genesis of tectonic inversion structures seismic evidence for the development of key structures along the purbeck isle of wight disturbance the interpretation of a densely spaced and well calibrated seismic grid sheds new light on the development and evolution of key regional and local structures in the wessex basin the results help to resolve long standing controversies concerning the tectonic significance of apparently anomalous outcrop patterns and the role of important local ancillary structures with respect to the major monoclinal folds with which they are associated although the structures are entirely consistent with the effects of contractional reactivation tectonic inversion of normal faults the subsurface data demonstrate the role that original extensional fault segmentation and associated relay ramps had on original depositional patterns subsequent inversion geometries and resultant outcrop patterns as well as illustrating regional controls on the formation of structures the new seismic based interpretations enable a reassessment of the lulworth crumple and the ballard down fault the lulworth crumple is interpreted as a parasitic fold complex generated by internal folding of the inverted incompetent syn rift fill in the immediate hanging wall to the purbeck fault a reactivated major normal fault the ballard down fault s origin is interpreted to result from the formation of a local late stage out of the syncline reverse fault which propagated southwards and upwards through a chalk succession as the chalk had already been rotated to form the northward dipping steep limb of the purbeck monocline at ballard down the structure cuts down stratigraphically the results stress the importance of understanding the nature of original extensional fault geometries and the competence of the sedimentary units incorporated in folds in gaining a full understanding of the genesis and evolution of structural styles in inverted basins

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

the relationship between rifting and magmatism in the northeastern arabian sea the rifting of the seychelles microcontinent from india involved two phases of extensional activity the initial separation of the laxmi ridge from india was accompanied by extensive magmatism but the later separation of the seychelles from the laxmi ridge was only weakly magmatic the causal mechanisms linking continental flood basalts lithospheric extension and mantle plumes as well as the relative timing of extension and volcanism are controversial1 2 3 4 the eruption of the deccan flood basalts was approximately contemporaneous with the separation of the seychelles microcontinent from india however between these continental blocks lies the enigmatic laxmi ridge and the sequence of extensional events that formed these various tectonic elements is poorly understood here we present wide angle seismic data along a profile across laxmi ridge that permit delineation of offshore igneous bodies associated with the deccan magmatism these bodies are similar to those associated with flood basalt volcanism and rifting in the atlantic region and elsewhere1 from the geometry of these bodies we infer that there were two periods of extension the first phase which involved extension between laxmi ridge and the indian subcontinent was accompanied by significant deccan related magmatism full development of a continental margin was achieved during the second phase of weakly magmatic extension between laxmi ridge and the seychelles we suggest that between these rifting events the region passed beyond the reach of lateral flow from the source region of the deccan flood basalts

geodynamic evolution of the offshore indus basin pakistan the western indian plate passive continental margin a complex meso cenozoic history characterizes the offshore indus basin pakistan including rifting drifting collision transpression and recent inverted extension that is poorly understood we use reflection seismic data to document the geometry of the structures that accommodated the deformation and offshore well data to correlate the sedimentary sequences which were affected by the different tectonic episodes synthetic seismograms have been prepared for calibrating seismic data with well information for identification of key reflections representative of major lithological horizons seismic stratigraphy helped in dividing the sedimentary successions into rifted margin and transpressive margin sequences additionally the basement represented by chaotic and semitransparent reflections and moho discontinuity by moderate amplitude bright reflections are also identified on seismic records syn depositional normal faulting in extensional regime cretaceous oligocene are identified on seismic profiles that converted into strike slip faults in miocene and listric faults in recent sediments forming inverted extensional regime we infer that the basin inversion from extension to inverted extension and transpression occurred in recent times number of depositional features including shelves deltaic deposits canyons and channel levee complexes have been identified that developed in different tectonic episodes five tectonic subsidence curves using back stripping procedure have been calculated in order to constrain the geodynamic evolution of the meso cenozoic sedimentary succession of the offshore indus pakistan the analysis of tectonic subsidence curves covering a time span of 140 ma allowed us to quantify the total subsidence and to document the activities of syn rift and post rift faulting systems different stages in terms of duration and magnitude of subsidence uplift trends have been identified in the evolution of the basin two uplift events are prominent in eocene similar to 40 ma and miocene time similar to 12 ma a rapid resumption of subsidence with non conformity in early palaeocene is seen along the margin

rift basins in western margin of india and their hydrocarbon prospects with special reference to kutch basin the western continental margin of india can be classed as a divergent or passive margin the western continental shelf is an extensive carbonate bank bombay offshore basin passing into clastic sediments on the north and south three craton margin embayed basins kutch cambay and narmada in the northern part of the shelf are filled predominantly with clastic sediments these basins occupy grabens bounded by faults diverging seaward the grabens were formed by three rift systems along major precambrian tectonic trends the rifting developed sequentially from north to south around the saurashtra horst kutch basin was formed in the early jurassic followed by cambay basin in early cretaceous time and the narmada in the late cretaceous it appears that these rifting ev nts occurred at successive stages during the northward migration of the indian plate after its break from gondwanaland in late triassic or early jurassic it is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the indian craton bombay offshore and cambay are two major oil producing basins in the western margin these basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region oil has not been found in kutch basin which is mainly an onshore mesozoic basin the basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the apennines italy the transient response of bedrock rivers to a drop in base level can be used to discriminate between competing fluvial erosion models however some recent studies of bedrock erosion conclude that transient river long profiles can be approximately characterized by a transport limited erosion model while other authors suggest that a detachment limited model best explains their field data the difference is thought to be due to the relative volume of sediment being fluxed through the fluvial system using a pragmatic approach we address this debate by testing the ability of end member fluvial erosion models to reproduce the well documented evolution of three catchments in the central apennines italy which have been perturbed to various extents by an independently constrained increase in relative uplift rate the transport limited model is unable to account for the catchments response to the increase in uplift rate consistent with the observed low rates of sediment supply to the channels instead a detachment limited model with a threshold corresponding to the field derived median grain size of the sediment plus a slope dependent channel width satisfactorily reproduces the overall convex long profiles along the studied rivers importantly we find that the prefactor in the hydraulic scaling relationship is uplift dependent leading to landscapes responding faster the higher the uplift rate consistent with field observations we conclude that a slope dependent channel width and an entrainment erosion threshold are necessary ingredients when modeling landscape evolution or mapping the distribution of fluvial erosion rates in areas where the rate of sediment supply to channels is low

sub bottom temperature anomalies detected by long term temperature monitoring at the tag hydrothermal mound during aug 1994 feb 1995 geothermal measurements were carried out on the tag hydrothermal mound 26 0 n 44 49 w of mid atlantic ridge two geothermal probes made reliable measurements six transient anomalies in subbottom temperatures were detected 20 m south of the central black smoker complex cbc having durations 40 to 400 hours and magnitude 0 2 c to 30 c these anomalies are probably evidence for short period fluctuations in the fluid circulation beneath the mound the time variability in subbottom temperatures near cbc suggests that instantaneous heat flow measurements may not represent the time average of heat flow there although it is difficult to specify the origin of thermal anomalies during the drilling period by the ocean drilling program at least one of them seems to correlate with the drilling events because it was also accompanied by transient temperature anomalies at cbc and 50 m east of cbc observed by other instruments

tidal current effects on temperature in diffuse hydrothermal flow guaymas basin a twelve day record of temperature collected from the diffuse flow area of a guaymas basin hydrothermal site exhibits variations from a minimum of 3 05 c to a maximum of 4 87 c whose periodicity is correlated with tides measured at the nearby town of guaymas a simple model based on the hypothesis that temperature variations result as changes in tidal bottom currents induce changes in the height of the thermal boundary layer is in good quantitative agreement with observed temperatures for most of the record the success of this model illustrates that the effects of tidal currents can be strong enough to dominate the time variability of a temperature signal at a fixed point in hydrothermal flow therefore tidal currents must be taken into account when using temperature measurements to estimate time varying heat fluxes from hydrothermal diffuse flow regions

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

tide related variability of tag hydrothermal activity observed by deep sea monitoring system and obsh abstract hydrothermal activities were monitored by an ocean bottom seismometer with hydrophone obsh and a composite measuring system manatee including ctd current meter transmission meter and cameras at a small depression on the tag hydrothermal mound in the mid atlantic ridge low frequency pressure pulses detected by the hydrophone with semi diurnal periodicity seem to correspond to cycles of hydrothermal upflow from a small and short lived smoker vent close to the observing site the peaks of pressure pulses are synchronous with the maximum gradient of areal strain decrease due to tidal load release microearthquakes with very near epicenters occur sporadically and do not appear to be directly correlatable to hydrothermal venting temporal variations in bottom water temperature also have semi diurnal periodicity but are more complicated than the pressure events temperatures may be affected both by upwelling of hot water and by lateral flow of the bottom current changing its directions with ocean tide

observation of pressure change associated with hydrothermal upwelling at a seamount in the south mariana trough using an ocean bottom seismometer an obsh ocean bottom seismometer with hydrophone was deployed just 1 m from a hot thermal vent on the summit region of a seamount located 18 km east of the south mariana trough axis to record continuous data on hydrothermal activity although the seismicity beneath the seamount was very weak numerous pulse shape pressure events showing sudden pressure decrease were observed on the hydrophone channel the characteristic period of the pressure events varied from 20 seconds to 1 minute after the submersible shinkai 6500 relocated the obsh 10 m south of the vent where no thermal vents existed the number and amplitude of pressure events drastically decreased this suggests that pressure events were generated by the hot water upwelling from the thermal vent the pressure events appear to be related to the ocean tide this experiment indicates that observations using obshs are an effective means of monitoring hydrothermal activity

correlations between silicic volcanic rocks of the st mary s islands southwestern india and eastern madagascar implications for late cretaceous india madagascar reconstructions abstract the st mary9s islands southwestern india expose silicic volcanic and sub volcanic rocks rhyolites and granophyric dacites emplaced contemporaneously with the cretaceous igneous province of madagascar roughly 88 90 ma ago the st mary9s islands rocks have phenocrysts of plagioclase clinopyroxene orthopyroxene and opaque oxide moderate enrichment in the incompatible elements e g zr 580 720 ppm nb 43 53 ppm la yb n 6 9 7 2 relatively low initial 87 sr 86 sr 0 7052 0 7055 and near chondritic initial 143 nd 144 nd 0 51248 0 51249 they have mineral chemical whole rock chemical and isotopic compositions very close to those of rhyolites exposed between vatomandry ilaka and mananjary in eastern madagascar and are distinctly different from rhyolites from other sectors of the madagascan province we therefore postulate that the st mary9s and the vatomandry ilaka mananjary silicic rock outcrops were adjacent before the late cretaceous rifting that split madagascar from india if so they provide a valuable tool to check and aid traditional cretaceous india madagascar reconstructions based on palaeomagnetism matching precambrian geological features and geometric fitting of continental shelves supplementary material mineral analyses mass balance calculations and locality information are available at http www geolsoc org uk sup18332

stable isotopes and archaeological geology the carrara marble northern italy abstract the principal marble quarries of italy for the past 2000 years have been those of carrara in roman times the marble was exported all over the ancient world renaissance quarries were opened up in seravezza both the seravezza and the carrara quarries have been exploited until today the correct identification of carrara marble has always been a problem because it was traded so widely in roman times and was later used to fabricate broken or lost pieces of classical statuary many types of geochemical analysis have been tried in order to find distinctive signatures for the recognition of classical marbles including trace elements esr spectroscopy of mn thermoluminescence and stable isotopic ratios to date the most promising has been isotopic ratio analysis of oxygen and carbon in this study stable isotopic signatures were investigated as a means for distinguishing 1 the principal quarry areas of the carrara commune fossacava miseglia and torano and the seravezza commune 2 or merely the largely classical quarries of carrara from the renaissance quarries of seravezza 3 and most important the carrara quarries from other quarries of greece turkey italy and tunisia in the classical marble data base discriminant analysis da of the isotopic analyses showed that the three quarry areas of a carrara could not be told apart but that they could be told easily from seravezza in addition a mineralized carrara quarry at mandria with buff to yellow rather than white marble was also distinctive da also suggests that carrara can be told from those quarries with which it is presently confused paros in the aegean sea marmara in turkey and pentelikon near athens sr isotopic analysis showed a range in the carrara and other marbles of hettangian lower jurassic age from 0 70778 to 0 70810 which is very different from values found in paros and pentelikon this suggests that sr might make a good third discriminator for classical marble

subsidence of the voring basin and the influence of the atlantic continental margin the post cretaceous subsidence history of the voring basin part of the atlantic passive margin offshore mid norway has been investigated extension and factors related to rifting and continental break up during the palaeocene have been quantified using both forward and reverse basin modelling techniques in the preferred geological model it is assumed that rifting occurred in the voring basin during the palaeocene prior to break up following an earlier rift event during the late jurassic during palaeocene rifting the basin may have been dynamically uplifted by the iceland mantle plume in the east of the basin there was no palaeocene extension subsidence analysis shows that in the centre of the basin forward and reverse models converge to predict a modest palaeocene stretching factor of c 1 15 in the west of the basin closest to the atlantic margin forward models of upper crustal faulting also predict a of c 1 15 but reverse backstripped models of subsidence predict a of up to 1 75 we suggest that lower crustal and mantle lithosphere thinning close to the margin were greater than the extension accommodated by upper crustal faulting and that some lower crustal mantle lithosphere stretching associated with continental separation was partitioned below the voring basin up to 150 km landwards of the margin

extension across the indian arabian plate boundary the murray ridge summary seismic reflection profiles from the murray ridge in the gulf of oman northwest indian ocean show a significant component of extension across the predominantly strike slip indian arabian plate boundary the murray ridge lies along the northern section of the plate boundary where its trend becomes more easterly and thus allows a component of extension the dalrymple trough is a 25 km wide steep sided halfgraben bounded by large faults with components of both strike slip and normal motion the throw at the seabed of the main fault on the southeastern side of the halfgraben reaches 1800 m the northwest side of the trough is delineated by a series of smaller antithetic normal faults wide angle seismic gravity and magnetic models show that the murray ridge and dalrymple trough are underlain by a crystalline crust up to 17 km thick which may be continental in origin any crustal thinning due to extension is limited and no new crust has been formed we favour a plate model in which the indian arabian plate boundary was initially located further west than the owen fracture zone possibly along the oman continental margin and suggest that during the oligocene early miocene indian ocean plate reorganization the plate boundary moved to the site of the present owen fracture zone and that motion further west ceased at this time deformation began along the murray ridge with both the uplift of basement highs and subsidence in the troughs tilting the lowest sedimentary unit qalhat seamount was formed at this time subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography the normal faulting was accompanied by hanging wall subsidence footwall uplift and erosion flat lying recent sediments show that the major vertical movements have ceased although continuing earthquakes show that some faulting is still active along the plate boundary

seismic reflections from the moho the effect of isostasy summary deep seismic reflection profiling has become an established technique for studying the continental lithosphere the base of the crust can be identified on many deep profiles and it correlates closely with the moho determined by refraction studies on many reflection profiles the moho appears at a rather constant two way travel time despite the highly variable structure of the crust above it is shown that the apparent flatness of the moho on reflection time sections is an artefact produced when the crust is in local isostatic equilibrium the true geometry of the moho can only be determined if accurate and spatially well resolved seismic velocities are obtained from the lower crust the degree of isostatic compensation can be assessed from deep reflection data and an equivalent elastic thickness calculated the elastic thickness around the uk is very small only 2 3 km it is suggested that the many major fault zones visible on the seismic data weaken the lithosphere to produce these low values

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

factors influencing magmatism during continental breakup new insights from a wide angle seismic experiment across the conjugate seychelles indian margins we present a model of the northern seychelles continental margin derived from controlled source wide angle seismic traveltime inversion and teleseismic receiver functions this margin has been widely cited as a classic example of rifting in association with a continental flood basalt province the deccan traps however we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts such as those of the north atlantic the oceanic crust formed immediately after breakup and throughout the first 3 ma of seafloor spreading is just 5 2 km thick less than half that typically seen at other volcanic margins the continent ocean transition zone is narrow and while two packages of seaward dipping reflectors are imaged within this transition they are weakly developed beneath the thinned continental crust there is an approximately 4 km thick layer of high velocity material 7 5 7 8 km s that we interpret as mafic material intruded and underplating the lower crust however we believe that this underplating most likely happened prior to the breakup overall the observations show that the rifting of india from the seychelles was characterized by modest magmatism the spatial extent of the deccan flood basalt province is therefore smaller than previously thought we speculate that either the lateral flow of deccan related hot material beneath the breakup region was hampered perhaps as the rifted margins did not intersect the center of the deccan source or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates if the latter explanation is correct then the rate of plate separation as indicated by the initial seafloor spreading rate is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized

the role of rock surface hardness and internal moisture in tafoni development in sandstone the development of cavernous weathering features such as tafoni remains poorly understood in particular the roles played by internal moisture and case hardening remain unclear in this study electric resistivity tomography ert has been used to map moisture distribution within inner walls of tafoni developed in sandstone and an equotip device used to measure rock surface hardness as a proxy measure of the degree of weathering and case hardening seven large tafoni in the golden gate highlands national park south africa varying in size and degree of development have been monitored a dynamic relationship between surface hardness degree of weathering and internal moisture regimes has been found we propose a new conceptual model which illustrates the complex interaction between case hardening and internal moisture and suggests a new direction for cavernous weathering research copyright 2011 john wiley sons ltd

deformation at plate boundaries around the gulf of oman abstract seismic studies during charles darwin cruise 18 86 focused on three zones of deformation the makran accretionary prism the murray ridge owen fracture zone system and the continental margin of east oman a multichannel seismic reflection profile across the makran margin clearly imaged the imbricate thrust slices and their bounding faults a bottom simulating reflector bsr 500 800 m beneath the seabed marking the base of a gas hydrate stability zone may be traced across most of the profile the heat flow variation estimated from changes in the depth of this reflector shows evidence for localised fluid expulsion reflection profiles across the murray ridge show evidence for active extension along a series of major normal faults the rift structure is segmented with the fault polarity changing between segments a combined reflection and wide angle study of the east oman margin shows a deep offshore basin underlain by crust only 5 km thick and bounded by a basement ridge interpreted as a tilted fault block the wide angle data suggest a sleeply dipping moho beneath the margin the large lateral changes in crustal thickness may be explained by episodes of margin parallel shear

a poroelastic model for the tidal modulation of seafloor hydrothermal systems 1 time series measurements of the temperature and exit velocity of hydrothermal effluent suggest that seafloor hydrothermal systems are modulated by tidal processes here we apply the theory of poroelasticity to predict the magnitude and phase of tidally induced changes in the temperature and flow rate of hydrothermal effluent at the seafloor we construct a model in which the steady state upwelling of buoyant fluid in the crust is modulated by tidal loading of a one dimensional seafloor by the overlying water column the nature of the solution is controlled by the relative magnitudes of three length scales these are 1 the depth h of the heat source below the seafloor 2 the skin depth d over which pore pressure signals can diffuse during one tidal cycle and 3 the advective length scale a over which the upwelling flow advects thermal signals during one tidal cycle we consider the likely magnitude of the parameters in a real system as well as the limitations of a one dimensional representation of that system we then discuss how observational data on the magnitude and phase lag of temperature and flow rate could be used to constrain the subseafloor parameters that govern hydrothermal circulation within the seafloor

presenting meso cenozoic seismic sequential stratigraphy of the offshore indus basin pakistan abstract preserved deformed depositional sequences along passive continental margins are important repositories of natural resources as well as key laboratories for understanding rift drift tectonics with better post signal processing schemes interpretation techniques and potential data imaging sedimentary basins along these margins has reached unique levels of refinement this in turn has led to considerable improvement in unfolding the complex tectono sedimentary history since rifting to basin development in this paper deep reflection seismic coupled with well data and biostratigraphic information have been used to decipher the third order depositional sequences of meso cenozoic succession in western parts of the indian plate passive continental margin offshore indus basin as pioneer research based on morpho tectonic considerations the depositional sequences are divided into syn rift passive margin and post rift stages deformed by steeply dipping normal faults using the principles of classic sequence stratigraphy five third order mappable depositional sequences have been recognized on seismic records that are controlled by regional tectonics and sea level fluctuations coupled with sedimentation type and rates in the region the associated system tracts were recognized and mapped based on genetic reflection terminations on seismic profiles and log motifs and constraint surfaces include sequence boundaries sbs maximum flooding surfaces mfss and transgressive surfaces tss the depositional systems encompass lowstand system tracts lsts transgressive system tracts tsts and highstand system tracts hsts the lsts are composed of shallowing upward lowstand deltas or terraces prograding in the basin ward direction while pinches out landward at preceding depositional shoreline breaks the sand units of tsts are interpreted as shoreface sands deposited in the shelfal part during rising sea levels and hsts are characterized by coarsening and shallowing upward intervals with both fluvial and laterally prograding deltaic deposits a hydrocarbon entrapment model is proposed suggesting an interesting petroleum trap based on the entire course of this research valuable recommendations have been made for answering past exploration failures and future efforts such studies can be applied to worldwide passive margins dominated by third order sequences with complex sediments tectonic interplay

modulus decomposition of compressional and shear velocities in sand bodies the advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies in sand bodies compressional and shear velocities depend predictably on porosity mineralogy grain contacts and fluid saturation an interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities careful experiments on pure materials i e pure quartz sandstones demonstrate two simplifying constitutive relationships first the bulk and shear frame moduli are simple functions of the porosity a comparison of the measured shear frame modulus to the prediction for the pure material distinguishes sand from shale second the ratio of the bulk and shear frame moduli is a constant 0 9 independent of the porosity the measured velocities are directly inverted to yield the bulk modulus of the pore fluid the fluid saturation effects are so dramatic at high porosit

a methodology for locating the original quarries used for constructing historical buildings application to malaga cathedral spain abstract it is often necessary to locate the original quarry which supplied the stone for a particular historical building this stones could be used for future restoration work and for testing in the laboratory artificial aging tests physical properties determination control of the efficacy of conservation treatments etc generally reviewing historical documentation gives information about the geographical setting of quarries and location of the stones in the monument but this information needs to be proved by field and laboratory studies the comparative study of stone from quarries and monuments should basically include the following 1 mineralogical and petrographical studies 2 the chemical analysis of major minor and trace elements 3 stable isotopes determinations 4 physical properties of quarry materials and unweathered building stone water absorption ultrasound transmission velocity porosity and porous system density bulk density compressive strength etc this methodology was applied to malaga cathedral stones represented in the main facade towers and the western zone of the terrace which according to historical literature came from almayate miocene pliocene limestones and cerro coronado permotriassic sandstone in malaga the conclusion of the comparative study carried out on quarries and building stones was consistent with the information available from the historical documentation

anomalous subsidence on the rifted volcanic margin of pakistan no influence from deccan plume abstract the role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern arabian sea is investigated using subsidence analysis of a drill site located on the seismically defined somnath volcanic ridge the ridge has experienced 4 km of subsidence since 65 ma and lies within oceanic lithosphere we estimate crustal thickness to be 9 5 11 5 km curiously 600 c especially considering the rapid northward drift of india relative to the deccan reunion hotspot we suggest that this reflects very slow lithospheric growth possibly caused by vigorous asthenospheric convection lasting 28 m y and induced by the steep continent ocean boundary post rift slow subsidence is also recognized on volcanic margins in the ne atlantic and se newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break up

in situ evidence for dextral active motion at the arabia india plate boundary the arabia india plate boundary also called theowen fracture zone is perhaps the least known boundary among large tectonic plates1 6 although it was identified early on as an example of a transform fault converting the divergent motion along the carlsberg ridge to convergent motion in the himalayas7 its structure and rate of motion remains poorly constrained here we present the first direct evidence for active dextral strike slip motion along this fault based on seafloor multibeam mapping of the arabia india somalia triple junction in the northwest indian ocean there is evidence for 12km of apparent strike slip motion along the mapped segment of the owen fracture zone which is terminated to the south by a 50 km wide pull apart basin bounded by active faults by evaluating these new constraints within the context of geodetic models of global plate motions we determine a robust angular velocity for the arabian plate relative to the indian plate that predicts 2 4mmyr 1 dextral motion along the owen fracture zone this transformfault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8 11 which induced a change in configuration of the triple junction infrequent earthquakes of magnitude 7 and greater may occur along the arabia india plate boundary unless deformation is in the formof aseismic creep

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

hydrocarbon prospects of southern indus basin pakistan the southern indus basin extends approximately between lat 23 and 28 31 primen and from long 66 e to the eastern boundary of pakistan of the 55 exploratory wells drilled 1955 1984 27 were based on results of multifold seismic surveys five commercial oil discoveries and one gas discovery in cretaceous sands three gas discoveries in paleocene limestone or sandstone and one gas condensate discovery from lower eocene limestone prove that hydrocarbons are present the main hydrocarbon fairways are mesozoic tilted fault blocks tertiary reefal banks and drape and compressional anticlines older reservoirs are accessible toward the east and northeast and younger mature source rocks are to the west including offshore of the badin block oil field area the indus offshore basin reflects sedimentation associated with mesozoic rifting of the pakistan indian margin superimposed by a terrigenous clastic depositional system comprised of deltas shelves and deep sea fans of the indus river

a preliminary tidal analysis of topex poseidon altimetry approximately 12 months of data from the topex poseidon satellite altimeter mission are analyzed for the major short period oceanic tides a harmonic analysis is performed on data captured within bins defined on a deep ocean grid which owing to tidal aliasing considerations must have a relatively coarse spatial resolution our analysis is in terms of corrections to the schwiderski and cartwright ray models and it confirms many of the schwiderski differences previously reported by cartwright and ray our differences with respect to the geosat based cartwright and ray model form a sectorial pattern in m2 with high low differences separated roughly 180 in longitude we suggest that these sectorial errors were most likely induced by geosat s relatively large orbit error comparisons to independent data validate the improved topex poseidon solutions in situ ground truth shows m2 rms differences of 4 10cm schwiderski 3 86cm cartwright and ray 2 63cm this paper global rates of energy dissipation confirm earlier estimates for m2 and show improved agreement with satellite tracking studies for k1 and s2 these preliminary exercises confirm that topex poseidon should result in a new generation of improved global tidal models

limits of tidal energy dissipation by fluid flow in subsea formations summary tidal loading causes fluid flow through permeable seafloor and between regions of contrasting elastic properties or porosity within subsea formations we examine theoretically the dissipation of energy by these flows and its global significance as a mechanism for tidal energy dissipation expressions are given for energy dissipation rates in layered formations due to vertical flow caused by tidal loading but the results can be used to constrain dissipation by other flow patterns we consider flow near the seafloor in gas bearing sediments and in highly fractured permeable igneous crust energy dissipation by the first two mechanisms is negligibly small globally although it may be locally significant under extreme conditions under favourable conditions flow in fractured crust may have greater energy dissipation but the total amount is limited by the thickness of the permeable layer based on our current understanding of subsea hydrogeology tidally induced flow in subsea formations appears to make little contribution to the observed global tidal energy dissipation

structure and early evolution of the arabian sea and east somali basin summary the laxmi ridge is a large scale basement high buried beneath the sediments of the indus fan the location of the ocean continent transition oct on this margin has previously been proposed at either the southern edge of the laxmi ridge or beyond it towards the india pakistan shelf the former explains the margin parallel laxmi basin as thinned continental crust the latter as a failed rift of earlier seafloor spreading to examine the structure of this margin a reassessment of marine magnetic data has detailed seafloor spreading magnetic anomalies prior to anomaly 24 in both the arabian and east somali basins the previously identified anomaly 28 is not interpreted as a seafloor spreading anomaly but as a magnetized basement feature adjacent to and merging with the ridge the laxmi spur new gravity models across the laxmi ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the laxmi ridge and basin and the location of the oct at the southern edge of the ridge the results are not compatible with the existence of a pre anomaly 28 phase of seafloor spreading although large scale intrusions may be the origin of some of the basement features in the laxmi basin the models also identify the laxmi spur as a low density feature with a natural remanent magnetization nrm compatible with serpentinization the laxmi ridge is mapped to the southeast where it appears to terminate at a point coinciding with the appearance of e w magnetic lineations and gravity anomalies at 15 5 n thereafter it becomes indistinct this is interpreted as necessary in the reconstruction to the mascarene plateau to avoid continental overlap

tidally driven effluent detected by long term temperature monitoring at the tag hydrothermal mound mid atlantic ridge abstract during aug 13 21 1994 temperatures and current velocity were simultaneously monitored on the tag hydrothermal mound three giant kelps gks vertical thermistor arrays of 50 m height were moored on the periphery of the central black smoker complex cbc a manatee multi monitoring system including current velocity was deployed 50 m east of cbc four daibutsu geothermal probes penetrated the sediment south to west of cbc compilation of all data revealed semi diurnal variations in water temperatures and current velocity and allowed us to discuss the source of these anomalies temperature anomalies of gks correlate well with current velocity and are interpreted to be caused by the main plume from cbc that was bent over by the tidal current we identified two types of asymmetric periodic temperature variations at daibutsu probes 2 and 8 located 20 m to the south of cbc by comparing temperatures and current velocity they are attributed to non buoyant effluents laterally advected by the tidal current the source of one variation is located east to ese of the probes and the source of the other is located to the north on aug 31 a new periodic anomaly emerged on probe 2 with its amplitude up to 0 8 c the 6 h offset between the new anomaly and the previous one suggests that the source of the new anomaly lies to the west of probe 2 the heat flux of these non buoyant effluents is estimated to range from 30 to 100 kw m 2 which is of the same order as direct estimates of diffuse flow at the tag mound it suggests that a significant amount of diffuse effluent is laterally advected by the prevailing current near the seafloor

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

temporal variations in diffuse hydrothermal flow at tag we have obtained direct measurements of diffuse hydrothermal effluent temperature velocity and chemistry from the tag hydrothermal mound 26 08 n 44 49 w mid atlantic ridge we report on the time series physical measurements obtained from a location on the eastern flank of the main summit complex several features were observed including strong tidal variability in flow a conspicuous elevation of effluent temperatures during the period 11 october 17 october 1994 and a long term increase in temperature superimposed upon all other trends the six day long temperature excursion is coincident with increases in sediment and effluent temperatures observed by instruments left in place on the tag mound by other workers we have established through time lapse seafloor video observations and through drilling records that the mid october effluent temperature excursion is correlated closely with local drilling activity by the ocean drilling program

flexural and stratigraphic development of the west indian continental margin subsidence curves for 27 wells from the western continental margin of india show a characteristic late oligocene to early miocene 24 5 ma rapid increase in subsidence rate superposed on the long lived slow subsidence typical of the thermal subsidence phase of passive continental margins by subtracting a best fit negative exponential subsidence from the observed subsidence curves we obtain an estimate of the distribution and magnitude of the excess subsidence affecting the neogene development of the west indian margin the magnitude of this excess subsidence increases seaward from the coast ranging from a few meters to 2000 m near the shelf edge we examine the following hypotheses to explain the distribution and timing of this excess subsidence 1 modification of basin stratigraphy due to the effects of lithospheric in plane compression 2 creation of accommodation space on the margin by flexural effects associated with indus fan loading and 3 rapid growth of the continental margin and associated flexural effects of the three hypotheses tested the least important mechanism to account for the observed excess subsidence is that of variations of lithospheric in plane force principally because maximum in plane compression within the indo australian plate was only achieved in the late miocene because indus fan sediment deposition began in the late oligocene to early miocene we investigated three dimensional flexural effects associated with fan loading as a cause of the excess subsidence beginning at 24 ma the distribution and magnitude of modeled flexural deflection however are not consistent with the observed excess subsidence interpretation of seismic reflection data indicates that the margin has aggraded and prograded by 100 km basin ward since the oligocene therefore we evaluate the flexural effects of this margin growth by estimating the amount of space infilled by margin progradation and aggradation since 24 ma and computing the resulting deflection this deflection matches the distribution and magnitude of observed excess subsidence along the margin in addition the distribution of the flexural bulge predicted from the combined deflections due to indus fan and margin loading is spatially coincident with the distribution of exposed marine terraces and drainage divides in the saurastra peninsula and the regions surrounding the gulfs of cambay and kutch respectively available gravity seismic reflection refraction and well data are consistent with our prediction of a 4000 to 5000 m thick sediment load developed during the neogene along the outer margin we propose that flexural deformation due to sedimentary loading provides a potential tectonic feedback mechanism that affects coastal and fluvial depositional processes as regions in close proximity to the load are depressed regions farther from the load experience uplift i e the peripheral bulge which is sufficient to cause subaerial exposure of large portions of the shelf and to modify existing drainage networks this feedback represents a mechanism for inducing relative sea level changes without invoking glacial eustasy

characterization usage and provenance of building rocks in the fortress of sao jose of macapa amazon brazil abstract the use of rocks in historical buildings is recurrent and in the amazon it denotes the use of local materials or even those from other continents in the military fortifications the most usual is the use of easily accessed rocks found in natural outcrops these structures were pioneered in american cities colonized by europeans historical building restoration involves knowledge of materials and the original quarry sources with respect to fortresses is important to understand how rock is used in different functions and treatments studying the fortress of sao jose de macapa in amapa brazil which is exemplary of the 18th century this paper aims to characterize the rocks in military fortifications according to their architectural applications and their raw material source for this characterization mapping and mesoscopic analyses were carried out in association with the analytical techniques of optical microscopy scanning electron microscopy sem with an analytical system eds x ray diffraction xrd x ray fluorescence xrf and compressive resistance the results indicated the use of sedimentary rocks and lateritic crusts from the geological environment of the implantation site and its surroundings however in the same geological context they are classified as ferruginized sandstones fss ferruginous lateritic crusts flc and iron stones ist all of which are primarily composed of goethite hematite quartz and kaolinite due to their compositional textural and physical characteristics it was possible to correlate their uses and functions in the fortress which indicated a formal and constructive logic that suited the raw materials available to the builders

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

characterisation of monzogranitic batholiths as a supply source for heritage construction in the northwest of madrid abstract the church of the assumption of our lady in valdemorillo madrid spain is almost entirely built with granitoids the studies and analyses were focused on the oldest part of the building ghosts corner which will be restored and rehabilitated in the near future homogeneous monzogranite is the predominant granitoid used in the ghosts corner followed by granitic porphyry porphyrytic monzogranite gneiss and leucogranite the original quarries that supplied homogeneous monzogranites for the first construction stages correspond to blocks present in the monument s surroundings quarries that supplied the granitic porphyries correspond to the dykes located to east southeast of valdemorillo which are oriented n s in relation to their intrusion direction the combined use of ultrasonic velocity and schmidt hammer techniques allowed first selection of the most representative blocks and ashlars for sampling this reduced sampling to a minimum but yielded representative results for the whole building second the combined values were used to compare results for the building stones of the church to rocks from the surrounding quarries stones from the building showed lower ultrasonic and schmidt hammer values and higher porosity and water saturation values compared to the original quarry rocks the anisotropy indices are directly related to the decay processes in the stone spalling of homogeneous monzogranite and grain disintegration in granitic porphyries finally the methods applied are inexpensive easy to use and non destructive techniques very advisable properties when working with built heritage

unfolding impacts of freaky tectonics on sedimentary sequences along passive margins pioneer findings from western indian continental margin offshore indus basin abstract the western passive continental margin of the indian plate wcmi serves as an elegant laboratory for understanding the syn sedimentary rift drift tectonics preserving a variety of episodic structural deformations and complex sedimentary infill here we present a multidisciplinary study on offshore indus basin oib that is less investigated compared to adjacent marginal basins along wcmi reflection seismic gravity borehole and biostratigraphic data have been used to reconstruct the tectono sedimentary history of the meso cenozoic succession in oib seismic based dynamic stratigraphy helped in mapping seven different depositional sequences grounded on morpho tectonic considerations the seismic expression of these sequences provided an idea of the depositional styles with different tectonic episodes since rifting in jurassic to basin inversion in miocene continued to present time the oib initially filled from the north with the sediments rapidly prograding southwards in general the basin deepens in the central and northwestern part where the formations achieve maximum depths the analysis of 3d structural models revealed that the region experienced poly phase deformation from the late cretaceous to recent times that include steeply dipping normal faults horst and graben half graben half graben with growth faulting flower structures anticline syncline and uplift with an angular unconformity the most remarkable tectonic activity happened at the early miocene and resulted in the reactivation of extensional faults into strike slip faults giving rise to a spectacular structural inversion due to transform movement between indian and arabian plates that continues to present day the most predominant faults are oriented in s20 50 e with azimuth as 120 160 and the maximum dip angles are 80 in the southwest flank of the basin the main fault trend is s20 e n20 w and dip angles ranging between 40 and 80 and above a thick crust and the ocean continent transition oct is imaged with massive felsic mafic and ultramafic intrusions by the help of forward gravity modeling an initial onset of syn rift tectonic subsidence between 90 and 68 ma with a post rift general increase between 68 and 54 ma is comprehended while two uplift events at 40 ma and 18 ma are documented through backstripping analysis it is inferred that the region is rapidly under complex structural inversion with growing folding faulting and flowering structures this study could help in explaining the unanswered reasons of structural complexities depositional styles and basin inversion along worldwide passive margins

near field variability of the tag non buoyant plume 26 n mid atlantic ridge we report the initial results of a nine hour monitoring of the non buoyant plume above the tag hydrothermal vent field at 26 n on the mid atlantic ridge a ctd conductivity temperature and pressure and nephelometer particle sensor were continuously raised and lowered through the non buoyant plume for nine hours at a site situated 100 m south of the hydrothermal mound this method permits a detailed record of the variability of the non buoyant plume close to the vent field to be made our results show that the particulate maximum of the non buoyant plume appears 150 m lower in the water column at the start and end of the record compared with profiles recorded towards the middle of the record a corresponding structure also appears in the temperature and salinity records with the result that the plume particle maximum resides on the 27 8035 isopycnal throughout modelling of the rise height of an axisymmetric plume can only account for 95 m of the variability in the final level of neutral buoyancy plume bending due to currents is a likely explanation for the recorded temporal variability

poisson s ratio structure of young oceanic crust we have applied full waveform inversion to wide aperture seismic reflection data from the southern east pacific rise near 14 s the data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the p and s wave velocities vp vs and hence poisson s ratio and seismic attenuation qp qs structure of seismic layer 2 layer 2a is highly attenuating qp 18 30 and qs 8 15 and layer 2b is moderately attenuating qp 30 50 and qs 20 25 our results show high at the seafloor and in layer 2a 0 48 across the top of the 2a b transition the rapid increase in vp is accompanied by a sharp drop in to 0 25 within just 200 m of the seafloor we perform simple calculations to gain an insight into the porosity and crack distribution with depth these calculations suggest that porosity is in excess of 30 in layer 2a but reduces to 6 7 at the top of the 2a b transition and to about 5 at a depth of 600 m below seafloor within layer 2b our results suggest that there is an increase in the average aspect ratio with depth across the 2a b transition the most likely explanation is that numerous thin cracks either mechanically close or are infilled at depth our results show there to be an abrupt change in the pore structure across the 2a b transition which is consistent with a lithologic transition from extrusives to dykes but is equally consistent with a transition mechanical or hydrothermal within the extrusive pile

tectonics of the bay of bengal new insights from satellite gravity and ship borne geophysical data abstract recently released satellite derived free air gravity anomalies and the existing ship board geophysical data provide new insights into the tectonics of the bay of bengal with respect to the structure and regional extension of the buried 85 e ridge and the tectonics of the eastern continental margin of india ecmi the 85 e ridge can be visualized extending inland via the mahanadi basin volcanics to the rajmahal traps a large volcanic province in eastern india encompassing the rajmahal and sylhet traps and volcanics in the bengal and mahanadi basins almost on the scale of the deccan volcanic province along the west coast can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 ma the 85 e ridge represents the deep ocean volcanic trace of this magmatic activity towards the south the ridge continues in an arcuate manner to the afanasy nikitin seamount at equatorial latitudes in the central indian ocean gravity models of the ridge are indicative of hotspot related crustal underplating processes beneath the ridge the ecmi can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data which bear similarities with the conjugate east antarctica margin

present day motion along the owen fracture zone and dalrymple trough in the arabian sea all plate motion data available for describing the present day motion between the arabian and indian plates are analyzed to understand the present day tectonic role of the presumed plate boundary the owen fracture zone and dalrymple trough in the northern arabian sea all prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary in particular published global plate motion models predict some convergence across the owen fracture zone and dalrymple trough convergence along the owen fracture zone is contradicted by earthquake focal mechanisms which show right lateral strike slip convergence along the dalrymple trough is contradicted by observed normal faulting in the trough and by normal and strike slip focal mechanisms for earthquakes occurring along or near the trough the sense and rate of motion is further constrained by spreading rates along the sheba ridge northwest of the owen fracture zone and along the carlsberg ridge southeast of the owen fracture zone to estimate the present day motion we determine 43 3 m y average spreading rates from sheba ridge carlsberg ridge and central indian ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the arabia india boundary we determine an arabia india euler vector that predicts 2 mm yr of right lateral strike slip motion along the owen fracture zone with a 95 confidence interval of 0 7 mm yr which excludes the faster rates of slips predicted by several prior models the direction of motion is 35 50 counterclockwise from that predicted by published global plate motion models if we omit the data along the arabia india boundary the motion differs insignificantly from zero suggesting that magnetic profiles from the carlsberg and sheba ridges give spreading rates too imprecise to detect the slow arabia india motion if the owen fracture zone and dalrymple trough are regarded as a plate boundary it is the slowest slipping plate boundary known on earth with motion about 100 times slower than the fastest slipping faults along the east pacific rise

structural modelling of regional depth profiles in the voring basin implications for the structural and stratigraphic development of the norwegian passive margin interpretation of key regional seismic lines across the voring basin has identified a number of seismic megasequences that have provided a framework for further basin analysis these megasequences have been classified as pre syn and post rift with respect to the late cretaceous early tertiary phase of extension forward and reverse structural and stratigraphic modelling of two of these regional lines has been performed in an attempt to test and validate the structural and stratigraphic interpretations of the syn and post rift megasequences reverse thermal subsidence modelling flexural backstripping of these lines to the base tertiary was performed using a laterally varying beta factor across the sections a number of features on each of the depth profiles are considered to be regional subaerial datums for the backstripped sections the backstripped templates were then forward modelled using the flexural cantilever model the forward models calculate a laterally varying beta profile along their length which can be compared to the reverse models thereby ensuring an internally consistent structural model between both the reverse and forward modelling techniques the modelling work has highlighted the existence of a significant discrepancy between the extension measured from the forward models fault heaves and the extension needed to produce geologically acceptable backstripped profiles this observation is of particular importance as it may place a significant constraint on the mode of lithospheric extension that can be used to explain the evolution of the norwegian continental margin

the tectonic evolution of western india and its impact on hydrocarbon occurrences an overview abstract the largest hydrocarbon accumulations of india were localized in western india by a unique convergence of tectonic events mesozoic rifting along the structural trends of proterozoic mobile belts formed the passive margin basins of the indian coast during the campanian rifting of madagascar from india north south striking normal faults propagated into the area of the cambay graben late maastrichtian doming over the incipient deccan reunion hotspot imparted extensional stresses to the northwestern indian coast and formed the fault block that became the bombay high after eruption of the deccan flood basalts in early paleocene extension continued in the weakened crust this resulted in enhanced subsidence of the cambay graben and surat danahu depression rifting of the seychelles microcontinent from india and reactivation of normal faults on the emergent bombay high the cambay graben and the surat danahu depression filled with organic rich source shales as they underwent thermal subsidence shallow water eocene to miocene carbonates developed on the slowly subsiding bombay platform and sea level oscillations fostered secondary porosity development late tertiary maturation of the surat danahu depression shales generated hydrocarbons that migrated into the carbonate reservoirs on the bombay high konkan kerala and the older basins of the east coast of india were unaffected by the post campanian tectonic events and lack the favorable play elements that characterize the giant fields

palaeocene uplift and eocene subsidence in the northern north sea basin from 2d forward and reverse stratigraphic modelling structural and stratigraphic basin modelling has been used to examine quantitatively the cretaceous and tertiary post rift subsidence history of the northern north sea basin following late jurassic extension 2d forward and reverse syn rift and post rift modelling has been used to determine the magnitude and timing of departures from the mckenzie post rift thermal subsidence trend for regional stratigraphic profiles in the outer moray firth the south viking graben and the north viking graben observed stratigraphy has been reverse post rift modelled using flexural backstripping combined with decompaction and reverse thermal subsidence calculations and forward modelled through the syn rift and post rift period using the flexural cantilever model of continental rift basin formation long term global eustasy has been included in the analysis only reliable palaeobathymetric markers such as erosion surfaces and coals have been used to constrain subsidence history both forward and reverse 2d modelling show that a palaeocene regional uplift followed by a rapid eocene subsidence must be superimposed on mckenzie post rift cretaceous and tertiary basement subsidence in order to generate the stratigraphy observed the magnitude of palaeocene uplift deduced from modelling was of the order of 375 m in the outer moray firth central north sea increasing to 525 m in the north viking graben northern north sea rapid early eocene subsidence was of the order of 160 310 m the most likely mechanism for regional palaeocene uplift and rapid eocene subsidence in the northern north sea was the palaeocene development of the iceland plume situated at a distance of 700 900 km from the study area in the early tertiary forward and reverse modelling predict axial bathymetries in the graben of up to 1300 m for the early cretaceous and 900 m for the upper cretaceous the large palaeobathymetries at the end of the cretaceous provided much of the accommodation space for tertiary sedimentation

tidal pumping of fluids within and from the oceanic crust new observations and opportunities for sampling the crustal hydrosphere abstract seven boreholes established by the ocean drilling program in sedimented ridge crest and ridge flank environments have been sealed to stop the exchange of formation water and seawater via the holes after drilling and instrumented to allow long term hydrologic monitoring records of seafloor and basement fluid pressures in these holes document tidal frequency formation pressure variations that are attenuated and phase shifted relative to variations at the seafloor spatial variations in the response to seafloor tidal loading create oscillatory pressure gradients which drive fluid flow within the crust and tidal pressure differentials between the crust and the water column modulate flow through the seafloor both may have significant geochemical and biological consequences the magnitude of the tidal pressure differential observed across sediment sections is in many instances larger than the average differential pressure created by hydrothermal buoyancy forces and this causes the pressure difference across fluid sampling ports incorporated in the borehole seals to reverse sign periodically this can create a problem for obtaining undiluted crustal fluid samples at the seafloor from moderately overpressured holes but it also provides an opportunity for fluids to be pumped passively from moderately underpressured holes through the use of uni directional valves potential production rates estimated using pre and post seal thermal fluid flow and pressure data indicate that significant fluid fluxes can be established of the order of 10 m 3 typically one hole volume per month

time series measurements of hydrothermal activity on northern juan de fuca ridge during august september 1984 the submersible alvin visited a large active hydrothermal system on the juan de fuca ridge a time lapse camera taking photographs every 42 minutes for six days was deployed at an active vent to characterize the dynamic nature of this system this report documents the growth rate of a low temperature measured at 56 c hydrothermal deposit and establishes that this growth is not constant periods of substantial erosion are superimposed upon the general growth pattern quantitative photographic interpretation of the suspended particulate load suggests a strong diurnal component to these fluctuations water flow manifest by the movement of attached worm tubes also shows strong variations but on a semi diurnal basis correlated to tidal motions