soil chronosequences in israel abstract chronosequences of israel soils are summarized soil development in the coastal plain is related to the time of the dune deposits while those of the inland valley is related to the formation of the sedimentological terraces soil development in the mountains is related to the features and stability of the slope the properties of young soils resemble those of the parent material with advancing development the soils are affected by various processes among them also the accretion of aeolian dust leaching is significant already at early stages in some cases like hamra and terra rossa soils the leaching stage is more advanced in earlier stages than in the final stage of soil development in advanced stages of soil development the effect of the underlying rock on soil properties is reduced at the final stage soils on moderate relief in the same climatic zone on different bedrocks resemble each other this is attributed mainly to the effect of the aeolian dust accretion the final stage of soil development on moderate relief consists of grumusols in the northern parts of israel grumic dark brown soils in the semi arid parts of central israel loessial light brown clay loam and loessial serozems in the arid parts of the northern negev and regs in the extremely arid areas of the southern negev and sinai petrocalcic horizons are developed on slopes and terraces where the rate of soil erosion equals that of accumulation of aeolian dust the formation of pertrogypsic horizons in the extremely arid zones is restricted to areas where erosion is negligible the final stages of soil development exhibit a clear climatic zonality this zonality differs from the soil zonality in the ussr and the usa due to the special features of dust accretion on the soils of israel

along strike segmentation in the northern caribbean plate boundary zone hispaniola sector tectonic implications abstract the north american noam plate converges with the caribbean carib plate at a rate of 20 0 0 4 mm yr towards 254 1 plate convergence is highly oblique 20 10 resulting in a complex crustal boundary with along strike segmentation strain partitioning and microplate tectonics we study the oblique convergence of the noam and carib plates between southeastern cuba to northern puerto rico using new swath multibeam bathymetry data and 2d multi channel seismic profiles the combined interpretation of marine geophysical data with the seismicity and geodetic data from public databases allow us to perform a regional scale analysis of the shallower structure the seismotectonics and the slab geometry along the plate boundary due to differential rollback between the noam oceanic crust north of puerto rico and the relative thicker bahamas carbonate province crust north of hispaniola a slab tear is created at 68 5 w the northern margin of puerto rico records the oblique high dip subduction and rollback of the noam plate below the island arc those processes have resulted in a forearc transpressive tectonics without strain partitioning controlled by the septentrional oriente fault zone sofz and the bunce fault zone bfz meanwhile in the northern margin of hispaniola the collision of the bahamas carbonate province results in high plate coupling with strain partitioning sofz and northern hispaniola deformed belt nhdb in the northern haitian margin compression is still relevant since seismicity is mostly associated with the deformation front whereas strike slip earthquakes are hardly anecdotal although in hispaniola intermediate depth seismicity should disappear diffuse intermediate depth hypocenter remains evidencing the presence of remnant noam subducted slab below central and western hispaniola results of this study improve our understanding of the active tectonics in the ne caribbean that it is the base for future assessment studies on seismic and tsunamigenic hazard

erosional troughs in deep sea carbonates and their relationship to basement structure abstract a detailed survey of a small area in the eastern equatorial pacific with the marine physical laboratory s deep tow instrument package reveals erosionally excavated troughs in an area of relatively rapid accumulation of carbonate the troughs lie on top of a basement high implying a relationship between the troughs and the basement feature differential accumulation and compaction may explain the origin of the troughs sediment deposited around the basement high preferentially accumulates on gentle slopes and in troughs this results in a thick sequence of sediment on the flanks of the basement high and a thin sequence above on top of the hill unstable sediment masses and differential compaction generate a tensile stress field above the peak of the basement high which causes slumping and faulting the steep slopes generated by faulting and slumping may then serve to intensify tidal currents or act as spillways and locally generate increased turbulence this increased turbulence results in local erosion by dissolution a trough will form above a basement high however only when a combination of 1 basement steepness 2 high sedimentation rate 3 sediment thickness and 4 sediment erodibility are present

deep earth structure major developments in earth structure in the last four years have been concentrated in the description of the earth s lateral heterogeneity the regions that are heterogenous and the per cent variation of velocity and density in each region most studies find that lateral variation is concentrated in the upper 400 and lower 200 km of the mantle a radially symmetric earth model has been defined that represents the best average fit to seismic data in a broad frequency band sampling many regions p and s velocity is found to increase in zones of 50 km or less at 400 and 650 to 700 km depth the model is transversely anisotropic in the upper mantle it possesses a vertical axis of symmetry such that the elastic constants are different for vertical propagation than for horizontal and intermediate angles of propagation the real earth generally exhibits azimuthal anisotropy as well but the azimuthal anisotropy cannot be resolved by a global average of data the nature and magnitude of the anisotropy agrees with that found in ultramafic samples of the upper mantle in attenuation models of intrinsic attenuation have included the dispersive properties of intrinsic anelasticity and constructed relaxation models consistent with an observed frequency dependence of q in the body wave band there has been progress in mapping the scattering properties of the lithosphere attenuation due to scattering in the crust and lithosphere has been shown to have strong effects on the amplitudes of seismic waves at local and teleseismic distances

do moderate magnitude earthquakes generate seismically induced ground effects the case study of the m w 5 16 29th december 2013 matese earthquake southern apennines italy seismically induced ground effects characterize moderate to high magnitude seismic events whereas they are not so common during seismic sequences of low to moderate magnitude a low to moderate magnitude seismic sequence with a m w 5 16 0 07 main event occurred from december 2013 to february 2014 in the matese ridge area in the southern apennines mountain chain in the epicentral area of the m w 5 16 main event which happened on december 29th 2013 in the southeastern part of the matese ridge field surveys combined with information from local people and reports allowed the recognition of several earthquake induced ground effects such ground effects include landslides hydrological variations in local springs gas flux and a flame that was observed around the main shock epicentre a coseismic rupture was identified in the sw fault scarp of a small sized intermontane basin mt airola basin to detect the nature of the coseismic rupture detail scale geological and geomorphological investigations combined with geoelectrical and soil gas prospections were carried out such a multidisciplinary study besides allowing reconstruction of the surface and subsurface architecture of the mt airola basin and suggesting the occurrence of an active fault at the sw boundary of such basin points to the gravitational nature of the coseismic ground rupture based on typology and spatial distribution of the ground effects an intensity i vii viii is estimated for the m w 5 16 earthquake according to the esi 07 scale which affected an area of at least 90 km2

deep sea fluid and sediment dynamics influence of hill to seamount scale seafloor topography abstract deep sea sediments play a central role in a wide range of subject areas a number of important controls on the formation of sedimentary deposits have been studied however to date the impact of submarine landscape geometry as a possible control has received comparatively little attention this seems to be particularly true for intermediate scale topographic features such as abyssal hills knolls and seamounts that can be found in many regions of the global seafloor recent estimates suggest that in the deep open oceans away from continental margins there might be as many as 25 106 abyssal hills knolls and seamounts despite this large number very little is known about how they influence environmental complexity and patchiness biogeochemical fluxes and the formation of sedimentary records this paper reviews the currently known types of fluid flow interactions with abyssal hills knolls and seamounts that could potentially influence the way sediments are formed the main types of relevant flow components are quasi steady to eddying background flow internal lee and near inertial waves barotropic and baroclinic tides and seamount trapped waves previous studies looking into systematic links between fluid dynamics and sediments at hills knolls and seamounts are reviewed finally a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid flow components leaves a detectable imprint in the recent sediments on and around a short seamount the main conclusions and implications are as follows 1 topographically generated flow field geometries that are composed of a number of different prevailing fluid flow components can be reflected and detected in properties of the underlying sediments 2 tidal and other higher frequency lee wave near inertial components of deep ocean currents can be essential for locally driving total current velocities across threshold values for non deposition erosion resuspension of freshly deposited deep sea sediments moreover there is evidence suggesting that not only maximum current speeds but also intensities of higher frequency tidal and or near inertial current direction variability might control sediment dynamics and sediment formation this relativises the view that current speed is the main or even only controlling factor for sediment dynamics and sediment formation 3 when it comes to the reconstruction of paleo flows these findings imply that certain sedimentary records may well reveal more about variability in the higher frequency flow components than about variability in the basin scale net flow component that often is the focus of paleoceanographic studies 4 single core paleo records from hill seamount or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features to arrive at unbiased paleo records for non fluid dynamic parameters the influence of the flow field geometry would have to be removed from the record first 5 it seems the mechanistic understanding of hill and seamount related flow topography interactions and their links to sediment dynamics is approaching a level that may a facilitate improved interpretation of topographically controlled sedimentary paleo records b help fill in the knowledge gap that exists for functional deep sea biodiversity at intermediate space scales and c improve predictive capabilities for exploration of economically relevant iron manganese fe mn crusts on seamounts

thin crust and active upper mantle beneath the southern sierra nevada in the western united states abstract analysis of seismic refraction wide angle reflection travel time and seismic amplitude data collected during the 1993 southern sierra nevada continental dynamics sscd project resolves a small crustal root 40 42 km thick centered 80 km west of the sierran topographic crest the sscd experiment consisted of a west east profile across the sierra nevada at approximately 36 5 n and a north south profile extending the length of owens valley located eastward of the sierran topographic crest two dimensional finite difference travel time inversion of p g and p n arrivals resolves upper crustal velocities of 6 0 to 6 4 km s 1 within the sierran batholith and basin and range an upper crustal west dipping wedge of higher velocities 6 8 7 2 km s 1 consistent with ophiolitic material underlying the great valley sedimentary sequence and higher velocities in the lower crust beneath the basin and range 6 8 7 0 km s 1 than those beneath the sierran batholith 6 6 km s 1 low average p n velocities 7 6 7 9 km s 1 and a laminated transitional moho imaged beneath the sierran batholith also differ from the higher p n velocities 7 9 8 0 km s 1 and sharp first order moho observed beneath the basin and range the crust decreases in thickness both westward of the root to 28 34 km beneath the great valley and eastward to 35 km beneath the highest sierran topography and decreases further to 27 30 km beneath the basin and range crustal thickness also appears to increase southeast to northwest from 29 30 km beneath the garlock fault in the south to 38 40 km beneath the north end of chalfant valley juxtaposition of the crustal model with previous p wave tomography models of the south ernmost sierra nevada upper mantle reveal that the thickest sierran crust on the west east profile overlies a pronounced upper mantle high velocity anomaly 5 whereas the region of laminated moho overlies a flanking upper mantle low velocity region 3 the upper mantle velocity anomalies relatively low p n and relatively flat moho observed beneath the sierran crest suggest that the recent uplift of the sierra nevada is due to asthenospheric flow and or lithospheric thinning beneath the southeastern sierra nevada and basin and range

significant southern ocean warming event in the late middle eocene a prominent middle eocene warming event is identified in southern ocean deep sea cores indicating that long term cooling through the middle and late eocene was not monotonic at sites on maud rise and the kerguelen plateau a distinct negative shift in 1 8 o values 1 0 is observed ca 41 5 ma this excursion is interpreted as primarily a temperature signal with a transient warming of 4 c over 600 k y affecting both surface and middle bathyal deep waters in the indian atlantic region of the southern ocean this isotopic event is designated as the middle eocene climatic optimum and is interpreted to represent a significant climatic reversal in the midst of middle to late eocene deep sea cooling the lack of a significant negative carbon isotope excursion as observed during the paleocene eocene thermal maximum and the gradual rate of high latitude warming suggest that this event was not triggered by methane hydrate dissociation rather a transient rise in pco 2 levels is suspected possibly as a result of metamorphic decarbonation in the himalayan orogen or increased ridge arc volcanism during the late middle eocene

kilometre scale polygonal seabed depressions in the hatton basin ne atlantic ocean constraints on the origin of polygonal faulting polygonal faulting is a widespread phenomenon in sedimentary basins worldwide it changes basin scale fluid flow patterns and alters the physical properties of the sediments making it important for hydrocarbon exploration and geohazard analysis it is generally accepted that polygonal fault patterns derive from dewatering and compaction of the host sediments but there is debate regarding the processes that control polygonal faulting new multibeam bathymetry data from the hatton basin ne atlantic show up to 10 m deep and 200 600 m wide troughs at the seabed they connect to each other forming polygons that are several hundred metres across i e of similar size as buried polygonal fault systems observed in 3d seismic data the troughs are symmetrical and resemble elongated pockmarks previously unpublished high resolution 2d seismic data from the same area show seismic disturbance zones similar to pipes observed under pockmarks elsewhere as well as faults that have all the characteristics of polygonal fault systems the observation of the wide disturbance zones is enigmatic as they appear to follow the polygonal seafloor pattern the observed extent of the polygonal sediment contraction system is substantial covering almost 37 000 km2 we calculate that some 2600 km3 of possibly carbon bearing fluids have been expelled from this system and we expect that this will affect the benthic ecosystems although so far there is only limited evidence for chemosynthetic habitats

three dimensional seismic characterization of a complex sediment drift in the south china sea evidence for unsteady flow regime this study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion infill cross section based on high resolution three dimensional two dimensional seismic data and bathymetric data the honeycomb structures cover an area of more than 5000km 2 and are developed within the late miocene to recent deep water sediments of the north western south china sea linear erosional troughs up to 10km long and 1km wide are widely developed in this area are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the late miocene the unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies considerable sea floor topography was generated as a direct result of tectonic movements in the area since the late miocene and this topography then influenced the pathways of strong bottom currents this study highlights that i an unsteady bottom current regime can be laterally extensive and persist for millions of years ii structurally controlled sea floor relief plays an important role in controlling the depositional pattern and iii the bottom currents were active since the late miocene flowing from the south east through the xisha guangle gateway and crossing the honeycomb structure zone this study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the south china sea deep water circulation which is at present still poorly understood

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

seismic interpretation of pelagic sedimentation regimes in the 18 53 ma eastern equatorial pacific basin scale sedimentation and infilling of abyssal valleys understanding how pelagic sediment has been eroded transported and deposited is critical to evaluating pelagic sediment records for paleoceanography we use digital seismic reflection data from an integrated ocean drilling program site survey amat03 to investigate pelagic sedimentation across the eastern central equatorial pacific which represents the first comprehensive record published covering the 18 53 ma eastern equatorial pacific our goals are to quantify 1 basin hill scale primary deposition regimes and 2 the extent to which seafloor topography has been subdued by abyssal valley filling sediments the eastern pacific seafloor consists of a series of abyssal hills and basins with minor late stage faulting in the basement ocean crust rarely outcrops at the seafloor away from the rise crest both hills and basins are sediment covered the carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data overall we recognized three different sedimentation regimes depositional high sedimentation rate transitional and minimal sedimentation low sedimentation rate regimes in all areas the sedimented seafloor mimics the underlying basement topography although the degree to which topography becomes subdued varies depositional regimes result in symmetric sedimentation within basins and subdued topography whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief regardless of sedimentation regime enhanced sediment deposition occurs within basins however we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills if this variation is due to sediment focusing the focusing factor in the basins as measured by 230th is no more than a factor of 1 3 of the total vertical particulate rain

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

fields of multi kilometer scale sub circular depressions in the carnegie ridge sedimentary blanket effect of underwater carbonate dissolution offshore ecuador the carnegie ridge is a volcanic ridge with a carbonate sediment drape during the salieri cruise multibeam bathymetry was collected across carnegie ridge with the simrad em120 of the r v sonne the most conspicuous features discovered on the carnegie ridge are fields of circular closed depressions widely distributed along the mid slope of the northern and southern flanks of the ridge between 1500 and 2600 m water depth these circular depressions are 1 4 km wide and typically 100 400 m deep most are flat floored and some are so densely packed that they form a honeycomb pattern the depressions were carved into the ridge sedimentary blanket which consists of carbonate sediment and has been dated from upper miocene to upper pleistocene several hypotheses including pockmark origin sediment creeping paleo topography of the volcanic basement effects of subbottom currents and both marine and subaerial karstic origins are discussed we believe that underwater dissolution process merits the most serious consideration regarding the origin of the closed depression

intraplate volcanism of the western pacific new insights from geological and geophysical observations in the pigafetta basin understanding intraplate volcanism is a key to deciphering the earth s magmatic history one of the largest intraplate volcanic events occurred during the mid cretaceous roughly 75 to 125 ma in the western pacific to investigate the origin and effects of this volcanism on various earth systems we present the first comprehensive study of volcanism in the pigafetta basin using seismic surveys magnetic and gravity modeling and ocean drilling program drill core and well log data from site 801 our results show that intraplate volcanism in the pigafetta basin coincides with the rest of the western pacific seamount provinces supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid cretaceous volcanic events we also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the jurassic ocean crust in the pigafetta basin and hence marine magnetic anomalies recorded in the jurassic basement are preserved also the formerly identified rough smooth boundary rsb is indistinguishable from any other rough smooth topographic boundaries throughout the survey area suggesting that the rsb is unlikely to be a cretaceous sill jurassic basement boundary lastly the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the pigafetta basin indicating that hydrothermal circulation was ongoing well past 100 ma this article is protected by copyright all rights reserved

large scale lateral advection of seawater through oceanic crust in the central equatorial pacific the existence of large scale lateral advection of water through basaltic crust in the central equatorial pacific ocean is demonstrated by the calcium magnesium strontium sulfate and strontium isotopic compositions of pore waters from the overlying sediments the advection is believed to extend throughout the region of the equatorial high productivity sediment bulge from about 110 to 160 w and from about 5 s to 8 n the corresponding crustal ages of this region vary from east to west from about 15 to about 70 ma respectively this advection is responsible for the low conductive heat flows previously observed throughout the region the flow is recognized by the following characteristic pore water compositional variations calcium and magnesium concentrations remain nearly constant downhole from the sediment water interface to basement species such as strontium sulfate and strontium isotopes which are more affected by diagenetic reactions in the sediment column depart from seawater values with increasing depth in the sediments but then they display concentration reversals near basement at the sediment basement interface pore waters are chemically and isotopically nearly indistinguishable from present day seawater fluid flow in basement is rapid having a calculated average residence time in oceanic crust of about 20 000 years and an inferred pore fluid velocity between 1 and 10 m y 1 because of the short reaction time between basement rocks and fluids as well as the low temperature of this fluid the chemistry of basement water remains similar to seawater as a result despite the important impact of this process on oceanic heat flow the flow may have little effect on the long term major element composition of seawater

hydrothermal pits in the biogenic sediments of the equatorial pacific ocean 1 a recent expedition to the equatorial pacific surveyed over 60 000 km2 of seafloor using swath map imaging and high resolution seismic profiling this survey revealed the common occurrence of large pits in the biogenic sediments that are usually associated with basement highs the pits range in size up to a few kilometers across and are on the order of 50 m 100 m deep they were found on seafloor ranging in age from 15 ma to 55 ma with overlying sediment cover up to 500 m thick we link their origin with hydrothermal discharge through conduits created by differential compaction and fracturing over basement highs and by basement faulting that penetrates the overlying sediments although carbonate dissolution is likely to be associated with the formation of the pits their occurrence shows no consistent relationship with the present calcite compensation depth ccd

mmi attenuation and historical earthquakes in the basin and range province of western north america earthquakes in central nevada 1932 1959 were used to develop a modified mercalli intensity mmi attenuation model for estimating moment mag nitude m for earthquakes in the basin and range province of interior western north america m is 7 4 7 5 for the 26 march 1872 owens valley california earthquake in agreement with beanland and clark s 1994 m 7 6 that was estimated from geologic field observations m is 7 5 for the 3 may 1887 sonora mexico earthquake in agreement with natali and sbar s 1982 m 7 4 and suter s 2006 m 7 5 both estimated from geologic field observations mmi at sites in california for earthquakes in the nevada basin and range appar ently are not much affected by the sierra nevada except at sites near the sierra nevada where mmi is reduced this reduction in mmi is consistent with a shadow zone produced by the root of the sierra nevada in contrast mmi assignments for earthquakes located in the eastern sierra nevada near the west margin of the basin and range are greater than predicted at sites in california these higher mmi values may result from critical reflections due to layering near the base of the sierra nevada

paleogene tropical pacific clues to circulation productivity and plate motion 1 stratigraphic data from 63 deep sea drilling project and ocean drilling program odp sites that sample the lower neogene and paleogene sediments of the tropical pacific have been compiled and put on the biostratigraphic and paleomagnetic timescale refined by odp leg 199 scientists sediment accumulation rates have been calculated for ten intervals ranging in age from 10 to 56 ma and have been plotted for the midpoint of each interval at the associated paleoposition for each site used a fixed hot spot model was used for reconstruction of the pacific lithospheric plate all such reconstructed intervals show the development of a tongue of high accumulation rates associated with the oceanographic divergence at the geographic equator the development of this equatorial band is weakest between 46 and 56 ma the time of the peak warmth in paleogene climate possible motion of the hawaiian hot spot or true polar wander between 46 and 56 ma appears to have had little effect on the plate rotation estimate of the position of the equator in addition to temporal changes in the calcite compensation depth and in productivity the biggest change in the patterns of sediment accumulation rates in the eastern tropical pacific was the development of a relatively strong divergence between 6 and 10 n near the region of divergence between the modern north equatorial current and the north equatorial counter current changes in the equatorial circulation appear to be associated in time with the opening and closing of oceanic gateways particularly the complex closing of the caribbean pacific gateway

reinterpretation by ray tracing of a transverse refraction seismic profile through the california sierra nevada part i abstract this study reanalyzes a refraction seismic profile from nts nevada to san francisco transverse to the high sierra nevada three models of the earth9s crust with an asymmetrical mountain root constructed by different authors are tested by the use of ray tracing model cqm carer et al 1970 with a maximum crustal thickness of 35 km centered about 50 km west of the crest of the sierra models a and b pakiser and brune 1980 with a mountain root extending to a depth of 55 km below the sierra in the latter guided and diffracted waves are used to explain seismic p n energy in the shadow zone of the root the preferred model a has a high velocity layer dipping from west to east in the upper crust while model b does not the present study demonstrates how fallacies in interpretation of complicated structures can occur if all ray types are not quantitatively traced the influence of the p wave gradient in the upper mantle on the length of the shadow zone is shown to be important and it is demonstrated that a modified cqm model explains the observed travel times when diffracted waves are included cqm does not however explain why the p n travel times on a separate refraction profile along the sierra nevada from a truckee earthquake are so great model b interprets the p n signals at 200 km as diffracted waves but the computed travel times do not fit the observations the dipping wave guide in model a does not work as expected and perhaps should be ruled out

biogenic silica and chert in the pacific ocean evidence for the dissolution of biogenic silica at the base of pelagic sections supports the hypothesis that much of the chert formed in the pacific derives from the dissolution and reprecipitation of this silica by hydrothermal waters as ocean bottom waters flow into and through the crust they become warmer initially they remain less saturated with respect to dissolved silica than pore water in the overlying sediments with the diffusion of heat dissolved ions and to some extent the advection of water itself biogenic silica in the basal part of the sedimentary section is dissolved upon conductively cooling these pore waters precipitate chert layers the most common thickness for the basal silica free zone 20 m lies below the most common height of the top of the chert interval above basement 50 m this mode of chert formation explains the frequent occurrence of chert layers at very shallow subbottom depths in pelagic sections of the pacific it is also consistent with the common occurrence of cherts 150 m above basement

influence of sedimentation local and regional hydrothermal circulation and thermal rebound on measurements of seafloor heat flux 1 we quantify the influence of environmental processes on measurements of seafloor heat flux with a new one dimensional thermal model that includes time varying sedimentation and boundary conditions represents several scales of hydrothermal and transient conductive processes within basement and allows fluid seepage through accumulating sediments variations in basement thermal conductivity the extent of hydrothermal mixing in upper basement and fluid seepage through sediments each influence seafloor heat flux by 2 8 conductive thermal rebound following the cessation of advective heat loss from the crust may lower seafloor heat flux values by 5 10 even after several to several tens of million years the new models indicate that thermal rebound takes much longer than suggested by earlier analytical calculations mainly because earlier models did not account for the heat capacitance of the conductive lithosphere application of the new model to 3 5 3 6 ma seafloor on the eastern flank of the juan de fuca ridge suggests that anomalously low heat flux in this area is best explained by incomplete conductive thermal rebound in the last 100 200 ka following the burial of numerous basement outcrops application of the new model to the eastern flank of the east pacific rise and extrapolation out to the age of some of the oldest remaining seafloor indicate that sedimentation corrections may be important even where accumulation rates are typical of global values model results also suggest that conductive thermal rebound through thick sediments may bias measurements made on moderate to old seafloor even where there is little evidence at present for ridge flank hydrothermal circulation

formation of hydrothermal pits and the role of seamounts in the guatemala basin equatorial east pacific from heat flow seismic and core studies we acquired seismic and heat flow data and collected sediment cores in three areas in the guatemala basin cocos plate eastern pacific to investigate the process by which depressions pits in the sedimentary cover on young oceanic crust were formed median heat flow of 55 mw m2 for the three areas is about half of the expected conductive cooling value the heat deficit is caused by massive recharge of cold seawater into the upper crust through seamounts which is inferred from depressed heat flow in the vicinity of seamounts heat flow inside of pits is always elevated in some cases up to three times max 300 mw m2 relative to background none of the geochemical pore water profiles from cores inside and outside of the pits show any evidence of active fluid flow inside the pits all three areas originated within the high productivity equatorial zone and moved northwest over the past 15 to 18 ma pits found in the working areas are likely dissolution structures formed by diffuse hydrothermal venting in a zone of high biogenic carbonate production which were sealed when they moved north it is likely that these pits were discharge sites of hydrothermal siphons where recharging seamounts could feed cold seawater via the upper crust to several discharging pits probably pit density on the whole cocos plate is similar to the three working areas and which may explain the huge heat deficit of the cocos plate this article is protected by copyright all rights reserved

equatorial ocean circulation in an extremely warm climate 1 the warm climates of the early paleogene and the associated diminished near surface winds should have resulted in a reduction in near surface ocean circulation one check on this deduction is the delineation of biogenic sediments associated with an equatorial current system of the early eocene pacific a latitudinal seismic reflection transect across the tropical pacific along early paleogene ocean crust reveals a basal high amplitude reflection package that we take to be the lower eocene section this unit varies in thickness by a factor of about two with the thickest portion forming a low mound some 3 4 north of the 56 ma paleoequator this mound may represent the position of a divergence generated in the frontal region between two currents flowing in opposite directions and its position suggests that the wind driven equatorial circulation of the early eocene was one without a pronounced equatorial divergence

sediment mixing in the tropical pacific and radiolarian stratigraphy 1 the reworking of older radiolarian microfossils into near surface sediments of the tropical pacific has long been the source of confusion for the development of radiolarian stratigraphy and of puzzlement over the mechanism s that could effect such pervasive reworking widespread dissolution pits in the sediments of the tropical pacific are believed to be associated with hydrothermal circulation cells in the older oceanic crust and are here linked to processes which expose older sections and inject older non carbonate material into near bottom waters discharging waters of these circulation cells tend to dissolve carbonate in near surface sediments thus only the non carbonate material including radiolarians is preserved and reworked into younger sediments results from the study of two sites in the tropical pacific indicate that reworked older stratigraphically important radiolarians are less than 2 of the total radiolarian assemblage this constitutes a minimum estimate of the amount of reworked non carbonate material in the younger sediments the oldest reworked radiolarians are no more than 10 m y younger than the underlying basement and radiolarians from the entire older section above that level can be found in the reworked material a time series of the flux of reworked material at one site is not constant but instead has varied by a factor of 3 to 4 over the past 2 5 m y during times when the flux of reworked material is particularly low the proportion of older more robust radiolarians is larger

earthquake refraction profiles of the root of the sierra nevada we examine the seismic structure of the sierra nevada using records of nine earthquakes and one explosion in and near the sierra recorded on stations in the sierra we first interpret travel times from these paths which are confined to a single tectonic block in terms of one dimensional structures the most nearly reversed pair of earthquakes the 1966 truckee and 1983 durrwood meadows earthquakes share refracted pn arrival times corrected to surface focus along a line t 8 75 0 25 8 0 suggesting that a nearly flat layer of 8 0 km s mantle material lies at depths of 46 48 km first arrivals from these events do not constrain velocities from 30 to 45 km depth secondary arrivals and some first arrivals from other earthquakes suggest that velocities in part of this region range between 6 9 and 7 8 km s the presence of this 7 x km s layer can help to explain previous contradictory observations the 7 x km s layer could be interpreted as either the mafic bottom of a silicic mesozoic magmatic arc or as accreted mafic underplating or rejuvenated mantle related to cenozoic arc volcanism or basin and range spreading arrivals at stations in the foothills and the crest of the sierra cannot be fit with a single longitudinal structure indicating a lateral variation of velocity structure these variations support previously inferred variations of lithospheric structure with higher velocity thinner crust to the west beneath the sierran foothills and slower velocity crust or possibly upper mantle beneath the high mountains in the eastern sierra rapid changes in arrival times between stations separated by short distances in the eastern sierra suggest that a sharp boundary exists between the sierra and the basin and range at moho depths we also present fresh evidence of the asymmetry of the root of the sierra wherein arrivals from earthquakes on the west of the sierra are delayed within the sierra and return to original values in the basin and range while arrivals from earthquakes and explosions from the sierra into the great valley we suggest that if the 7 x km s material occurs in a wedge above the moho then the asymmetry can be explained by arrivals from the west being delayed by the dipping 8 0 km s moho while those from the east may be entering the root along a 7 x km s layer that is near the depth of the basin and range moho

late cretaceous apparent polar wander of the pacific plate evidence for a rapid shift of the pacific hotspots with respect to the spin axis paleomagnetic data from the pacific plate show that the average azimuth of the pacific plate decreased by 12 between 90 ma and 81 ma whereas trends of late cretaceous seamount chains suggest that the pacific plate moved northwestward relative to the hotspots with little change in azimuth during the same time interval this difference is used to infer a 13 4 shift of the spin axis relative to the hotspots in the pacific ocean basin this shift occurred within 5 m y of the occurrence of anomalous behavior of the paleomagnetic field recorded in anomalies 33 and 34 this coincidence in timing suggests there may be a causal link between these three phenomena

shrinkage and atterberg limits in relation to other properties of principal soil types in israel abstract soil samples collected from 32 sites across israel representing major types were analyzed for their pedological characteristics and mechanical properties correlative relationships between shrinkage measured by the coefficient of linear extensibility cole atterberg limits liquid and plastic and the physical and chemical properties were established and indicate possibilities of estimating mechanical properties from known pedological data strong correlations were noted between mechanical properties and the pedological characteristics reflecting clay mineralogy and texture e g cation exchange capacity specific surface area hygroscopic moisture and clay content these relationships are useful inasmuch as clay mineralogy is usually evident from soil classification sodicity and salinity common in dry soils of semi arid regions may introduce complicating factors such as raising or lowering atterberg limits respectively shrinkage is similarly affected being accentuated at high sodium low salt levels and reduced in highly saline sodic soils organic matter was correlated with liquid and plastic limits but no relationship with shrinkage was noted calcium carbonate also had little influence on atterberg limits the relationships presented may serve as first approximations mechanical properties of the soil groups illustrate the range of values encountered among regions with differing environmental conditions

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

crustal and upper mantle structure of the northern and central sierra nevada supplement available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 2009 document b83 002 2 50 payment must accompany order teleseismic data were recorded within the sierra nevada to look for lateral variations in the upper mantle the data were collected at both temporary and permanent stations and p wave residuals were computed after correcting the p residual data for crustal and topographic effects there is still a variation of as much as 0 5 0 6 s from the north end of the sierra nevada to mono lake located east of the central part of the range in addition there are significant variations in travel time patterns depending on the azimuth of wave arrivals two simple modeling approaches have been used to infer the upper mantle velocity structure from the observed variations in travel time in the first it is assumed that the velocity variations are distributed throughout a depth range of about 60 160 km one model that fits the data indicates a north to south trend in upper mantle p velocities from about 7 9 km s at the north end of the sierra to 7 6 km s near mono lake superimposed is a west to east trend from 7 85 km s under the sierra nevada crest to 7 7 km s east of lake tahoe in the second modeling approach a fixed velocity contrast is assumed between the lithosphere and asthenosphere with variations in travel time resulting from variations in lithospheric thickness one model consistent with the observations indicates a thinning of the lithosphere from 110 km at the north end of the sierra to 60 km near mono lake

significance of primary hilltop loess in reconstructing dust chronology accretion rates and sources an example from the negev desert israel 1 although high latitude loess sequences have been studied extensively for reconstructing past climates loess in desert margins has been mostly overlooked moreover many low latitude loess studies have focused on secondary reworked sequences for which paleoclimatic inferences are complicated owing to postdepositional processes acting under local control in this respect the spatial distribution of the loess and its different types are crucial in this study we differentiate primary and secondary loess using remote sensing and field criteria we present remote sensing and geographic information system procedures for mapping loess distribution in general and for identifying pristine hilltop loess sequences in particular then we present the mineralogical and textural analyses combined with optically stimulated luminescence ages of one such primary hilltop loess sequence in the negev desert that show that the late pleistocene negev loess started accumulating at 95 ka much earlier than previously proposed 70 ka it was preceded by a previously unknown middle pleistocene period of loess formation at 180 130 ka the ages also suggest that the threshold of dust accretion rate needed for loess formation is 0 02 mm a 1 30 g m 2 a 1 a factor of 2 or more lower than previously thought 0 04 to 1 0 mm a 1 our results also provide insights into causes of late pleistocene loess accretion and its close association with regional dune field activity the methodology for mapping the loess distribution may potentially be generalized to help understanding loess formation in other more remote desert areas

a honeycomb seafloor morphology in carbonate sediment of the carnegie ridge offshore ecuador formation and potential geodynamic significance based on swath bathymetry two dimensional high resolution seismic reflection profiles and ocean drilling program deep sea drilling project odp dsdp data we describe a seafloor honeycomb pattern and propose a model for its formation in pliocene miocene carbonate deposited on the uneven oceanic basement of the carnegie ridge offshore ecuador hydrothermal fluids derived from the basement aquifer fractured and dissolved carbonate sediment creating seafloor pits above basements highs fluids expelled along polygonal faults may have assisted the nucleation of seafloor depressions at the pliocene pleistocene boundary strong bottom currents scoured previously damaged sediments enlarging the initial depressions and producing the seafloor honeycomb pattern this regional erosive episode was contemporaneous with the final closing of the isthmus of panama and the clogging of the ecuador trench by the subduction of the carnegie ridge so that the honeycomb pattern may be viewed as a regional marker of these two geodynamic events

eocene el nino evidence for robust tropical dynamics in the hothouse much uncertainty surrounds the interactions between the el nino southern oscillation enso and long term global change past periods of extreme global warmth exemplified by the eocene 55 to 35 million years ago provide a good testing ground for theories for this interaction here we compare eocene coupled climate model simulations with annually resolved variability records preserved in lake sediments the simulations show pacific deep ocean and high latitude surface warming of approximately 10 degrees c but little change in the tropical thermocline structure atmosphere ocean dynamics and enso in agreement with proxies this result contrasts with theories linking past and future hothouse climates with a shift toward a permanent el nino like state

latitudinal shift of pacific hotspots during the late cretaceous and early tertiary two frames of reference the geomagnetic field and hotspots have been widely used to measure the past motions and interactions of lithospheric plates several studies have found that these reference frames have undergone as much as 19 22 of relative motion often called true polar wander tpw in the past 200 myr1 4 moreover it also appears that this offset did not accumulate at a uniform rate some evidence suggests that spurts of tpw can occur at rates as high as 1 2 per myr3 5 rates even an order of magnitude less could pose serious problems for tectonic studies based on the hotspot reference frame because velocities of this magnitude cannot be considered insignificant compared to those of the plates unfortunately the imprecision of most palaeomagnetic data is such that large error bounds are common for many estimates of tpw rates in this report we present a record of tpw derived from pacific palaeomagnetic data that are relatively well constrained in age and palaeolatitude they show that offset between the two reference frames accumulated at 0 3 per myr 1 from late cretaceous to early tertiary

volcanism jump and propagation on the sheba ridge eastern gulf of aden segmentation evolution and implications for oceanic accretion processes the rifting between arabia and somalia which started around 35 ma was followed by oceanic accretion from at least 17 6 ma leading to the formation of the present day gulf of aden bathymetric gravity and magnetic data from the encens sheba cruise are used to constrain the structure and segmentation of the oceanic basin separating the conjugate continental margins in the eastern part of the gulf of aden between 51 degrees e and 55 5 degrees e data analysis reveals that the oceanic domain along this ridge section is divided into two distinct areas the eastern area is characterized by a shorter wavelength variation of the axial segmentation and an extremely thin oceanic crust in the western segment a thicker oceanic crust suggests a high melt supply this supply is probably due to an off axis melting anomaly located below the southern flank of the sheba ridge 75 km east of the major alula fartak transform fault this suggests that the axial morphology is produced by a combination of factors including spreading rate melt supply and the edge effect of the alula fartak transform fault as well as the proximity of the continental margin the oceanic domains have undergone two distinct phases of accretion since the onset of seafloor spreading with a shift around 11 ma at that time the ridge jumped southwards in response to the melting anomaly propagating ridges were triggered by the melting activity and propagated both eastward and westward the influence of the melting anomaly on the ridges decreased stopping their propagation since less than 9 ma from that time up to the present the n025 degrees e trending socotra transform fault developed in association with the formation of the n115 degrees e trending segment 2 in recent times a counter clockwise rotation of the stress field associated with kinematic changes could explain the structural morphology of the alula fartak and socotra hadbeen fracture zones

chert in the pacific biogenic silica and hydrothermal circulation abstract data on sections drilled by dsdp and odp in the pacific have been compiled for 187 sites 105 of which contain chert the spatial pattern of siliceous deposits in the pacific generally follows the pattern of biologic productivity there is some evidence that enhanced vertical diffusivity in the ocean during the eocene warm climate gave rise to siliceous deposits on and near bathymetric highs and basin margins the most common occurrence of chert is in eocene and older sections however cherts do occur in sections as young as 7 ma in the sites investigated over half of the youngest chert occurrences in sedimentary sections are within 150 m of basement nearly all the chert occurrences on crust younger than 60 ma are within 150 m of basement the deficit in heat flow over ocean crust younger than 60 ma is explained as the result of heat advection by hydrothermal waters circulating in the upper part of the ocean crust these waters appear to dissolve biogenic silica and deposit it as chert in the lower parts of sedimentary sections diffusion of these waters into the lower sediment section is enhanced by small scale faulting this faulting may be associated with intra plate stresses that develop during ridge jumps and reorientations of plate motion the relatively warm bottom waters of the early cenozoic may have also enhanced the dissolution of biogenic silica during hydrothermal circulation in the upper oceanic crust and basal sediment section

seismic characteristics of fluid escape pipes in sedimentary basins implications for pipe genesis abstract fluid escape pipes were first documented from 3d seismic data over a decade ago and have subsequently been identified in many petroliferous basins worldwide they are characterized on seismic data by vertical to sub vertical zones of reduced reflection continuity that have a columnar geometry in three dimensions the upper terminations of these pipes commonly coincide with pockmarks or palaeo pockmarks signifying a close connection of pipe formation with a high flux fluid expulsion process dimensions range from tens to hundreds of metres in diameter and hundreds to over a thousand metres in height and the slenderness ratio defined as height diameter ranges from 0 8 to over 20 pipes are frequently associated with sub vertical clustering of amplitude anomalies on seismic data related either to the presence of free gas or to cementation linked to the passage of hydrocarbons three mechanisms have been suggested to explain pipe genesis 1 hydraulic fracturing 2 erosional fluidisation and 3 capillary invasion we suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation we review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts fluid escape pipes may be far more widespread that currently appreciated and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

active sand seas and the formation of desert loess abstract loess is a widespread eolian deposit dominated by coarse silt and very fine sand quartz grains 20 80 m that serve as an important archive of information on quaternary climate change despite the intensive research during the past century on desert loess formation its origin remains poorly understood and is still a fundamental problem in sedimentology the ongoing debate is focused on the first stage of loess formation the production of coarse silt quartz grains although laboratory experiments indicate the potential of dunes with their abundant quartz sand grains as a primary source for generating coarse silt grains this concept has been generally rejected because field based evidence of abrasion from sand grains eolian abrasion is scarce here we have adopted a global view of the sand sea loess association and examine in detail several well known late pleistocene loess regions in different subtropical deserts africa the middle east and arabia this analysis demonstrates that all these loess regions are located downwind of adjacent sand seas together with evidence of mineralogical similarity between the loess and the sand dunes and the contemporaneous deposition of the loess and movement of the dunes these observations suggest that sand seas are an important source for desert loess because there is only limited silt storage in sand dunes we postulate that the silt grains comprising most of the loess are not reworked from the dunes but are generated probably through active eolian abrasion of the sand grains under past climates characterized by intensified winds as a result the role of sand dunes and eolian abrasion in the formation of desert loess may be more important than previously thought

initiation and propagation of polygonal fault arrays by thermally triggered volume reduction reactions in siliceous sediment abstract polygonal fault arrays have been documented in sedimentary basins from around the world and several theories exist as to how they initiate and propagate three dimensional seismic data from polygonal fault arrays from offshore norway are used to develop a new process model for polygonal fault development we propose that in siliceous sediment polygonal fault arrays can be triggered thermally due to the conversion of opal a to opal ct at depths of 100 1000 m this conversion causes differential compaction and shear failure and therefore fault initiation the location of the earliest faults is dependent on where opal a to opal ct conversion and compaction occur first this is controlled by which strata have a favourable bed composition local fluid chemistry and temperature or because the strata reach the depth of the reaction front first due to the presence of pre existing structural relief folds or faults subsidence of biosiliceous sediment through the opal a to opal ct reaction front causes fault propagation because of continued localised differential compaction fault initiation and propagation due to silica conversion generate polygonal fault arrays at significantly deeper burial depths than previously thought possible

crustal structure of the lomonosov ridge and the fram and makarov basins near the north pole reversed refraction surveys were conducted along and across the lomonosov ridge as part of the 1979 lomonosov ridge experiment interpretation of the strike profiles indicates a 5 km thick upper crustal layer with a velocity of 4 7 km s overlying a 15 to 20 km thick layer of 6 6 km s material an upper mantle velocity of 8 3 km s is indicated by a few pn arrivals high amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km ray trace modeling of the dip profiles suggests a root structure extending to about 28 km depth flanked by crust thinning to a depth near 13 km beneath the makarov basin and a more gradual thinning to near 16 km beneath the fram basin the similarity between the crust of the lomonosov ridge and that beneath the barents and kara seas supports the suggestion that the ridge is a slice rifted from the baltic shelf

erosion and reworking of pacific sediments near the eocene oligocene boundary 1 the eocene oligocene e o boundary interval marks one of the largest and most rapid changes in climate during the last 50 myr because of a very shallow calcium carbonate compensation depth in the eocene as well as the reworking of sediments and hiatuses in the boundary zone it has also been one of the most difficult stratigraphic boundaries to study in deep water marine sections especially in the pacific ocean recently three drill sites have recovered complete sections of the e o boundary interval in the tropical pacific a detailed study of these sections shows a series of pulses of reworked older radiolarians in the upper eocene and lowermost oligocene the two largest pulses are coincident with the two sharp steps in carbon and oxygen isotope values that bracket the e o boundary several smaller peaks in reworked material precede these two maxima it is proposed that immixing of the older radiolarian species results from erosion and redeposition possibly linked to pulses of new bottom water formation and the formation of a deep pycnocline

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

modeling cenozoic sedimentation in the central equatorial pacific and implications for true polar wander high productivity along the central pacific equator has left thick deposits of calcareous and siliceous ooze west of 102 w these have deposited on the pacific plate which has been drifting northwards and therefore the thickest deposits are displaced north of the equator this study attempts to simulate the sediment distribution using information from deep sea drilling project dsdp cores and models for pacific plate motion the dsdp core data are used to account for accumulation rate variations with time paleolatitude and paleodepth accounting for variable carbonate dissolution the model predicts the distribution on 35 50 ma seafloor relatively well and demonstrates that the sedimentary bulge is distorted by variable dissolution over a regional bathymetry gradient due to the northern edge of the south pacific superswell a westerly deepening of the carbonate compensation depth and sedimentary lysocline evolution are proposed which improve the model for seafloor 30 ma and older the degree of match between model and data provides a test of the amount of northward drift of the pacific plate and assuming a model for pacific hotspots motion of true polar wander tpw motion of the hotspot frame relative to the earth s spin axis assuming the duncan and clague 1985 model for pacific hotspots motion the sedimentary modeling favors tpw models which have little or no effect on the central pacific paleoequator over the past 30 ma we also explore implications of interhotspot motion and the suggestion that the bend in the hawaiian volcanic chain represents an abrupt change in pacific hotspot motion within the last 5 ma

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires

enhanced hydrothermal activity on an ultraslow spreading supersegment with a seismically detected melting anomaly abstract seafloor hydrothermal venting fields occur on all ocean spreading ridges osrs regardless of spreading rates however the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow spreading osrs are poorly known chinese dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system with an array of turbidity sensors and a near bottom camera along the ultraslow spreading southwest indian ridge here we report the discovery of multiple hydrothermal plumes overlying segments 28 29 and 30 between the indomed and gallieni fracture zones from these data and earlier explorations in segments 25 27 we identify nine active venting sites the spatial density fs sites 100 km of active sites along the 394 km of ridge axis in our study area is thus 2 8 nearly 3 higher than predicted by the global trend of fs for ultraslow osrs in the interridge database previous studies concluded that an enhanced magma supply to the central indomed gallieni supersegment 11 8 ma is now limited to segment 27 our results indicate that although hydrothermal activity may be most concentrated in segment 27 the discoveries of active venting in segments 25 30 implies the presence of additional magma bodies across a broad extent of the indomed gallieni supersegment

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the cocos plate 1 new thermal data from 18 24 ma lithosphere on the cocos plate delineate contrasting subsurface thermal conditions in adjacent sections of crust heat flow through seafloor created at the east pacific rise is generally suppressed by 70 relative to conductive lithospheric cooling models whereas heat flow through adjacent similarly aged lithosphere generated at the cocos nazca spreading center is consistent with these models the transition between thermal regimes is remarkably abrupt only 2 5 km wide indicating a shallow hydrothermal origin the transition is more closely associated with differences in the distribution of basement outcrops than with tectonic boundaries demonstrating the importance of the former in extracting heat from the lithosphere on a regional basis

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated