on the alaskan north slope during late campanian early maestrichtian time about 66 to 76 million years ago in a deltaic environment dominated by herbaceous vegetation the high ground terrestrial plant community was a mild to cold temperate forest composed of coniferous and broad leaf trees the high paleolatitude about 70 to 85 north implies extreme seasonal variation in solar insolation temperature and herbivore food supply great distances of migration to contemporaneous evergreen floras and the presence of both juvenile and adult hadrosaurs suggest that they remained at high latitudes year round this challenges the hypothesis that short term periods of darkness and temperature decrease resulting from a bolide impact caused dinosaurian extinction

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

hydrothermal seismicity beneath the summit of lucky strike volcano mid atlantic ridge abstract we present two years july 2007 july 2009 of earthquake locations from a local seismological network on the lucky strike volcano mid atlantic ridge whose summit hosts one of the most active and largest deep sea hydrothermal fields known two clusters of small m l

hyposat an enhanced routine to locate seismic events a program package called hyposat has been under development that attempts to use the maximum information possible to estimate the hypocenter of a seismic source the standard input parameters can be used arrival times of first and later onsets with backazimuths and ray parameters or apparent velocities in addition travel time differences between different phases observed at the same station can be optionally used the observed standard deviations are used to weight all input parameters and the inversion is done with a generalized matrix inversion code

microearthquake seismicity of the mid atlantic ridge at 5 s a view of tectonic extension we report measurements made with an ocean bottom array which was operated for 10 days on the mid atlantic ridge just south of the 5 s transform fault fracture zone a total of 148 locatable earthquakes with magnitudes 0 5 2 8 were recorded seismic activity appears to be concentrated within the western half of the median valley the median valley seismic zone is bounded in along axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south a few scattered events occurred within the inside corner high on the transform fault and in the western sidewall close to the segment center earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle although a few crustal earthquakes also occurred the presence of earthquakes in the mantle indicates that it is not strongly serpentinized we infer the median valley seismic activity to primarily arise from normal faulting

seismicity and structure of the 85 e volcanic complex at the ultraslow spreading gakkel ridge from local earthquake tomography accretion mechanisms at ultraslow spreading ridges are still only poorly understood due to difficult survey conditions for seismic experiments at these ridges melts gets focused in distinct magmatic centres with thin crust in between at the 85 e 85 n volcanic complex at gakkel ridge arctic ocean where a recent spreading episode has been observed perennial sea ice cover challenges traditional investigations of seismic structure and microseismicity we used an unusual survey set up to gather seismological data during 16 days in july 2007 using seismometer arrays mounted on ice floes despite only 12 stations the drift of the ice floes over the survey area resulted in a multitude of crossing rays in the rift valley the data included 303 microearthquakes of which 248 events could be confidently located we compiled a 1d velocity model by localizing a subset of these earthquakes with a suite of randomly created velocity models in this model the moho is placed at 7 km depth below seafloor inferring a thick basaltic crust using 124 events which were recorded by at least two arrays we inverted for p wave velocity structure in a local earthquake tomography resolution tests indicate reliable results in the central rift valley illuminating the thermal structure underneath the asgard volcanic chain in the aftermath of its most recent spreading episode our results show microearthquake activity down to 16 km beneath seafloor inferring a cold lithosphere most hypocentres cluster at the centre of the rift valley at the site the asgard volcanic chain this may mean that existing thermal models for this class of ridges have to be refined an area of decreased seismic velocities crosses the rift valley at this location and microearthquake activity is located at its eastern fringe where the velocity gradient is highest we therefore speculate that the reduced velocities may be caused by warm intruded material and that the observed seismicity predominatly reflects the relaxation of thermal stresses following recent intrusion

influence of pressure and mineralogy on seismic velocities in oceanic gabbros implications for the composition and state of the lower oceanic crust 1 we have analyzed the relationship between p wave velocities measured at pressures of 40 100 and 200 mpa and modal mineralogy in oceanic gabbro samples from ocean drilling program holes 735b 894g and 923a at all pressures increasing velocities correspond to increasing pyroxene contents and decreasing alteration phyllosilicate and amphibole content but indicate little or no variation of olivine content in part because the olivine compositions are in the range fo65 to fo73 a voigt reuss hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low even relative to fo73 possibly because the olivine grains contain ubiquitous networks of cracks even if the cracks are not present in situ on average gabbros with velocities typical of seismic layer 3 6 7 7 0 km s 1 contain 5 15 alteration products including 5 15 amphibole and 0 5 phyllosilicates these results suggest that the lower crust is on average slightly to moderately altered

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

what settlements leave behind pxrf compositional data analysis of archaeological layers from tell el fara in buto egypt using machine learning abstract modern portable and handheld xrf devices pxrf allow quick measurement of large geochemical datasets without the necessity for laboratory facilities such facilities are rare in egypt and modern dating techniques which are indispensable in geoarchaeology to establish a robust chronology are not available as sample transport is restricted and egypt does not provide osl or ams 14c laboratories with these preconditions we evaluate the usability of pxrf geochemical data for the dating of archaeological sediments with machine learning techniques the sample material was collected via sampling of archaeological sections and profile walls from archaeological excavations in the northwestern nile delta at the settlement site buto tell el fara in and kom el gir additionally samples were taken from sediments and cultural layers uncovered from their surroundings using vibracore corings with open steel auger heads in this methodological approach we examine the applicability of pxrf methods and test the sample data for distinct geochemical differences between the main settlement phases with multivariate statistical methods the dating is based on the training of artificial neural networks with pxrf data from archaeological material of well dated context to date test data of cultural layers within the vibracores this allows us to link fundamental changes in the landscape with the settlement history of buto and neighboring tells

characteristics of tectonomagmatic earthquake swarms at the southwest indian ridge between 16 e and 25 e summary the ultraslow spreading southwest indian ridge swir is a prominent end member of the global mid ocean ridge system it spreads with a full rate of 14 16 mm y 1 and shows several segments of various obliquities the western swir consists of the oblique and orthogonal supersegments lying at an epicentral distance of 21 to the vna2 seismic array operated by the german neumayer station in east antarctica the array monitors backazimuth apparent velocity and signal to noise ratio of arriving waves and provides a data set of seismicity from the western swir over several years compared to the global seismological network its detection threshold for earthquakes occurring at the western swir is more than 0 5 mb lower enabling a more comprehensive study of mid ocean ridge processes than the teleseismic earthquake catalogues we identified a total number of 743 earthquakes occurring at the western part of the swir and calculated the body wave magnitudes mb from p wave amplitude picks on the vna2 broad band sensor obtaining a magnitude range from mb 3 18 to mb 5 34 in the years of 2001 2004 2005 and 2008 significantly increased event rates indicated four earthquake swarms with up to 164 events lasting for several days all swarms had strong events registered in the international seismological centre catalogue the relocalization of these events confirmed that all swarms occurred in the same region on the orthogonal supersegment we analysed event and moment release rate histories b values and aftershock decay rates modified omori law finding that the swarms of 2001 2004 and 2005 have similarities in the temporal distribution of seismic moment and event numbers the swarm of 2008 is smaller with high magnitude events at the swarm s onset which represent shear failure on normal faults the application of the modified omori law and the b value show that the earthquakes of the swarms do not follow the classical main shock aftershock pattern of purely tectonic earthquake sequences at the orthogonal supersegment a continuous positive magnetic anomaly along the rift axis a negative mantle bouguer anomaly basalts at the seafloor and potentially volcanic edifices indicate robust magmatic crustal accretion the high resolution bathymetry shows ubiquitous rift parallel ridges with steep flanks towards the rift axis indicating high angle normal faults the high magnitude earthquakes detected teleseismically during the swarms are generated there we interpret that the swarms are caused by magmatic accretion episodes at a suggested volcanic centre of the orthogonal supersegment and that possible magma injection activates the steeply dipping fault planes

pressure temperature estimates of the lizardite antigorite transition in high pressure serpentinites serpentine minerals in natural samples are dominated by lizardite and antigorite in spite of numerous petrological experiments the stability fields of these species remain poorly constrained this paper presents the petrological observations and the raman spectroscopy and xrd analyses of natural serpentinites from the alpine paleo accretionary wedge serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub greenschist p 20 kbar t 460 c along a subduction geothermal gradient we used the observed mineral assemblage in natural serpentinite along with the tmax estimated by raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures we show that below 300 c lizardite and locally chrysotile are the dominant species in the mesh texture between 320 and 390 c lizardite is progressively replaced by antigorite at the grain boundaries through dissolution precipitation processes in the presence of sio2 enriched fluids and through a solid state transition in the cores of the lizardite mesh above 390 c under high grade blueschist to eclogite facies conditions antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460 c

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

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

emag2 a 2 arc min resolution earth magnetic anomaly grid compiled from satellite airborne and marine magnetic measurements 1 a global earth magnetic anomaly grid emag2 has been compiled from satellite ship and airborne magnetic measurements emag2 is a significant update of our previous candidate grid for the world digital magnetic anomaly map the resolution has been improved from 3 arc min to 2 arc min and the altitude has been reduced from 5 km to 4 km above the geoid additional grid and track line data have been included both over land and the oceans wherever available the original shipborne and airborne data were used instead of precompiled oceanic magnetic grids interpolation between sparse track lines in the oceans was improved by directional gridding and extrapolation based on an oceanic crustal age model the longest wavelengths 330 km were replaced with the latest champ satellite magnetic field model mf6 emag2 is available at http geomag org models emag2 and for permanent archive at http earthref org cgi bin er cgi s erda cgi n 970

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

young off axis volcanism along the ultraslow spreading southwest indian ridge the growth of oceanic plates at mid ocean ridges crustal accretion occurs by a combination of magmatic and tectonic processes magmatic processes along ridges spreading at fast intermediate and slow rates continually add volcanic material to a centrally located spreading axis this creates a narrow band of young volcanic rocks however at ridges spreading at ultraslow rates diminished volcanism allows entire blocks of mantle to spread on the sea floor by tectonic processes remote imaging has advanced our observational understanding of crustal accretion but temporal constraints are required to quantitatively understand ultraslow spreading ridge construction here we use u series eruption ages of volcanic rocks collected from the ultraslow spreading southwest indian ridge unexpectedly we find young volcanic eruption ages that are broadly dispersed throughout the rift valley indicating that crustal accretion of young volcanic rocks is not confined to a narrow central spreading axis as areas of young volcanism are observed close to distinct fault surfaces we propose that the widely dispersed volcanism may result from magma rising along faults our results indicate that axial centric spreading models may not accurately describe crustal accretion at ultraslow spreading ridges prompting the re evaluation of these models mid ocean ridges grow through tectonic and volcanic processes uranium series dating of volcanic rocks at the ultraslow spreading southwest indian ridge reveals widely dispersed young off axis volcanism that is spatially coincident with fault surfaces faults may therefore provide a mechanism for the wide dispersal of magma at ultraslow spreading ridges

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

performance of localization algorithms for teleseismic mid ocean ridge earthquakes the 1999 gakkel ridge earthquake swarm and its geological interpretation in 1999 an unusual earthquake swarm at the 85 e 85 n volcanic centre on the ultraslow spreading gakkel ridge arctic ocean was detected teleseismically the swarm lasted over 9 months and counted 252 events with mb 3 1 it represents the strongest and largest ever recorded mid ocean ridge earthquake swarm and it occurred at a site where spreading rates are only about 10 mm yr 1 we relocated the earthquake swarm comparing the performance of three different localization algorithms 1 the absolute least squares routine hyposat 2 the absolute probabilistic routine nonlinloc and 3 the relative least squares routine mlocate the epicentres as calculated by each algorithm mostly did not agree within their error ellipses thus the choice of location algorithm proved more critical than for example the choice of a local velocity model we compiled a set of well localized events which closely agree in at least two routines mostly mlocate and nonlinloc we conclude that the earthquake swarm of 1999 was related to a spreading episode and shows a complex interplay of tectonic and magmatic events our geological interpretation revealed three phases in swarm activity in the first phase from january 17 up to february 1 fracturing of the crust took place either as a result of or enabling magmatic intrusion seismicity in the second phase from february 2 to april 6 expanded along and across axis it showed signs of magmatic interaction but a clear dyke migration pattern is absent at the beginning of the third phase a distinct break in the event rate suggested a change in the physical process either an adjustment of the stress field to the new regime or a transition to an effusive stage