geochemical and mineralogical control of different genetic types of deep sea nodules from the pacific ocean deep sea nodules from the northeast pacific nodule belt and the southeast pacific sonne basin being formed in areas bordering the equatorial zone of high biological productivity accumulate by two basically different growth processes a early diagenetic growth by supply from pore water and b hydrogenetic growth by supply from near bottom sea water these growth processes lead to different genetic types of nodules early diagenetic type a hydrogenetic type b and mixed type ab a further type ac very rich in mn is being formed by increasing influence of early diagenesis these types can clearly be distinguished by their shapes surface textures mineral constituents of oxide fraction internal microstructures and geochemistry a genetical classification is being proposed on the basis of statistically computed interelement relationships todorokite very poor in fe is the main mn phase in the early diagenetic substance mno2 intimately intergrown with feooh xh2o is the main phase in the hydrogenetic substance consequently an important difference can be pointed out the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of me2 e g mn2 ni2 cu2 zn2 whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles mobilization of mn2 and fractionation from fe is controlled by the amount of decomposing organic matter in the peneliquid layer of the sediments the main factor controlling the intensity of early diagenesis is the biological productivity in surface waters the crucial point of reversal at a mn fe ratio of about 5 obtained by hyperbolical regression of the analyses of nodules from the southeast pacific represents best concentrations in ni and cu mn fe quotients greater than 5 cause a decrease of ni and cu content nodules from the northeast pacific nodule belt generally contain higher concentrations in cu than nodules from the southeast pacific this can be explained by an additional supply of cu transported below ccd by siliceous plankton

growth rate variations of manganese nodules and crusts induced by paleoceanographic events time markers at about 3 ma 6 5 ma and 14 ma are common to 16 manganese nodules and crusts from different localities in the world oceans these time markers correspond to variations in the growth rates in the inner structure and in the chemistry of the samples the observed time markers could be related to 1 changes of the bottom water velocity and 2 variations in the manganese and iron contents in the deep ocean the time markers were determined by high resolution 10be dating applying the accelerator mass spectrometry facility at the eidgenossische technische hochschule zurich

a low seismic wavespeed anomaly beneath northwestern india a seismic signature of the deccan plume abstract seismic tomography of the indian region using p wave arrival times reveals an approximately cylindrical region of lowered seismic velocities in the upper mantle to the north of the gulf of cambay and the present exposure of the deccan flood basalts this anomaly with low velocities interrupts the characteristically high seismic velocities in the lithosphere beneath peninsular india and extends from shallow depth down to a more extensive low velocity zone beneath 200 km the centre of the low velocity anomaly lies just below the earliest alkaline magmatism which has been directly associated with the deccan plume in the northern portion of the cambay graben these magmatic events precede the main deccan flood eruptions by at least 3 myr the lowered seismic wavespeeds link into the ancient narmada lineament and to some extent to the south but here unfortunately the tomographic image runs out of resolution synthetic tests indicate that such a zone of lowered wavespeeds can be recovered with some loss of amplitude towards the south the association of the current seismic anomaly with the deccan traps is intriguing since the cambay graben remains a place of raised heat flow one explanation would be that the seismic anomaly represents the signature of the initial conduit through which plume material forced its way through the lithosphere beneath the cambay rift the zone of lowered seismic velocities beneath the indian lithosphere might then represent the remains of the initial plume head which would need to have moved with india over the 65 myr since the deccan eruptions

seismic imaging of the aftershock zone of the 2001 mw 7 7 bhuj earthquake india 1 detailed seismic tomography reveals that the 2001 mw 7 7 bhuj earthquake was associated with a 10 14 increase in vp and vs and a 10 decrease in vp vs in the 10 to 35 km depth range covering a 2750 km2 area beneath the aftershock zone this anomaly could be attributed to the existence of a lithological heterogeneity or a pluton of mafic composition that might have intruded during the rifting in early jurassic 160 ma the banni basin and the wagad uplift are found to be associated with high velocity intrusive bodies extending from 5 to 35 km depth a few patches of low vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to a fluid filled fractured rock matrix

radiochemical observations on manganese nodules from three sedimentary environments in the north pacific abstract decay series isotopes were measured on manganese nodules from three sedimentary environments characterized by substrates of red clay r siliceous ooze s and hemipelagic clay h growth rates of nodules are shown to be site dependent 1 2 mm myr at site r 3 8 mm myr at sites s and 20 50 mm myr at site h correlation between growth rate and the parameter mn fe2 suggests that regional diagenetic variations control the compositions and growth rates of the nodules the frequency of nodule turnover and the period of their growth are assessed from the top vs bottom distributions of several nuclides based on the 230th 231pa and 226ra data turnover times of 103 to 105 years are estimated and they vary with relative size and shape of the specimens at a given site the presence of unsupported 210pb and 228th in the top surfaces but not in the bottom surfaces of the surface nodules suggests active growth during the last decades or years despite their old ages the data also indicate that turnover rates are not more frequent than once every several years or decades the 230th 226ra 226ra 210pb and 232th 228th disequilibrium relationships in the nodules allow the migratory behavior of 226ra 222rn and 228ra and their fluxes to be deduced thus radium leaves the top sides but enters the bottom sides of the surface nodules at sites r and s at site r there is a net loss of radium to the sea the opposite is true at site s surface nodules at site h trap radium from both sides probably due to more intense diagenetic input of radium from sediment pore water the effective diffusivities of 226ra in nodules vary from 3 10 12 to 8 10 14 cm2 s dependent on the textural variation of the nodule material which crudely reflects the growth rate and hence sedimentary environment 222rn diffuses out of nodules from all sides with an effective diffusivity of ca 2 6 10 8 cm 2 s at all sites nodules serve as a more effective source of 222rn to sea water than their adjacent sediments the outward flux of 222rn from nodules relative to that from adjacent sediments tends to be higher in more reduced environments an effect caused by the fact that mn rich nodules from more reduced environment act as a more efficient trap for 226ra

evidence of coupling in ocean atmosphere dynamics over the north atlantic coupling between the ocean and the atmosphere is investigated in reanalysis data sets projecting the data sets onto a dynamically defined subspace allows one to isolate the dominant modes of variability of the coupled system this coupled projection is then analyzed using multichannel singular spectrum analysis the results suggest that a dominant low frequency signal with a 25 30 year period already mentioned in the literature is a common mode of variability of the atmosphere and the ocean a new score for evaluating the internal nature of the common variability is then introduced and it confirms the presence of coupled dynamics in the ocean atmosphere system that impacts the atmosphere at large scale the physical nature of this coupled dynamics is then discussed

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

the sao vicente earthquake of 2008 april and seismicity in the continental shelf off se brazil further evidence for flexural stresses summary the continental margin and shelf of most stable intraplate regions tend to be relatively more seismically active than the continental interior in the southeast continental margin of brazil a seismiczoneextendsfromriograndedosultoespeismicactivityoccurring mainly along the continental slope and suggesting a close relationship with flexural stresses caused by the weight of the sediments in this region earthquakes with magnitudes larger than 5m b occurevery20 25yr onaverage thefocalmechanismsolutionsofpreviousearthquakes in this zone indicated reverse faulting on planes dipping approximately 45 with horizontal p axes the recent 5 2m b earthquake of 2008 april 23 occurred 125 km south of s ao vicente and was well recorded by many stations in se brazil as well as at teleseismic distances in north america and africa its focal depth was 17 km locating the hypocentre in the lower crust a well determined focal mechanism solution shows one vertical nodal plane and one subhorizontal nodal plane thep a ndt axes exhibit large dips which were confirmed by ar egional moment tensor inversion this unusual orientation of the fault mechanism can be attributed to a rotation of the principal stress directions in the lower crust caused by flexural effects due to the load of recent sedimentation

fossil manganese deposits buried within dsdp odp cores legs 1 126 abstract probable in situ manganese deposits larger than 1 cm in diameter buried in odp dsdp cores were selected for study after examining previous descriptions of the manganese deposits in site reports and the odp data base most of the selected samples from 11 cores occur at or just above sedimentary hiatuses or in slowly deposited sediments and are overlain by rapidly deposited sediments of biogenic terrigenous or volcanogenic origin the changes in sedimentation recorded in the lithostratigraphic sections around these deposits are closely related to changes in tectonic evolution deep water circulation or biological productivity at the sites the similarity in composition and structure of the buried deposits to those of the modern manganese nodules and crusts with no evidence of post depositional change suggest that buried manganese deposits may be used as indicators of past sedimentary conditions during which they formed their major components are hydrogenetic and early diagenetic manganese minerals as well as detrital minerals the characteristics of these manganese deposits suggests that similar processes of deposition have taken place since the paleogene or older

tectonics and crustal structures related to bhuj earthquake of january 26 2001 based on gravity and magnetic surveys constrained from seismic and seismological studies abstract gravity and magnetic data of the kachchh basin and surrounding regions have delineated major e w and nw se oriented lineaments and faults which are even extending up to plate boundaries in the north arabian sea and western boundary of the indian plate respectively the epicentral zone of bhuj earthquake and its aftershocks is located over the junction of rann of kachchh and median uplifts viz kachchh mainland and wagad uplifts which are separated by thrust faults gravity data with constraints from the results of the seismic studies along a profile suggest that the basement is uplifted towards the north along thrust faults dipping 40 60 south similarly gravity and magnetic modeling along a profile across wagad uplift suggest south dipping 50 60 basement contacts separating rocks of high susceptibility and density towards the north one of these contacts coincides with the fault plane of the bhuj earthquake as inferred from seismological studies and its projection on the surface coincides with the e w oriented north wagad thrust fault a circular gravity high in contact with the fault in northern part of the wagad uplift along with high amplitude magnetic anomaly suggests plug type mafic intrusive in this region several such gravity anomalies are observed over the island belt in the rann of kachchh indicating their association with mafic intrusions the contact of these intrusives with the country rock demarcates shallow crustal inhomogeneities which provides excellent sites for the accumulation of regional stress a regional gravity anomaly map based on the concept of isostasy presents two centers of gravity lows of 11 to 13 mgal 10 5 m s2 representing mass deficiency in the epicentral region their best fit model constrained from the receiver function analysis and seismic refraction studies suggest crustal root of 7 8 km deep crustal inhomogeneity under them for a standard density contrast of 400 kg m3 it is therefore suggested that significant amount of stress get concentrated in this region due to a buoyant crustal root b regional stress due to plate tectonic forces and c mafic intrusives as stress concentrators and the same might be responsible for the frequent and large magnitude earthquakes in this region including the bhuj earthquake of january 26 2001

variations of seismic velocities in the kachchh rift zone gujarat india during 2001 2013 abstract we herein study variations of seismic velocities in the main rupture zone mrz of the mw 7 7 2001 bhuj earthquake for the time periods 2001 05 2006 08 2009 10 and 2011 13 by constructing dvp dvs and d vp vs tomograms using high quality arrival times of 28 902 p and 28 696 s waves from 4644 precise jhd joint hypocentral determination relocations of local events differential tomograms for 2001 05 reveal a marked decrease in seismic velocities low dvp low dvs and high d vp vs in the mrz at 5 35 km depths during 2001 10 which is attributed to an increase in crack fracture density higher pore fluid pressure resulted from the intense fracturing that occurred during the mainshock and post seismic periods while we observe a slight recovery or increase in seismic velocities 2011 13 this could be related to the healing process lower pore fluid pressure due to sealing of cracks of the causative fault zone of the 2001 bhuj mainshock the temporal reduction in seismic velocities is observed to be higher at deeper levels more fluid enrichment under near lithostatic pressure than that at shallower levels fluid source for low velocity zone lvz at 0 10 km depths with high d vp vs could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content while fluid source for lvz at 10 35 km depths could be due to the presence of brine fluids released from the metamorphic dewatering and volatile co2 emanating from the crystallization of carbonatite melts in the asthenosphere in fractures and pores we also imaged two prominent lvzs associated with the katrol hill fault zone and island belt fault zone extending from shallow upper crust to sub crustal depth which might be facilitating the deeper circulation of metamorphic fluids volatile co2 thereby the generation of lower crustal earthquakes occurring in the kachchh rift zone

variations in the fine scale composition of a central pacific ferromanganese crust paleoceanographic implications a 47 to 60 mm thick fe mn crust from horizon guyot water depth 1800 1780 m central pacific was used to evaluate the potential of crusts as recorders of neogene paleoceanographic and paleoclimatic conditions the chemical composition was determined by microprobe for 16 elements from a polished thin section three analyses were made per millimeter and averaged to give the composition of each millimeter the age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the tertiary seawater curve the crust represents 18 5 m y of growth of fe and mn oxyhydroxides the crust is composed of alternating botryoidal and laminated layers the botryoidal layers formed during the same time intervals that widespread neogene deep sea hiatuses were forming in bottom sediments the botryoidal layers represent growth during times of intensified deepwater flow whereas the laminated intervals represent more quiescent conditions the correspondence between the botryoidal layers and the neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust chemical changes in the crust take two forms the first is represented by broad changes in the composition defined chiefly by fourth order polynomial fits to the chemical profiles of each element with depth in the crust the second is high frequency changes in composition the broader changes occurred primarily at about 15 11 5 7 4 6 4 5 2 and 4 6 ma which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles the periods of the high frequency changes may reflect climatic changes that resulted from orbital forcing these high frequency changes may correspond to the high order eccentricity periods of 3 47 2 04 and 1 31 m y

three dimensional seismic velocity imaging of the kachchh rift zone gujarat india implications toward the crustal mafic pluton induced intraplate seismicity abstract three dimensional tomographic images of seismic velocities have been constructed for the 2001 mw7 7 bhuj earthquake region using data from a dense 58 station seismograph network during 2001 16 two distinct prominent high velocity anomalies 10 14 increase in vp and vs and 10 11 decrease in vp vs have been mapped at 2 40 km depths below the banni and wagad uplift regions which could be attributed to mafic plutons we also image several high velocity zones intrusive mafic rocks and some low velocity patches meteoric water metamorphic fluids volatile co2 within the main rupture zone of the 2001 bhuj earthquake where maximum confinement of seismicity is noticed we propose that stresses induced by mafic plutons in response to the n s compression of the indian plate might have resulted in a stress regime near to critical for thesub parallel rift associated fault system within the main rupture zone thereby generating small to large earthquakes through small stress perturbations caused by crustal fluid flows and stress transfer

the influence of the carbonate dissolution rate on the growth and composition of co rich ferromanganese crusts from central pacific seamount areas abstract the metal composition of oceanic ferromanganese deposits occurring in seamount regions line islands chain and mid pacific mountains varies with water depth and age the results of metal determinations of carbonate plankton samples suggest that carbonate dissolution in the water column might have an important influence on the accretion and composition of hydrogenetic precipitates two ferromanganese crust generations of different age have been observed the precipitation of the older crust took probably place during early oligocene the younger crust began to form during middle miocene between the two crust generations periods of carbonate sedimentation and of phosphorite deposition occur the hydrogenetic formation of the crusts is controlled by the metal supply from the water column according to the laws of colloidal surface chemistry dissolution experiments with carbonate plankton samples show that the main fe source for the hydrogenetic crust formation are colloidal fe hydroxide particles being released in the water column from the dissolution of carbonate plankton skeletons in the case of mn maximum dissolved mn occurs in the oxygen minimum zone as the result of in situ break down of organic matter and the in situ reduction of mn bearing solid phases closely beneath the oxygen minimum zone a fe supply mobilized within the oxygen minimum zone has also to be taken into account in the water column below the oxygen minimum zone a mixture of colloidal particles of mn fe oxyhydroxide and colloidal al fe silicate precipitate together on the surface of substratum rocks the mixing ratio of these colloidal phases controlling the metal composition of the ferromanganese precipitates is depth dependent and shows also temporal variations in general mn fe ratio ni and co contents decrease with depth down to the calcite compensation depth the most probable mechanism for the ultimate removal of co and ni from the water column might be a surface reaction mno2 is specifically able to absorb hydrous co2 and ni2 ions because of the surface enrichment of co and the strong electrical field of mn iv a subsequent oxidation of co2 to co3 takes place leading to higher enrichment of co in comparison to ni the most important factor governing the high co enrichment in the ferromanganese crusts is the growth rate the lower the growth rate the higher the co content maximum values of up to 2 co occurring in samples from water depths between 1500 and 1100 m 1 are related to lower carbonate dissolution rates and corresponding lower fe supply the metal supply from the water column is strongly related to distinct environmental factors such as bio productivity range of lysocline and calcite compensation depth rate of carbonate dissolution and activity of the antarctic bottom water thus our model shows that the growth periods and the metal composition of hydrogenetic seamount crusts are controlled by changes in the paleoceanography and reflect distinct environmental conditions

on the cenozoic oceans evidence of the calcium carbonate content δ13c and δ18o abstract the present pattern of the vertical circulation of the oceans was established c 6 2 ma ma million years ago the vertical circulation in the north atlantic was intensified at the onset of the northern glaciation c 2 4 ma as well as during each of the following glaciations particularly during the last three elsterian saalian and weichselian the last two interglacials eemian and holocene were probably cooler than those prior to the saalian it is suggested that the northern glaciations resulted from the opening of the bering strait which started the fresh water cycle atlantic n pacific arctic ocean n atlantic this inflow together with river water created the salinity stratification in the arctic ocean the prerequisite for a sea ice cover there the albedo change produced the temperature drop necessary to initiate glaciations the fixture of the greenland ice sheet probably during the saalian has resulted in an intensified vertical circulation of the oceans prior to 6 2 ma the southern ocean was the only source of deep water at the eocene oligocene transition there was a drastic change in the ocean circulation caused by the cooling from the ice sheet which then formed over eastern antarctica the 13 c is useful for tracing the source and the paths of deep water the 18o in pre quaternary sequences is greatly influenced by a non constant 18 o of the sea water caused by the water circulation through the oceanic crust

element accumulation rates in and growth histories of manganese nodules from the southwestern pacific basin abstract accretion rates and element accumulation rates of seven manganese nodules collected from the southwestern pacific basin during r v sonne cruise so 14 on a tahiti east pacific rise new zealand transect have been determined the accretion rates of the nodules lie in the range 3 5 6 0 mm 10 6 yr the highest rates correspond to areas with higher biogenic silica contents in the sediment suggesting a higher diagenetic contribution to the nodules in one nodule 143gb dated with 10 be nodule growth began at about 14 5 ma there is a change in structure of the nodule at 6 ma and concentric layers filled with detritus occur at 3 2 ma and 1 3 ma there is also a major change in the flux rates of the elements at the 6 ma time marker with the authigenic elements mn cu and ni showing greater accumulation rates in the outer zone and the detrital fraction al showing a greater accumulation rate in the inner zone the variations in the element accumulation rates probably reflect a change of abyssal circulation pattern at 6 2 ma however the precise nature of this change remains a matter for speculation

shear wave velocity structure in the northern basin and range province from the combined analysis of receiver functions and surface waves a new method based on the joint inversion of receiver functions and surface wave phase velocities results in well determined shear velocity structures that are consistent with the compressional wave structure gravity heat flow and elevation data in the northern basin and range this new inversion method takes advantage of average velocity information present in the surface wave method and differential velocity information contained in the receiver function method thus min imizing the nonuniqueness problem that results from the velocity depth trade off an unusually thick 38 kin and relatively faster crust and upper mantle are found in central and eastern nevada compared to the thin 28 to 34 km and slower crust and upper mantle of the western basin and range we interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less cenozoic ex tension relative to the surrounding regions to the west and north the thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern nevada simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface indicating a near classical airy type of compensation in the province we analyze in detail the shear wave s wave velocity model derived from the receiver functions at station bmn and compressional wave p wave velocity models derived from the 1986 passcal experiment in northwestern nevada the most interesting feature of these models is the presence of negative velocity gradients in the s wave model with no corresponding velocity decrease in the p wave models between depths of 10 and 24 km this combined velocity model may be explained by high pore fluid pressures at these depths this model favors a layered fluid porosity model proposed in the literature to explain extensive middle to lower crust conti nental seismic reflections and high electrical conductivity an upper mantle grada tional low velocity zone is present between 32 and 38 km in the s wave model this upper mantle shear wave low velocity zone is consistent with partial melt which may be the source material for magmatic underplating in this region

petrogenesis of ferromanganese nodules from east of the chagos archipelago central indian basin indian ocean abstract deep sea ferromanganese nodules occur over a large area and on many different sediment types of the central indian basin indian ocean selected samples were studied to determine their chemical and mineralogical compositions and microstructural features repeated laminations of variable thickness alternately dominated by todorokite and vernadite are characteristic of these nodules these laminae show on electron microprobe line scans corresponding interlaminar partitioning of mn cu ni and fe co the bulk chemical compositions of these nodules plot in both the hydrogenetic and early diagenetic fields on the fe mn ni cu co 10 ternary diagram the binary diagram depicting the covariation of mn ni cu against fe co shows two distinct parallel regression lines one delineated by nodules from terrigenous siliceous ooze and siliceous ooze terrigenous sediments and the other by nodules from red clay siliceous ooze red clay and calcareous ooze red clay an increasing diagenetic influence in the nodules with the nature of the host sediment types was observed in the sequence terrigenous siliceous ooze and red clay siliceous calcareous ooze red clay a negative correlation between mn fe ratio and co and a positive correlation between mn fe ratio and ni cu was established the nodules show dendritic laminated and globular microstructures formed by primary growth of fe mn oxide laminae depositional hiatuses in the primary microstructures indicate that the growth of these nodules was episodic the oxide laminations show extremely complex growth patterns scattered biogenic remains and mineral grains acted as accessory seeds for growth of oxide layers in addition to the main nuclei none of the primary microstructures can be uniquely linked to a particular growth process or growth rate radial cracks cutting across primary microstructures are often filled by todorokite of a later generation post depositional modifications of the nodules were largely controlled by accreted biogenic remains as indicated by their progressive dissolution with increasing depth from nodule surfaces their pseudomorphic replacement by todorokite and the later growth of phillipsite and todorokite in the microfossil molds the growth patterns of the in filled oxides are often controlled entirely by the cavity walls and are discordant with the primary growth fabric primary todorokite was recrystallized to coarser grains of different chemical composition later generation veins of todorokite cut across and chaotically disrupted primary laminae

crustal and lithospheric thinning beneath the seismogenic kachchh rift zone gujarat india its implications toward the generation of the 2001 bhuj earthquake sequence a high resolution passive seismic experiment in the kachchh rift zone of the western india has produced an excellent dataset of several thousands teleseismic events from this network 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions for the 30 310 back azimuth ranges which show a positive phase at 4 5 6 1 s delay time and a strong negative phase at 8 0 11 0 s these phases have been modeled by a velocity increase at moho i e 34 43 km and a velocity decrease at 62 92 km depth the estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3 7 km in crustal thickness and 6 14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un rifted parts of the kachchh rift zone on an average the kachchh region is characterized by a thin lithosphere of 75 9 5 9 km the marked velocity decrease associated with the lithosphere asthenoshere boundary lab observed over an area of 120 km 80 km and the isotropic study of xenoliths from kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of co2 rich lherzolite beneath the kachchh seismic zone that might have caused by rifting episode at 88 ma and the associated deccan thermal plume interaction at 65 ma episodes thus the coincidence of the area of the major aftershock activity and the moho as well as asthenospheric upwarping beneath the central kachchh rift zone suggests that these pockets of co2 rich lherzolite partial melts could perhaps provide a high input of volatiles containing co2 into the lower crust which might contribute significantly in the seismo genesis of continued aftershock activity in the region it is also inferred that large stresses in the denser and stronger lower crust at 14 34 km depths induced by ongoing banni upliftment crustal intrusive marked lateral variation in crustal thickness and related sub crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the kachchh seismic zone

ferromanganese nodules from manop sites h s and r control of mineralogical and chemical composition by multiple accretionary processes abstract the chemical composition of ferromanganese nodules from the three nodule bearing manop sites in the pacific can be accounted for in a qualitative way by variable contributions of distinct accretionary processes these accretionary modes are 1 1 hydrogenous i e direct precipitation or accumulation of colloidal metal oxides in seawater 2 2 oxic diagenesis which refers to a variety of ferromanganese accretion processes occurring in oxic sediments and 3 3 suboxic diagenesis which results from reduction of mn 4 by oxidation of organic matter in the sediments geochemical evidence suggests processes 1 and 2 occur at all three manop nodule bearing sites and process 3 occurs only at the hemipelagic site h which underlies the relatively productive waters of the eastern tropical pacific a normative model quantitatively accounts for the variability observed in nearly all elements zn and na however are not well explained by the three end member model and we suggest that an additional accretionary process results in greater variability in the abundances of these elements variable contributions from the three accretionary processes result in distinct top bottom compositional differences at the three sites nodule tops from h are enriched in ni cu and zn instead of the more typical enrichments of these elements in nodule bottoms in addition elemental correlations typical of most pelagic nodules are reversed at site h the three accretionary processes result in distinct mineralogies hydrogenous precipitation produces mno2 oxic diagenesis however produces cu ni rich todorokite and suboxic diagenesis results in an unstable todorokite which transforms to a 7 a phase birnessite upon dehydration the presence of cu and ni as charge balancing cations influence the stability of the todorokite structure in the bottoms of h nodules which accrete dominantly by suboxic diagenesis na and possibly mn 2 provide much of the charge balance for the todorokite structure limited growth rate data for h nodules suggest suboxic accretion is the fastest of the three processes with rates at least 200 mm 106 yr oxic accretion is probably 10 times slower and hydrogenous 100 times slower since these rates predict more suboxic component in bulk nodules than is calculated by the normative analysis we propose that suboxic accretion is a non steady state process variations in surface water productivity cause pulses of particulate flux to the sea floor which result in transient mn reduction in the surface sediments and reprecipitation on nodule surfaces

variations of element distribution in ferromanganese nodules and its bearing on growth rates mineralogical chemical and radiochemical analyses of nodules sampled at various distances from the east pacific rise indicate that the mineralogical composition is not clearly linked to distance but chemical composition seems to be the pelagic nodules accrete at a lower rate and are on the whole richer in manganese uranium and thorium and poorer in lead and silicon than nodules from the ridge the deep seated nodules display a marked decrease in thorium concentration toward their centers which can be explained in several ways one hypothesis involves a decrease in the growth rate as the plate moves away from the ridge and manganese flux diminishes as suggested in previous studies some dissolution of the bottom of the nodules may occur and contribute to abnormal element distributions

manganese nodules in the miocene bahia inglesa formation north central chile petrography geochemistry genesis and palaeoceanographic significance abstract manganese nodules recovered from two stratigraphic horizons of tortonian messinian late miocene age in the bahia inglesa formation of north central chile were studied using xrd sem and geochemical analysis the dominant mineral in the nodules is todorokite which suggests a diagenetic marine environment this is supported by field observations of nodules replacing ophiomorpha burrows preliminary traditional statistical analysis of the nodule geochemistry including single element binary and ternary ratios suggests that the nodules are of the supergene deep marine type as also indicated by the presence of foraminifers typical of the upper continental slope as well as debris flow and turbidity current deposits in an associated submarine palaeocanyon however abnormally low cu concentrations seem to contradict this interpretation so that additional analyses were carried out this included multiple discriminant analysis mda as well as a new technique applied for the first time to manganese nodules namely artificial neural network analysis ann in both methods central log ratio clr normalization was applied to the raw data the results in particular those of the ann analysis suggest that the bahia inglesa nodules present a chemical signature distinct from that of nodules described to date a new class is therefore proposed namely supergene intermediate marine partially restricted basin

10 be dating of a manganese crust from central north pacific and implications for ocean palaeocirculation one of the most promising applications of the new 10be detection technique using accelerator mass spectrometry is the determination of growth rates of mn nodules and crusts1 5 because the half life of cosmogenic 10be 1 5 myr is of the same order as the time needed for a few millimetres of nodule material to grow 10be is a useful tool for unravelling such evolution during the late tertiary when most nodules are assumed to have started their growth6 mass spectrometric measurement of a 10be profile across the manganese crust va 13 2 from the central pacific using a tandem van de gr a ff accelerator yields growth rates of 2 7 and 4 8 mm myr 1 for the layers of the crust accumulated between recent and 6 myr bp and between 6 and 11 myr respectively using these measurements as well as 230th dating we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them these ages are 0 12 2 9 3 4 5 7 6 7 7 9 10 12 and 13 16 myr bp all of these boundaries apparently coincide with the times reported for quaternary and late tertiary palaeoclimatic events suggesting that the crust growth has been strongly influenced by palaeoclimate

uranium thorium isotopes and transition metal fluxes in two oriented manganese nodules from the central indian basin implications for nodule turnover transition metal fluxes to the top and bottom of two oriented manganese nodules ss 657 and sk 176 were determined by combining radiochemical and geochemical analyses distinct differences in transition metal fluxes 230th contents 230th232th and mnfe ratios and silicate mineralogy in the top and bottom of nodule ss 657 suggest that since the time of inception of oxyhydroxide accretion 1 m y b p it has been growing with a fixed orientation on the seafloor in nodule sk 176 in contrast nearly uniform accumulation rates 230thexc and 230thexc232th extrapolated to surface activities transition metal fluxes and increasing diagenetic influence outwards in the top and bottom are thought to be suggestive of rotation of the nodule several times over time intervals which are smaller than the time resolution involved in u th isotope dating techniques

growth response of a deep water ferromanganese crust to evolution of the neogene indian ocean abstract a deep water ferromanganese crust from a central indian ocean seamount dated previously by 10 be and 230 th excess was studied for compositional and textural variations that occurred throughout its growth history the 10 be 9 be dated interval upper 32 mm yields an uniform growth rate of 2 8 0 1 mm ma frank m o nions r k 1998 sources of pb for indian ocean ferromanganese crusts a record of himalayan erosion earth planet sci lett 158 pp 121 130 which gives an extrapolated age of 26 ma for the base of the crust at 72 mm and is comparable to the maximum age derived from the co model based growth rate estimates this study shows that fe mn oxyhydroxide precipitation did not occur from the time of emplacement of the seamount during the eocene 53 ma until the late oligocene 26 ma this paucity probably was the result of a nearly overlapping palaeo ccd and palaeo depth of crust formation increased early eocene productivity instability and reworking of the surface rocks on the flanks of the seamount and lack of oxic deep water in the nascent indian ocean crust accretion began older zone with the formation of isolated cusps of fe mn oxide during a time of high detritus influx probably due to the early miocene intense erosion associated with maximum exhumation of the himalayas op cit this cuspate textured zone extends from 72 mm to 42 mm representing the early miocene period intense polar cooling and increased mixing of deep and intermediate waters at the close of the oligocene might have led to the increased oxygenation of the bottom water in the basin a considerable expansion in the vertical distance between the seafloor depth and the ccd during the early miocene in addition to the influx of oxygenated bottom water likely initiated fe mn crust formation pillar structure characterises the younger zone which extends from 40 mm to the surface of the crust i e 15 ma to present this zone is characterised by 25 higher content of oxide bound elements than in the older zone possibly corresponding to further increased oxygenation of bottom waters increased stability of the seamount slope and gradually reduced input of continental detritus from the erosion of the himalayas middle miocene antarctic glaciation which peaked 12 13 ma ago increased the oxic bottom water influx to the basin resulting in accretion of the crust with low detritus therefore the younger crust started to accrete in response to a shift in bottom water circulation towards the contemporary pattern which produced a uniform growth rate and pillar structure up to the present

sediment thicknesses and qs vs qp relations in the kachchh rift basin gujarat india using sp converted phases delineation of the top sedimentary structure and its qs vs qp relationship using the travel time difference of direct s and converted sp phase is key to understanding the seismic hazard of any sedimentary basin area we constructed filtered displacement waveforms from local etna episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 bhuj earthquake recorded by the kachchh seismological network of the national geophysical research institute ngri hyderabad india during 2001 2004 stations are within 15 70km of epicenters and the resulting displacement waveforms are generally simple displaying prominent p sp and s wave pulses particle motion of p and s waves suggest near vertical raypaths consistent with preliminary depth estimates the direct s wave on the horizontal component is characterized by lower frequency content than the converted sp phase on the vertical component this difference in frequency content between s and sp phases can be explained in terms of different attenuation effects for p and s waves in the unconsolidated sediments the sp phase is generated by s to p phase conversion at the base of mesozoic sediments of the kachchh basin travel time inversion velest of 2565 p and 2380 s arrivals from 658 well located aftershocks recorded at 8 14 three component local seismic stations led to 1 d velocity models indicated very slow sediments in the upper 0 2 km depth range vp 2 92 km s and vs 0 90 km s and an increasing trend of velocities with depth at 2 40 km depth the estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel time difference between s and converted sp phases reaches a maximum of 1 534 0 117 km beneath bandri near the location of 2001 bhuj mainshock and attains a minimum sediment thickness of 0 858 0 104 km beneath ramvav and burudia the spectral ratios between sp and s from 159 three component accelerograms have been used to study seismic wave attenuation in the kachchh rift basin the estimated qs vs qp relations for 12 accelerograph sites vary from qs 0 184 qp at chobari to qs 0 505 qp at dudhai for stations chobari chopdwa jahawarnagar vondh and tapar the spectral ratio slopes and hence the calculated qs vs qp relations are effectively the same and the correlation coefficients are quite high 0 91 0 93 stations adhoi manfara new dudhai dudhai and sikara have similar qs vs qp relationships to each other and also have high correlation coefficients 0 78 0 87 the spectral ratios for stations anjar and ramvav are small and poorly constrained resulting in less reliable qs vs qp relations this could be due to noisy data fewer available waveforms or scattering due to velocity heterogeneities and or interface irregularities

a possible origin of intraplate earthquakes in the kachchh rift zone india since the 2001 mw7 7 bhuj earthquake abstract we herein use the joint inversion of p receiver functions and fundamental mode group velocity dispersion data 9 70 s of rayleigh and love waves to estimate crustal and lithospheric thicknesses at twenty three component mobile broadband stations in kachchh gujarat modeled moho depths range from 35 to 43 km while lithospheric thicknesses vary from 64 to 106 km the main result of our modelling is the delineation of a marked crustal 2 4 km as well as lithospheric 10 20 km thinning and a 2 6 drop in vs across the lithosphere asthenosphere boundary lab within the samkhiali graben associated with gravity high underlying the central kachchh rift zone krz where 95 of the continued aftershock activity took place since 2001 such a large drop in vs could be attributed to the presence of carbonatite melts in the upper mantle our modeling reveals a 4 km crustal thinning below the central krz and a 4 km crustal thickening below the surrounding riftless regions this kind of crustal structure is inferred to induce large flexural deviatoric stresses 50 100 mpa in the upper crust thereby these stresses in the presence of regional plate tectonic stresses can bring the samkhiali graben below the central krz near to the critical stress level while stress transfer meteoric water in the upper crust and metamorphic fluid as well volatile co2 flows in the lower crust provide the required triggering effect to the critically stressed graben structure down to 35 km depth for generating continued aftershock activity in the kachchh rift zone since 2001 we also propose that the deeper circulation of volatile co2 through the inferred conduit related to the 65 ma deccan plume activity extending from lower crust down to asthenosphere plays a key role in the generation of uninterrupted occurrence of earthquakes in the kachchh rift zone gujarat india

atmospheric summer circulation and coastal upwelling in the arabian sea during the holocene and the last glaciation accumulation rates of biogenic and lithogenic components were studied in 39 turbidite free well dated sediment cores from the northern indian ocean to define the proportions of fluvial and eolian input and to reconstruct quaternary patterns of coastal upwelling the majority of dust deposited in the western arabian sea during the holocene about 100 106 t yr 1 is advected from arabia by northwesterly winds which overlie the low level southwest monsoon the glacial increase in dust flux to 160 106 t yr 1 culminated in the northern arabian sea most probably due to i entrainment of dust rich in chlorite dolomite and lithogenic carbonate in the then dry persian gulf and ii a southward shift of the mean position of the southwest monsoon during glacial summer this shift is recorded in reduced accumulation rates of biogenic opal and increased rates of marine carbonate off somalia and oman both the terrigenous and biogenic sediment records show that the northwesterly winds and the southwest monsoon persisted over the last 27 000 yr as well as the asian continental summer heat low however the glacial seasonal time span of the southwest monsoon season was much reduced most likely because of a delay in the seasonal onset of the southwest monsoon

intra plate seismicity and flexural stresses in central brazil 1 explaining intra plate seismicity is a challenging task different models have been proposed combining weak zones and stress concentration mechanisms here we propose that flexural deformation is a major factor to explain seismicity in central brazil a sw ne oriented seismic zone between the amazon and the sao francisco cratons coincides with high gravity anomalies possibly due to a sw ne belt of thin crust the load from the high density shallow mantle rocks causes upper crustal compressional stresses up to 100 mpa in the 200 km wide seismic zone away from the central zone of horizontal compression extensional stresses in the peripheral bulge balance the regional compression explaining the aseismic areas three other seismic clusters in brazil also correlate with high gravity anomalies suggesting that flexural deformation contributes significantly to explain mid plate seismicity in brazil

paleoceanographic implications of miocene deep sea hiatuses miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages the subsequent changes in oceanic circulation and development of major antarctic glaciation the consequences and timing of these events can be observed in variations in the distribution of deep sea hiatuses sedimentation patterns and biogeographic distribution of planktic organisms the opening of the drake passage in the latest oligocene to early miocene 25 20 ma resulted in the establishment of the deep circumpolar current which led to thermal isolation of antarctica and increased global cooling this development was associated with a major turnover in planktic organisms resulting in the evolution of neogene assemblages and the eventual extinction of paleogene assemblages the erosive patterns of two widespread hiatuses ph 23 0 22 5 ma and nh 1 20 18 ma indicate that a deep circumequatorial circulation existed at this time characterized by a broad band of carbonate ooze deposition siliceous sedimentation was restricted to the north atlantic and a narrow band around antarctica a major reorganization in deep sea sedimentation and hiatus distribution patterns occurred near the early middle miocene boundary apparently resulting from changes in oceanic circulation beginning at this time deep sea erosion occurred throughout the caribbean hiatus nh 2 16 15 ma suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents and or intensification of the gulf stream sediment distribution patterns changed dramatically with the sudden appearance of siliceous ooze deposition in the marginal and east equatorial north pacific by 16 0 to 15 5 ma coincident with the decline of siliceous sedimentation in the north atlantic this silica switch may have been caused by the introduction of norwegian overflow water into the north atlantic acting as a barrier to outcropping of silica rich antarctic bottom water the main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13 5 to 12 5 ma hiatus nh 3 coincident with the establishment of a major east antarctic ice cap antarctic glaciation resulted in a broadening belt of siliceous ooze deposition around antarctica increased siliceous sedimentation in the marginal and east equatorial north pacific and indian oceans and further northward restriction of siliceous sediments in the north atlantic periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep sea erosion at 12 to 11 ma nh 4 10 to 9 ma nh 5 7 5 to 6 2 ma nh 6 and 5 2 to 4 7 ma nh 7

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

upper mantle shear velocity structure below northwestern india based on group velocity dispersion abstract we derive a well resolved 1d model of shear wave velocity v s through both linear and nonlinear joint inversions of rayleigh and love wave group velocity dispersion data our best model from the deterministic approach suggests a two layered crust over a half space the upper crust is 13 8 km thick with a shear velocity of 3 22 km s for the lower crust they are 24 9 km and 3 62 km s respectively finally we use a global optimization method very fast simulated annealing to estimate shear wave velocity multiple models give acceptable fits to the observed data a common feature of these models is a low velocity zone lvz with its upper boundary at 60 70 km depth at this depth the temperature depth profile of kachchh cuts the wet peridotite geotherm resulting in increased water content due to partial melting and thus the decreased shear velocity this lvz extends down to a depth of 120 130 km where the temperature depth profile of kachchh cuts the dry peridotite geotherm that leads to a sharp decrease in water content thereby a sharp increase in shear velocity the maximum v s perturbation of 3 in the upper mantle can be explained by the presence of 100 150 k excess temperature beneath northwestern india at 70 120 km depth relative to the adjacent mantle along with the presence of 1 penny shaped melt inclusions probably co 2 rich carbonatite melts in the upper mantle this is likely related to the imprints of the initial deccan reunion plume head that might have moved with india since the deccan volcanism at 65 ma

occurrences of large magnitude earthquakes in the kachchh region gujarat western india tectonic implications abstract moderate to large damaging earthquakes in the peninsular part of the indian plate do not support the long standing belief of the seismic stability of this region the historical record shows that about 15 damaging earthquakes with magnitudes from 5 5 to 8 0 occurred in the indian peninsula most of these events were associated with the old rift systems our analysis of the 2001 bhuj earthquake and its 12 year aftershock sequence indicates a seismic zone bound by two linear trends nnw and nne that intersect an e w trending graben the bouguer gravity values near the epicentre of the bhuj earthquake are relatively low 2 mgal the gravity anomaly maps the distribution of earthquake epicentres and the crustal strain rate patterns indicate that the 2001 bhuj earthquake occurred along a fault within strain hardened mid crustal rocks the collision resistance between the indian plate and the eurasian plate along the himalayas and anticlockwise rotation of the indian plate provide the far field stresses that concentrate within a fault bounded block close to the western margin of the indian plate and is periodically released during earthquakes such as the 2001 m w 7 7 bhuj earthquake we propose that the moderate to large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain hardened environment

paleodepth distribution of neocene deep sea hiatuses the depth of formation of miocene to middle pliocene deep sea hiatuses nh1 to nh8 has been determined using 152 deep sea drilling project sites in the atlantic pacific and indian oceans megahiatuses representing maxima in deep sea erosion occur at three water depths below 3800 m at intermediate depths between about 2000 and 3000 m and in shallower waters above 1500 m paleodepth both the paleodepths and distribution of these hiatus maxima suggest that flow of antarctic bottom water antarctic intermediate water and north atlantic deep water masses largely caused this erosion correlation of the hiatuses with oxygen isotope carbonate and sea level records indicates that prior to 11 ma brief hiatuses including those resulting from nondeposition in high productivity equatorial regions largely correlate with global cooling episodes high carbonate content and lowstands of sea level during the last 11 m y hiatuses also seem to correlate with cooling episodes but carbonate dissolution is characteristic and sea levels may have been rising or at a lowstand during the nh4 nh7 and nh8 intervals it is possible that a rise in sea level corresponds with polar cooling but there is uncertainty in correlation brief hiatuses during rising sea levels can be explained by basin shelf fractionation of carbonates hiatuses during sea level lowstands and cooling episodes may result from intensified bottom water circulation and increased corrosiveness of bottom water due to higher levels of co2 and increased productivity during increased upwelling supplement appendix 1 is availablewith entire article on microfiche orderfrom american geophysical union 2000florida avenue n w washington dc20009 document p87001 2 50 paymentmust accompany order

widespread miocene deep sea hiatuses coincidence with periods of global cooling high resolution biostratigraphic analyses of miocene deep sea cores reveal eight intervals of widespread hiatuses in the world ocean at 23 0 22 5 20 0 18 0 16 0 15 0 13 5 12 5 12 0 11 0 10 0 9 0 7 5 6 2 and 5 2 4 7 m y ago in complete sections these hiatuses correspond to intervals of cool faunal and floral assemblages rapid enrichment of 18 o and sea level regressions these factors suggest that miocene deep sea hiatuses result from an increased intensity of circulation and corrosiveness of bottom currents during periods of increased polar refrigeration

neogene oceanographic variations recorded in manganese nodules from the somali basin abstract a detailed study of a nodule from the somali basin dated by 230 th excess was correlated with the paleoceanographic events recorded in site 236 leg 24 deep sea drilling project dsdp cores tentative indications are that the phase of nodule accretion starting with the development of pillar structure at a depth of 20 mm in the nodule around 13 ma coincides with increased antarctic bottom water aabw flow and an elevated calciumcarbonate compensation depth ccd the late miocene lowering of the ccd is represented by the mottled zones between 8 and 18 mm in the nodule is characterised by an abundant silicate component 20 of aeolian origin the miocene pliocene boundary 5 ma occurs at a depth of about 8 mm and is represented by the development of pillar structure and a minimum of aeolian dust 10 3 the increased biological productivity of the somali surface water since the middle miocene is demonstrated by the increasing c org content of the nodule from 0 11 to 0 19 towards its surface

magnetostratigraphy of hydrogenetic manganese crusts from northwestern pacific seamounts abstract fine scale dating is crucial to understanding the growth and environments of formation of marine manganese deposits the paleomagnetic method of dating of manganese nodules and crusts has been attempted but with no success so far we measured remanent magnetization rm on up to 75 mm thick hydrogenetic crusts at intervals of 2 5 mm after mineralogical and chemical examination careful separation and alternative field demagnetization stable sharp rm and well correlated polarity change patterns between four samples made it possible to identify the pliocene quaternary magnetic chrons in them these data show that the three crusts have grown continuously at rates of 14 17 mm m y since the early pliocene this is three or four times faster than those for central pacific seamount hydrogenetic crusts estimates of growth rate based on determination of co flux are within this range magnetostratigraphy therefore appears to be a powerful method for dating hydrogenetic manganese crusts when supported by conventional dating methods these results encourage finer scale investigations which promise more detailed paleoceanographic reconstruction

passive source seismic imaging of the crust and upper mantle beneath the 2001 mw 7 7 bhuj earthquake region gujarat india abstract variations in crustal and lithospheric thicknesses below the kachchh rift kr zone have been investigated employing the joint inversion of p receiver functions rf and rayleigh wave and love wave group velocity dispersion 5 70 s on the broadband waveforms of teleseismic events estimated crustal and lithospheric thicknesses at 15 broadband sites led to the delineation of an updoming of 4 7 km in the moho and 6 12 km in the asthenosphere beneath the central kr relative to the surrounding unrifted regions the moho depths vary from 35 to 42 km and lithospheric thicknesses range from 62 to 78 km the observed marked shear velocity reduction 2 6 at the lithosphere asthenosphere boundary lab over an area of 120 km 80 km suggests the presence of patches of partial melt in the asthenosphere below the kr evidencing the possible presence of imprints of the deccan reunion mantle plume the coincidence of the area of updoming of the moho and asthenosphere and the confined aftershock activity below the central kr implies that these patches could provide a high input of volatile co 2 into the lower crust which might play a key role in generating the continued aftershock activity in the region since the occurrence of the 2001 m w 7 7 bhuj mainshock

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