the oceanic crust in 3d paleomagnetic reconstruction in the troodos ophiolite gabbro the troodos complex cyprus provides an opportunity to study the structural configuration along a fossil intersection of a spreading axis and a transform fault we complement studies at troodos that have reconstructed the brittle deformation of the upper crust by new paleomagnetic data from the gabbro suite the gabbro suite is exposed at the extinct spreading axis continuing the solea graben toward the intersection with the fossil arakapas oceanic transform this is a unique exposure of deep crustal rocks formed at both an inside corner and an outside corner of a ridge transform intersection remanence directions from gabbros 23 sites were used as indicators for rigid body rotation the spatial distribution of rotation axes allow recognition of three regions to which deformation is partitioned 1 a western region outside corner that experienced primarily tilt about horizontal axis 2 a central region with minor rotation and 3 an eastern area inside corner where vertical axis rotations are dominant the absence of significant rotation in the 6 km wide central domain together with its location between the inside and the outside corner uncover the root of a fossil axial volcanic zone a zone sufficiently hot so the upper crust can decouple from the substrate clockwise rotation in the gabbro increases from the axial zone eastward similar to that in the overlying dikes indicating coupling of the lower crust with the brittle upper oceanic crust the transition from the decoupled layers of sheeted dikes and gabbro in the axial zone to the dikes gabbro coupling in the inside corner is in keeping with deepening of the brittle ductile transition from the dike gabbro boundary into the lower crust away from the axial zone our conclusions are consistent with one of the previous reconstructions in which the solea spreading axis was orthogonal to the arakapas trunsform fault and with recent studies of the present day lower oceanic crust however the newly inferred surface trace of the solea spreading axis is further to the east probably reflecting the tilt of axial upper crust rotated blocks c 2006 elsevier b v all rights reserved

vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

tectonic rotations of dikes in fast spread oceanic crust exposed near hess deep oceanic crust that initially formed at the fast spreading 130 mm yr east pacific rise at 1 ma is exposed in the walls of hess deep rift at the northern edge of the galapagos microplate steep fault scarps expose a dismembered vertical sequence of pillow basalts sheeted diabase dikes and gabbroic rocks diabase dikes in these units are dominantly vertical however extensive outcrops of steeply to moderatley dipping dikes exist in some places the nonvertical attitude of the dikes may be the result of tectonic rotations attending sea floor spreading at the east pacific rise or the result of complex rotations of fault blocks during the opening of the hess deep rift at 0 65 ma

temporal and spatial variability in the composition of lavas exposed along the western blanco transform fault the northern scarp of the western blanco transform bt fault zone provides a tectonic window into crust generated at an intermediate rate spreading center exposing a 2000 m vertical section of lavas and dikes the lava unit was sampled by submersible during the blancovin dive program in 1995 recovering a total of 61 samples over vertical distances of 1000 m and a lateral extent of 13 km major element analyses of 40 whole rock samples exhibit typical tholeiitic fractionation trends of increasing feo na2o and tio2 and decreasing al2o3 and cao with decreasing mgo the lava suite shows a considerable range in extent of crystallization including primitive samples mg 64 and evolved feti basalts feo 12 tio2 2 on the basis of rare earth element and trace element data all of the lavas are incompatible element depleted normal mid ocean ridge basalts n morb la smn 1 the geochemical systematics suggest that the lavas were derived from a slightly heterogeneous mantle source and crystallization occurred in a magmatic regime of relatively low magma flux and or high cooling rate consistent with magmatic processes occurring along the present day southern cleft segment the bt scarp reveals the oceanic crust in two dimensional space allowing us to explore temporal and spatial relationships in the horizontal and vertical directions as a whole the data do not appear to form regular spatial trends rather primitive lavas tend to cluster shallower and toward the center of the study area while more evolved lavas are present deeper and toward the west and east considered within a model for construction of the upper crust these findings suggest that the upper lavas along the bt scarp may have been emplaced off axis either by extensive off axis flow or off axis eruption while the lower lavas represent axial flows that have subsided with time a calculation based on an isochron model for construction of the upper crust suggests that the cleft segment requires at least 50 kyr to build the lower extrusive section consistent to first order with independent estimates for the construction of intermediate spreading rate crust

spreading evolution of the troodos ophiolite cyprus some recent developments particularly the recognition of transform tectonized and nontransform tectonized fabrics close to the southern troodos transform fault provide a framework for a new evolutionary model of the troodos ophiolite this model proposes that most of the ophiolite formed at the solea axis against a dextrally slipping southern troodos transform spreading was largely accommodated by magmatic accretion the ridge then jumped to the east a last phase of amagmatic stretching driving normal faulting above a detachment horizon in the solea graben a ridge jump boundary separates the old crust formed at the solea axis from the new crust generated at the outside corner of the new ridge with a dextrally slipping continuation of the southern troodos transform the final episode involved extension of the crust generated at the new ridge to form the larnaca graben in the eastern part of the ophiolite

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

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

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

pito and orongo fracture zones the northern and southern boundaries of the easter microplate southeast pacific abstract the easter rapanui microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on earth is occurring today recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets during the january 1987 rapanui expedition on the n o jean charcot we conducted a sea beam magnetics gravity survey of a portion of the microplate boundaries the method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships the northern terminus of the east rift or eastern boundary of the microplate is expressed as a series of parallel nw se trending valleys including what appears to be with 5890 m depth the deepest active rift axis mapped in the pacific today pito rift the northern end of the pito rift merges with an e w to 083 narrow band of linear faults interpreted to be a transform fault between the nazca and easter rapanui plates the northern triple junction between the easter rapanui nazca and pacific plates is a rff type with the two transform faults colinear along an approximately e w direction the southwestern boundary of the easter rapanui microplate is marked by a series of en echelon offsets outlined by depressions which merge into an approximately e w zone where shear must be predominant the southern triple junction is a rrf junction with an overlapping ridge system the structural data acquired during the survey provide strong constraints for kinematic models of the microplate the structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate

extreme avalanche runout in space and time the distribution of avalanche runout varies in space and time for individual avalanche paths and from mountain range to mountain range in this paper such variations are considered based on the assumption supported by data worldwide that the spatial distribution of extreme avalanche runout follows a gumbel distribution and that the arrival rate of avalanches can be modelled as a poisson process the input required is a set of extreme avalanche runout distances for the mountain range and a knowledge of avalanche frequency at the beginning of the runout zone for the path in question such information allows theoretical estimation of the effective return period as a function of position which is very important in zoning applications in addition general expressions are derived to relate gumbel parameters for different mountain ranges to a frequency index to explore general frequency implications from one mountain range to another the estimated gumbel parameters imply consistent relationships for expect

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

constraining crustal emplacement processes from the variation in seismic layer 2a thickness at the east pacific rise abstract a stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the east pacific rise epr in ocean drill holes and in ophiolites in this model the location of successive dike intrusion events relative to the ridge axis is determined by a gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a gaussian shaped lava pile we interpret wide angle seismic reflections from the steep velocity gradient at the base of seismic layer 2a to be the extrusive sheeted dike contact seismic data from the northern and southern epr place constraints on the on axis extrusive layer thickness 230 50 m the distance over which the thickening of the extrusive layer occurs width of the accretion zone 1 3 km and its off axis thickness 300 800 m ophiolites and ocean drill holes dsdp hole 504b provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from extrusives to sheeted dikes 100 200 m a simple stochastic emplacement model where the lavas are described by one mean flow length fits the thickening of the extrusive layer off axis inferred from the deepening of seismic layer 2a but the predicted transition from sheeted dikes to extrusives is too thick in order to match the dimensions and flat topped shape of the seismic layer 2a boundary as well as the thickness of the extrusive sheeted dike transition a bimodal distribution of lava flows is used short flows confined within the axial summit caldera asc build up approximately half the extrusive volume occasional voluminous flows spill out of the asc or erupt outside of the asc and pond at a considerable distance off axis to build up the remainder of the extrusive section the upper part of the final extrusive section will be dominated by the off axis flows while the lower portions will be primarily composed of short flows erupted within the asc magnetic transition widths predicted from the overlap of lavas 2 km in this model are similar to those measured in deep tow studies assuming a smoothing function which acts over one seismic wavelength the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the epr the ages of seafloor lavas in this model are younger than the tectonic spreading model ages by 30 70 kyr in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the epr

downhole magnetic measurements of odp hole 801c implications for pacific oceanic crust and magnetic field behavior in the middle jurassic 1 downhole horizontal and vertical magnetic field measurements within the 474 m thick jurassic crustal section drilled at odp hole 801c in the western pacific show anomalies that are in phase indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18 6 n the inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion this is compatible with the observed dips in the formation micro scanner fms data which show up to 42 of rotation toward the fossil spreading axis we restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39 9 6 6 corresponding to a paleolatitude of 22 7 5 s the magnetic logging data document six polarity units within the drilled volcanic extrusive crust the upper 132 m thick logged section of lavas contains four polarity units which apparently formed 7 myr after the formation of the underlying basement the lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone more than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust 45 000 years at a fast spreading rate of 66 km myr previous results from a contemporaneous section in spain suggest rapid reversals throughout the late bajocian early bathonian stage 170 165 ma nevertheless the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801c crustal section

alteration processes at deep sea drilling project ocean drilling program hole 504b at the costa rica rift implications for magnetization of oceanic crust magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504b during ocean drilling program leg 111 our results combined with those from previous studies show abrupt first order changes in magnetic properties at alteration boundaries within the 504b crustal section changes in style and degree of alteration fall near the boundaries of the three well defined lithologic units the extrusive basalts the transition zone and the sheeted dike complex this postemplacement alteration heavily influences magnetic properties and is observed to change in both style and degree with depth within each lithologic unit our results indicate that at 504b the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration the upper more permeable basalts above this critical depth have not experienced this reheating within the sheeted dike complex the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth the changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth overall these observations suggest that the effective penetration of water into layer 2c decreased with increasing depth and resulted in a lower rate of convective cooling magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration the upper dikes have become magnetically more stable than the lower dikes a review of all magnetic properties indicate that the dike section at 504b carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict we suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions despite a magnetization which is lower than expected it appears that the sheeted dike complex at hole 504b is capable of making a substantial contribution to the overlying marine magnetic anomaly because of the systematic decrease in hydrothermal alteration of magnetic minerals the ability of the dike section to contribute decreases as a function of depth

dense avalanche friction coefficients influence of physical properties of snow the values of the voellmy friction parameters of 735 historical avalanches that have occurred along 26 paths in the chamonix valley france since 1958 are back analyzed with a depth averaged hydraulic model including sub models for erosion entrainment and deposition for each path the longitudinal and crosswise topographic profiles were derived from a high resolution digital elevation model acquired by laser scanning the initial snow depth and snow cohesion as well as various physical properties of snow were computed from numerical simulations of the detailed snowpack model crocus fed by the safran meteorological analysis for each event the full ranges of the two friction parameters were scanned and the pairs of friction parameters for which the run out altitude is found close enough to the observed one with an uncertainty of 5 m were retained statistical class analysis was used to investigate the correlation between the obtained friction coefficients and the snow physical properties no evident trend with the snow parameters was found for the inertial friction coefficient for the static friction coefficient an increasing trend with temperature and density was observed as well as a decreasing trend with liquid water content and initial snow depth although modeling assumptions and limitations regarding data and the calibration procedure should be kept in mind these trends are worth noting allowing avalanche simulations to be refined to take into account prevailing weather and snow conditions

origin of the sheeted dike complex at superfast spread east pacific rise revealed by deep ocean crust drilling at ocean drilling program hole 1256d 1 superfast spread 22 cm a upper oceanic crust drilled at ocean drilling program site 1256 comprises a thick sequence of extrusive lavas underlain by a thin region of sheeted dikes the sheeted dikes at this site are characterized by the intimate association of in situ hyaloclastic fragmentation and hydrothermal alteration lithostatic and magmastatic pressure estimates reveal that there is no effective level of neutral buoyancy within the extrusive layer to trap dikes in the crust this results in a situation that favors the extrusion rather than intrusion of magma fractures created by magmas rising in dikes were the loci of intense hydrothermal circulation and provided the pathways for subsequent dike intrusions magma rich conditions expected for fast spreading ridge segments give rise to a rapid increase in magmatic pressure which can result in dike intrusion even under only small deviatoric stress this allows for the emplacement of narrow dikes in the upper crust with magmas more likely to extrude and hence the high extrusive intrusive ratio for site 1256

equatorial paleosecular variation of the geomagnetic field from 0 to 3 ma lavas from the galapagos islands abstract complete progressive thermal demagnetization of nearly 400 oriented samples from 58 sites lava flows from the galapagos islands of santa cruz san cristobal and floreana provide data for the statistical characterization of the time averaged geomagnetic field near the equator for the past few million years estimates of vgp dispersion due to paleosecular variation range from 9 2 to 11 8 depending on site selection criteria our preferred estimate based on 64 site vgps 51 accepted from this study and 13 from the 1971 study by cox is 11 4 95 confidence interval 10 2 13 0 consistent with previous estimates from the galapagos islands as well as paleosecular variation model g and confirming that angular dispersion of vgps near the equator is relatively low the mean direction is not significantly different from a geocentric axial dipole field when account is taken of southward plate motion over the galapagos hotspot preliminary paleointensity results from a comparison of the natural remanence with a total thermal remanence produced in a lab field of 15 t on a subset of 321 samples from 48 sites that had relatively small changes in magnetic susceptibility after laboratory heating suggest that the time averaged field was about 21 t or only two thirds the present strength in agreement with some other recent estimates

evaluation of slope stability with respect to snowpack spatial variability the evaluation of avalanche release conditions constitutes a great challenge for risk assessment in mountainous areas the spatial variability of snowpack properties has an important impact on snow slope stability and thus on avalanche formation since it strongly influences failure initiation and crack propagation in weak snow layers hence the determination of the link between these spatial variations and slope stability is very important in particular for avalanche public forecasting in this study a statistical mechanical model of the slab weak layer wl system relying on stochastic finite element simulations is used to investigate snowpack stability and avalanche release probability for spontaneously releasing avalanches this model accounts in particular for the spatial variations of wl shear strength and stress redistribution by elasticity of the slab we show how avalanche release probability can be computed from release depth distributions which allows us to study the influence of wl spatial variations and slab properties on slope stability the importance of smoothing effects by slab elasticity is verified and the crucial impact of spatial variation characteristics on the so called knock down effect on slope stability is revisited using this model finally critical length values are computed from the simulations as a function of the various model parameters and are compared to field data obtained with propagation saw tests

extreme avalanche runout a comparison of empirical models the prediction of snow avalanche runout distances and the probability of exceeding the predicted positions is the first and most important step required before making decisions about placement of facilities or control structures in snow avalanche prone terrain there are two main prediction methods for calculating runout distances 1 procedures linked to the selection of friction coefficients in avalanche dynamic models and 2 empirical statistical prediction based on terrain parameters for a set of extreme runout distances for the mountain range in question within the second method there are presently two competing empirical approaches to prediction i ordinary least squares regression analysis related to angles measured for the path profile in question and ii extreme value prediction of runout based on a gumbel distribution of a dimensionless terrain parameter in this paper a comparison of the two empirical methods with emphasis on the slope steepness in the runout zone is provided the comp

modelling of snow avalanche dynamics influence of model parameters the three parameter hydraulic model of snow avalanche dynamics including the coefficients of dry and turbulent friction and the coefficient of new snow mass entrainment was investigated the domestic avalanche site in elbrus region caucasus russia was chosen as the model avalanche range according to the model the fixed avalanche run out can be achieved with various combinations of model parameters at the fixed value of the coefficient of entrainment me we have a curve on a plane of the coefficients of dry and turbulent friction it was found that the family of curves me is a parameter are crossed at the single point the value of the coefficient of turbulent friction at the cross point remained practically constant for the maximum and average avalanche run outs the conclusions obtained are confirmed by the results of modelling for six arbitrarily chosen avalanche sites three in the khibiny mountains kola peninsula russia two in the elbrus region and one idealized site with an exponential longitudinal profile the dependences of run out on the coefficient of dry friction are constructed for all the investigated avalanche sites the results are important for the statistical simulation of avalanche dynamics since they suggest the possibility of using only one random model parameter namely the coefficient of dry friction in the model the histograms and distribution functions of the coefficient of dry friction are constructed and presented for avalanche sites nos 22 and 43 khibiny mountains and domestic with the available series of field data

internal structure of uppermost oceanic crust along the western blanco transform scarp implications for subaxial accretion and deformation at the juan de fuca ridge 1 the uppermost 2 km of oceanic crust created near the southern end of the juan de fuca ridge 60 mm yr full spreading rate is exposed in a tectonic window created along the north wall of the blanco transform fault a total of 27 submersible dives transect this escarpment over a distance of 30 km corresponding to 1 m y of spreading history in this exposure weakly deformed basaltic lavas grade downward into much more intensely fractured lavas cut by basaltic dikes lava flow surfaces dip persistently to the northwest toward the spreading center and the dikes dip southeast away from the spreading center the underlying sheeted dike complex is composed of subparallel dikes that dip moderately to the southeast discrete faults and more distributed cataclastic deformation zones separate the panels of sheeted dikes from one another disruption of the lava and dike units are interpreted as a result of dramatic subaxial vertical mass transport that created the accommodation space for the accumulation of a thickness of 1000 1500 m of basaltic lava the observed patterns of tilting of lavas and dikes imply subsidence beneath the region of active volcanism 3 km wide that is most rapid near the axis and diminishes quickly off axis deformation that accommodates this subsidence has generated extensive fracture porosity in the subaxial region that could strongly influence upper crustal hydrothermal pathways alteration history seismic properties and the distribution of subsurface biological communities

structural pattern of the galapagos microplate and evolution of the galapagos triple junctions a 13 000 km2 microplate intervenes to prevent the junction of the nazca cocos and pacific plates in the eastern equatorial pacific ocean west of the galapagos islands the microplate is rotating clockwise at 6 m y about an absolute pole 400 km southeast of its center so that although its northern margin has a similar motion to adjacent parts of the cocos plate its southern margin is 50 slower the perimeter of the galapagos microplate is best defined by bathymetry and has been surveyed with sea beam and examined by deep tow two of the three triple junctions on its margins are ridge ridge ridge rrr junctions 160 km apart on the crest of the east pacific rise epr the most like a classic rrr junction is the pacific nazca galapagos intersection at 1 10 n where the epr axis is joined by a narrow volcanic ridge 70 km long and 1 km high the cocos nazca galapagos junction at the west end of the cocos nazca spreading center is inferred to be of the rff type but the transform fault branches are represented by large nontransform offsets lack of stable transform faults is related to rapid westward migration of the junction caused by lengthening of the cocos nazca axis at 60 70 mm yr this rate which was once required to maintain a stable nazca cocos pacific rrr junction has persisted even after birth of the microplate about 1 ma continuing propagation of the cocos nazca axis has opened up the hess deep rift basin in the eastern epr flank in advance of the eruptive propagator tip this rift valley has a relief of more than 3 km from its uplifted rims to the floor where severe attenuation exposes basal layers of the epr crust the similar dietz deep rift basin at the southeastern margin of the microplate 70 km from hess deep was formed when a lengthening galapagos nazca spreading axis rifted through another part of the epr flank

edge driven microplate kinematics it is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate s motion relative to the bounding major plates are frequently located close to its margins with those plates close to the tips of propagating rifts we propose a class of edge driven block models to illustrate how slip across the microplate margins block rotation and propagation of rifting may be related to the relative motion of the plates on either side an important feature of these edge driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates according to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time pseudofault geometries and matching rotations of the easter microplate show that for most of its 5 m y history block rotation could be driven by the drag of the nazca and pacific plates on the microplate s edges rather than by a shear flow of mantle underneath

towards a conceptual approach to predetermining long return period avalanche run out distances investigating snow avalanches using a purely statistical approach raises several issues first even in the heavily populated areas of the alps there are few data on avalanche motion or extension second most of the field data are related to the point of furthest reach in the avalanche path run out distance or altitude as data of this kind are tightly dependent on the avalanche path profile it is a priori not permissible to extrapolate the cumulative distribution function fitted to these data without severe restrictions or further assumptions using deterministic models is also problematic as these are not really physically based models for instance they do not include all the phenomena occur ring in the avalanche movement and the rheological behaviour of the snow is not known consequently it is not easy to predetermine extreme event extensions here in order to overcome this problem we propose to use a conceptual approach first using an ava lanche dynamics numerical model we fitted the model parameters friction coefficients and the volume of snow involved in the avalanches to the field data then using these parameters as random variables we adjusted appropriate statistical distributions the last steps involved simulating a large number of fictitious avalanches using the monte carlo approach thus the cumulative distribution function of the run out distance can be com puted over a much broader range than was initially possible with the historical data in this paper we develop the proposed method through a complete case study comparing two different models

dyke complex of the east pacific rise exposed in the walls of hess deep and the structure of the upper oceanic crust abstract detailed observations have been made of the extrusive and intrusive units of the uppermost crust emplaced at the axis of the east pacific rise during dives conducted with the submersible nautile on the walls of hess deep at four widely separated locations the same sequence was found an upper thin 100 200 m unit of extrusives pillow lavas underlain by a mixed zone of variable thickness 50 500 m averaging 200 m where extrusives are found and intrusives and massive subhorizontal layers which may represent sills the mixed zone itself is underlain by a sheeted dyke unit that may be up to 1200 m thick where dykes are subvertical i e untilted and parallel to the east pacific rise fabric this sequence may be a general feature of medium to fast spreading crust because it is similar to that observed in hole 504b of the costa rica rift and is consistent with images of the upper oceanic crust derived from seismic experiments on the axis of the east pacific rise dyke thickness is a good measure of the depth to the roof of the axial magma chamber the observed sections provide much needed ground truth for east pacific rise crust the thickness of the lava sequence and the overall thickness of layer 2 at most 1400 m appear to have been overestimated in previous studies of the oceanic crust

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

the development of two oceanic microplates easter and juan fernandez microplates east pacific rise in this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid ocean ridge microplates on the east pacific rise we quantify their development in terms of the principal dimensions of the microplates and their boundaries and the temporal changes in these dimensions the results are discussed in the framework of the roller bearing microplate model of schouten and others which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates we test the specific predictions of this model for the geometry of the pseudofaults traces of the propagating tips of the microplates spreading boundaries and the histories of rotation of the microplates both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling no slip or deformation between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries juan fernandez microplate was also rotating at the predicted rate just before 2 ma when it was driven by the nazca pacific couple rather than the nazca antarctic couple that currently applies however earlier rotations were slower than predicted implying some degree of decoupling or deformation at earlier times the microplates appear to have grown from moderate sized c 100 km probably left stepping offsets of the east pacific rise since about 5 ma the nature of these offsets is currently uncertain but they may have been propagating rifts which changed from shearing by bookshelf faulting to rigid block microplate rotation on reaching a critical size once rigid block rotation began the microplates grew by seafloor spreading accompanied by propagation of their bounding spreading centres the effects of the neighbouring easter mantle plume and pacific nazca antarctic triple junction can be discerned in the bathymetry and ridge propagation rates but do not appear to have had a major influence on the microplates development

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

dike orientations fault block rotations and the construction of slow spreading oceanic crust at 22 40 n on the mid atlantic ridge the first paleomagnetic results from oriented dike samples collected on the mid atlantic ridge shed new light on the complex interplay between magmatic accretion and mechanical extension at a slow spreading ridge segment an upper crustal section about 1 5 km thick is exposed along a west dipping normal fault zone that defines the eastern median valley wall of the southern segment of the mid atlantic ridge south of the kane fracture zone mark area two distinct groups of dikes are differentiated on the basis of orientation and paleomagnetic characteristics one group on the basis of the paleomagnetic data appears to be in its original intrusion orientation this group includes both ridge parallel vertical dikes as well as dikes in other orientations calling into question assumptions about uniform dike orientations at oceanic spreading centers the second group consists of dikes that have paleomagnetic directions that are distinct from the predicted dipole direction and we interpret them to have been tectonically rotated these also occur in many orientations the spatial relations between rotated and nonrotated dikes indicate that intrusion faulting and block rotation were contemporaneous beneath the median valley floor nonrotated dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmatic construction ceased hence slip and associated uplift probably occurred in the fault zones present orientation these results provide the basis for a general model of mechanical extension and dike intrusion for this segment of the mid atlantic ridge initially a portion of crust forms beneath the median valley by synkinematic dike intrusion into laterally discontinuous fault blocks slip and associated uplift along a cataclastic normal fault zone later exposes this crustal section on the valley margin as spreading continues this valley bounding cataclastic normal fault zone is abandoned in favor of a new fault system thus passively moving the exposed crustal section away from the median valley

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

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

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

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

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

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

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

textural character of gabbroic rocks from pito deep a record of magmatic processes and the genesis of the upper plutonic crust at fast spreading mid ocean ridges

implications for crustal accretion at fast spreading ridges from observations in jurassic oceanic crust in the western pacific 1 downhole logging data and basement stratigraphy interpretations are used to determine the spreading environment and crustal accretion history of the ocean basement cored at odp hole 801c located in the jurassic magnetic quiet zone of the western pacific high resolution microresistivity data obtained with the formation microscanner are used to measure the dip of the extrusive layers and indicate a 10 30 increase in dip down the drill hole with lava flow contacts dipping back toward the original ridge axis this structural pattern and the high proportion of massive flows relative to pillow units are consistent with prevailing crustal accretion models proposed for faster spreading ridges e g 60 km m y full rate a detailed analysis of the age data basement lithology related geochemistry and structural attitudes suggest the shallowest 100 m of the drilled section e g sequences i iii were emplaced just off the ridge sequence iii or significantly farther off axis up to 5 15 m y later sequences i and ii the remainder of the drilled section e g sequences iv viii has geochemical lithological and physical trends that are assumed to be representative of crust created at faster spreading ridges the pattern of dipping lava flow contacts from this deeper section of the drill hole suggests lava flows emanating from the ridge axis are limited to 1 2 km off axis our results suggest ocean crust drilled at hole 801c was created at faster spreading rates however caution should be used when incorporating sequences i iii into geochemical reference sections for faster spreading ridges

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

structural contribution from the oman ophiolite to processes of crustal accretion at the east pacific rise a comprehensive model for the activity of the elementary accretion segment at fast spreading ridges relies on integration of structural data from the oman ophiolite and geophysical results from the east pacific rise epr around 9 n which are of comparable size and spreading rates the axial melt lens at shallow crustal level provides a link between deval segmentation at the seafloor and a lower melt sill at moho level imaged at the epr as a crustal melt zone cmz and mapped in oman as the moho transition zone mtz both are attached to a mantle upwelling at the epr and to a frozen diapir in oman the physical link between diapiric mantle uprising at the moho and devals segmentation at the seafloor is the melt being injected from the mantle into the lower mtz ponding there and then being released by powerful injections into the upper melt lens the magma chamber covers the diapir at a distance of 5 km from the ridge axis

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

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

possible erosion mechanisms in snow avalanches snow erosion and entrainment processes in avalanches are classified according to their mechanisms the flow regimes in which they occur and their spatial position within the avalanche simple but process specific models are proposed for erosion by impacts abrasion plowing and blasting on the basis of order of magnitude estimates the first three mechanisms are clearly expected to be important the fourth mechanism stipulates that the compaction of the snowcover ahead of the avalanche leads to the flow of escaping air just in front of the avalanche that may disrupt the snowcover and support formation of a saltation layer the effects of this hypothetical mechanism resemble those of the plowing mechanism all mechanisms depend strongly on the snow properties but with plausible parameter values erosion rates at or above the experimentally found rates are obtained the entrainment rate of an avalanche is most often limited by the shear stress needed to accelerate the eroded snow to avalanche speed

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

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

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

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

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

integration of 4d seismic data and the dynamic reservoir model reveal new targets in gannet c two sets of 4d seismic data gave major new insights into the structure and dynamic behavior of the gannet c oil and gas reservoir in the u k central north sea the 4d data revealed major extensions of reservoir units previously presumed to be absent or thin over much of the reservoir furthermore in a subsea field with significant uncertainty of production allocation 4d also proved an invaluable history matching parameter for the dynamic model together the dynamic model and the 4d data gave rise to the identification of one recompletion opportunity and two infill well opportunities to produce oil volumes in existing and newly identified reservoir sands these now have been realized roughly trebling oil production from the field

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

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

a record of eruption and intrusion at a fast spreading ridge axis axial summit trough of the east pacific rise at 9 10 n 1 high resolution side scan sonar near bottom multibeam bathymetry and deep sea photo and bathymetry traverses are used to map the axial summit trough ast at the east pacific rise between 9 and 10 n we define three ridge axis morphologic types no ast narrow ast and wide ast which characterize distinct ridge crest domains spanning tens of kilometers along strike near bottom observations modeling of deformation above intruding dikes and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of ast genesis and evolution this model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years the grabens in the narrow ast domain 9 43 53 n are consistent with deformation above the most recent 10 diking events beneath the ridge crest frequent high effusion rate extrusive volcanism in this domain several eruptions every 100 years overprints near axis deformation and maintains a consistent ast width the most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the ast the grabens in the wide ast domain 9 23 43 n originated with similar dimensions to the narrow ast spreading driven primarily by the intrusion of shallow dikes within a narrow axial zone causes the initial graben bounding faults to migrate away from the axis infrequent extrusive volcanism several eruptions every 1000 years fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the ast outside of these domains lower effusion rate constructional volcanism without efficient drain back fills and erases the signature of the ast the relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified however over longer time scales 10 25 ka domain specific intrusive to extrusive ratios do not appear to be fixed in space resulting in a fairly consistent volcanic accretion over the length scale of the second order ridge segment between 9 n and 10 n

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

field experiments and numerical modeling of mass entrainment in snow avalanches 1 snow entrainment alters the speed and hence the run out distance of avalanches yet little is known about this significant process we studied entrainment in snow avalanches using observations from 1 the swiss vallee de la sionne test site 2 the italian pizzac site 3 catastrophic avalanches that occurred during the winter 1998 1999 in switzerland and 4 a medium sized spontaneous avalanche that occurred in 2000 in davos switzerland we determined mass and energy balances for 18 avalanche events on average the mass increased by a factor of 4 the primary mode of entrainment appeared to be frontal ploughing although entrainment behind the avalanche front was also observed step entrainment where a snow cover layer fractures and is entirely consumed by the avalanche also occurred basal erosion was negligible mass availability and snow cover structure were the limiting factors governing entrainment other factors such as track topography and avalanche dimension played a secondary role using the experimental results we introduced an entrainment model into a saint venant type flow model where the internal shear deformation of the avalanche is governed by a bagnold law and the shear stress at the basal layer is treated as a voellmy fluid the model with entrainment not only improves the prediction of the velocities and flow heights in comparison to measurements but also reproduces the variations in run out distances which characterize avalanches with similar terminal velocities but different masses

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

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

4d seismic for oil rim monitoring in the central north sea gannet a field a 50 ft oil rim is overlain by a gas cap of variable thickness oil is produced from horizontal wells which initially produced dry oil but as the field became more mature a significant water cut was seen in several wells a dedicated 4d seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves by forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations using 2d and 3d wedge models and comparing the results with real seismic data we are able to decipher the contact movements across the field it is shown that in one part of the field the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap this oil rim displacement produces a very different 4d seismic response from the case of a static gas oil contact and rising oil water contact normal production as a result of these observations we are able to optimize field production by both re perforation of existing wells and by drilling sidetracks into the displaced rim a brown field development opportunity that might otherwise be missed

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

avalanche risk mapping by simulation in this paper we present a simulation approach to mapping avalanche risk with application to settlements in iceland two simulation models are developed to calculate the probability of avalanches travelling a certain distance and of the flow being a specific width these two simulation models in combination with knowledge of the average frequency of avalanche occurrence the variability in avalanche direction and the degree of loss caused by an avalanche permit risk values to be determined for the areas of concern

adaptive scaling for an enhanced dynamic interpretation of 4d seismic data in this study importance is drawn to the role of engineering principles when interpreting dynamic reservoir changes from 4d seismic data in particular it is found that in clastic reservoirs the principal parameters controlling mapped 4d signatures are not the pressure and saturation changes per se but these changes scaled by the corresponding thickness or pore volume of the reservoir volume that these effects occupy for this reason pressure and saturation changes cannot strictly be recovered by themselves this being true for all data interpretation this understanding is validated both with numerical modelling and analytic calculation interestingly the study also indicates that the impact of gas saturation on the seismic can be written using a linear term but that inversion for gas saturation can yield at best only the total thickness pore volume of the distribution the above provides a basis for a linear equation that can readily and accurately be used to estimate pressure and saturation changes quantitative updates of the static and dynamic components of the simulation model can be achieved by comparing thickness or pore volume scaled changes from the simulator with the corresponding quantities on the inverted observations

skewness of marine magnetic anomalies created between 85 and 40 ma in the indian ocean we have performed a detailed analysis of the skewness of marine magnetic anomalies in indian ocean basins created between 85 and 40 ma as a result of the northward motion of india visual and semiautomated methods of skewness determination were applied to the data both provide consistent results but the visual method is prefered for its ability to deal with noisy data plots of apparent effective remanent inclination or skewness corrected for present geomagnetic field inclination versus time for conjugate basins display the combination of three effects a gradual increase with time related to the northward motion of the ridges attached to india in the geomagnetic reference frame a gap between conjugate curves which represents anomalous skewness senso stricto and short period fluctuations which represent the sequence effect i e the effect of neighboring magnetic sources on the skewness of a given anomaly the anomalous skewness decreases with faster spreading rate and completely disappears above 50 km m y an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness

determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge electrofacies analyses of odp iodp hole 1256d 1 the objective of this study is to construct a representative volcanostratigraphy of ocean drilling program hole 1256d the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate an accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater but this is rarely achieved due to very low recovery rates in most basement holes we used two approaches to determine the rock types that form the wall rocks in the basement sections of hole 1256d 1 user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools and 2 the use of an artificial neutral network to objectively classify the responses of all available logging information great availability of formation microscanner fms images provided superior coverage of the borehole wall compared to previous attempts at core log integration this has resulted in more confident and detailed lithologic classifications such as with the distinction between pillows and different styles of breciation ten lithology types are suggested for a volcanostratigraphy model massive flows ponded lava fractured massive flows fragmented flows thin flows or thick pillows pillows breccias dikes in dike complex isolated dikes and gabbros three major lithology types in the extrusive section are massive flows both massive and fragmented massive flow 22 fragmented flows 32 and breccias 19 pillow lavas make up only 1 9 of the volcanic section and are confined to a 100 m interval below the extrusive section subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts the dikes dip steeply to the northeast indicating slight rotation away from the ridge axis we used an artificial neural network ann approach to determine a quantitative lithostratigraphy the ann is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by fms texture as opposed to lithologic interpretation the ann thus provides a porosity based stratigraphy of the basement rather than the traditional lithology based stratigraphy

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

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

influence of weak layer heterogeneity on snow slab avalanche release application to the evaluation of avalanche release depths slab avalanches are generally caused by the collapse of a weak layer underlying a cohesive slab the two key ingredients for the mechanical description of such slab avalanche releases are the heterogeneity of the weak layer and the redistribution of stresses via the elasticity of the slab the finite element code cast3m is used to build a complete full scale mechanical model of the slab weak layer system including inertial effects the weak layer is modeled as a cohesive mohr coulomb interface with cohesion softening which accounts for shear induced collapse the overlying slab is represented by an elastic layer the system is loaded by increasing the slope angle until rupture we first study the influence of a single weak spot in the weak layer in order to validate the model against analytical solutions the interaction between two weak spots is also analyzed the case of heterogeneous weak layers represented through gaussian stochastic distributions of the cohesion with a spherical spatial covariance is then studied several simulations for different realizations of the heterogeneity of the weak layer are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is examined in particular a heterogeneity smoothing effect caused by slab elasticity and characterized by a typical length scale of the system is evidenced finally the model is coupled with extreme snowfall distributions belonging to the gev class which allows to recover with very good accuracy release depth distributions obtained from field data

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies