the somali current in autumn 1984 before the onset of the north east monsoon the somali current undergoes vigorous variations during the course of the south west monsoon although the initial phases of this response were fairly intensively studied during the fgge indian ocean experiment index in 19791 2 relatively little is known about what happens in the late summer monsoon and the transition to the winter monsoon in october 1984 we carried out measurements in the somali current regime with a shipboard acoustic doppler current profiler which showed that a continuous somali current did not exist at this time instead the cross equatorial flow with a transport of 12 l06 m3 s 1 in the upper 100 m identified by relatively low surface salinities was found to turn offshore south of 2 n between 6 and 11 n a large closed anticyclonic gyre marked by relatively high surface salinities was found with a transport of 32 106 m3 s 1 in the upper 250 m which is significantly greater than previously estimated from geostrophic calculations

the combined effects of continents and the 660 km depth endothermic phase boundary on the thermal regime in the mantle abstract we explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km depth in simple two dimensional convection calculations phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection our study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary we show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering unlike the free slip case where the surface heat flow decreases substantially with increasing magnitude of the clapeyron slope surface heat flow is shown to be almost independent of the clapeyron slope for mixed boundary condition cases although very different when not layered models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the clapeyron slope we also calculate the critical boundary layer rayleigh number as a measure of the thermal resistance of the surface boundary layer our results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no slip cases respectively we find that even for purely basally heated models the mantle becomes significantly subadiabatic in the presence of partial continental coverage this is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells

multiple scale convection in the earth s mantle a three dimensional study laboratory experiments suggest that a convective regime characterized by two length scales of motion is a reasonable model for circulations in the earth s upper mantle the flows of largest horizontal scale represent a likely plate driving mechanism required by some theories of plate tectonics it is also suggested that the small scale circulations could influence the chemical evolution of the mantle by extracting primitive mantle material that is otherwise entrained in the large scale flow

development of the flow field during the onset of the somali current 1979 abstract during the spring and summer of 1979 a multi ship survey studied the changes in currents along the east african coast in response to the transition from the northeast to the southwest monsoon the somali current in late april and early may flowed southwestward along the coast from northern somalia to about the equator surface currents were 50 100 cm s 1 and the transport was 3 106 m3 s 1 south of the equator the east african coast current eacc flowed northward the two currents met in the vicinity of the equator and turned offshore to the southeast surface velocities in the eacc were about 200 cm s 1 and its transport in the top 100 m was 15 106 m3 s 1 with the initial onset of southerly winds in early may the somali current reversed by mid may at 3 n surface speeds of 200 cm s 1 were observed the flow did not continue up the coast but turned offshore by 4 n the second onset of the monsoon took place in mid june shortly after this surface currents 350 cm s 1 were observed at the tur

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

gravity anomalies the deep structure and dynamic processes beneath the tien shan bouguer gravity anomalies over the tien shan and pamir are large and negative and therefore consistent with crustal thickening beneath both mountain ranges gravity anomalies over the western tien shan are within about 10 mgal of those expected for local airy isostatic equilibrium but those over the eastern tien shan are up to 50 mgal too large and negative thus they require static or dynamic support of an uncompensated mass deficit these gravity anomalies cannot be matched using regional compensation of the weight of the range on a continuous elastic plate a simple model of two separate elastic plates extending north and south from the axis of the tien shan can allow an adequate fit of observed and calculated anomalies the flexural rigidity of the northern plate must be quite small d 1023 n m corresponding to an equivalent effective elastic thicknesste 25 km but that of the southern plate must be larger d 1024 n m te 50 60 km the data for the eastern tien shan cannot be fit without a force per unit length of roughly 1 4 1012 n or by a bending moment applied to the end of the southern plate thus either a deep mass per unit length of about 1 4 1011 kg m must underlie the eastern part of the range or dynamic deviatoric tensional normal stresses of up to about 5 mpa due to flow in the asthenosphere must pull the tien shan down probably a combination of them resulting from the formation and sinking of a thickened lithospheric root causes the deflection of the moho and the deviations from local isostatic equilibrium

repeatability issues of 3 d vsp data increased repeatability is recognized as one major issue for improving the time lapse seismic technology as a reservoir management tool a 3 d vertical seismic profiling vsp data set acquired over a period of two days is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning it is found that for a frequency range between 3 5 and 50 hz the difference root mean square rms level between two recorded traces belonging to two different shots is about 8 this fact shows that there is a potential for acquiring very accurate time lapse seismic data by using a permanently installed downhole geophone array repeatability variation with increasing shot separation distances is analyzed showing a rapid decrease in repeatability as the accuracy of the positioning of the repeat survey decreases horizontal geophone components show approximately the same degree of repeatability compared to the vertical component but horizontal geophone data is slightly more sensitive to positioning errors the results show that repeated 3 d vsp surveys preferably using permanently installed geophone arrays might be an efficient tool for detailed and precise monitoring of fluid and pressure changes within a hydrocarbon reservoir

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

the influence of second scale convection on the thickness of continental lithosphere and crust abstract the effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies newtonian non newtonian and temperature and pressure dependent heat fluxes and heating modes bottom versus internal heating the stagnant regions near the top of the models are identified with the lithosphere we distinguish a seismic and thermal lithosphere controlled by temperature an elastic lithosphere controlled by viscosity and a mechanical lithosphere controlled by strain rate strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop the thickness of the elastic lithosphere is about two thirds of the mechanical lithosphere while the ratio of the elastic to the thermal lithosphere varies between 0 4 and 0 8 the time dependence of some models with internal heating and a surface heat flow of 20 mw m 2 is characterized by long periods of the order of 1 ga close to steady state and short periods 100 ma of changing cell patterns and lithospheric thicknesses models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold slowly downwelling convective flow rather than being firmly attached to the lithosphere the missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the moho observed lithospheric thickness variations in europe and their minimum to maximum spacing agree well with the models suggesting sublithospheric convection to be the cause compared to other observed features thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection relatively low viscosities in the lower crust may lead to a long term decoupling of surface topography from moho topography and mantle dynamics the moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 ma while surface topography may flatten out such a mechanism may be important in regions with a deep moho such as fennoscandia or with a shallow moho such as the pannonian basin neither region showing a pronounced surface topography

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

the netherlands indian ocean expedition 1992 1993 first results and an introduction the netherlands indian ocean programme was carried out with the rv tyro in the nw indian ocean the arabian sea and off kenya and the seychelles during 1992 1993 with the objective to study the effect on both spatial and temporal scales of the monsoon on the climate system in the northern indian ocean the programme included five research foci studied in different projects project a was directed towards the effects of the seasonally reversing monsoonal system on the coastal ecosystems in kenya especially to gain a clear understanding of the physical chemical and biological processes occurring in the coastal zone this includes the development of the marine flora and fauna in response to the changing conditions in the coastal zone the second project b aimed at a study of the dynamics of the pelagic system including a quantification of the carbon flux and biogeographical studies with the emphasis on distribution and taxonomy of plankton part of this project was designed as pilot project for the jgofs northwest indian ocean study carried out in 1994 95 covering most of the jgofs core measurements these studies were planned in close cooperation with the geological project c tracing a seasonal upwelling system aimed at determining the spatial and temporal variability of biological chemical and geological parameters that define an upwelling system and recognizing these in the sedimentary record as proxies for upwelling in the waters off somalia and yemen project d concentrated on two main issues the first was the late quaternary productivity and the dynamics of the oxygen minimum zone in the northeast arabian sea the second was the depositional architecture and sediment facies of the middle and lower indus fan the biology of oceanic reef systems of the seychelles formed the last project e within the netherlands indian ocean programme and was mainly directed towards biogeographical taxonomical and ecological studies of the reefs the studies comprised within these projects were and are being carried out at a range of different institutes and preliminary results were discussed at an arabian sea workshop held at nioz texel the netherlands from 13 16 february 1995 the papers in this special form the first group of results from the progamme and as a consequence the articles have a strongly variable character the first paper by kromkamp et al describes temporal and spatial variability of primary production along the kenyan coast and their relationship to the development of the cyanobacterium trichodesmium

the great whirl observations of its seasonal development and interannual variability 1 in situ measurements are used together with sea surface height data to study the development and variability of the great whirl gw a large quasi stationary anticyclone that appears off the coast of somalia during the southwest monsoon season we find that anticyclonic circulation indicative of the gw appears on average in april almost two months before the onset of the southwest monsoon winds this early initiation is coincident with the arrival of annual rossby waves at the western boundary with the onset of the monsoon winds in early june the gw proper intensifies quickly remaining at its peak throughout july august and september and dissipating about one month after the winds have died the gw is present for an average 166 30 days per year and the position of its northern flank close to 9 n coincides with the latitude of zero wind stress curl the intraseasonal variability of the gw is intense as a result of mutual advection with one to three flanking cyclones which accompany the gw 70 of the time and tend to circulate clockwise around it there is no consistent seasonal pattern for the development or dissipation of the gw although movement to the southwest is common toward the end of the season the gw of 1995 deepened from 200 m in june to over 1000 m in september and strengthened from a swirl transport of 10 to 60 sv cool waters in its core resulted from advection via the somali current and some local vertical mixing

gulf of aden eddies and their impact on red sea water 0 5 m s 1 while the equally energetic anticyclone appeared to be decoupled from the surface circulation all three eddies reached nearly to the 1000 2000 m deep sea floor with speeds as high as 0 2 0 3 m s 1 extending through the depth range of rsw comparison of salinity and direct velocity measurements indicates that the eddies advect and stir rsw through the gulf of aden anomalous water properties in the center of the anticyclonic eddy point to a possible formation site in the somali current system index terms 4520 oceanography physical eddies and mesoscale processes 4536 oceanography physical hydrography 4512 oceanography physical currents citation bower a s d m fratantoni w e johns and h peters gulf of aden eddies and their impact on red sea water geophys res lett 29 21 2025 doi 10 1029 2002gl015342 2002

development of near surface flow pattern and water mass distribution in the somali basin in response to the southwest monsoon of 1979 abstract near surface observations of temperature salinity and current are used to describe the seasonal reversal of the somali current during 1979 in response to the onset of the southwest monsoon winds during april prior to the reversal of the winds north of the equator the northward flowing east african coastal current eacc and the southward flowing somali current sc converged near the equator the eacc was characterized by surface waters with salinities less than 35 1 and the sc by salinities greater than 35 3 the winds reversed north of the equator during the first week of may and the boundary current intruded in the form of an anticyclonic gyre to 2 5 n most of the low salinity water was recirculated back south of the equator by the offshore limb of the gyre it did not flow continuously at the surface into the eastward equatorial jet which was present farther offshore during may and june that current was fed by high salinity water from the region to the north of the low latitude gy

time dependent surface topography in a coupled crust mantle convection model summary recent geodynamic research has shown that convective flow in the mantle may have an important role in the development of long wavelength surface topography this flow induced dynamic topography is usually derived from mantle convection models by computing the vertical component of hydrodynamic stress at the top of the model and assuming the stress is compensated by deflection of the surface however these models have generally ignored the presence of an overlying buoyant crust and its deformational response to the mantle flow we consider the effects of horizontal convective forcing on the crust and investigate how this crust lithosphere deformation interacts with the vertical component of mantle flow induced subsidence uplift at the surface in particular we test the response of various rheologies of the crust and mantle lithosphere to an episode of mantle downwelling the evolution of crustal thickness and topography is tracked using thermomechanical numerical models of the crust mantle system with a free surface upper boundary a strong crust c 2 5 10 25 pa s does not experience significant internal deformation and subsides above a mantle downwelling for a weaker crust c 10 23 pa s descending mantle flow initially induces subsidence but there is an inversion to surface uplift as crustal thickening induced by convergent mantle flow overcomes the dynamic subsidence the presence of a strong mantle lithosphere however may effectively shield a weak crust from deformation imposed by the underlying horizontal mantle convective stresses with temperature dependent rheologies in the model the interplay of the vertical horizontal mantle forcings with the thermal evolution of the crust results in a highly time dependent signal of topography subsequent to crustal thickening and uplift the system may undergo rapid topographic subsidence and crustal thinning by localized channel flow in the hot and weak lower crust these results suggest an alternative interpretation for the development of mantle flow induced topography in certain tectonic environments

currents and transports of the great whirl socotra gyre system during the summer monsoon august 1993 from august 11 to 22 1993 a conductivity temperature depth acoustic doppler current profiler survey was carried out in the somali socotra region to investigate currents and transports associated with the great whirl and socotra gyre circulation during the height of the summer monsoon the monsoon circulation was confined to the upper 300 m depth with intense surface currents up to 2 2 m s 1 in the great whirl and up to 1 4 m s 1 in the socotra gyre deeper reaching flow was found in the northwestern part of the somali basin and in the passage between the shelf of somalia and abd al kuri the great whirl transport was 58 sv of which nearly 25 were due to ageostrophic flow components the northern part of the great whirl thereby appeared as a closed circulation cell in which the offshore transport was balanced by a southward transport of the same magnitude upwelled water was advected from the cold wedge of the upwelling regime at the somali coast along the edge of the gyre the water in the center of the gyre had the characteristics of indian equatorial water iew the socotra gyre carried 23 sv of modified arabian sea water asw with the transports in the two anticyclonic gyres nearly balanced the exchange of water masses between the somali basin west of the carlsberg ridge and the arabian sea occurred in two areas about 16 sv of warm and saline surface water of southern offshore origin entered the northern somali basin within a 120 km wide swift current between the great whirl and the socotra gyre the other key region for the exchange of water masses was the passage between somalia and abd al kuri there the total northward transport was 13 sv with contributions of iew of upwelled water close to the surface and asw underneath

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

a 90 kyr upwelling record from the northwestern indian ocean using a novel long chain diol index abstract presently upwelling is of major importance for driving primary productivity in the arabian sea but its intensity in the past is not well constrained here we used long chain 1 14 alkane diols specific lipids of diatoms of the genus proboscia as new proxies to reconstruct upwelling conditions in the arabian sea variations in the seasonal lipid fluxes were determined using sediment traps in the somalia upwelling system deployed 80 km off the coast on the somali continental slope niop 905 10 45 444 n 51 56 655 e at 1265 m water depth 268 m above the sea floor and 270 km off the coast in the deep somali basin south of socotra niop 915 10 43 068 n 53 34 422 e at 3047 m depth 1000 m above the sea floor highest fluxes of c 28 and c 30 1 14 diols up to almost 600 g m 2 day 1 were only observed during nutricline shoaling at the onset of the southwest monsoon swm prior to massive upwelling by contrast fluxes of c 30 1 15 diols derived from as yet undefined biological sources only increased marginally during the swm and also during the northeast monsoon nem when instead of upwelling enhanced vertical mixing led to a second productivity pulse sediment core niop 905 taken at the continental slope site showed strong fluctuations in relative concentrations of long chain 1 14 and 1 15 diols with time which we quantified as the summed concentrations of c 28 and c 30 1 14 diols divided by the summed concentrations of c 28 and c 30 1 14 diols and c 30 1 15 diols this diol index follows the same trend as other upwelling intensity records from the arabian sea that are based on sea surface temperature reconstructions organic carbon content barium aluminium ratios and abundance and stable isotope composition of specific foraminiferal species the diol index was relatively high during the holocene ca 0 7 but much lower during the late glacial maximum ca 0 2 it was generally low during the last glacial but elevated values were found during the first half of marine isotope stage 3 between 60 and 45 ka and at the end of marine isotope stage 5 1 approximately 80 ka suggesting intensified glacial upwelling our data shows that long chain diols are suitable proxies to reconstruct past upwelling intensities in the arabian sea

the volume transport of the somali current during the 1995 southwest monsoon abstract during the summer monsoon of 1995 two hydrographic and direct velocity sections were occupied over the continental shelf and slope off the coast of somalia in order to capture the flux of the western boundary current the first section was in early june just after the onset of the monsoon winds when no current was observed instead there was an anticyclonic eddy at the boundary with a small net northward transport of 3 5 1 5 sv the second section was occupied in mid september when a strong somali current was evident the resulting snapshot represents the only direct velocity section of the jet structure as it flows along the western boundary the strength of the somali current was 37 5 sv this is in general agreement with previous observations given the temporal variability of the current and represents a peak value according to contemporary model simulations the somali current of september extended far into shallow coastal waters of less than 200 m depth with speeds over 150 cm s 1 resulting in a considerable transport of 14 sv over the shelf northward flow of more than 10 cm s 1 penetrated to over 2000 m depth along the continental slope so that the subthermocline flux contributes another 13 sv to the total volume transport model simulations and satellite images of sea surface height illustrate that the somali current system exhibits large spatial variability on short timescales and thus differences in measured transports owe as much maybe more to the changing path of the current as to variability in its strength

abyssal circulation in the somali basin the bottom and deep circulation in the somali basin are investigated on the basis of hydrographic and direct velocity profiles from three shipboard surveys carried out during the southwest monsoon in 1995 and of velocity time series from the woce mooring array icm7 the inflow of bottom water into the somali basin through the amirante passage drives a thermohaline circulation which may be modulated by the monsoon wind forcing details of the abyssal circulation have been discussed controversially deep velocity records from the mooring array in the northern somali basin are dominated by fluctuations with periods of 30 50 days and amplitudes above full size image 1 k despite this strong variability annual record averages indicate the existence of a deep western boundary current dwbc below full size image 1 k at the base of the continental slope south of socotra island as part of a cyclonic bottom circulation the southwestward dwbc transport off socotra island is estimated to full size image 1 k the bottom and deep water exchange between the somali and arabian basin north of 7 n is estimated from two cross basin geostrophic velocity sections referenced by vertically averaged ladcp currents for the bottom water an eastward transport into the arabian basin of full size image 1 k and full size image 1 k was determined in june and august respectively while for the deep water layer above full size image 1 k eastward transports of full size image 1 k in june and full size image 1 k in august were obtained

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

the role of passive microseismic monitoring in the instrumented oil field with the current industry trend toward instrumented oil fields and smart well completions the permanent deployment of geophones or other acoustic sensors to complement standard engineering gauges is being promoted as a way to map reservoir dynamics the biggest push is from the time lapse seismic practitioners although the deployment of permanent seismic instrumentation is also potentially an ideal route to monitor passive seismicity

somali current rings in the eastern gulf of aden 1 new satellite based observations reveal that westward translating anticyclonic rings are generated as a portion of the somali current accelerates northward through the socotra passage near the mouth of the gulf of aden rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm s are comparable in overall diameter to the width of the gulf of aden 250 km and translate westward into the gulf at 5 8 cm s ring generation is most notable in satellite ocean color imagery in november immediately following the transition between southwest boreal summer and northeast winter monsoon regimes the observed rings contain anomalous fluid within their core which reflects their origin in the equator crossing somali current system estimates of socotra passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in may following the winter to summer monsoon transition cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together recent shipboard and lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern gulf of aden

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

intraseasonal variability in the southwestern arabian sea and its relation to the seasonal circulation an analysis of topex poseidon altimeter data and in situ current and temperature data obtained between april 1995 and october 1996 from a moored array shows strong intraseasonal fluctuations in the southwestern arabian sea an oceanic region where the great whirl gw a predominantly wind generated very energetic anticyclone is present during the southwest monsoon fluctuation periods between 30 and 50 days up to 100 days during some years are observed in the 8 year altimetric dataset mostly during late summer and fall these fluctuations are largest in a 1000 km wide region off the somali omani and yemeni coasts north of 5 n suggesting a local generation mechanism the in situ data at different moorings show strong and coherent fluctuations that are characterized by southwestward phase propagation and northward energy propagation their periods range from 30 to 60 days and increase steadily from july 1995 to january 1996 in the first stage these periods are at and below the cut off period of freely propagating first baroclinic mode rossby waves but approach this theoretical limit later in the year instabilities of the flow in the transition region between the southern gyre and the gw are likely sources of these fluctuations

more thoughts on convergent crustal plateau formation and mantle dynamics with regard to tibet evidence suggests that some 10 m y ago a large portion of thickened mantle lithosphere below the tibetan plateau was rapidly removed removal appears to have been abrupt and delayed from the time of substantial thickening fluid dynamical models of convective thermal boundary layer instability have had difficulties explaining abrupt lithospheric removal long after initial thickening such models have implied that either removal should be synchronous with thickening or should not occur at all due to boundary layer stabilization resulting from high lithospheric viscosity we present simple thermal chemical boundary layer convection models that suggest a compromise between previous end member results the models allow for delayed instability of a thickened thermal boundary layer under geologically reasonable parameter conditions instability delay relies on high viscosity of thickened mantle lithosphere stabilizing it against free convective removal and on lateral extent of thickened crust exceeding that of mantle lithosphere the latter condition leads to a peripheral region of thickened crust and relatively unthickened mantle lithosphere and an interior plateau region of both thickened crust and mantle lithosphere if the thermal resistance and or the rate of internal heat production of crust is greater than that of mantle then mantle lithosphere at the peripheral region can be thermally eroded to the point that the base of the crust comes into contact with bulk mantle this creates a peripheral weak zone that allows thickened mantle lithosphere of the interior plateau region to become mechanically detached and sink into bulk mantle the instability is akin to delamination as originally defined and its delay time is roughly the thermal diffusion time across the thinnest portion of mantle lithosphere

the korudag anticlinorium in the south thrace basin northwest turkey a super giant petroleum trap complex the sedimentary sequence of the south thrace basin northwest turkey comprises upper cretaceous holocene sediments in this basin the korudag anticlinorium which is the subject of this study is located between the aegean sea in the west and the sea of marmara to the east the anticlinorium which is approximately 300 km 186 mi long and 40 km 25 mi wide was formed by effects of the neotethys subduction accretion complex and istranca massif collision during the late early miocene the korudag anticlinorium was deformed to its present day structure by the oldest splay of the north anatolian fault upper middle miocene and the northern branch of the north anatolian fault naf n not earlier than 200 ka organic geochemical analysis oil and gas to source rock correlation and basin modeling studies suggest that the korudag anticlinorium should be charged by hydrocarbons generated from the karaagac ceylan regionally and mezardere formations gas and oil are being produced from the korudag anticlinorium and its subparallel anticlines in the north marmara degirmenkoy seymen cayirdere karacali yulafli tekirdag sevindik and vakiflar gas plus oil fields mapping done as part of this study indicates that the korudag anticlinorium has not yet been tested and explored comprehensively

a method for determining significant structures in a cloud of earthquakes the interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task locations are subject to uncertainties which will not be the same for every earthquake the data should therefore not be interpreted simply by inspection which is difficult in the case of three dimensional data anyway instead we propose using the location uncertainties as a guide in processing the data earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained which can then be interpreted in terms of some simplified structure such as faults the aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data the method is applied to three synthetic sets of data these illustrate the potential and limitations of the method application to a real earthquake data set from rabaul caldera in papua new guinea gives an image of the caldera ring fault that suggests departures from the simple ring fault structure previously assumed sensitivity analysis on the rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it

three dimensional numerical simulations of crustal deformation and subcontinental mantle convection abstract 3 d simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large scale mantle flow simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 1 1 cartesian domain convective stresses cause crust to thicken above a sheet like mantle downwelling for mild convective vigor an initial crustal thickness variation is required to induce 3 d lithospheric instability below the zone of crustal convergence the amplitude of the required variation decreases with increasing convective vigor morphologically instability is manifest in formation of drip like thermals that exist within the large scale roll associated with initial crustal thickening a strong surface signature of the drips is their ability to cause deviations from local airy compensation of topography after the initial thickening phase the crustal accumulation that forms serves as a model analog to a continent its presence leads to mantle flow patterns distinctly different from the steady state roll that results in its absence large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small scale thermal instabilities eventually these instabilities induce a restructuring of large scale mantle flow with the roll pattern being replaced by a square cell although preliminary and idealized the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2 d mountain chain by the formation of the chain itself the ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low order despite the seemingly trivial volume of crust relative to mantle

vorticity and vertical circulation at an ocean front abstract density and velocity data with 4 km horizontal resolution from a survey of a front during fasinex frontal air sea interaction experiment are combined to describe the structure of the top 300 m of the ocean the geostrophic velocity field is derived and is used to examine the relative importance of stratification relative vorticity and twisting terms in ertel s potential vorticity q tenfold isopycnic changes in q are found across a horizontal scale of only 10 km these changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer the ageostrophic velocity field is quantified by solution of the omega equation and vertical velocities of up to 40 m day 1 are found small 40 km surface trapped top 200 m eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer

the next wave in reservoir monitoring the instrumented oil field the next wave in reservoir monitoring will be the instrumented oil field the defining concept is twofold first arrays of seismic and other geophysical sensors will be permanently deployed at or near the surface and in boreholes associated with a producing hydrocarbon reservoir these permanent arrays will provide on demand imaging capability of large scale reservoir fluid flow second injecting and producing wells will be instrumented with downhole sensors such as pressure temperature and saturation gauges to monitor small scale fluid flow local to the wells

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

speculations on the thermal and tectonic history of the earth summary the connection between the earth s thermal history and convection in the mantle is exploited to elucidate the early evolution of the earth it appears probable that convection extending over almost all of the mantle has dominated vertical heat transport throughout the whole of the earth s history only in boundary layers at the surface and at a depth of 650 700 km is conduction likely to be important the resulting evolution appears to be consistent with geological observations on early precambrian rocks various arguments are put forward in favour of two horizontal scales of convective flow in the mantle at depths less than 650 km the large scale flow is related to the motion of major plates and must be ordered over distances of more than 5000 km its evolution and energetics are discussed and there are no obvious problems in maintaining the proposed convective motions small scale flow with an extent of the order of 500 km appears necessary both to explain the heat flow through older parts of the earth s surface and to reconcile the geophysical observations with the results of numerical experiments though the existence of the small scale flow is at present speculative various tests of its presence are proposed

thermal effects of continental collisions thickening a variable viscosity lithosphere abstract based on data that indicate that the lithosphere and the crust are anomalously hot in some convergence zones simple calculations of the thermal effect of crustal thickening are presented the results indicate that post tectonic melting of the lower crust requires high levels of mantle heat flux it has been suggested that this high heat flux can be explained by the convective removal of thermal boundary layers thickened by convergence to test this hypothesis a series of numerical experiments were carried out on the removal of thickened thermal boundary layers whose viscosity depends on temperature and pressure it is found that the boundary layers can be removed quickly 10 m y in some cases however these layers are too thin to greatly alter the thickness of the lithosphere or the heat flux at the top of the mantle a set of calculations was done with internal heat sources which give equilibrium lithospheric thicknesses half that of the thickened lithospheres these indicate that the time scale for convective thinning by one half is extremely long at least 100 m y for initial lithospheric thicknesses of 100 km therefore the lithosphere must be anomalously hot and thin before deformation for extensive crustal melting to occur after thickening in observed times 40 m y for tibet finally it is suggested that subduction induced convection may raise the general mantle temperatures over a broad region behind a convergence zone this effect can inhibit the cooling of the lithosphere for more than 50 m y after the subduction induced convection has ceased

the fate of upwelled waters in the great whirl august 1995 abstract the great whirl is a large anticyclonic gyre that develops off the northern somali coast during the southwest monsoon in august 1995 the noaa ship malcolm baldrige surveyed the seaward edge of the upwelling zone associated with this gyre the fate of recently upwelled water was followed by mapping surface property distributions along a cool surface feature that extended seaward along the northern edge of the great whirl surface properties t s and chlorophyll a surface velocity adcp and xbt and ctd casts were interpreted in relation to the trajectories of three instrumented surface drifters deployed in the feature cool surface waters correspond in space to the shoaling of the upper thermocline and offshore advection from the coast surface chlorophyll a concentrations decreased from 2 to 3 g l 1 in the upwelling zone to 0 5 1 5 g l 1 in the surface feature and contiguous waters maximum surface velocities in the great whirl were 250 cm s 1 with velocities 100 cm s 1 along the northern perimeter of the gyre decorrelation time scales for u and v velocity components and chlorophyll a fluorescence from the drifters were on the order of 4 to 7 days these times are comparable to those over which the drifters were ejected from the great whirl into the socotra gyre decorrelation times for sea surface temperature were somewhat longer 10 days all three platforms passed between the somali coast and socotra within a week of their deployment and then traveled east into the northern arabian sea

intraplate tectonics feedback between radioactive thermal weakening and crustal deformation driven by mantle lithosphere instabilities abstract two dimensional viscous plastic numerical experiments show that weak radioactive crust may be susceptible to significant tectonic activity by the effect of rayleigh taylor type instability of the sub crustal lithosphere below a continental interior in particular crust having a localized or regional enrichment of radioactive elements can respond to an underlying mantle lithosphere downwelling by initial subsidence then thickening and uplift owing to the positive feedback among thickening heating and weakening such models may later undergo orogenic deflation as spontaneous crustal extension thinning and surface subsidence occur while mantle downwelling continues strong homogeneous crust subsides dynamically above mantle downwellings and undergoes essentially no internal deformation a uniform distribution of radioactive elements through the crust or a concentration at depth is much more effective in causing thermal weakening of the crust and subsequent intraplate deformation than with radioactive material concentrated nearer the surface for wide regions of high heat production in the crust the length scale of the intracrustal tectonic zone is controlled by the width of the mantle downwelling in contrast with narrow heat production regions which control their own length scale in general the numerical experiments demonstrate that the presence of radioactive elements may make the crust vulnerable to intraplate tectonic deformation and represent a primary control on the way the strength of the crust evolves during sub crustal forcing the results may explain first order features of intraplate tectonics including the subsidence mechanism for intracontinental basins orogenic crustal contraction thickening and surface uplift and subsequent extension and crustal deflation the processes have particular implications for regions of high radioactive heat production and juvenile crust which would more likely have a higher concentration of radioactive elements in the lower crust

mesoscale dynamics in the arabian sea and a focus on the great whirl life cycle a numerical investigation using roms the great whirl gw is a persistent anticyclonic mesoscale eddy that is observed seasonally in the arabian sea during a period embedding the 3 months of the southwest monsoon june july august at a quasi steady location its dynamics remain unclear despite it being one of the largest coherent vortices in the world ocean realistic regional numerical experiments using roms are performed to investigate the life cycle of the gw which is not well resolved by sparse available in situ measurements in the region using a set of sensitivity experiments and an accurate temporal characterization of the eddy properties including position radius depth and vorticity we i confirm the role of basin scale downwelling rossby waves in the gw generation ii clarify the role of the monsoonal strong anticyclonic wind in its maintenance and barotropization and iii suggest a connection between basin scale rossby wave dynamics and gw collapse

sedimentation in the western arabian sea the role of coastal and open ocean upwelling monsoon induced coastal and open ocean upwelling explain 84 of the variations of the organic carbon fluxes measured in the deep western arabian sea in this paper sea level measurements satellite derived wind speeds sea surface temperatures and nutrient profiles are used to discern the relative importance of these factors on fluxes measured during nine years of continuous sediment trap deployments this exercise shows i the increase in fluxes observed during the initial stages of the sw monsoons are caused by open ocean upwelling which develops faster than the coastal upwelling ii coastal upwelling triggers diatom blooms from nutrients from subsurface water and sediment resuspension and more importantly by injecting resting stages of diatoms back into the euphotic zone iii silica depletion resulting from diatom blooms in laterally advecting water masses leads to a replacement of diatoms by other nitrate limited organisms iv organic carbon fluxes to the deep arabian sea increase in response to an intensification of both coastal and open ocean upwelling weak coastal upwelling and strong open ocean upwelling also increase organic carbon fluxes the varying dominance of their influence is reflected in the timing and the composition of the peak fluxes v the link between organic carbon flux and monsoon strength is non linear probably due to changes in the surface currents and to vigorous turbulence in the surface water during strong sw monsoons these processes could reduce the organic carbon flux in the western arabian sea by about 65

when 4d seismic is not applicable alternative monitoring scenarios for the arab d reservoir in the ghawar field abstract ghawar the largest oilfield in the world produces oil from the upper jurassic arab d carbonate reservoir the high rigidity of the limestone dolomite reservoir rock matrix and the small contrast between the elastic properties of the pore fluids i e oil and water are responsible for the weak 4d seismic effect due to oil production a feasibility study was recently completed to quantify the 4d seismic response of reservoir saturation changes as brine replaced oil the study consisted of analysing reservoir rock physics petro acoustic data and seismic modelling a seismic model of flow simulation using fluid substitution concluded that time lapse surface seismic or conventional 4d seismic is unlikely to detect the floodfront within the repeatability of surface seismic measurements thus an alternative approach to 4d seismic for reservoir fluid monitoring is proposed permanent seismic sensors could be installed in a borehole and on the surface for passive monitoring of microseismic activity from reservoir pore pressure perturbations reservoir production and injection operations create these pressure or stress perturbations reservoir heterogeneities affecting the fluid flow could be mapped by recording the distribution of epicentre locations of these microseisms or small earthquakes the permanent borehole sensors could also record repeated offset vertical seismic profiling surveys using a surface source at a fixed location to ensure repeatability the repeated vertical seismic profiling could image the change in reservoir properties with production

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

shear wave splitting and small scale convection in the continental upper mantle the deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt but this deformation is difficult to observe seismic azimuthal anisotropy manifested by shear wave splitting provides the best geophysical evidence of deformation in the upper mantle here we use observations of shear wave splitting to investigate deformation in the upper mantle beneath the tien shan in central asia we find that underneath most of the tien shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt but it deviates by nearly 90 where the upper mantle is anomalously hot the former relationship previously observed in other collisional belts seems to be usual for this environment the anomalous directions however provide evidence of a rising plume beneath the range suggesting more generally that the along strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection