anisotropy and crustal thickness of northern tibet new constraints for tectonic modelling in 1993 a seismological array was installed on a profile along the golmud lhasa highway from the qaidam basin to central tibet a travel time analysis for local and regional earthquakes was performed in order to better constrain the crustal structure obtained by teleseismic tomography more information about the crustal thickness and anisotropy comes from the analysis of moho converted p to sv waves the two methods supplement each other and confirm the existence of crust thinner than expected accross the northern edge of the tibetan plateau and qaidam p to sv splitting analysis yields a striking similarity between the fast polarization direction and the velocity field of present day deformation as inferred from kinematic modelling the two observations the thinning of the crust and the clockwise rotation of the fast polarization direction emphasize the leading role of eastward extrusion for the absorption of the large amount of crustal shortening occuring in central asia

influence of sedimentation on enrichment of manganese and growth of ferromanganese micronodules bengal fan india abstract based upon down core variations in o mn ratio in sediment and in micronodules mn content organic carbon and abundance of micronodules in two box cores from high and low sedimentation regions of the bay of bengal 0 20 and 0 40 cm levels were identified as mn oxidation zone in the western and the eastern regions of the bay despite similarity in bottom water o 2 and organic carbon contents of the region a higher abundance of micronodules down to the deeper levels was observed in the area of low sedimentation but in the upper surfaces increased enrichment of mn was observed in core 1 located in the area of higher sediment accumulation rapid recycling of mn faster burial transfer to mn reduction zone mn dissolution and its upward diffusion re precipitation increasing upward flux of mn ii and associated higher consumption of o 2 limits the width of mn oxidation zone and appears to be responsible for enhanced enrichment of mn in the area of high sedimentation the results of the present study also suggest that though the availability of mn controls the formation of micronodules their resident time in mn oxic zone regulates their growth and influences their morphology

magnitude of late quaternary left lateral displacements along the north edge of tibet images taken by the earth observation satellite spot of the quaternary morphology at 18 sites on the 2000 kilometer long altyn tagh fault at the north edge of tibet demonstrate that it is outstandingly active long term left lateral strike slip offsets of stream channels alluvial terrace edges and glacial moraines along the fault cluster between 100 and 400 meters the high elevation of the sites mostly above 4000 meters in the periglacial zone suggests that most offsets resulted from slip on the fault since the beginning of the holocene these data imply that slip rates are 2 to 3 centimeters per year along much of the fault length and support the hypothesis that the continuing penetration of india into asia forces tibet rapidly toward the east

heavy rare earth element enrichment in archean felsic veins examples of heavy rare earth element ree enrichment are described from granitic and trondhjemitic veins from the lewisian complex at scourie and gruinard bay the observed enrichment results from the accumulation of phases rich in heavy ree such as garnet zircon and allanite during the emplacement of the veins and is probably controlled by filter press fractionation the melt composition exerts a strong influence on the partitioning of the heavy ree into these phases therefore heavy ree enrichment may only be seen in high silica melts

the composition and diagenesis of interstitial solutions ii fluxes and diagenesis at the water sediment interface in the high latitude north and south atlantic studies of the major element composition of in situ sampled pore waters are reported for the north atlantic and southern ocean between africa and antarctica the pattern of diagenetic modification of pore water composition is similar throughout the entire atlantic enrichment of na ca2 and alkalinity and depletion of mg2 and k are nearly universal only siliceous oozes consistently provide very limited evidence of cation diagenesis the changes observed and the calculated fluxes across the seawater sediment interface are much the same as those reported previously for other areas of the atlantic and caribbean fluxes of the major cations across the interface continue to be indicated as a major factor in the geochemical cycling of these elements particularly na mg2 and k diagenetic modelling indicates that aerobic oxidation of organic matter and consequent dissolution of caco3 is a dominant reaction throughout the north atlantic the data indicate that o2 oxidation to at least 30 cm is prevalent at nearly all stations dissolution of caco3 in response to the introduction of metabolic co2 can lead to significant post depositional modification of the sediments the modelling also indicates a 1 1 stoichiometric relationship between na release and mg2 uptake by sedimentary components although dissolved silica concentrations in biogenic siliceous sediments are among the highest yet reported 700 m calculations demonstrate that solubility control cannot be through equilibria with the mineral sepiolite further the influence of cation silicate surface phases generally upon solubility is contraindicated by si oh 4 mg2 h relationships evaluation of the influence of fluid advection on pore water profiles indicates that at the stations studied in the north atlantic it is small a more general consideration of the potential contribution of fluid advection to shaping interstitial water profiles demonstrates that advection can be a dominant factor in such circumstances serious misinterpretation of the nature of diagenetic reactions may result from a lack of knowledge and consideration of fluid advection

a new report on the occurrence of zeolitites in the abyssal depths of the central indian basin abstract forty two indurated slabs of zeolites collected from the abyssal depths of the central indian basin have been studied the slabs show characters similar to harmotome rich scorias of the society ridge and phillipsite slabs in the pacific ocean these slabs are described as zeolitites in the present paper as they represent dominantly the phillipsite harmotome group of minerals ferromanganese oxides occur as specks and dendrites over these slabs also the zeolitic material form nuclei for ferromanganese nodules the zeolitites exhibit various degrees of weathering which seems to have no effect on the mineralogy pumice clasts are entrapped within the zeolitites bioturbation is evident from the intense macro and micro burrowing structures and porous nature of the samples geographical distribution of zeolitites in the basin is restricted to south of 10 s further they closely occur in the vicinity of major fracture zones and seamounts the transformational conditions from volcanic precursors to zeolitites are suggested

post depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east atlantic abstract two cores from a ne atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age thickness and composition diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes the diagenetic process is thought to be driven by bacterially mediated organic oxidation and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process in one core a long turbidite section emplaced 330 000 years ago is overlain by clay organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a u concentration peak below the oxidation front marked by a colour change the u source is believed to be u initially associated with the organic matter in the oxidised section of the turbidite vanadium and cu behave similarly but upward migration of the redox sensitive metals mn and ni is also seen in the second core an 8 cm turbidite section was emplaced about 170 000 years ago in a clay column in this case organic consumption now appears complete but evidence for diagenetic effects is found in a 16 cm compositional alteration halo of the underlying pelagic clay fe ii has been enriched and the hydrogenous component of mn co ni and cu removed from the pelagic clay to form the halo

characteristics of manganese nodules from the central indian basin relationship with the sedimentary environment the hydrogenetic and diagenetic manganese nodules from the cib show an unequivocal genetic relationship with neogene sedimentological patterns hydrogenetic nodules were observed at the top of the sedimentary column and interstratified in thin sequences of biosiliceous sediments and underlying miocene red clays in cores from topographic highs early to late pliocene radiolarian fauna were found at the base of the biosiliceous deposits these condensed biosiliceous sediments are less than 4 0 m thick and periods of non deposition and or erosion may have occurred in the central deep area 11 15 s significant biogenic deposition began during the early to late pliocene as a result of the development of a high biogenic productivity belt to the south producing the favourable conditions for diagenetic accretionary processes for mn nodules this indicates growth of hydrogenetic nodules prior to the late pliocene and the subsequent inception of diagenetic accretion formation of the most recent diagenetic nodules is associated with higher rates of sedimentation in the deep sea troughs where the siliceous sediments are much thicker this should promote the diagenetic mobilization of mn and its precipitation under oxic and suboxic conditions nevertheless diagenetic nodules are restricted to the upper 40 cm of sediments younger than 400 000 years suggesting fast growing diagenetic nodules and the dissolution of buried nodules below the first 50 cm of siliceous oozes hydrogenetic nodules are formed by dense layers of mno2 fe oh 3 and amorphous silica and they show smooth texture and botryoidal shapes analysis of bulk and selected portions of layered crust or rind shows very low mnfe ratios 2 and relatively high co and ti contents in most cases the cores of hydrogenetic nodules are fragments of altered ash layers from the indonesian volcanic arc which is located more than 2500 km to the east the ash layers are also interstratified with miocene red clays diagenetic nodules show a granular texture and comprise concentric layers with dendritic or columnar structures mainly of 10 a manganate higher mn ni and cu contents are observed in this phase late diagenetic enrichment of these nodules is continuous within the upper 40 cm of recent ni that enhances the stability of early diagenetic 10 a manganate buserite and c the formation of todorokite some nodules referred to as mixed in the literature show hydrogenetic internal layers which grade outward to a diagenetic crust this transitional character suggests slow changes in the sedimentary environment as growth advances the nodules of the cib and the east equatorial pacific eep are similar in terms of both metallic composition and accretionary patterns however the nodules of the cib are considerably younger and present a continuous accretionary history the history of the manganese nodules from the cib is related to the post late miocene sedimentary and geodynamic evolution of this basin

abyssal sediment erosion in the central indian basin evidence from radiochemical and radiolarian studies abstract radiochemical analyses of three spade cores collected around 78 e between 10 s and 12 s from the abyssal depths in the central indian basin yield an average accumulation rate of 2 mm ka the ratios of 230th flux in the sediments to its production rate in the overlying water column fa fp are extremely low 0 2 this fact and the absence of collosphaera invaginata first appearance datum 150 200 ka an index radiolarian species of neogene radiolarian zone 1 indicate intense erosion and lateral transport of younger sediments from this region the radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent antarctic bottom water entering the central indian basin through the northern saddles of the ninety east ridge

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

unmelted cosmic metal particles in the indian ocean fe ni metal is a common constituent of most meteorites and is an indicator of the thermal history of the respective meteorites it is a diagnostic tool to distinguish between groups subgroups of meteorites in spite of over a million micrometeorites collected from various domains reports of pure metallic particles among micrometeorites have been extremely rare we report here the finding of a variety of cosmic metal particles such as kamacite plessite taenite and fe ni beads from deep sea sediments of the indian ocean a majority of which have entered the earth unaffected by frictional heating during atmospheric entry such particles are known as components of meteorites but have never been found as individual entities their compositions suggest precursors from a variety of meteorite groups thus providing an insight into the metal fluxes on the earth some particles have undergone heating and oxidation to different levels during entry developing features similar to i type cosmic spherules suggesting atmospheric processing of individual kamacites taenite grains as another hitherto unknown source for the i type spherules the particles have undergone postdepositional aqueous alteration transforming finally into the serpentine mineral cronstedtite aqueous alteration products of kamacite reflect the local microenvironment therefore they have the potential to provide information on the composition of water in the solar nebula on the parent bodies or on surfaces of planetary bodies our observations suggest it would take sustained burial in water for tens of thousands of years under cold conditions for kamacites to alter to cronstedtite

manganese micronodules a possible indicator of sedimentary environments abstract manganese micronodule distribution size internal features and chemical composition as well as the composition of associated sediments have been determined in eight sediment cores taken from the siliceous ooze area of the central indian basin cib micronodule abundance expressed as the number of micronodules per gram of dried sediment 0 375 n g and size 75 1000 m show considerable variation with core depth and are directly related to each other the morphology surface texture and internal features of the micronodules are similar to those of surface macronodules from the same basin micronodules have high contents of mn 35 02 cu 1 6 ni 1 4 and zn 0 37 high mn fe ratios 5 101 and very low fe 1 8 al 2 o 3 1 3 tio 2 0 30 sio 2 2 7 and rare earth element 522 ppm contents compared to surface and buried macronodules micronodule abundance size and composition depend on the redox characteristics of the sedimentary environment and can therefore be used as indicators of the sedimentary environment

a study of azimuthal p residuals and shear wave splitting across the kunlun range northern tibetan plateau abstract the kunlun fault system is an east west transcurrent fault bordering the tibetan plateau that has experienced left lateral motion since 3 ma a linear array of one component and three component seismological stations was deployed across the kunlun range for 4 months p travel time residuals define three basic units along the profile the north kunlun unit the south kunlun unit and the bayan har unit p residuals are a function of azimuth p residuals are fast in the southern stations for southern azimuths and in the northern stations for northern azimuths measurements of shear wave splitting of the s scs and sks phases have been performed on deep events delay times and fast polarization directions have been computed by finding the maximum of the cross correlation between the two horizontal components rotated at different angles delay times from 0 40 s to 2 52 s remain in the range of those found by mcnamara et al 1994 j geophys res 99 13655 13665 at two sites along the same profile p residual travel times and the splitting of s waves are discussed in relation to models of the geological evolution of this region the northern tibetan plateau appears as a juxtaposition of different terranes the azimuthal p residual pattern implies that p wave anisotropy is about 3 in the lithosphere on both sides of the kunlun fault and that fast velocities are oriented outwards from the kunlun fault plane s wave splitting is observed far from the fault and implies that the lithosphere beneath northern tibet has undergone an important deformation

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

joint interpretation of upper mantle anisotropy based on teleseismic p travel time delays and inversion of shear wave splitting parameters abstract we inverted shear wave splitting parameters and simultaneously analysed delays of teleseismic longitudinal waves to obtain a self consistent three dimensional 3 d image of the anisotropic upper mantle beneath the continents efficiency of the simultaneous 3 d analyses of p residual spheres and shear wave polarizations is demonstrated on data from two regions southern sweden and the western usa the anisotropic inferences of the subcrustal lithosphere are to a first approximation represented by homogeneous hexagonal or orthorhombic media with plunging symmetry axes

geochemical evidence of terrigenous influence in deep sea sediments up to 8 s in the central indian basin abstract thirty two representative deep sea sediment samples have been studied from three manganese nodule bearing environments in the central indian basin diatomaceous radiolarian ooze clay pelagic brown clay and calcareous ooze they have been analysed for major and minor elements such as si al ca mg na k fe mn p ti ba cu ni co v zn and cr r mode factor analysis reveals five important sources for these elements 1 detrital loaded with fe ti al mg mn p and k 2 combined hydrogenetic diagenetic mn ni cu co and fe 3 biogenic si 4 seasalts na and mg and 5 dissolution residue ba factor scores distributional trends of detrital elements al ti and fe concentrations and al al fe mn ratios provide new geochemical evidence for the effect of the terrigenous influx from the ganges brahmaputra rivers into the northern part of the basin up to 8 s which inhibits the growth of manganese nodules in this area in contrast incorporation of transitional metals into sediments and manganese nodules in the other two areas of the basin is governed by hydrogenetic and diagenetic processes

on the occurrence of ferromanganese micronodules from the sediments of the bengal fan a high terrigenous sediment input region studies of 13 samples 10 surface sediments and 3 cores from across the middle fan region of the bay of bengal reveal the presence of micronodules area 0 22 million km2 over the entire bay these micronodules have two dominant morphological forms botryoidal in the eastern bay and spheroidal in the central and the western regions of the bay the abundance of the micronodules was found to be higher in the eastern region of the bay in the cores the micronodules were generally found to decrease in abundance down core the core from the eastern region had micronodules down to deeper levels than the western side todorokite is the manganese mineral present in the micronodules associated mineral phases are quartz feldspar and phillipsite these micronodules have high mn 19 20 32 13 rather low fe 1 20 1 83 ni 0 10 1 14 zn 0 10 0 30 and co 0 01 0 04 and their mnfe ratio ranges from 10 55 to 25 42 the contents of mn ni cu and zn and the mnfe ratio decrease down core based upon the variations in the abundance morphology mnfe ratio of the micronodules and the elemental distributions and the omn ratio in the sediments remobilization of mn from deeper levels and its precipitation at the upper surfaces of the sediment column in an oxic environment is suggested the results of the present study also suggest that a high rate of sedimentation is not a limiting factor for the formation of micronodules although it might affect their growth

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

pore waters of the central pacific ocean nutrient results interstitial waters from the manop siliceous and calcareous ooze sites in the central pacific ocean were recovered by shipboard squeezing and centrifuging techniques and an in situ harpoon sampler the pore waters were analyzed for nutrients carbonate system species mn 2 ca 2 total alkaline earth metals and dissolved oxygen siliceous ooze site only in calcareous sediments the co 2 alkalinity po 4 3 and ca 2 results obtained from shipboard techniques are significantly different than those recovered in situ the other species measured as well as all of the data from the siliceous ooze site reveal no significant dependence on the method used to sample the pore water the relative changes in o 2 no 3 mn 2 alkalinity and co 2 indicate that respiration using o 2 no 3 and mno 2 as electron acceptors is the dominant reaction affecting the pore water species we develop a model which simulates the fate of pore water oxygen and nitrate and particulate organic carbon in sediments this model includes the effects of pore water diffusion oxygen and nitrate respiration sediment accumulation sediment mixing and changes in sediment porosity we observe that factor of 2 variations in the rates of sediment mixing and accumulation have only a minor effect on the pore water profiles the major factor controlling the pore water profiles is the flux of organic carbon across the sediment water interface we conclude that the large intra site variability observed in the interstitial water nitrate profiles is probably caused by small scale variations in the flux of organic material

interstitial water sediment geochemistry of n p and fe and its response to overlying waters of tropical estuaries a case from the southwest coast of india the concentrations of n p and fe in surface sediments and interstitial and overlying bottom and surface waters of the ashtamudi estuary located in the southwest coast of india are reported along with the various chemical species of n no2 n no3 n nh3 n and total n and p organic p inorganic p and total p in interstitial and overlying waters and discussed in terms of the physico chemical environment of the system the interstitial water exhibits higher salinity values compared to bottom and surface waters indicating the coupled effects of salt wedge phenomena and gravitational convection of more saline denser marine water downward through surface sediments n p and fe as well as their chemical forms are enriched in the interstitial water compared to bottom and surface waters however the dissolved oxygen do shows an opposite trend the marked enrichment of nh3 n in the interstitial water and its marginal presence in bottom and surface waters together with the substantial decrease in the do concentrations of bottom water and consequent increase in the concentrations of no2 n and no3 n in interstitial and bottom waters points to the nitrification process operating in the sediment water interface of the ashtamudi estuary the enrichment of total n p and fe in the interstitial water compared to the overlying counterparts and the positive correlation of sediment n p and fe with mud contents as well as organic carbon indicate that these elements are liberated during the early diagenetic decomposition of organic matter trapped in estuarine muds

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

early diagenetic processes affecting nutrients in the pore waters of central indian ocean cores abstract pore water nutrients nitrite nitrate phosphate silicate ph and solid phase organic carbon were analysed for one core from the arabian sea and three cores from the manganese nodule area in the central indian ocean basin possible denitrification and unoxidized organic carbon are observed in the arabian sea core while nitrification with intermediate low nitrate and high organic carbon are found in the mottled zones of the pelagic cores mobilization of iron host oxyhydroxides and peaked profiles of organic carbon are related to a gradual increase in the phosphate profiles the asymptotic silica values are less than the 100 saturation levels of amorphous silica and they fall outside the stability field of sepiolite

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

anisotropy beneath 9 stations of the geoscope broadband network as deduced from shear wave splitting polarization of sks scs and s waves has been analyzed on digital 3 components seismograms recorded at 9 stations of the recently installed geoscope broadband network splitting was observed for about 15 records and is interpreted as being due to seismic anisotropy beneath the stations from every record it is possible to retrieve the fast direction at the station and the time difference between the fast and slow polarization directions some 30 observations of quasi linear s pulses also help constrain the direction of fast velocity synthetic seismograms were built to test the validity of the method used to retrieve the parameters of anisotropy typical time differences are about 1 sec corresponding for example to a 80 km thick zone of perfectly oriented pyrolite the fast directions obtained at 3 continental stations in america westford santa cruz heredia could be related to plate tectonic processes on the other hand the fast directions found in 3 oceanic islands kipapa papete la reunion differ strongly from the oceanic plate velocity direction there

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

silica distribution in interstitial waters and sediments from the southeastern pacific abstract analyses for silica in the interstitial water of five cores from the southeast pacific are presented silica is enriched in these interstitial waters resulting in a vertical flux of silica of between 10 and 50 mol cm 2 yr 1 from the sediment into the overlaying seawater this flux is generated by the dissolution of biogenic silica the dissolution of which is increased in areas of bottom water turbulence the si al and calculated opal leinen 1977 contents of the bulk sediment of these cores are also presented small scale variations over depth intervals of tens of centimetres are present as a result of chaning conditions of sedimentation

evidence of himalayan erosional event at 0 5 ma from a sediment core from the equatorial indian ocean in the vicinity of odp leg 116 sites abstract a sediment core collected from an area 100 miles south of the odp leg 116 distal bengal fan in the equatorial indian ocean was investigated for microfossils mineralogy mineral chemistry magnetic susceptibility grain size major minor and rare earth element geochemistry organic carbon and total nitrogen contents in the bulk sediments distinct changes in depositional characteristics including presence of abundant sand sized micas and other detrital minerals occur at two sub surface depths corresponding to 0 5 and probably 0 8 ma time periods the detrital mineral suite of this core resembles that of turbidite unit i sediments of odp cores in the distal bengal fan the core site has received an increased supply of terrigenous sediments at these two time periods the older pulse 0 8 ma stronger than the younger pulse several lines of evidence such as the nature of the mineral suite lower magnetic susceptibility values si al in mica mineral separates major element composition discrimination plots of ca ti versus k ti and k2o al2o3 and la yb ratios suggest a highly metamorphosed source such as higher himalayan crystalline hhc series indicating two events of increased physical weathering and erosion in the himalayan region while the erosional event of 0 8 ma is well known the episode of 0 5 ma was not reported earlier

u pb zircon systematics at gruinard bay northwest scotland implications for the early orogenic evolution of the lewisian complex the gruinard bay area of the mainland lewisian complex comprises a metamorphosed suite of archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves the u pb zircon dating yields ages of 2731 14 ma and 2728 2 ma for two trondhjemite and 2732 4 ma for one granite sample also revealing the presence of large amounts of inherited xenocrystic zircons although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies the textural relations between biotite hornblende quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high ti amphibole formed in hornblende granulite facies structural relations link this metamorphism to a steep northeast trending fabric coeval with the intrusion of the trondhjemites dated at 2730 ma dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 ma this age range reflects new growth during the 2730 ma metamorphic metasomatic events superimposed on older zircon phases which include combinations of xenocrystic cores and magmatic and or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging techniques e g cathodoluminescence alone a pegmatitic vein that escaped the d3 strain and related isotopic disturbances yields a precise age of 2792 2 ma which constrains to some degree the earliest orogenic events in the area age relationships displayed in the central block at scourie badcall and in the gruinard bay area indicate that petrogenetic events in both areas were comparable about 2800 ma and that both areas underwent trondhjemitic magmatism about 2730 ma in contrast at gruinard bay there is no isotopic evidence for a period of high grade metamorphism and magmatism at 2490 2480 ma that drastically affected the scourie block indicating that at this stage the two regions occupied different levels of the crust

growth of diagenetic ferromanganese nodules in an oxic deep sea sedimentary environment northeast equatorial pacific abstract to study the accretion mechanism of diagenetic ferromanganese nodules formed in an oxic deep sea sedimentary environment manganese nodules sediments and pore waters were sampled in the northeast equatorial pacific diagenetic ferromanganese nodules are ellipsoidal to discoidal in shape rough in surface texture enriched in cu ni and 10 a manganite and are ubiquitous on the abyssal plain hydrogenetic nodules are polynucleated irregular in shape smooth in surface texture enriched in co and mno 2 and occur dominantly on seamounts numerous microlayers with diagenetic or hydrogenetic characteristics occur successively in the nodules concentration of nitrate in sediment pore waters indicates that overall redox conditions are oxic however mn oxide grains on the walls of burrows and worm tubes are interpreted to be reprecipitated metal ions supplied from oxygen depleted microenvironments that formed in the oxic sediment column by decomposition of organic matter active mixing of bottom sediments by benthic fauna and bottom currents is indicated by ubiquitous burrows worm tubes and tracks and trails high values in sediment mixing coefficients 1 cm 2 yr and scattering of data points in depth profiles of pore water nitrate concentrations we suggest that diagenetic ferromanganese nodules in an oxic deep sea sedimentary environment grew from remobilized metal ions as well as reprecipitated mn oxide grains which were supplied to the nodules episodically during the stirring of bottom sediments by benthic fauna and intermittent strong bottom current flow

shear wave splitting beneath the central tien shan and tectonic implications 1 shear wave splitting analyses were performed at 30 seismic stations in the central tien shan and its vicinity fast orientation at most of the stations on the tien shan range is ene wsw parallel to the strike of the tien shan it largely reflects a coherent deformation of the lithosphere in response to the ns shortening anisotropy with nne ssw fast axes is observed within adjacent basins around the issyk kul and near the northwestern tarim where the lithosphere is presumably strong less deformed the nne ssw orientation which coincides with the surface velocity of the central tien shan can be best explained by shear between the lithosphere and asthenosphere on a regional scale rather than small scale convection as proposed in previous studies the dextral talasso fergana fault seems to have little effect on anisotropy at its nearby stations suggesting this fault is probably confined to the shallow part of the lithosphere

the oxidation state of manganese in ferromanganese nodules and deep sea sediments from the central indian ocean abstract ferromanganese nodules from different sediment facies including siliceous ooze pelagic clay sediments from transition zone of siliceous and pelagic clay and seamount crusts from the central indian ocean were analysed for chemical composition mineralogy and bulk oxidation state of mn by the iodometric method despite considerable variation in chemical composition mn fe ratio times from 1 35 to 7 42 and mineralogy the o mn ratio or mno x solid hydrous manganese oxide does not vary much from 1 73 to 1 81 suggesting most of the mn in the manganese nodules in mn iv the oxidised upper zone in the sediment cores decreases in thickness from 15 cm in the north 5 s to 11 cm in the southern part 14 s of the central indian ocean iodometric titration of deep sea sediments yields o mn ratios of 1 29 1 95 sediments from the manganese nodule field have maximum solid mn concentrations and lower o mn ratios in the core top 0 3 cm with minimum mn and o mn ratios at 11 cm depth and thus may reflect the mechanism controlling the supply of diagenetic mn to the nodules in contrast sediments from the non manganese nodule field have comparatively low solid phase mn concentrations in the core top 0 3 cm and almost uniform o mn ratios with depth indicating that mn is not preserved in the sediments but may be remobilised and deposited somewhere else

low al and high al trondhjemites in the huai an complex north china craton geochemistry zircon u pb and hf isotopes and implications for neoarchean crustal growth and remelting abstract voluminous tonalite trondhjemite granodiorite ttg magmas were emplaced in the huai an complex of north china craton ncc during the neoarchean here we present the geochemical characteristics of these rocks and identify low al lal and high al hal types of trondhjemites within the ttg suite the lal group displays high silica sio2 77 80 wt and low al2o3 contents 11 06 11 89 wt a strongly peraluminous a cnk 1 16 and 1 19 composition and enrichment in hree lan ybn 4 and 5 gdn ybn 0 35 and 0 9 their low rb 13 and 28 ppm th 0 3 and 7 0 ppm relatively high na2o k2o values 3 6 and 4 1 and mgo 0 63 and 0 68 wt with moderately high mg 36 and 53 suggest crystallization from a primitive source the hal group is characterized by relatively low sio2 67 22 71 57 wt and high al2o3 contents 16 35 16 41 wt with trace element contents and distribution patterns closely comparable to those of archean high al ttgs our geochemical data do not favor a direct genetic link between the lal and hal trondhjemite groups the zircon u pb data reveal that the lal and hal groups represent coeval intrusions at ca 2 55 2 5 ga in situ analysis of hf isotopes in the zircons ehf 5 6 to 10 and tdm 2462 2595 ma together with the geochemical features of the low al trondhjemites suggest that they were generated under relatively low pressure

teleseismic shear wave splitting and lithospheric anisotropy beneath and across the altyn tagh fault sixteen 3 component 5 sec stations along a profile crossing the altyn tagh fault atf at 90 e were used to measure shear wave splitting beneath tibet s northern edge splitting delay times t of 0 3 to 1 3 sec show a marked increase in a narrow zone centered on the fault implying anisotropy in the lithospheric mantle fast polarization directions fpd swing counterclockwise from a mean azimuth of 110 e in the qaidam basin south of the fault to a mean azimuth of 60 e in the altyn mountains this swing occurs in less than 40 km both the t increase and fpd swing are consistent with ductile sinistral shear along the 40 km wide low p wave velocity anomaly 6 8 imaged to a depth of 140 km beneath the fault by teleseismic tomography this confirms that the atf guides the northeastwards extrusion of the tibet qaidam s lithosphere relative to the tarim s

bacterial response to contrasting sediment geochemistry in the central indian basin in order to investigate whether geochemical physiographic and lithological differences in two end member sedimentary settings could evoke varied microbe sediment interactions two 25 cm long sediment cores from contrasting regions in the central indian basin have been examined site tvbc 26 in the northern siliceous realm 10 s 75 5 e is organic c rich with 0 3 0 09 total organic carbon site tvbc 08 in the southern pelagic red clay realm 16 s 75 5 e located on the flank of a seamount in a mid plate volcanic area with hydrothermal alterations of recent origin is organic c poor 0 1 0 07 significantly higher bacterial viability under anaerobic conditions generally lower microbial carbon uptake and higher numbers of aerobic sulphur oxidizers at the mottled zones characterize core tvbc 26 in the carbon poor environment of core tvbc 08 a doubling of the 14 c uptake a 250 times increase in the number of autotrophic nitrifiers a fourfold lowering in the number of aerobic sulphur oxidizers and an order higher of denitrifiers exist when compared to core tvbc 26 this suggests the prevalence of a potentially autotrophic microbial community in core tvbc 08 in response to hydrothermal activity microbial activity at the northern tvbc 26 is predominantly heterotrophic with enhanced chemosynthetic activity restricted to tangreen mottled zones the southern tvbc 08 is autotrophic with increased heterotrophic activity in the deepest layers notably the bacterial activity is generally dependent on the surface productivity in tvbc 26 the carbon rich core and mostly independent in tvbc 08 the carbon poor hydrothermally influenced core the northern sediment is more organic sink controlled and the southern more hydrothermal source controlled hydrothermal activity and associated rock alteration processes may be more relevant than organic matter delivery in these deep sea sediments thus this study highlights the relative importance of hydrothermal activity versus organic delivery in evoking different microbial response in the central indian basin sediments

the indian ocean nodule field petrotectonic evolution and ferromanganese deposits abstract the indian ocean nodule field ionf is significant from several points of view roughly bordered by 10 s to 16 30 s and 72 e to 80 e and located within the central indian ocean basin ciob the field hosts the world s second largest and second high grade manganese nodule deposit after the equatorial nodule belt in the north pacific ocean moreover the crust underlying this field is characterised by unique morphotectonic signatures owing to its formation between 60 and 49 ma under three variable spreading conditions fast intermediate and slow from the indian ocean ridge system iors the nodule field has been surveyed both extensively more than 0 4 million km2 area and intensively comprising of a large geophysical data set and geological sample inventory during the last two decades several morphotectonic features such as seamounts hills ridge normal lineaments and ridge parallel lineations have disturbed the apparently smooth topographic gradient 1 7000 of the seafloor here variations in the rate of spreading and formation of new oceanic crust along the ridge crest during more than one episode of india eurasia collision are imprinted in the ionf based on the nature of the ridge parallel lineations which are related to the rate of crustal accretion the field is divided in to four sectors a b c and d from north to south sectors a and c were formed at a fast rate of spreading 90 95 mm year half rate and sectors b and d were formed at an intermediate 55 mm year and slow 26 mm year rates respectively the predominance of tensional stress in sectors a and c caused asymmetrical flexuring of the seafloor resulting in widely spaced faults and folds with low amplitude and large wavelength in contrast the seafloor flexuring in sector d are closely spaced long symmetrical and of high amplitude the timing and intensity of the collision of india with eurasia is constrained by the variable intensity of these flexures suggesting probably a soft touch at 58 ma and the hard collision at about 51 ma the nodule field hosts several seamounts both as isolated entities and in linear chains which are arranged parallel to the flow lines along the direction of absolute motion of the indian plate the distribution morphology and growth patterns of a majority of these seamounts are related to spreading rate suggesting their formation at the ridge crest however many of the seamounts show more than one stage of growth with local intraplate volcanism contributing to the enlargement of the larger ancient seamounts varieties of volcanics such as tholeiitic basalts spilites ferrobasalts and pumice occur within the ionf the alteration of some of these volcanics has resulted in palagonitisation of the glass and formation of zeolites subsequently during its journey away from the ridge crest to the abyssal areas the crust underlying the nodule field witnessed intraplate volcanism this is evident from the addition of younger rocks at the base of the ancient seamounts inconsistent growth of volcanoes eruption of ferrobasalt corresponding to the areas of high amplitude magnetic zones and presence of volcanogenic hydrothermal materials vhm of 10 ka age these findings collectively hint that the ionf is geodynamically unstable and may have been volcanically active in the recent past during the last 8 ma growth of authigenic ferromanganese deposits in the form of manganese nodules and crusts has occurred in the nodule field the deposits occur at an average water depth of 5000 m the basinal geomorphology intraplate tectonic deformations and volcanic eruptions considerably influence the formation development morphology mineralogy and composition of these deposits the data show that the large seamounts reverse faults and fracture zones fzs supply rock fragments as seeds for the nodule formation the hydrogenous precipitation from the overlying water column is the primary source of metal accumulation in the nodules the secondary relatively weak intraplate eruptions along the base of ancient seamounts or lineaments subsurface igneous activity and diagenetic remobilisation of metals have also played significant role in the growth and enrichment of the deposit based on a large data set we estimate the contribution of various physico chemical parameters and model the probable conditions of formation of the ferromanganese deposits in the ionf the model also hints at the location where the resource exploitation should be rewarding

rare earth element patterns of the central indian basin sediments related to their lithology rare earth element ree concentrations have been determined in terrigenous siliceous nodule barren and nodule bearing calcareous and red clay from the central indian basin the bulk distribution of ree and in particular the relative cerium fractionation in these surface sediments has been studied in relation to bottom water conditions ree fractionations within a single depositional environment are found to be characteristic for each sediment type with flat shale normalized patterns associated with terrigenous sediments positive ce anomalies with siliceous sediments negative ce and positive eu anomalies with calcareous sediments and lree depleted patterns with pelagic red clays there is no relation of any ree fractionation to bottom water redox conditions our results indicate that the ree signatures in marine sediments are not only related to depositional setting but also to the lithological variations and surficial diagenetic processes therefore these further constraints on understanding ree behaviour indicate that ree fractionations for paleotectonic and paleoredox reconstructions should be used with caution

interstitial water and hydrochemistry of a mangrove forest and adjoining water system south west coast of india eh ph salinity total alkalinity dissolved o2 no2 po4 3 sio2 and nh4 of waters from a mangrove forest an estuary and a creek connecting the mangrove forest and the estuary have been measured further the chemistry of interstitial waters of surficial and core sediments from the mangrove forest have been analyzed for the above parameters except dissolved oxygen to understand the flux of nutrients from the mangrove forest to the adjoining estuary creek waters were monitored during tidal phases po4 3 sio2 and nh4 were found to be at elevated levels in mangrove waters whereas no2 shows no variation compared to the estuary dissolved o2 is low in mangrove waters po4 3 nh4 and sio2 are several times higher in interstitial waters than in overlying waters several fold enrichment of po4 3 nh4 and to some extent sio2 were measured in creek waters during ebbing relative to flooding indicating that mangroves act as a perennial source for the above nutrients

seismic anisotropy beneath tibet evidence for eastward extrusion of the tibetan lithosphere strong seismic anisotropy beneath tibet has recently been reported from the study of sks shear wave splitting the fast split waves are generally polarized in an easterly direction close to the present day direction of motion of the tibetan crust relative to stable eurasia as deduced from holocene slip rates on the major active faults in and around tibet this correlation may be taken to suggest that the whole tibetan lithosphere is being extruded in front of indenting india and that the anisotropic layer is the deforming asthenosphere that accommodates the motion of the tibetan lithosphere relative to the fixed mantle at depth uncertainties about this motion are at present too large to bring unambiguous support to that view assuming that this view is correct however a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer the observed delay times would require a 50 100 km thick anisotropic layer beneath south central tibet and an over 200 km thick layer beneath north central tibet where particularly hot asthenosphere has been inferred this study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

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

width of mantle deformation across a continental transform evidence from upper mantle pn seismic anisotropy measurements seismic anisotropy v p of at least 6 5 2 5 in the upper mantle pn phase is reported 200 km east of the alpine fault in south island new zealand when these new data are combined with a previous pn study and published sks splitting results two distinct domains of mantle deformation are inferred within south island in southern south island the deforming zone in the mantle lithosphere is as wide as 335 km whereas in mid south island the deforming zone narrows to 200 km both regions display wider zones of seismic anisotropy than found at other continental transforms of the world the width and orientation of mantle anisotropy in southern south island are consistent with a model of simple shear in a zone that has undergone 800 200 km of right lateral shear displacement a value close to that predicted by geological reconstructions hence displacement can be accommodated by distributed shear without faulting being required in the mantle

the mechanics of continental transforms an alternative approach with applications to the san andreas system and the tectonics of california article i nfo displacement on intracontinental transforms is commonly distributed over a zone several hundred km wide and may incorporate large regions of transtensional and transpressional strain but no consensus exists on what controls the distribution and style of this deformation we model the transform boundary as a weak shear zone of finite length that exerts shear stress on the deformable continental lithosphere on either side strain rate decreases away from the shear zone on a scale related to its length force balance in this system requires lateral gradients in shear strain rate to be balanced by longitudinal gradients in stretching rates which create zones of lithospheric thickening and thinning distributed anti symmetrically about the shear zone simple analytical estimates two dimensional 2d spectral models and 2d 3d numerical models are employed to study the spatial scales and magnitudes of the zones of uplift and subsidence using reasonable parameter values the models yield geologically relevant rates strain rate components inferred from the gps determined 2 d velocity field and analysis of seismicity using kostrov s method taken together with the geological data on the distribution of active faults uplift and subsidence suggest that the distribution and rates of active deformation in california are consistent with our predictions this validates the assumptions of the continuum approach and provides a tool for predicting and explaining the tectonics of california and of other intracontinental transform systems

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones