seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

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

crustal structure of the southwest indian ridge at the atlantis ii fracture zone the southwest indian ridge is a slow spreading end member of the mid ocean ridge system the deepest borehole penetrating the lower oceanic crust ocean drilling program hole 735b lies on the eastern transverse ridge of the atlantis ii fracture zone at 57 e a wide angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous to the east of atlantis bank on which hole 735b is located the crust consists of a 2 2 5 km thick high velocity gradient oceanic layer 2 and a 1 2 km thick low velocity gradient layer 3 the transform valley has a 2 5 3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks the seismically defined crust is thickest beneath the borehole where layer 2 is thinner and the lower crust is inferred to contain 2 3 km of partially serpentinized mantle the seismic velocity models are consistent with gravity data which show weak residual mantle bouguer anomalies because the regions of thinner crust have lower crustal densities stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically the variation of crustal thickness with spreading rate defined by data from the southwest indian ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates resulting in reduced melt generation under the spreading axis large segment scale variations in crustal thickness suggest subcrustal along axis migration of melt toward segment centers

three dimensional gravity and bathymetry analysis of the mid atlantic ridge between 20 n and 24 n flow geometry and temporal evolution of the segmentation bathymetry and gravity data covering 10 m y on both flanks of the mid atlantic ridge south of the kane fracture zone reveal a complex segmentation pattern well correlated with short scale temporal variations in crustal production segment and paleosegment centers are associated with crustal thickening while their extremities appear as elongated bands of thin crust subsidence rates vary between centers and extremities of a segment with higher than world average values at segmert midpoints and closer to average values at the edges this is consistent with segment centers being the site of focused mantle upwellings with associated thin lithosphere and with thick crust a pronounced circular minimum in the long wavelength of the mantle bouguer anomaly associated to a maximum in the long wavelength of the bathymetry marks the site of a mantle upwelling associated with a particularly robust segment located north of a small fracture zone 21 20 n an offset is often observed between extreme values of topography and crustal thickness resulting in a lack of isostatic equilibrium for off axis short wavelength features an obliquity between maxima of crustal thickening associated with active segments and the topographic axial domain suggests that the shift between topography and crustal structure may arise partly from a decoupling between crust and topography formation processes

spatial and temporal distribution of seismicity along the northern mid atlantic ridge 15 35 n 1 a detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern mid atlantic ridge between 15 and 35 n is presented two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones the seismically active zone extends 20 km to either side of the ridge axis consistent with earlier results from studies of fault morphology along the axis hydrophone recorded activity shows important variations areas with intense and persistent seismic activity stripes stand in sharp contrast to areas that lack seismicity gaps the regions of persistent activity are a new observation at mid ocean ridges in general the patterns of seismically active inactive regions are also recognized in the 28 year teleseismic record implying that these patterns are maintained at timescales between a few years and a few decades we find no simple relationship between individual segment variables e g length or trend of the segment maximum offset of discontinuities or along axis change in mantle bouguer anomaly mba and water depths and number of hydrophone recorded events there does appear to be a correlation between axial thermal structure and seismicity regions of low and high numbers of events would thus correspond to thinner hotter and thicker colder lithosphere respectively seismicity may reflect thermal structure at short timescales decadal or longer while relief and inferred crustal thickness may integrate this structure over longer periods of time order of 1 myr

an ocean bottom gravity study of the southern juan de fuca ridge we use seafloor and sea surface gravity data to model density structure along the southern juan de fuca ridge we determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models of 63 seafloor gravity measurements obtained 42 observations were distributed over the 3 km left lateral overlapping rift zone orz located at latitude 45 03 n and separating the juan de fuca ridge into the cleft and vance segments a 21 measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44 52 n a region of linear ridge geometry on the cleft segment these seafloor gravity measurements which are sensitive to shallow crustal density variations were augmented by roughly 800 km of sea surface gravity measurements using a bathymetry bouguer anomaly analysis of the seafloor gravity data we determined the average density of the shallow roughly upper 2 km oceanic crust at these two locations to be 2630 kg m 3 50 kg m 3 within the uncertainties of the density determinations there is no difference between the average shallow oceanic crustal density at the linear cleft segment and at the cleft vance orz using the seafloor measurements we modeled the study area s underlying density to provide constraints on its fine scale structure localized porosity of up to 17 within the upper 500 m of oceanic crust layer 2a provide a possible explanation for the observed gravity anomalies at the cleft segment two dimensional density models of the crust underlying the linear cleft segment 44 52 n show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis for the cleft vance orz three dimensional modeling suggests low density material between the rifts in agreement with thickened layer 2a from seismic observations in contrast layer 2a variations do not explain the low density inferred at the northern end of the cleft segment since seismic observations suggest a thin layer 2a the magmatic activity recently observed in this region may have a distinct deeper source

themis observations of substorms on 26 february 2008 initiated by magnetotail reconnection 1 on 26 february 2008 the themis satellites observed two substorms that occurred at about 0405 and 0455 ut angelopoulos et al 2008 made a comprehensive study of the second event in this paper we display detailed features of the two substorms with emphasis on the first in both substorms a distinct auroral intensification occurred during the earliest stage of onset about 1 to 2 min after midtail reconnection began this initial intensification was weak and localized and thus had the signatures of a pseudobreakup in both substorms a second major intensification occurred next in the substorm onset sequence followed by rapid and extensive poleward expansion this second intensification had the features of the major expansion onset and was nearly coincident with observations of earthward flows and magnetic dipolarization in the near earth tail during the growth phase of the two substorms open magnetic flux accumulated in the polar cap in the expansion recovery phase the polar cap open flux was quickly reduced these observations are in agreement with the assertion that tail reconnection initiates the initial pseudobreakup and the ensuing major expansion and releases and transports energy to eventually cause near earth dipolarization and the expansion phase onset of these two substorms

reply to comment on the poleward leap of the auroral electrojet as seen in auroral images by edward w hones jr un decrochement du cote pole de l electrojet auroral est il une signature des dernieres etapes de la phase d expansion d un sous orage

seismic velocities of the uppermost igneous crust versus age summary seismic refraction velocity data from the acoustic basement called layer 2a have been compiled for different mid ocean ridges the data from post 1970 studies show a strong correlation between velocity and basement age importantly velocities double in less than 10 myr for all ridges but for older crust up to 160 myr in age velocities do not increase significantly

westward traveling surges sliding along boundary arcs and distinction from onset arc brightening 1 after onset auroral brightening rapidly spreads azimuthally along the onset arc typically but not always extending substantially farther westward than eastward expansion phase auroras also expand poleward and can contact an arc along the auroral poleward boundary for such events we find that poleward expansion of the region of active aurora accelerates after contact forming a bulge region that protrudes into the preexisting polar cap and expands westward as the westward traveling surge this westward motion is slower than and initiates later than the azimuthal spreading of the onset arc brightening the poleward boundary arc remains connected to the surge as the surge slides along the equatorward moving edge of the boundary arc and this connection appears to set off the westward traveling poleward protruding activity of the surge via localized polar cap flow adjacent to the boundary arc ground magnetic signatures show a relatively weak h component depression moving westward with the onset arc brightening and a delayed substantially larger depression moving with the westward propagating surge that must be associated with a substantially larger westward ionospheric current the clear distinction between onset arc brightening and the westward traveling surge seen in auroral and magnetometer current signatures implies that their underlying processes are very different evidence suggests that flow channels adjacent to the boundary arc possibly feed surge development these results are consistent with current wedge formation being more associated with post onset activity than with onset and with flow channels from the polar cap being important for surge bulge development

estimation of gas hydrate concentration and free gas saturation from log and seismic data when no direct measurements are available detailed knowledge of the compressional and or shear wave velocity fields is essential for quantitative estimation of gas hydrate and free gas in bottom simulating reflectors bsr discrepancies between experimental velocity profiles and the velocity for water filled sediments reveal the presence of gas hydrate positive anomalies and free gas negative anomalies

stepwise tailward retreat of magnetic reconnection themis observations of an auroral substorm auroral stepwise poleward expansions were clarified by investigating a multiple onset substorm that occurred on 27 february 2009 five successive auroral brightenings were identified in all sky images occurring at approximately 10 min intervals the first brightening was a faint precursor the second brightening had a wide longitude thus it represented the akasofu substorm onset other brightenings expanded poleward thus they were interpreted to be auroral breakups these breakups occurred stepwise that is later breakups were initiated at higher latitudes corresponding reconnection signatures were studied using time history of events and macroscale interactions during substorms themis satellite observations between 8 and 24 re down the magnetotail the akasofu substorm onset was not accompanied by a clear reconnection signature in the tail in contrast the three subsequent auroral breakups occurred simultaneously within a few minutes with three successive fast flows at 24 re thus these were interpreted to be associated with impulsive reconnection episodes these three fast flows consisted of a tailward flow and two subsequent earthward flows the flow reversal at the second breakup indicated that a tailward retreat of the near earth reconnection site occurred during the substorm expansion phase in addition the earthward flow at the third breakup was consistent with the classic tailward retreat near the end of the expansion phase therefore the tailward retreat is likely to have occurred in a stepwise manner we interpreted the stepwise characteristics of the tailward retreat and poleward expansion to be potentially associated by a stepwise magnetic flux pileup

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

crustal structure of a rifted oceanic core complex and its conjugate side at the mar at 5 s implications for melt extraction during detachment faulting and core complex formation we present results of a seismic refraction experiment which determines the crustal and upper mantle structure of an oceanic core complex occ and its conjugate side located south of the 5 s ridge transform intersection at the mid atlantic ridge the core complex with a corrugated surface has been split by a change in location of active seafloor spreading resulting in two massifs on either side of the current spreading axis we applied a joint tomographic inversion of wide angle reflected and refracted phases for five intersecting seismic profiles the obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness an abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall the observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface the thickness of the high velocity body is constrained by pmp reflections to vary along and across axis between 3 and 5 km the thickest crust is associated with the central phase of detachment faulting at the higher elevated northern portion of the massif beneath the breakaway of the occ the crust is 2 5 km thick and reveals significantly lower velocities this implies that the fault initially exhumed low velocity material overlying the gabbro plutons in contrast crust formed at the conjugate side during occ formation is characterized by an up to 2 km thick seismic layer 2 overlying a 1 7 km thick seismic layer 3 obtained upper mantle velocities range from 7 3 to 7 9 km s 1 and seem to increase with distance from the median valley however velocities of 7 3 7 5 km s 1 beneath the older portions of the occ may derive from deep fluid circulation and related hydrothermal alteration which may likely be facilitated by the subsequent rifting our velocity models reveal a strongly asymmetric velocity structure across the ridge axis associated with the accretion of gabbros into the footwall of the detachment fault and upper crustal portions concentrated at the conjugate side our results do not support a substantial increase in the axial ridge s melt supply related to the final phase of detachment faulting hence the footwall rifting at 5 s may be a generic mechanism of detachment termination under very low melt conditions as predicted by recent numerical models of tucholke et al

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

hydrothermal activity and the evolution of the seismic properties of upper oceanic crust in order to investigate the impact of off axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust layer 2a we performed an extensive geophysical survey on the eastern flank of the east pacific rise at 14 s seismic refraction and heat flow data were obtained along a 720 km long and 25 to 40 km wide corridor covering thinly sedimented seafloor created since 8 5 ma the seismic data yield a seismic velocity of 2 9 km s at the top of 0 5 m y old basement rocks within about 8 m y the velocity increases gradually to a value of mature oceanic crust 4 3 km s heat flow data derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements suggest that an open hydrothermal circulation system persists for at least 6 7 m y in crust older than 7 ma regional heat flow is close to values predicted by plate cooling models suggesting that hydrothermal circulation is going to cease considering published dating of alteration minerals it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10 15 m y this idea may explain why seismic velocities in the pacific ocean have not changed significantly in igneous crust older than 8 10 ma in regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different velocities increase rapidly and reach values of mature oceanic crust within 1 2 m y we therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust stephen and harding 1983 rohr 1994

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

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

seismic evidence for large scale compositional heterogeneity of oceanic core complexes long lived detachment faults at mid ocean ridges exhume deep seated rocks to form oceanic core complexes occs using large offset 6 km multichannel seismic data we have derived two dimensional seismic tomography models for three of the best developed occs on the mid atlantic ridge our results show that large lateral variations in p wave velocity occur within the upper 0 5 1 7 km of the lithosphere we observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology and we use these correlations to show that gabbros are heterogeneously distributed as large tens to 100 km2 bodies within serpentinized peridotites neither the gabbros nor the serpentinites show any systematic distribution with respect to along isochron position within the enclosing spreading segment indicating that melt extraction from the mantle is not necessarily focused at segment centers as has been commonly inferred in the spreading direction gabbros are consistently present toward the terminations of the detachment faults this suggests enhanced magmatism during the late stage of occ formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults and it therefore may explain eventual abandonment of detachments that form occs detailed seismic studies of the kind described here when constrained by seafloor morphology and geological samples can distinguish between major lithological units such as volcanics gabbros and serpentinized peridotites at lateral scales of a few kilometers thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced

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

multistation observations of auroras polar cap substorms double wavelength 630 0 and 427 8 nm all sky imagers were operated at the automatic geophysical observatory ago sites p1 and p4 magnetic latitude 80 south pole station magnetic latitude 74 and p3 magnetic latitude 72 during the austral winter of 1994 and 1995 a new data format was developed to present the 24 hour auroral displays from the station arrays which provided 24 hour optical data during the antarctic winter night for several months auroras at these stations exhibit the typical magnetic local time mlt features of high latitude precipitation on the nightside mlt substorm poleward expansions were observed to reach very high latitudes a typical event june 3 1994 0400 0500 ut started equatorward from the south pole and propagated to the high latitude limit of the field of view at p1 and p4 the morphological appearance of these high latitude auroral substorm features was similar to that of substorm auroras observed at lower latitudes the intense 427 8 nm emission accompanied by a weak 630 nm emission showed that the particles were energetic the interplanetary magnetic field imf bz component was slightly negative before and during this event emphasizing that these were substorm auroras and not polar cap auroras which tend to occur during times of prolonged positive imf bz statistical analysis of the 1995 ago optical data shows that over 20 of substorms are seen to penetrate to regions greater than 80 of latitude

centroid depths and mechanisms of mid ocean ridge earthquakes in the indian ocean gulf of aden and red sea we have determined the centroid depths and source mechanisms of 14 earthquakes along spreading centers in the indian ocean gulf of aden and red sea the earthquakes are all characterized by normal faulting on planes dipping at about 45 and generally striking parallel to the local trend of the spreading center for the ridge axis earthquakes in the indian ocean gulf of aden and central red sea seismic moments range from 3 to 9 1024 dyn cm source time functions are all of simple form and centroid depths of events with adequate azimuthal coverage are all very shallow between 1 and 4 km beneath the seafloor if the centroid depth typically marks the mean depth of fault slip then earthquake faulting extended to 2 8 km beneath the seafloor during these earthquakes the centroid depths for two earth quakes near the mouth of the gulf of suez support the view that seismic faulting accompanying continental rifting generally extends to greater depth than does seismic faulting along the axes of oceanic spreading centers when these findings are combined with our earlier results from the mid atlantic ridge the greatest centroid depths of earthquakes with well constrained source mechanisms tend to shoal with increasing spreading rate this result supports a general decrease with spreading rate in the maximum thickness of the zone of brittle behavior for slow spreading ridges

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

seismic reflection imaging of an oceanic detachment fault atlantis megamullion mid atlantic ridge 30 10 n abstract we present multichannel seismic reflection data collected over the atlantis megamullion at the eastern ridge transform intersection of atlantis fracture zone on the northern mid atlantic ridge and over its conjugate crust these data image for the first time the internal structure of a young well developed megamullion dome formed by tectonic extension across a long lived oceanic detachment fault the exposed corrugated detachment fault surface exhibits a sharp coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor at the termination of the megamullion the fault is imaged 13 km along strike beneath a volcanic hanging wall block at a sub seafloor depth of 0 2 0 5 s two way travel time reaching north as far as 30 19 n the eastward dipping of the fault beneath the hanging wall block is estimated to be 6 14 the corrugated fault surface is underlain by a continuous strong and relatively smooth reflection d at 0 2 0 25 s sub bottom below the central axis of the dome this reflection deepens up to 0 6 s sub bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope we suggest that atlantis massif formed by sequential slip on two different detachment faults that merged at depth with breakaways as little as 2 km apart the initial detachment is represented by reflection d and the second corresponds to the presently exposed fault surface in this interpretation much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity it lies in contact with less altered higher velocity mantle below the first detachment resulting in the strong smooth character of reflection d mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism in conjugate crust corresponding to termination of the megamullion observed sub bottom reflections are interpreted as base of seismic layer 2a this layer is as thick as or thicker 570 900 m than layer 2a in normal atlantic crust and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

the poleward leap of the auroral electrojet as seen in auroral images this paper discusses a recently published series of 12 successive images of the northern auroral oval obtained by the university of iowa s auroral imager on de 1 during a substorm on november 8 1981 the images showed a sudden within 12 min thickening and poleward expansion of the midnight sector of the oval that occurred at the maximum epoch of the substorm i e as auroral zone bays reached their peaks and began to subside about 90 min after the expansive phase onset we propose that this was the auroral manifestation of the poleward leap of the auroral electrojet that has been reported on many occasions to coincide with thickening of the plasma sheet in the outer magnetotail beyond 15 re late in a substorm the plasma sheet thickening is thought to be associated with a tailward retreat of the substorm magnetic neutral line we note evidence that the neutral line s tailward retreat influences auroral currents and particle precipitation even at low auroral latitudes modifications of the present auroral substorm model are suggested

detachment shear zone of the atlantis massif core complex mid atlantic ridge 30 n near bottom investigations of the cross section of the atlantis massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex integrated direct observations sampling photogeology and imaging define a mylonitic low angle detachment shear zone dsz along the crest of the massif the shear zone may project beneath the nearby corrugated upper surface of the massif the dsz and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme localized tectonic extension during seafloor spreading over the past 2 m y the dsz is characterized by strongly foliated to mylonitic serpentinites and talc amphibole schists it is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction the dsz foliation arches over the top of the massif in a convex upward trajectory mimicking the morphology of the top of the massif kinematic indicators show consistent top to east toward the mar axis tectonic transport directions foliated dsz rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop scale foliation the dsz and underlying basement rocks are cut by discrete anastomosing normal slip shear zones widely spaced steeply dipping normal faults cut all the older structures and localize serpentinization driven hydrothermal outflow at the lost city hydrothermal field a thin few meters sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the dsz and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present gently dipping orientation during lateral spreading and uplift

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

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

oman diopsidites a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres abstract mafic ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts in the frame of a global survey of these melt migration relics in the oman mantle harzburgites we discovered a peculiar lithology made essentially of pure diopside 0 95 m g m g f e total 1 whose characteristics do not match a magmatic or mantle origin arguments against a magmatic or mantle origin for these diopsidites combine compositional and textural evidence in spite of their refractory composition they are strongly depleted in cr cr2o3 0 2 wt by the same way other minor elements al ti na and rare earth elements have peculiarly low abundances and plot away from magmatic differentiation trends in a few samples the paragenetic association includes pure anorthite an up to 0 99 minor amounts of forsterite fo 0 95 or traces of andradite when not deformed the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and or carbonate diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust they frequently appear as dykes former cracks with a few mm to few tens of cm wide transitional zones which contain high amounts of hydrous minerals between the diopsidite facies and its host rock the diopsidites are not randomly distributed in the oman ophiolite being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere we interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids 800 c which may have leached plagioclase rich lithologies before penetrating the mantle as shown by a well developed positive eu anomaly our data confirm the prediction of mccollom and shock t m mccollom e l shock fluid rock interactions in the lower oceanic crust thermodynamic models of hydrothermal alteration j geophys res 103 b1 1998 547 575 who proposed that common anhydrous minerals like pyroxene plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water this confirms that there is no clear cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations

detailed analysis of a substorm event on 6 and 7 june 1989 1 growth phase evolution of nightside auroral activities and ionospheric convection toward expansion phase onset 1 we have analyzed in detail the evolution of nightside auroral activities and ionospheric convection during the growth phase of an isolated substorm the following three characteristic auroral activities were identified by meridian scanning photometers msp at ground stations 1 proton main oval 2 fast equatorward moving fem arc and 3 near plasma sheet boundary layer npsbl aurora the proton main oval corresponds to the main part of the proton auroral region and was located at the equatorward most region of the electron auroral emissions and gradually moved equatorward during the growth phase the fem arc was a discrete auroral arc having a clear longitudinally elongated form it appeared about 20 min before the onset on the high latitude side of the auroral region and moved equatorward toward the proton main oval its equatorward motion gradually slowed as it approached the proton main oval as the fem arc moved equatorward the location of the peak velocity in the nightside return flow of the two cell convection also moved equatorward the auroral breakup occurred in a localized area around the demarcation region between the two return flows close to the latitude of the fem arc this was observed by the uv imager aboard the akebono satellite the npsbl aurora appeared a few minutes before the onset around the poleward most region of the aurora and continued even after the onset our detailed analysis of these phenomena can give us important clues as to how the growth phase proceeds toward the expansion phase onset

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

geometry of extensional faults developed at slow spreading centres from pre stack depth migration of seismic reflection data in the central atlantic canary basin we present depth images from portions of profiles that are close to flow lines of cretaceous oceanic crust in the eastern central atlantic compared with post stack time migrations the images illustrate the improvement resulting from the application of pre stack depth migration the images document the scale and geometry of normal faulting in oceanic crust formed over 25 myr at a half spreading rate of less than 10 mm yr 1 and the variation in extensional style with position within the spreading segment away from major fault zones fzs most faults are subplanar dip more than 35 are associated with moderate basement relief 0 2 1 km relief and may penetrate to deep crustal levels these faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains also observed away from fzs are gently dipping to subhorizontal reflections in the upper crust which resemble detachment faults in contrast the inside corner crust is more rugged with basement highs rising up to 2 km above the intervening basins this larger scale topography is associated with a different style of faulting the depth images reveal gently dipping 35 faults that are rooted in the deep crust and that project to the ridgeward flank of the dome shaped large basement highs 1 2 km vertical relief the faults seem to continue as the ridge facing flank of these highs and some may extend over the crest of the high to breakaways beyond in this case the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes these controlling faults are up to 20 km long and have a heave of 10 km sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks we suggest that similar faults can explain the structure and lithologies found at megamullion structures oceanic core complexes at inside corners near the present day spreading ridge

structure and ageing of oceanic crust at 14 s on the east pacific rise a wide angle reflection refraction survey was conducted along a flow line on the eastern flank of the east pacific rise at 14 s the data were acquired with a single 32 litre airgun and ocean bottom hydrophones obh and consist of six profiles on 0 5 8 3 myr old seafloor in total 17 obhs provided seismic records of excellent quality to generate 2 d velocity models traveltimes and amplitudes have been calculated using asymptotic ray theory a comparison of the traveltimes and amplitudes of p wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust the uppermost crust is composed of a 0 4 0 7 km thick low velocity 2 9 4 6 km s 1 surficial layer layer 2a and is bounded at its base by a sharp velocity increase to 5 5 km s 1 at the top of layer 2b overall layer 2 is 1 4 1 8 km thick small seamounts frequently superposed on fast spreading crust are characterized by velocities lower than within the adjacent upper crust velocities 6 0 km s 1 exist throughout the edifices suggesting that the seamounts are dominantly extrusive decreasing amplitudes beyond 8 10 km offset constrain a moderate gradient lower crust layer 3 with velocities of 6 8 7 2 km s 1 crustal thickness is found to be remarkably uniform at 6 11 0 14 km nevertheless the seismic data provide evidence for age dependent variations in both the low velocity surficial layer and the upper mantle we find that velocities in layer 2a increase from 2 9 to 4 3 km s 1 velocities in the middle and lower crust show uniform values while velocities within the uppermost mantle increase gradually from 7 5 km s 1 to 8 0 km s 1 the most viable explanation for increasing velocities in the upper crust is that off axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces thereby decreasing the porosity and increasing the velocity low mantle velocities beneath young crust are suggested to be a temperature controlled phenomenon and may indicate the presence of partial melt in the upper mantle

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data

seismic structure of an oceanic core complex at the mid atlantic ridge 22 19 n we present results from a seismic refraction and wide angle experiment surveying an oceanic core complex on the mid atlantic ridge at 22 19 n oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections tomographic inversion of our seismic data yielded lateral variations of p wave velocity within the upper 3 to 4 km of the lithosphere across the median valley a joint modeling procedure of seismic p wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle a gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks caused by un roofing lower crust rocks under the core complex crust is approximately 40 thinner than in the median valley and under the conjugated eastern flank clear pmp reflections turning under the western ridge flank suggest the creation of a moho boundary and hence continuous magmatic accretion during core complex formation

a rifted inside corner massif on the mid atlantic ridge at 5 s the structure of the mid atlantic ridge at 5 s was investigated during a recent cruise with the fs meteor a major dextral transform fault hereafter the 5 s fz offsets the ridge left laterally by 80 km just south of the transform and to the west of the median valley the inside corner ic the region bounded by the ridge and the active transform is marked by a major massif characterized by a corrugated upper surface fossil ic massifs can also be identified further to the west unusually a massif almost as high as the ic massif also characterizes the outside corner oc south of the inactive fracture zone and to the east of the median valley this oc massif has axis parallel dimensions identical to the ic massif and both are bounded on their sides closest to the spreading axis by abrupt steep slopes an axial volcanic ridge is well developed in the median valley both south of the ic oc massifs and in an abandoned rift valley to the east of the oc massif but is absent along the new ridge axis segment between the ic and oc massifs wide angle seismic data show that between the massifs the crust of the median valley thins markedly towards the fz these observations are consistent with the formation of the oc massif by the rifting of an ic core complex and the development of a new spreading centre between the ic and oc massifs the split ic massif presents an opportunity to study the internal structure of the footwall of a detachment fault from the corrugated fault surface to deeper beneath the fault without recourse to drilling preliminary dredging recovered gabbros from the scarp slope of the rifted ic massif and serpentinites and gabbros from the intersection of this scarp with the corrugated surface this is compatible with a concentration of serpentinites along the detachment surface even where the massif internally is largely plutonic in nature

magnetic mapping effects of substorm currents leading to auroral poleward expansion and equatorward retreat magnetotail fast flows magnetic field dipolarization and its relaxation are linked to auroral brightening poleward expansion and equatorward motion during substorm onset expansion and recovery respectively while auroral brightening is often attributed to the field aligned currents produced by flow vorticity and pressure redistribution the physical causes of auroral poleward expansion and equatorward retreat are not fully understood simplistically such latitudinal changes can be directly associated to the tailward motion of the flux pileup region and the earthward flux transport toward the dayside that depletes the near earth plasma sheet however because the equatorial magnetic field profile and the magnetospheric field aligned current system change significantly mapping is severely distorted to investigate this distortion we superimpose a substorm current wedge model dynamically driven by ground based observations on the global tsyganenko model t96 during an isolated substorm on 13 february 2008 observed by the time history of events and macroscale interactions during substorms and goes 10 spacecraft and by ground all sky imagers we validate our model by showing that the timing and ionospheric projection of the flux pileup region and flow bursts observed at the spacecraft match auroral activations we then use the improved mapping enabled by the model to demonstrate that in this event auroral poleward expansion and equatorward retreat are mainly caused by substorm current wedge induced mapping changes

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

upper crustal evolution across the juan de fuca ridge flanks 1 recent p wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2a doubles or reaches 4 3 km s found in mature crust in 10 ma after crustal formation this velocity change is commonly attributed to precipitation of low temperature alteration minerals within the extrusive rocks associated with ridge flank hydrothermal circulation sediment blanketing acting as a thermal insulator has been proposed to further accelerate layer 2a evolution by enhancing mineral precipitation we carried out 1 d traveltime modeling on common midpoint supergathers from our 2002 juan de fuca ridge multichannel seismic data to determine upper crustal structure at 3 km intervals along 300 km long transects crossing the endeavor northern symmetric and cleft ridge segments our results show a regional correlation between upper crustal velocity and crustal age the measured velocity increase with crustal age is not uniform across the investigated ridge flanks for the ridge flanks blanketed with a sealing sedimentary cover the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks 60 increase versus 28 over the same crustal age range of 5 9 ma extrapolation of velocity age gradients indicates that layer 2a velocity reaches 4 3 km s by 8 ma on the sediment blanketed flanks compared to 16 ma on the flanks with thin and discontinuous sediment cover the computed thickness gradients show that layer 2a does not thin and disappear in the juan de fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust however layer 2a on the fully sedimented ridge flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks 330 80 versus 430 80 m the change in thickness occurs over a 10 20 km distance coincident with the onset of sediment burial our results also suggest that propagator wakes can have atypical layer 2a thickness and velocity impact of propagator wakes is evident in the chemical signature of the fluids sampled by odp drill holes along the east endeavor transect providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration

detachment faulting at ocean core complexes intracrustal reflections from a cretaceous inside corner are interpreted as a detachment fault that dips gently within the basement and can be followed up the flank of a domed high the fault heave is sufficient for ridge normal extension to have exhumed deep crustal and probably mantle rocks as sampled at some inside corner domed massifs the morphology of these massifs closely resembles that of the cretaceous high implying that the structures are analogous and supporting the notion that modern inside corner massifs are oceanic core complexes formed by unroofing the footwalls of similar detachment faults partially exposed as corrugated slip surfaces this is the first demonstration of the overall geometry of an oceanic core complex detachment fault

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined