isotopic evidence for late cretaceous plume ridge interaction at the hawaiian hotspot when a mantle plume interacts with a mid ocean ridge both are noticeably affected the mid ocean ridge can display anomalously shallow bathymetry excess volcanism thickened crust asymmetric sea floor spreading and a plume component in the composition of the ridge basalts the hotspot related volcanism can be drawn closer to the ridge and its geochemical composition can also be affected here we present sr nd pb isotopic analyses of samples from the next to oldest seamount in the hawaiian hotspot track the detroit seamount at 51 degrees n which show that 81 myr ago the hawaiian hotspot produced volcanism with an isotopic signature indistinguishable from mid ocean ridge basalt this composition is unprecedented in the known volcanism from the hawaiian hotspot but is consistent with the interpretation from plate reconstructions that the hotspot was located close to a mid ocean ridge about 80 myr ago as the rising mantle plume encountered the hot low viscosity asthenosphere and hot thin lithosphere near the spreading centre it appears to have entrained enough of the isotopically depleted upper mantle to overwhelm the chemical characteristics of the plume itself the hawaiian hotspot thus joins the growing list of hotspots that have interacted with a rift early in their history

magmatic underplating of crust beneath the laccadive island nw indian ocean summary we investigate the crust and uppermost mantle velocity structure beneath the laccadive island through the inversion of teleseismic receiver functions following the neighbourhood algorithm the velocity structure suggests that the 16 km thick oceanic crust is underplated by 8 km thick high velocity layer vs 4 25 4 4 km s 1 vp 7 35 7 6 km s 1 the uppermost mantle shear velocity is 4 6 km s 1 vp 8 18 km s 1 the moho at 24 km is possibly formed due to underplating of mantle derived picritic melt at the base of oceanic crust produced during the interaction of the indian plate with the reunion hotspot continuity of crustal underplated material beneath the laccadive could be traced to the laxmi ridge in the north and to the reunion island in the south suggesting this to be possible trace of the reunion hotspot

indications of a link between seismotectonics and ch4 release from seeps off costa rica measurements of ch4 concentrations in the bottom water during two discrete sampling periods in subsequent years above different cold seeps at the pacific margin off costa rica indicate large scale variations of ch4 release ch4 is emitted from mud extrusions and a slide scar at 1000 2300 m water depth maximum ch4 concentrations were found to be lower above all investigated sites in autumn 2003 than in autumn 2002 although seep sites are up to 300 km apart tidal and current changes were observed but found to apply only to individual seep sites increased seismic activity connected to the moment magnitude m w 6 4 earthquake offshore costa rica in june 2002 could have had an impact on all seep sites and thereby caused an increase in ch4 emission this is supported by the largest variations of ch4 concentration found above mud extrusions located above faults likely more strongly affected by tectonic movements even though our data indicate a relation between seismicity and ch4 seepage the relation is not proven and future work is needed to comprehensively test this hypothesis

great earthquakes on canada s west coast a review this article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geological survey of canada contribution 20090196 the first lithoprobe transect in 1984 across vancouver island had primary objectives to define the structure associated with subduction and constraints on the potential for great thrust earthquakes

self consistent modeling of crustal thickness at chagos laccadive ridge from bathymetry and gravity data the chagos laccadive ridge clr is an aseismic ridge in the indian ocean that formed by the action of the reunion mantle plume we use modeling of bathymetry and gravity data to constrain the crustal thickness variations at three locations from 0 s to 6 s where the plume was likely interacting directly with the carlsberg spreading center ambiguities in the modeling were reduced by requiring both the density and thickness of the crust to be consistent with a single set of mantle melting conditions in this case using a passive upwelling model in which mantle potential temperature is the controlling factor mean crustal thicknesses beneath the ridge at the three locations predicted from the gravity modeling were 16 6 12 5 and 27 0 km these thicknesses are up to 50 greater than would be obtained had we not required self consistency in the modeling if the thick crust is produced entirely by elevated potential temperatures in the melting region mantle temperatures would have to be 100 250 c higher than normal abrupt decreases in crustal thickness at a large offset fracture zone are consistent with previous models for plume spreading center interactions

influence of total organic carbon deposition on the inventory of gas hydrate in the indian continental margins abstract total organic carbon toc content of marine sediments represents residual carbon originally derived from terrestrial and marine sources which has survived seafloor and shallow subseafloor diagenesis ultimately its preservation below the sulfate reduction zone in marine sediments drives methanogenesis within the gas hydrate stability zone ghsz methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate in this paper we examine the inventory and sources of toc in sediments collected from four regions within the ghsz along the indian continental margins the recovered sediments vary in age from oligocene to recent mean toc abundance is greatest in the krishna godavari k g basin and decreases progressively to the mahanadi basin andaman wedge and kerala konkan k k basin this decrease in toc is matched by a progressive increase in biogenic caco3 and increasing distance from terrestrial sources of organic matter and lithogenic materials organic carbon sources inferred from c n and 13ctoc range from terrestrial k g basin to mixed marine and terrestrial mahanadi basin to marine dominant andaman wedge and k k basin in the k g basin variation in the bulk 13ctoc is consistent with changes in c3 and c4 vegetation driven by monsoon variability on glacial interglacial timescales whereas in the mahanadi basin a shift in the 13ctoc likely reflects the onset of c4 plant deposition in the late miocene a large shift the 13ctoc in the k k basin is consistent with a change from c3 to c4 dominated plants during the middle miocene we observe a close relationship between toc content and gas hydrate saturation but consider the role of sedimentation rates on the preservation of toc in the zone of methanogenesis and advective flow of methane from depth although toc contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data seismic borehole log pressure core and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions although toc content may be a first order indicator for gas hydrate potential the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur

heterogeneity in southern central indian ridge morb implications for ridge hot spot interaction between the rodrigues triple junction and the marie celeste fracture zone basalts from the central indian ridge cir exhibit an enrichment in incompatible elements that increases in intensity northward in addition h2o tio2 al 8 and dy yb ratios increase while na 8 remains unchanged and fe 8 decreases evolution of the enriched magma appears to be affected by elevated water contents which lower the mantle solidus thereby increasing the initial depth of melting as well as delaying plagioclase crystallization however the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization instead of trending toward a small melt fraction from the mantle as predicted by hydrous melting models the cir samples lie on a mixing line between n morb and a source component that closely resembles present day reunion hot spot lavas thus it appears that while hydrous melting and crystallization affect the cir the enriched and wet mantle originates from the reunion hot spot where it migrates eastward toward the cir against the direction of motion of the lithosphere

the giant tortoise aldabrachelys and its bearing on the biogeography and dispersal of terrestrial biota in the western indian ocean abstract the evolution of species of giant tortoises and the nature of processes governing their dispersal in the western indian ocean have been matters of interest for some time historical records indicate that these animals were once widely distributed in the region from madagascar to the seychelles in the north and to the mascarene islands in the south new data on the status of aldabrachelys and related or supposed species and controversies regarding genetic diversity and distribution have prompted an evaluation of the islands and submerged banks in the region that during the later cenozoic might have provided refuges and or stepping stones for the spread of populations in what is now open ocean given the apparent stability of the crust in much of the area for at least the quaternary period sea level driven by climatic change and transport by ocean currents appear to be the principle factors controlling the access not only of populations of giant tortoises but also other terrestrial biota to potential refuge areas in the region the distributions of living and fossil tortoise communities are discussed four types of terrestrial oceanic refuges for migrants have been identified at the present day with the exception of aldabra high limestone islands offer only relatively small areas for colonization and all would have been drowned during interglacial sea level high stands although they have the potential for some increases in land area during the early stages of transitions to low stand as sea level fell significantly they would have become waterless pinnacles only generally accessible to organisms capable of flight volcanic islands have the same general limitations but may have remained as smaller entities when sea level was higher than at present but providing only limited land access and restricted vegetation low islands and sand cays typically of more recent origin could not have existed during periods of high sea levels but many are situated on extensive submerged banks that would have formed relatively large land areas when sea level was low major banks most of which are regarded as fragments of continental crust are currently submerged but during glacial periods would together have offered land areas in excess of 130 000 km2 providing a diverse range of terrestrial environments and vegetation access to all of these was nevertheless restricted by the distribution of ocean currents the only plausible natural means of dispersal which probably varied during glacial interludes but also by the abilities of the animals to survive long periods adrift in the ocean the distribution of these tortoises in both the fossil record and historical data provides a model for the dispersal of other terrestrial biota in the region although the present distribution and genetic characteristics of the tortoises probably reflects recent human intervention

hydroacoustic methodology for detection localization and quantification of gas bubbles rising from the seafloor at gas seeps from the eastern black sea 1 detailed acoustic investigation of bubble streams rising from the seafloor were conducted during r v meteor cruise m72 3a at a deep submarine hydrocarbon seep environment the area is located offshore georgia eastern part of the black sea at a water depth between 840 m and 870 m the sediment echosounder parasound ds 3 p70 was used for detecting bubbles in the water column that causes strong backscatter in the echographs flares employing the swath echsounder kongsberg em710 flares in the water column were mapped along the entire swath width of approximately 1000 m at high spatial resolution the exact location of the flares could be extracted manually subsequently the horizontally looking sonar kongsberg digital telemetry ms1000 mounted on a remotely operated vehicle rov was utilized to quantify the flux of bubbles a model was developed that is based on the principle of finding the acoustic mass in order to quantify the bubble flux at various seeps the acoustic approach from the backscatter data of the rov sonar resulted in bubble fluxes in the range of 0 01 to 5 5 l min corresponding to 0 037 to 20 5 mol ch4 min at in situ conditions 850 m water depth 9 c independent flux estimations using a funnel shaped device showed that the acoustic model consistently produced lower values but the offset is less than 12 furthermore the deviation decreased with increasing flux rates a field of bubble streams was scanned three times from different directions in order to reveal the reproducibility of the method flux estimations yielded consistent fluxes of about 2 l min 7 4 mol ch4 min with variations of less than 10 although gas emissions have been found at many sites at the seafloor in a range of geological settings the amount of escaping gas is still largely unknown with this study presenting a novel method of quantifying bubble fluxes employing a horizontally looking sonar system it is intended to contribute to the global effort of better constraining bubble fluxes at deep sea settings

sedimentation rates from calcareous nannofossil and planktonic foraminifera biostratigraphy in the andaman sea northern bay of bengal and eastern arabian sea abstract in this study we determined calcareous nannofossil and planktonic foraminifera events in sediments recovered during the 2006 indian national gas hydrate program nghp expedition 01 selected bioevents permitted the assignment of orbitally calibrated ages derivative sedimentation rate estimates and identification of sedimentary discontinuities in the andaman sea at hole nghp 01 17a a late miocene to recent record was recovered but with a significant hiatus during the latest miocene and pliocene sedimentation rates here vary from 50 m myr during the pleistocene to 130 m myr for the late miocene in the northern bay of bengal at hole nghp 01 19a the base of the cored record also reaches the late miocene and extends to recent but the same hiatus missing the pliocene is present at this hole a the early pleistocene at hole nghp 1 19a shows sensible variation in sedimentation rates ranging between 130 m myr and 10 m myr during the last 0 5 ma for the late miocene the sedimentation rate was ca 50 m myr in the western bay of bengal at hole nghp 01 10b d calcareous nannofossils were sparse and only foraminifera datums are available the base of the section is latest early pliocene to late pliocene and the majority of the record is considered younger than late pliocene due to the scarcity of calcareous nannofossils and the few foraminifera datums sedimentation rates were not determined here however hole nghp 01 16a in the same region recorded the highest occurrence of pseudoemiliania lacunosa an event that confirms a pleistocene age allowing us to estimate a sedimentation rate between 110 m myr and 450 m myr along the western peninsular indian margin at hole nghp 01 01a a continuous record from the holocene to the early oligocene indicates sedimentation rates of ca 25 m myr until the middle miocene and a sensible reduction downward reaching 4 m myr in the early miocene to the early oligocene no hiatuses were observed the large changes in sedimentation rates observed in these cores reflect long timescale changes in the environment likely induced by changes in the strength of the indian monsoon system

temporal variability of in situ methane concentrations in gas hydrate bearing sediments near bullseye vent northern cascadia margin 1 to assess the temporal variability in the methane fluxes from marine sediments that overly gas hydrate bearing sediments and the factors that might control its rate in situ methane concentrations were measured near bullseye vent on the northern cascadia continental margin a long term sampling device collected overlying water and pore fluid samples from 25 cm above seafloor at the sediment water interface swi and 7 cmbsf centimeters below seafloor over a 9 month period august 2009 may 2010 these samples provide a record at 4 day resolution of in situ methane ethane propane sulfate and chloride concentrations as well as stable carbon isotope ratios of methane 13c ch4 and dissolved inorganic carbon 13c dic we show that pore fluids near the swi are saturated or supersaturated with respect to methane 80 mm and the methane flux from the seabed is variable over time we hypothesized that regional seismic activity controlled this variable ch4 flux in the northern cascadia continental margin setting however we found no direct correlation between earthquakes and ch4 flux we also posited alternative controls on ch4 flux variability such as storms regional oceanography and microbial activity again no direct correlation was seen this study takes first steps toward exploring which physical factors play a role in methane flux from hydrate bearing sediments

turbine tent measurements of marine hydrocarbon seeps on subhourly timescales 1 three turbine seep tents simultaneously measured marine seep gas fluxes with high time resolution 0 2 s at multiple locations tents were inverted polyvinyl cones 2 m diameter 1 m tall and weighted on their lower skirt edges rising gas bubbles induce vertical fluid motions which were measured by laboratory calibrated turbines in chimneys on top of the tents initial deployment was at an active seep area in the coal oil point seep field in the santa barbara channel california the three tents simultaneously collected data for continuous time periods of 2 hours in both the morning and afternoon seabed temperature and pressure were acquired every 3 s over the same time periods as the flux measurements from a conductivity temperature depth ctd mounted on one tent results strongly suggest that oceanic swell had a significant forcing effect on the flux at a subhourly timescale there was an inverse relationship between effect of swell height on the flux and flux swells from 1 to 4 m height and periodicities of 7 and 12 s caused variations of 1 to 4 from the average flux proposed mechanisms to explain the observations are diffusion with surrounding sediments termed gas charging swell induced changes in fracture size termed fracture forcing and swell induced vent activation deactivation termed pore activation on the basis of the seep frequency response we propose pore activation was dominant

latitudinal evolution of the reunion hotspot deduced from paleomagnetic results of leg 115 during odp leg 115 in the indian ocean four holes penetrated the basement paleomagnetic measurements on basement cores yield good quality results but their interpretation is limited by possible but often undetectable subsequent tectonic tilting and in one case by insufficient averaging of secular variation two sites 64 and 55 60 ma yield apparently reliable results results from sediments of another site 36 ma studied by schneider and kent also appear to be reliable the three corresponding paleolatitudes are roughly in agreement with values predicted from the synthetic apparent polar wander paths of the african and indian plates together with results from the deccan 67 ma they indicate smooth and rather slow 8mm yr northward drift of the reunion hotspot since its inception at the deccan this motion is consistent with the hotspot models of morgan and duncan

ocean circulation promotes methane release from gas hydrate outcrops at the neptune canada barkley canyon node 1 the neptune canada cabled observatory network enables non destructive controlled experiments and time series observations with mobile robots on gas hydrates and benthic community structure on a small plateau of about 1 km2 at a water depth of 870 m in barkley canyon about 100 km offshore vancouver island british columbia a mobile internet operated vehicle was used as an instrument platform to monitor and study up to 2000 m2of sediment surface in real time in 2010 the first mission of the robot was to investigate the importance of oscillatory deep ocean currents on methane release at continental margins previously other experimental studies have indicated that methane release from gas hydrate outcrops is diffusion controlled and should be much higher than seepage from buried hydrate in semipermeable sediments our results show that periods of enhanced bottom currents associated with diurnal shelf waves internal semidiurnal tides and also wind generated near inertial motions can modulate methane seepage flow dependent destruction of gas hydrates within the hydrate stability field is possible from enhanced bottom currents when hydrates are not covered by either seafloor biota or sediments the calculated seepage varied between 40 400 mol ch4 m 2 s 1 this is 1 3 orders of magnitude higher than dissolution rates of buried hydrates through permeable sediments and well within the experimentally derived range for exposed gas hydrates under different hydrodynamic boundary conditions we conclude that submarine canyons which display high hydrodynamic activity can become key areas of enhanced seepage as a result of emerging weather patterns due to climate change

three dimensional lithospheric structure of the western continental margin of india constrained from gravity modelling implication for tectonic evolution this paper describes a 3 d lithospheric density model of the western continental margin of india wcmi based on forward modelling of gravity data derived from satellite altimetry over the ocean and surface measurements on the indian peninsula the model covers the north eastern arabian sea and the western part of the indian peninsula and incorporates constraints from a wide variety of geophysical and geological information salient features of the density model include 1 the moho depth varying from 13 km below the oceanic crust to 46 km below the continental interior 2 the lithosphere asthenosphere boundary lab located at depths between 70 km in the southwestern corner under oceanic crust and about 165 km below the continental region 3 thickening of the crust under the chagos laccadive and laxmi ridges and 4 a revised definition of the continent ocean boundary the 3 d density structure of the region enables us to propose an evolutionary model of the wcmi that revisits earlier views of passive rifting the first stage of continental scale rifting of madagascar from india at about 90 ma is marked by relatively small amounts of magmatism a second episode of rifting and large scale magmatism was possibly initiated around 70 ma with the opening of the gop rift subsequently at around 68 ma the drifting away of the seychelles and formation of the laxmi ridge was a consequence of the down faulting of the northern margin during this second episode of rifting the northern part of the wcmi witnessed massive volcanism attributed to interaction with the reunion hotspot at around 65 ma subsequent stretching of the transitional crust between about 65 and 62 ma formed the laxmi basin the southward extension of the failed gop rift as the interaction between plume and lithosphere continued the chagos laccadive ridge was emplaced on the edge of the nascent oceanic crust rifted continental margin in the south as the indian plate was moving northwards

temporal variation of methane flares in the ocean above hydrate ridge oregon abstract while bubble plumes have been acoustically imaged in the water column above marine gas hydrate deposits in many studies little is known about the temporal variation in plume intensity in july 2008 we conducted surveys using 3 5 and 12 khz echosounders and a 75 khz acoustic doppler current profiler adcp over the northern and southern summits of hydrate ridge on the cascadia continental margin our study included multiple surveys at both sites including a survey of the northern summit that was repeated 16 times in 19 h seafloor depth at the northern summit is 600 m well within the hydrate stability zone hsz which was below 510 m during our survey based on ctd data three distinct flares a term used to denote the acoustic signature of bubble plumes were detected at northern hydrate ridge nhr and one was detected at southern hydrate ridge shr coincident with where flares were observed a decade ago indicating that the supply of gas is stable on this time scale high resolution bathymetric surveys of nhr and shr acquired with an autonomous underwater vehicle auv flown 50 m above the seafloor indicate that flare locations are correlated with a distinctive pattern of short wavelength seafloor roughness supporting the inference of long term stability in the location of bubble expulsion sites as in previous studies at hydrate ridge flares were not detected with the 3 5 khz echosounder but were clearly imaged at 12 khz by reprocessing routine shipboard adcp data we show that they are also observed in at 75 khz indicating that a wide range of bubble sizes is present the intensity of the flares varied strongly with time two primary sources for flares were observed one located on the regional topographic high showed continuous activity with two times periods of particularly strong flares that are not correlated with tidal height the other located on a local topographic high shows a pulse of increased backscatter that occurred on a falling tide while the time period of observation is not enough to constrain the effect of tidal changes in seafloor pressure on venting the data suggest that tides are not the dominant factor controlling release of bubbles from the seafloor the data support previously suggested models in which temporary sealing of vents by gas hydrate formation and breaking of these barriers as gas pressure builds up is responsible for burp like pulses of gas expulsion we also report the first observations of flares originating within the hsz that extend well above the hsz with little loss of backscatter intensity these represent a new source of methane injection into the upper ocean and possibly the atmosphere flare extension above the hsz may be due to coating of some bubbles by oil or biofilms or to inclusion of particulate matter possibly including floating pieces of hydrate in the plumes although this study provides tantalizing new information on both short term variability in gas expulsion rate and long term stability of vent sites longer well calibrated observations that integrate bubble flux over entire vent fields as a function of time are needed to develop accurate models for the flux of methane into the ocean and atmosphere from seafloor methane vents

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

reunion hotspot magma chemistry over the past 65 m y results from leg 115 of the ocean drilling program leg 115 of the ocean drilling program recovered basalts from four locations along the hotspot track that leads from the deccan flood basalts in india to reunion island in the western indian ocean sites 706 707 713 and 715 the drilled basalts range in age from 35 ma site 706 to 64 ma site 707 and including the deccan basalts 66 to 68 ma mauritius island 0 2 to 8 ma and reunion island 0 to 2 ma seven sites are provided for sampling the volcanic record of the 5 000 km long hotspot track chemical and age comparisons indicate that site 707 lavas correlate with basalt units near the top of the deccan flood basalt sequence the lavas of site 715 55 to 60 ma are most similar to the islands of mauritius and reunion site 713 basalts 48 ma are similar to the earliest lavas of the deccan province and site 706 basalts are intermediate in chemistry between those of central indian spreading ridge basalts and reunion differences in lava compositions along the hotspot track can be related to variable mixing of plume and asthenospheric mantle depending on the changing position of spreading ridge segments and the hotspot during the opening of the indian more ocean alternatively time dependent changes in the composition of hotspot melts may be due to a decrease in partial melting of a heterogeneous plume or to intrinsic changes in the composition of material supplied by the plume less

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

flood basalts and large igneous provinces from deep mantle plumes fact fiction and fallacy abstract flood basalts large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature in the starting plume model flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively and the deccan flood basalt province of india and the laccadives maldives chagos reunion island volcanic chain in the indian ocean are widely thought to be an excellent example supporting this model however the existing geodynamic thermal petrological geochemical and geochronological constraints on deccan volcanism do not require or support a mantle plume origin a new interpretation made here rules out any role for the reunion plume in deccan volcanism and even questions the notion that the reunion hotspot is the manifestation of an underlying reunion plume the laccadives reunion hotspot track is viewed here as caused by a southward propagating fracture based on the published radiometric age data and the ultimate cause of deccan volcanism is argued to be mega scale lithospheric rifting the appreciation of the great original areal extent of the deccan nearly as far as the southern tip of peninsular india solves the hitherto unsolved problem of the cause of the uplift of the 1500 km long western ghats escarpment this cause being combined surface erosion and magmatic underplating hypothesized mantle plumes do not appear responsible for most large igneous provinces instead their very existence is questionable no geological evidence of any kind geochemical petrological thermal topographic requires mantle plumes the self contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts based on various grounds that a globe encircling enriched mantle layer the perisphere may overlie a deeper depleted mantle which is the source of morb the systematic geochemical variations along the laccadive reunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding morb mantle as in the current plume model hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere as originally proposed decades ago the locations of large igneous provinces hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history they are not placed randomly on the globe indifferent to the surface geology neither are large igneous provinces sudden chance events within the vastness of geological time which plume theories typically allege

changes in the onset and intensity of wind driven upwelling and downwelling along the north american pacific coast 1 the timing duration and intensity of wind driven upwelling and downwelling along the north american pacific coast play an integral role in coastal circulation and basinwide ecosystem composition it has been suggested that global warming will cause changes in these winds here we develop a new set of objective criteria to unambiguously determine the onset duration and intensity of upwelling and downwelling seasons due to local wind forcing we use these criteria to examine and better characterize temporal trends in wind driven coastal currents over the previous 60 years and relate them to global warming and large scale climate oscillations in the coastal ocean between northern california and vancouver island 37 n and 51 n we find an exceptionally variable onset of upwelling at all locations some significant temporal trends are found in summer onset and upwelling intensity time series near the juan de fuca strait and off the coast of oregon positive phases of the pacific decadal oscillation are correlated to later and shorter upwelling seasons with weaker upwelling warm phases of the el nino southern oscillation are associated with a later onset of summer upwelling south of oregon and with more intense downwelling throughout the study area our analysis identifies strong interannual to interdecadal variability and emphasizes the importance of time series length when isolating physical temporal trends influenced by large scale oscillatory behavior of the climate

tidally controlled gas bubble emissions a comprehensive study using long term monitoring data from the neptune cabled observatory offshore vancouver island long term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore vancouver island british columbia data from the north east pacific time series underwater networked experiment observatory operated by ocean networks canada were used the site is equipped with a 260 khz imagenex sonar collecting hourly data conductivity temperature depth sensors bottom pressure recorders current meter and an ocean bottom seismograph this enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity three periods of gas emission activity were observed a short activity phases of few hours lasting several months b alternating activity and inactivity of up to several day long phases each and c a period of several weeks of permanent activity these periods can neither be explained by oceanographic conditions nor initiated by earthquakes however we found a clear correlation of gas emission with bottom pressure changes controlled by tides gas bubbles start emanating during decreasing tidal pressure tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides these pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load we propose a model for the fluid system at the seep fueled by a constant sub surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean transferable to other gas emission sites in the world s oceans

volcanogenic sediment distributions in the indian ocean through the cretaceous and cenozoic and their paleoenvironmental implications abstract volcanogenic sediments are significant components of the cores that have been drilled over the past two decades by the deep sea drilling project dsdp and the ocean drilling program odp the spatial distribution of these sediment components were used in connection with paleogeographical reconstructions to determine the primary dispersal processes direct air fall input surface circulation or atmospheric circulation for volcanic material across the indian ocean the identification of volcanic rich intervals in cores is based on the recognition of primary and secondary authigenic components or where sediment samples have not been obtained from analysis of downhole logging results e g spectral gamma ray and whole core magnetic susceptibility the data generated from dsdp cores by vallier and kidd 1977 have been upgraded in this study to fit a new integrated timescale that has been adopted for synthesis of odp and dsdp drilling data kidd et al 1992 it is now possible to map the distributions of volcanic rich sediments on paleogeographic reconstructions at 1 ma intervals through a review of its tectonic development the timing of accumulations of volcanic components in the indian ocean can be related to rifting of continents formation of new crust at ridge crests and at hotspots and to landmass volcanism linked to subduction along the indonesian arc the kerguelen and reunion hotspots were the dominant sources of volcanogenic sediments to the indian ocean since the early cretaceous the main accumulations of volcanogenic sediments occur within the basal sequences of drillholes immediately above basement controls on the transport of volcanic components were linked to atmospheric surface and possibly deeper water circulation patterns the opening indian ocean experienced a change from westerly surface currents generated by the high latitude westerly winds during the mesozoic to a gyral circulation pattern in the cenozoic the northwest indian ocean has also experienced seasonal reversal of winds since the development of the monsoon system during the late miocene to recent these changes in the wind and water circulation patterns are reflected in the distribution patterns of volcanogenic sediments in the indian ocean the kerguelen hotspot led to the development of kerguelen plateau which was formed above sea level during the albian 110 ma and remained above sea level for the next 30 40 ma volcaniclastic material from this source was mainly distributed to the northeast in contrast the reunion hotspot has contributed volcanic material mainly to the northwest during much of its evolution volcanism related to the reunion and kerguelen hotspots has apparently influenced volcanogenic sediment distributions in the developing indian ocean basins more than the regional rates of sea floor spreading which were previously thought to exert a major control valluer and kidd 1977

origin and compensation of chagos laccadive ridge indian ocean from admittance analysis of gravity and bathymetry data abstract the chagos laccadive ridge clr is a prominent aseismic volcanic ridge in the northern indian ocean the ridge together with the southern mascarene plateau smp to which it is genetically related is considered as a volcanic trace of the reunion hotspot we have examined the isostatic compensation of the clr through transfer function analysis of gravity and bathymetry data along seven profiles the analysis suggests that the clr is compensated locally with an airy crustal thickness t c of 20 km the rather low elastic plate thickness t e of about 4 km implies that the volcanism of the ridge took place very near a spreading centre the proximity of the chagos fracture zone indicates that the emplacement was probably near a spreading centre transform junction

variation of effective elastic thickness and melt production along the deccan reunion hotspot track we estimate the effective elastic thickness te along the deccan reunion hotspot track using admittance analysis of seafloor topography and the free air gravity field both corrected for the thermal effects of a cooling lithosphere our results reveal that the volcanic edifices saya de malha bank chagos maldives laccadives ridge formed in the first 30 myr after the deccan volcanism not vert similar 65 myr on lithosphere with te values of 4 2 km while the younger volcanic edifices on the african plate reunion mauritius nazareth bank were emplaced on lithosphere with te values of 17 9 km these estimates suggest that the hotspot volcanism occurred on juvenile lithosphere in the first 30 myr implying that the mid ocean ridge remained near the hotspot for not vert similar 30 myr in contrast in the last 30 myr volcanism occurred on aged lithosphere in an intraplate setting which might indicate that the mid ocean ridge migrated rapidly to the north after the african plate moved over the hotspot this conclusion of a rapid shift from plume influenced mid ocean ridge mor volcanism to intraplate plume volcanism is supported by geochemical major and trace element interpretations of data from ocean drilling program odp leg 115 an estimate of the melt production rate shows a striking increase in the small te region relative to the large te region of the hotspot track which suggest a strong interrelation between te and melt production however there is also variation of melt emplacement rates within the region of low te that may be due to unknown changes in the rates of plate motions or somewhat episodic melt production

cenozoic tectonics on the hautes fagnes plateau belgium abstract morphological features under which deformations of oligocene surfaces are included demonstrate the varied tectonic activity of the hautes fagnes plateau during cenozoic times beyond the general uplift of the region a local movement raised the baraque michel massif by about 100 m above the nearby crest of la vecquee moreover several zones of radial nw se faulting are active on the plateau they are related to the opening of the lower rhine embayment and represent the ultimate sw extension of the fault system associated with this opening