sediment dewatering in the makran accretionary prism abstract sediments within the makran accretionary prism are considerably more compacted than the material entering the subduction zone due to the additional tectonic stresses resulting from plate convergence and accretionary processes tectonic compaction of the offscraped sediments has been documented by deriving in situ porosities in the upper 4 km from their seismic velocities wide angle seismic reflection refraction profiles were analysed from a transect crossing both the makran accretionary prism and the undeformed abyssal plain sediments to the south the 6 7 km thick abyssal plain sediments exhibit normal compaction with depth the sediments incorporated into the accretionary prism have higher seismic velocities at all depths corresponding to reductions in porosity of 30 40 mapping the porosity depth gradient across the prism shows that there is a marked change in porosity at the seaward toe of the prism the frontal fold there is then little further change until further folding begins to occur 70 km landward of the toe the dewatering and consolidation of the sediments is an important control on deformation occurring within the sedimentary prism

sedimentary characteristics and origin of the late pleistocene ice complex on north east siberian arctic coastal lowlands and islands a review abstract the origin of late pleistocene ice rich fine grained permafrost sequences ice complex deposits in arctic and subarctic siberia has been in dispute for a long time corresponding permafrost sequences are frequently exposed along seacoasts and river banks in yedoma hills which are considered to be erosional remnants of late pleistocene accumulation plains detailed cryolithological sedimentological geochronological and stratigraphical results from 14 study sites along the laptev and east siberian seacoasts were summarized for the first time in order to compare and correlate the local datasets on a large regional scale the sediments of the ice complex are characterized by poorly sorted silt to fine sand buried cryosols toc contents of 1 2 4 8 wt and very high ground ice content 40 60 wt absolute a second large constituent of the ice complex are large syngenetic ice wedges resulting in a total ice content of the ice complex of up to 80 vol ice complex deposits were mostly formed during the middle weichselian interstadial and or during the late weichselian stadial periods a conceptual model of nival lithogenesis of ice complex deposits was developed that integrates various other formation hypotheses a combination of various local and regional paleogeographical geological and paleoclimate conditions controlled the formation of ice complex sequences during the late pleistocene in northern east siberia they are chronologically and stratigraphically but not genetically equivalent to eurasian and alaskan loess deposits

preliminary paleoenvironmental analysis of permafrost deposits at batagaika megaslump yana uplands northeast siberia a megaslump at batagaika in northern yakutia exposes a remarkable stratigraphic sequence of permafrost deposits 50 80 m thick to determine their potential for answering key questions about quaternary environmental and climatic change in northeast siberia we carried out a reconnaissance study of their cryostratigraphy and paleoecology supported by four rangefinder 14c ages the sequence includes two ice complexes separated by a unit of fine sand containing narrow syngenetic ice wedges and multiple palaeosols overall the sequence developed as permafrost grew syngenetically through an eolian sand sheet aggrading on a hillslope wood remains occur in two forest beds each associated with a reddened weathering horizon the lower bed contains high amounts of larix pollen 20 plus small amounts of picea and pinus pumila and is attributed to interglacial conditions pollen from the overlying sequence is dominated by herbaceous taxa 70 80 attributed to an open tundra landscape during interstadial climatic conditions of three hypothetical age schemes considered we tentatively attribute much of the batagaika sequence to marine isotope stage mis 3 the upper and lower forest beds may represent a mid mis 3 optimum and mis 5 respectively although we cannot discount alternative attributions to mis 5 and 7

basement drilling in the western atlantic ocean 2 a synthesis of construction processes at the cretaceous ridge axis petrologic geochemical and geophysical studies of cores from dsdp sites 417 and 418 show that the cretaceous eruptive crust was formed at a stable slow spreading axis similar to the present mid atlantic ridge probably not close to major transform fractures interpretations of whole rock and glass chemical variation suggest that magma fractionation occurred in temporally and spatially transient storage reservoirs at various depths between the zone of magma segregation and the ridge crest we suggest that melt generation uprise and eruption are part of a related episode of activity involving rapid upward propagation of fractures eruptive episodes are recorded by stratigraphic sequences of contiguous mostly cogenetic lavas separated by breccias sediment and or extreme alteration zones suggestive of lengthly quiescent intervals the section drilled at 417d comprises three such episodes in 365 m of basement whereas seven are recorded in 418a in 540 m patterns of downhole changes in magnetic inclinations and polarities strongly support the suggestion that iterative burial of successive lava episodes occurs with crustal spreading from the axis this model predicts that the eruptive layer is imbricated and that it shows increasing complexity of lithology chemical compositions and magnetic character with depth studies of crack distribution and laboratory measurements of core physical properties indicate preferential zones of water circulation which decrease with depth permeability of the lava pile is largely controlled by the imbricate eruptive structure

a comparison of statistical and deterministic wiener deconvolution of deep tow seismic data wiener spiking deconvolution of seismic traces in the absence of a known source wavelet relies upon the use of digital filters which are optimum in a least squares error sense only if the wavelet to be deconvolved is minimum phase in the marine environment in particular this condition is frequently violated since bubble pulse oscillations result in source signatures which deviate significantly from minimum phase the degree to which the deconvolution is impaired by such violation is generally difficult to assess since without a measured source signature there is no optimally deconvolved trace with which the spiked trace may be compared a recently developed near bottom seismic profiler used in conjunction with a surface air gun source produces traces which contain the far field source signature as the first arrival knowledge of this characteristic wavelet permits the design of two sided wiener spiking and shaping filters which can be used to accurately deconvolve the remainder of the trace in this paper the performance of such optimum lag filters is compared with that of the zero lag one sided operators which can be evaluated from the reflected arrival sequence alone by assuming a minimum phase source wavelet results indicate that the use of zero lag operators on traces containing non minimum phase wavelets introduces significant quantities of noise energy into the seismic record signal to noise ratios may however be preserved or even increased during deconvolution by the use of optimum lag spiking or shaping filters a debubbling technique involving matched filtering of the trace with the source wavelet followed by optimum lag wiener deconvolution did not give a higher quality result than can be obtained simply by the application of a suitably chosen wiener shaping filter however cross correlation of an optimum lag spike filtered trace with the known actual output of the filter when presented with the source signature is found to enhance signal to noise ratio whilst maintaining improved resolution

pedogenic modification of loess significance for palaeoclimatic reconstructions abstract this review considers the role of pedogenic processes in modifying wind blown dust loess concentrating particularly on the ways that resulting properties may be interpreted as indicators of past climatic conditions and changes emphasis is placed on the sequences of palaeosols developed within loess deposits that are frequently regarded as some of the best terrestrial equivalents of marine sediment records of long term global climatic change a palaeosol is generally interpreted in terms of the broad pedogenic processes and environments assumed to be currently responsible for that type of soil forming at the present surface even the very presence of a palaeosol may have palaeoclimatic significance however in that it is often taken to indicate a period of relative land surface stability and warmer and or moister conditions between cold and or arid phases of loess accumulation in reality it may be more useful to consider many loess palaeosol sequences in terms of changing balances between pedogenesis and loess accumulation over geological time in most regions it seems that the balance swings towards pedogenesis during interglacials or interstadials when sediment supply and transport are limited and the climate is warmer and or wetter where accumulation rates are still appreciable during these soil forming intervals however the soils and palaeosols may be accretionary with surface build up keeping pace with pedogenesis welding may also occur where covering sediments are insufficiently thick to isolate an underlying palaeosol from the effects of pedogenesis active at a new land surface further complications occur due to reworking of palaeosols and syndepositional pedogenic alteration of loess units generally such pedocomplexes can only be deciphered if the different pedogenic geomorphic and sedimentary processes are identified and ordered within a pedosedimentary reconstruction a recent trend has been to treat some loess palaeosol sequences as quasi continuous time series particularly when comparing depth functions of climatic proxy properties such as magnetic susceptibility and grain size with marine and ice core isotope curves

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

geochemistry of the pliocene red clay formation in the chinese loess plateau and implications for its origin source provenance and paleoclimate change abstract recent field observations and sedimentological studies suggest the eolian origin of the late tertiary red clay formation underlying the pleistocene loess soil sequence in the loess plateau thereby extending the wind blown record from 2 6 ma down to 7 0 ma in north central china to address the source provenance of the red clay major and trace element concentrations were analyzed on the samples from a north south loess transect of the loess plateau and a red clay section at jiaxian results show that there is a good agreement of both major and trace element compositions between the last glacial interglacial loess soil units and the jiaxian red clay record the ree distribution patterns of the loess soil and red clay samples are remarkably similar in shape with enriched lree and fairly flat hree profiles and clear negative eu anomaly the geochemical features of the red clay are also identical to those of upper continental crust these chemical characteristics provide further evidence for eolian sedimentation of the red clay from the relations both between na2o al2o3 and k2o al2o3 ratios and between u pb and th pb ratios the eolian loess and red clay materials must have been subject to thorough sedimentary differentiation and moderate chemical weathering in the source area the chemical similarity between soil unit s1 and the red clay implies that such an environment as occurred in the last interglacial period may have already formed in northwestern china during the late tertiary comparison of down section variations in elemental ratios with field observations of pedogenic development of the red clay suggests that cao al2o3 sr rb and sr ba are more sensitive to climate changes than magnetic susceptibility and can be used to reconstruct the climate history of the late cenozoic

global changes during carboniferous permian glaciation of gondwana linking polar and equatorial climate evolution by geochemical proxies the most prevalent phanerozoic glaciation occurred during the carboniferous permian on the southern hemisphere gondwana supercontinent sediments from the pennsylvanian dwyka group deposited in the karoo basin of south africa provide a complete record of glaciation and deglaciation phases the direct correlation of glaciation events in southern gondwana basins with the well studied climate evolution of equatorial regions was previously hampered by lack of precise radiometric dating as dating has now become available for the karoo basin the gondwana glaciation can be viewed in a global paleoclimatic framework with high temporal resolution element geochemical proxies cia chemical index of alteration zr ti rb k v cr record three confined shifts in climate and paleoenvironment of the karoo basin these shifts were induced by changes in sea level weathering rate provenance and redox conditions because of the low availability and diagenetic overprint of carbonates ocean and atmosphere pco 2 variations had to be reconstructed from 1 3 c o r g values of marine organic matter the 1 3 c o r g signatures are affected by variable proportions of marine versus terrestrially derived organic matter and its state of preservation organic geochemical investigations toc total organic carbon c n lipid biomarkers indicate the organic matter in the central karoo basin was primarily of algal origin in agreement with element proxies the varying 1 3 c o r g values mirror shifts in pco 2 rather than variations of organic matter type a covariation trend between carbon isotope signatures of equatorial carbonates and 1 3 c o r g values from the karoo basin argues against local forcing factors and instead implies a global climate control mechanism the 5 7 m y duration of a complete glacial cycle is not in tune with any known orbital frequency processes such as changes in equator pole temperature gradients or newly developing atmosphere ocean circulation pathways can be regarded as controlling factors

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

soils as indicators of quaternary climatic change in mid latitude regions abstract soils result from changes to uppermost layers of the earth s crust occuring beneath stable land surfaces soil properties are determined by five factors or groups of factors climate organisms relief parent material and time quantitative climatic interpretation of soils depends upon mathematical relationships between individual soil properties and climatic factors i e climofunctions these are difficult to establish and often cannot be applied to diagenetically modified buried soils so most climatic interpretations of soils are qualitative or semi quantitative soil features formed by frost action are the most reliable climatic indicators most soils continue to develop over time however many soil dating techniques provide only a single date some time during the period of soil formation the beginning and end of a soil development period are best determined by reference to dated deposits over as large an area as possible but stratigraphical interpretation of soils is still impeded by imprecise definitions and procedures the soil chronosequences developed in the major loess deposits of europe and asia provide the best evidence for climatic change throughout the quaternary however only major glacial interglacial cycles are well expressed to obtain more detailed evidence of weaker interstadial cycles it is necessary to study chronosequences in mid latitude regions which were generally moister for much of the quaternary however these soils are less well preserved and present additional difficulties of interpretation because of greater variation in parent material relief and time as soil forming factors

molecular fossils in a pleistocene river terrace in southern china related to paleoclimate variation abstract the red paleosols oxisols in southern china provide a record of global climate oscillation during at least the last 0 80 ma here we present a correlation between the marine oxygen isotope record stage 11 21 and the paleosol molecular fossil signature in the fourth xiushui river terrace in southern china temporal variations seen in two molecular ratios c15 21 c22 33n alkanes and pristane to phytane pr ph down the paleosol profile are postulated to have arisen from variations in biological input and subsequent pedogenesis in response to the changing strength of the east asian summer monsoon climate the preliminary findings suggest strong couplings among the soil vegetation and climate and provide a basis for a correlation between the terrestrial and oceanic records

chemical weathering of the loess deposits in the lower changjiang valley china and paleoclimatic implications abstract loess deposits xiashu loess are widely distributed in the lower changjiang valley east china and are considered to be of eolian origin with a similar provenance to the well documented loess of north central china the xiashu loess is characterized by strong dissolution of carbonate and considerable weathering of some silicate minerals especially plagioclase chemical mobility of ca sr na mg was observed in the xiashu loess sediments whilst most transition metals are relatively enriched chemical index of variation cia and element ratios of k na and k ca record several paleoclimate events in the lower changjiang valley during the mid late quaternary corresponding with isotopic records in deep sea sediments east china might be governed by stronger summer monsoons than north central china during interglacial periods of the mid late quaternary which may somewhat obscure the record of winter monsoons during glacial periods

deep tow studies of the vema fracture zone 1 tectonics of a major slow slipping transform fault and its intersection with the mid atlantic ridge the vema transform fault which slips at a rate of 24 mm yr displaces the mid atlantic ridge mar 320 km in a left lateral sense high resolution deep tow studies of the vema ridge transform intersection rti and the eastern 130 km of the active transform fault reveal a complex pattern of dip slip and strike slip faults which evolve in time and space at the intersection both the neovolcanic zone and the west wall of the mar rift valley curve counterclockwise toward the transform fault along trends approximately 30 oblique to the regional north south trend of the spreading axis the curving of extensional structures in the rift valley such as normal faults and the axial zone of dike injection appears to be related to transmission of transform related shear stresses into the spreading center domain intermittent locking of the american and african lithospheric plates across the rti causes shear stresses to penetrate up to 4 km into the mar axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block faulted west wall of the rift valley where the lithosphere is thicker the degree of shear coupling across the rti may vary with time due to changes in the thickness of the lithosphere along the axis 0 10 km the strength of a mantle weld at depth and the presence or absence of an axial magma chamber so that extensional structures at the rti may be either spreading center parallel when coupling is weak or oblique when coupling is strong oblique extension across the rti in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin the easternmost 20 km of the active transform fault zone near the rti displays a braided network of three to nine tectonically active grabens and v shaped furrows in a zone 2 4 km wide interpreted to consist of interwoven riedel shears p shears and oblique normal faults clay cake deformation experiments and deep tow observations suggest that p shears and r shears which are 10 20 oblique to the transform slip direction develop during the initial stages of transform faulting near the rti as the newly accreted lithosphere accelerates to full plate velocity some of the r shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the rti subsequent to this merging of the two fault types some of the r shears develop a significant component of dip slip while other r shears merge with p shears creating a complex anastomosing fault pattern up to 4 km wide a continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the rti twenty kilometers west of the rti the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays this narrow groove cuts through thinly sedimented basalt 20 40 km west of the rti and continues as a narrow furrow less than 100 m wide through up to 1 5 km of layered turbidite fill most of the way to the western rti such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old thick and rigid and has completed its acceleration to full plate velocity the transform fault zone is closely associated with a partially buried median ridge and widens to 1 2 km where it transects exposed portions of the ridge the transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro metagabbro and basalt

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

gloria investigations of oceanic fracture zones comparative study of the transform fault zone the tectonic patterns of some twenty spreading centre offsets have been determined using the long range sidescan sonar gloria and are reviewed in this paper offsets of less than about 20 km fail to produce transform faults but are accommodated by short sections of oblique spreading with overlapping spreading centres in the fast spreading case at slow slip rates such offsets can be associated with topographic fracture zones that are indistinguishable from those produced at true transform faults such features may account for a significant proportion of spreading centre offsets on slowly spreading mid ocean ridges offsets larger than about 30 km produce a true 9transform fault9 this comprises a 1 to 5 km wide band of individual faults that are parallel or subparallel to the spreading direction the band corresponds to the 9transform fault zone9 tfz of continental wrench faulting it includes the 9principal transform displacement zone9 ptdz and secondary structures such as riedel shears a narrow tfz is normally associated with a prominent ptdz which appears to be either a narrow furrow or a single scarp on the other hand wider zones often fail to show a clear ptdz on gloria both types are common on the slowly spreading mid atlantic ridge and may even occur at different points along the same large offset transform such as romanche although the narrow type appears to occur preferentially where the local ridge axis is oblique to the spreading direction narrow tfzs and prominent ptdzs appear to be more common on the fast spreading east pacific rise it is suggested that broader tfzs and less prominent ptdzs may form in response to the complicating effects of small components of compression across the transform fault perhaps as a result of departure of the transform fault trace from a true small circle such compression would be more easily supported in the thicker stronger lithosphere of slowly spreading ridges but could be relieved by tension due to the ridge push force where the ridge is oblique to the spreading direction

bimodal grain size distribution of chinese loess and its palaeoclimatic implications grain size analysis indicates that chinese loess generally shows a bimodal distribution with a coarse and a fine component the coarse component comprising the main part of the loess has pronounced kurtosis and is well sorted which is interpreted to be the product of dust storms generated by low altitude northwesterly winds its grain size reflects the strength of the low altitude circulation in the dust seasons of the year and its percentage provides an indicator of the source area aridity and the frequency of dust storms conversely the fine component has a wide grain size range and is poorly sorted sedimentary illustrations based on the grain size distribution characteristics of bulk samples and of detrital quartz suggest that the fine component probably represents the background dust load of the atmosphere and is mainly transported by high altitude westerly airstreams its grain size provides an estimate of the westerly air stream intensity the coarse and fine components of a loess sample can be mathematically separated by fitting a designated mathematical distribution function to the measured grain size data and this procedure constitutes an approach for reconstructing the palaeowind system of northern china c 2003 elsevier b v all rights reserved

multisensory satellite observations of the expansion of the batagaika crater and succession of vegetation in its interior from 1991 to 2018 on site monitoring in large areas located in inaccessible regions can be difficult and costly thus remote sensing is an essential tool for mapping and monitoring changes in such regions therefore this paper describes long term multisensory satellite observations of the expansion of the batagaika crater in northern siberia and natural succession of vegetation in its interior from 1991 to 2018 landsat 5 tm landsat 7 etm landsat 8 oli tirs imageries were mainly used as a data source for analyses although sentinel 2a imagery and dem image from aster satellite were also employed for calculating a vegetation index and expansion in the crater area the observations were conducted in years 1991 2018 and were made in a summer season the results reveal that the crater area increased by almost three times during these 27 years and that the fastest expansion took place between 2010 and 2014 with 22 7 increment the analysis of elevation of the crater revealed that in 2018 its maximum depth was ca 70 m and that depth was decreasing towards its north east tail additionally the satellite imagery of land surface temperature which is a driving force of crater expansion was visualized for chosen hot days within the time frame 2010 2018 the study of temporal and spatial changes in ndvi spatial distributions inside the crater revealed also a high rate of the succession of vegetation which may reduce melting of permafrost inside the batagaika crater and its further expansion

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

magnetic susceptibility evidence of monsoon variation on the loess plateau of central china during the last 130 000 years the magnetic susceptibility of loess and paleosols in central china represents a proxy climate index closely related to past changes of precipitation and vegetation and thus to summer monsoon intensity time series of magnetic susceptibility constructed for three loess paleosol sequences in the southern part of the chinese loess plateau document the history of summer monsoon variation during the last 130 000 yr they correlate closely with the oxygen isotope record of stages 1 to 5 in deep sea sediments soils were forming during intervals of strong summer monsoon whereas loess units were deposited at times of reduced monsoon intensity the chinese loess paleosol sequence can thus be viewed as a proxy record of asian monsoon variability extending over the last 2 5 myr

deep tow seismic profiles from the vema transform and ridge transform intersection a recently developed near bottom reflection profiling system has been used to obtain high resolution seismic sections across the vema transform fault at the western ridge transform intersection deep towed 3 5 khz echo sounder data show evidence of fresh lava flows at the seabed within the nodal deep over one kilometre of sediment fills the bottom of the ridge transform intersection deep many strong discontinuous reflectors within the sedimentary section caused by lava layers indicate that the intersection deep has been the site of sustained igneous activity on the eastern side of the deep is a large uplifted fault block created as the newly formed crust moved out of the spreading centre dipping strata within the fault block document back tilting as the new crust is extended by normal faulting profiles across the active transform fault reveal that the zone of deformation throughout the one kilometre thick sediment column is narrow suggesting that the location of the fault has remained stable throughout at least the last million years

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

neodymium and strontium isotopic constraints on soil sources in barbados west indies abstract neodymium and strontium isotopic compositions and sm nd ratios are used to constrain the sources of silicate rich soils developed on uplifted pleistocene coral reef limestones on barbados west indies the geographic and geologic setting of barbados facilitates the application of these tracers to the evaluation of the following soil sources 1 pleistocene reef limestone regolith 2 tertiary carbonate rocks sandstones and mudstones that are exposed in northeastern barbados 3 volcanic ash erupted from the lesser antilles arc 4 saharan dust transported by trade winds and 5 fertilizer the soils have nd values that range from 6 6 to 1 9 87 sr 86 sr values of 0 70890 to 0 71067 and sm nd ratios of 0 223 0 260 the pleistocene limestone component is the most significant source of sr in the soils and a negligible source of nd comparison of sm and nd concentrations and neodymium isotopic compositions of soil samples that are weathered to varying extents indicates that sm and nd are relatively unfractionated and retained in the soils during weathering nd and sm nd variations in the soils therefore primarily reflect the compositions and proportions of the silicate sources mass balance calculations based on sm nd systematics require that the silicate soil components contain between 30 85 volcanic ash with the remaining silicate fraction comprised of old continentally derived sediment in contrast to sm and nd sr is mobilized and removed from the soils during weathering strontium from volcanic and carbonate sources is preferentially removed relative to continental silicate sources the strontium isotopic compositions of the soils therefore reflect the combined effects of the degree of weathering and the compositions and proportions of the soil sources mass balance calculations indicate that at least 35 60 of the initial sr in the soils has been removed by weathering these results illustrate 1 the utility of radiogenic isotopes in identifying and quantifying soil sources and weathering processes 2 the compositional influence of numerous sources on soils even those developed in a relatively isolated area such as barbados and 3 the domination of barbados soil sm nd systematics by nonregolith eolian components

margin segmentation and volcano tectonic architecture along the volcanic margin off argentina uruguay south atlantic abstract some 25 000 kilometers of regional multichannel seismic data acquired by bgr along the continental margins off argentina and uruguay document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusives manifested in the seismic data by huge wedges of seaward dipping reflectors sdrs the emplacement of the deeply buried 60 120 km wide sdrs was probably episodic as documented by at least three superimposed sdrs units distinct along margin variations in architecture volume and width of the sdrs wedges are probably related to margin segmentation it is suggested that the margin can be divided at least in four compartments bounded by the falkland fracture zone falkland transfer the colorado transfer the ventana transfer and the salado transfer the individual margin segments reflect in the distribution and thickness of the post rift sediments the individual transfer zones may have acted as rift propagation barriers selectively directing rift segments in left stepping patterns along the western south atlantic margin although we found extensive variations in the architecture style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the sdrs wedges decreases northward within the individual margin segments we suggest that mainly adiabatic decompression and melt generation from shallow sources can explain distinct along margin variations in the volcano tectonic architecture and volumes of extruded magmas

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

serpentinization faults and their role in the tectonics of slow spreading ridges two major families of faults dominate the tectonics of slow spreading ridges the transform faults paralleling the spreading direction and the gloria faults observed by large scale side scan sonar normal to it the observation of microearthquakes to 8 km depth beneath the mid atlantic ridge axis implies that not only the transform faults but the gloria faults as well penetrate into the mantle since large horizontal temperature gradients are often associated with them both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle under these conditions serpentinization of upper mantle rocks will occur in a predictable way its influence on the tectonic development of slow spreading ridges is discussed

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

the nature origin and accumulation of loess abstract loess can be defined simply as a terrestrial clastic sediment composed predominantly of silt size particles which is formed essentially by the accumulation of wind blown dust there are three fundamental requirements for its formation 1 a sustained source of dust 2 adequate wind energy to transport the dust and 3 a suitable accumulation site most loess has been modified to some degree by local reworking bioturbation and syn depositional weathering but a process of loessification is not necessary for a dust deposit to qualify as loess the terms primary loess and secondary loess have previously been used to described wind deposited loess and redeposited loess respectively but it is more accurate to describe material reworked and redeposited by non aeolian processes as loess derived colluvium or loess derived alluvium during the quaternary loess formed in three main situations 1 mid continental shield areas beyond the limits of ice sheets periglacial loess 2 on the margins of high mountain ranges perimontane loess and 3 on the semi arid margins of some lowland deserts peridesert loess peridesert loess is relatively poorly developed partly due to the limited efficiency of silt generating mechanisms in low land deserts compared with glaciation but also important is the tendency in arid areas for dust to be dispersed over a wide area and to be rapidly reworked after deposition particularly in areas which experience frequent climatic changes the remarkably thick loess of china and central asia results from an unusual combination of conditions which has persisted for much of the last 2 3 million years namely rapid uplift of the tibetan plateau and surrounding mountain ranges high rates of sediment production and supply to adjacent basins a strong northwesterly and westerly wind regime and the existence of effective dust traps downwind of the source regions

tectonics of slow spreading mid ocean ridges and consequences of a variable depth to the brittle ductile transition geophysical evidence especially microearthquakes that extend into the upper mantle beneath the inner floor of the mid atlantic and gorda ridges indicates that the axial magma chamber episodically freezes beneath slow spreading ridges freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences most importantly large near axis faults will intersect as the brittle ductile transition deepens causing some of the faults to become locked the topographic effect of fault locking will be a variation in the width of the inner floor from narrow 10 km when a magma chamber is absent the deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle the nature of these faults at depth is unknown but the following hypotheses are presented 1 the faults may be planar and pass downward into aseismic ductile shear zones and 2 the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow such low angle normal faults are common in continental extensional domains large scale tilting 50 70 is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks similar to toppling dominoes calculations of temperatures at the brittle ductile transition are made assuming olivine and diabase rheologies for the mantle and crust respectively the temperature is approximately 800 c when the transition is at 8 km depth and approximately 600 when the transition is at 2 km depth the strength of the lithosphere varies dramatically with the rise and fall of the brittle ductile transition as magma chambers form and subsequently freeze strong lateral temperature gradients will be present when a magma chamber is formed resulting in localization of faulting within thin weak crust above the chamber when the magma chamber freezes lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area this results in active faulting occurring over a width of as much as 60 km some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges the entire crustal section of the josephine ophiolite was tilted by approximately 50 prior to deposition of overlying pelagic sediments this tilting probably took place by faulting suggesting that oceanic faults extended into the upper mantle possible oceanic faults have been identified in the josephine ophiolite but their oceanic origin cannot be established beyond doubt

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

loess stratigraphy in central china abstract the loess deposits in central china record world wide climate changes of the last 2 5 ma numerous climatic oscillations are marked by alternating loess and soil units with continuous coverage of the matuyama and brunhes epochs magnetic susceptibility of the deposits correlates closely with the oxygen isotope record of the deep sea sediments and provides an independent measure of climate and time the key marker bed of the chinese loess sequence is the paleosol s5 the time equivalent of the mid brunhes oxygen isotope stages 13 14 and 15 it marks a prolonged interval of warm humid climate lasting from approximately 615 to 470 thousand years ago several episodes of river downcutting coincide with deposition of the exceptionally thick loess units l1 oceanic oxygen isotope stages 2 to 4 l2 stage 6 l5 stage 12 l6 stage 16 l9 stage 22 l15 stage 38 about 1 15 ma and ws4 about 2 3 ma these erosional events are interpreted as a result of episodic uplift of the loess plateau the deposition of the earliest loess layers between 2 5 and 2 3 ma ago marks a first order shift from warm and humid environments toward harsh continental steppes comparable to those of the middle and late pleistocene little if any lithologic or paleontologic changes were noted within or above the olduvai magnetozone so that the proposed plio pleistocene boundary at approximately 1 65 ma has no lithostratigraphic and biostratigraphic representation in the chinese loess series comparison with the deep sea drilling program core 552a in the north atlantic and with the santerno river section near bologna shows that the occurrence of the earliest loess in china coincides with the timing of the first significant ice rafting in the north atlantic and with the appearance of cold water foraminifers in the marine deposits of northern italy an extension of the stage system of the oxygen isotope signal extended back to the gauss matuyama boundary is proposed

magnetostratigraphy of the baoji loess paleosol section in the north central china loess plateau the baoji section is judged to be the most complete accessible quaternary loess paleosol section in the north central china loess plateau it is located 5 km north of the city of baoji it contains 37 paleosols and was recently named the type pedostratigraphic section the pedostratigraphic units geosols of the baoji section can be correlated throughout the loess plateau by a combination of magnetostratigraphy and character position and association of pedostratigraphic units magnetic properties were determined from 483 samples taken from 159 m of loess that overlies the red clay formation all major reversals that occurred in the last 2 5 ma were recorded the brunhes matuyama boundary occurs at a depth of 58 5 m within loess unit l8 the jaramillo subchron between 72 0 m in loess unit l10 and 76 4 m in paleosol s11 the olduvai subchron between 125 5 m in loess unit l25 and 131 5 m in loess unit l27 and the gauss matuyama boundary at 157 8 m within loess unit l33 just above the contact with the red clay formation this is the only known site where all major magnetic reversals are clearly defined in distinct field identifiable loess soil units in one section the magnetostratigraphy of the baoji section is thus recommended as the type section for the loess plateau

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

major and trace elements δ13c and polycyclic aromatic hydrocarbons in the late pleistocene ice wedges a case study of batagay yedoma central yakutia abstract we studied the ionic composition of ice the content of suspended and dissolved trace and major elements in ice values of 13 and the composition of pahs of inclusions in the late pleistocene syngenetic ice wedges of the batagay yedoma for the first time the complex geochemical study of batagay megaslump was carried out the ice of the upper and lower complex of different ages was studied mineralisation of the late pleistocene ice wedges of batagay yedoma ranges from 66 56 to 424 8 mg l 1 from ultrafresh to desalinated the ionic composition of the ice is bicarbonate calcium and corresponds to snow formed under the influence of continental air masses the proportion of ca contained in dissolved form is not more than 20 30 as for the other elements the proportion of mg k na al fe mn sr ba in suspended form is greater than 90 the content of trace elements in inclusions in ice wedges and soils is equal which confirms the pedogenic origin of the inclusions in ice according to our data values of 13 in lipids of the pedogenic material in ice decrease with depth the lowest values values of 13 less than 30 were observed in the ice wedge of the upper complex at a depth of 7 4 8 6 m the increase of 13c values of lipids from pedogenic material in ice wedges with depth is due to the landscape change from grassland ecosystems to forest ecosystems and may also be influenced by the increased frequency of fires that occurred during the transition to forest ecosystems and the disappearance of large herbivores

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

seismic evidence for hydrothermally altered mantle beneath old crust in the tydeman fracture zone abstract to study the structure of old atlantic fracture zones seismic refraction profiles were obtained using ocean bottom seismometers and two ship multichannel expanding spread experiments on two inactive sections of the tydeman fracture zone the results indicate that a crustal seismic velocities are low compared with normal oceanic crust b there is a marked absence of an identifiable layer 3 c the depth to the moho is shallow though not exceptionally so and d anomalously low seismic velocities of 7 2 7 5 km s 1 are observed over ranges at which upper mantle velocities are normally detected s wave arrivals corresponding to both crustal and mantle refractions are also observed which enable the first estimates of poisson s ratio within a fracture zone to be made using these results and those from other surveys we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age these findings suggest that hydrothermal circulation which can lower the upper mantle velocity by serpentinization persists under fracture zones for at least 70 ma and occurs at greater depths than for normal oceanic crust the serpentinization process is not sufficiently rapid to obscure the location of the original crust mantle boundary

in situ study of the eastern ridge transform intersection of the vema fracture zone abstract fourteen dives of the submersible nautile have been carried out at the eastern intersection of the vema fracture zone with the mid atlantic ridge direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley the present day displacement zone corresponding to an extremely narrow less than 300 m transform fault zone is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform parallel 30 100 m high ridges of basaltic fragments and pelagic sediments active splays of the transform fault zone lie generally between these ridges creating steep v shaped furrows despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it the surface manifestations of the tectonic activity along the present day displacement zone include r riedel shears low angle shears p shears en echelon structures open fissures small pull apart structures and funnel alignments in the sedimentary cover the transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge the transform parallel topography defines a 10 km wide zone with dip slip motion the ridge transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the kane and oceanographer fracture zones recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area the present day displacement zone cross cuts the neovolcanic zone and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the african plate the extensive volcanism is suggestive of a magmatic production phase in the intersection area extremely fresh pillowed lava forms have been observed at the rti inside corner high indicating possible off axis volcanism

crustal structure of atlantic fracture zones ii the vema fracture zone and transverse ridge summary the crustal structure beneath the vema fracture zone and its flanking transverse ridge was determined from seismic refraction profiles along the fracture zone valley and across the ridge relatively normal oceanic crust but with an upwarped seismic moho was found under the transverse ridge we suggest that the transverse ridge represents a portion of tectonically uplifted crust without a major root or zone of serpentinite diapirism beneath it a region of anomalous crust associated with the fracture zone itself extends about 20 km to either side of the central fault gradually decreasing in thickness as the fracture zone is approached there is evidence to suggest that the thinnest crust is found beneath the edges of the 20 km wide fracture zone valley under the fracture zone valley the crust is generally thinner than normal oceanic crust and is also highly anomalous in its velocity structure seismic layer 3 is absent and the seismic velocities are lower than normal the absence of layer 3 indicates that normal magmatic accretionary processes are considerably modified in the vicinity of the transform fault the low velocities are probably caused by the accumulation of rubble and talus and by the extensive faulting and fracturing associated with the transform fault this same fracturing allows water to penetrate through the crust and the apparently somewhat thicker crust beneath the central part of the fracture zone valley may be explained by the resultant serpentinization having depressed the seismic moho below its original depth

recent faulting and microearthquakes at the intersection of the vema fracture zone and the mid atlantic ridge new air gun reflection profiles 3 5 khz reflection profiles and microearthquake data recorded by an array of ocean bottom seismographs form the basis for this study of the transition from a spreading center to a major transform fault disturbances of the thick normally flat lying turbidite deposits provide indications of recent vertical motions at the western intersection of the fracture zone with the median valley there is a depression in the sediments that represents the southerly extension of the median valley into the fracture zone valley the depression is terminated abruptly on the south by the active transform fault which acts as a locus for vertical as well as horizontal displacement flat lying undisturbed sediments terminate abruptly at the fault the western boundary of the depression is much broader and is characterized by a series of slumplike steps to the west there is little or no evidence for uplift or tilting of sediments which might indicate vertical recovery of the crust as it spreads away from the depression this suggests that uplift and recovery out of the depression is episodic in nature and has been inactive over the last million years along the western boundary to the east there is clear evidence of uplift and tilting of sedimentary layers a basement ridge emerging from the sediments is currently being uplifted and rotated in a manner analogous to processes responsible for the creation and cancellation of median valley relief the transition between the spreading center and the transform fault appears to take place within 1 2 km the width of the transform fault just east of the depression is less than a kilometer microearthquakes were located and displayed by new methods that directly account for nonlinearities associated with small arrays microearthquakes located by three or more ocean bottom seismometers show that the greatest seismic activity occurs along the eastern walls of the median valley at the basement ridge in the eastern portion of the depression and in the crestal mountains very little activity is associated with the western edge of the transform depression and the trace of the transform fault

mechanism for generating the anomalous uplift of oceanic core complexes atlantis bank southwest indian ridge atlantis bank is an anomalously uplifted oceanic core complex adjacent to the atlantis ii transform on the southwest indian ridge that rises 3 km above normal seafloor of the same age models of flexural uplift due to detachment faulting can account for 1 km of this uplift postdetachment normal faults have been observed during submersible dives and on swath bathymetry two transform parallel large offset hundreds of meters normal faults are identified on the eastern flank of atlantis bank with numerous smaller faults tens of meters on the western flank flexural uplift associated with this transform parallel normal faulting is consistent with gravity data and can account for the remaining anomalous uplift of atlantis bank extension normal to the atlantis ii transform may have occurred during a 12 m y period of transtension initiated by a 10 change in spreading direction ca 19 5 ma this extension may have produced the 120 km long transverse ridge of which atlantis bank is a part and is consistent with stress reorientation about a weak transform fault

regional distribution of the quaternary red clay with aeolian dust characteristics in subtropical china and its paleoclimatic implications abstract grain size and geochemical characteristics of 18 quaternary red clay qrc profiles mainly located in the lower and middle reaches of the yangtze river in subtropical china were studied the results showed that all the qrc profiles can be classified into three types type a mainly distributed between 29 n and 31 n share the similar characteristics with the xiashu loess with the uniformity of grain size and elemental compositions and the enrichment of the basic dust fraction 10 m 63 m which may originate from aeolian dust deposits and is hence called a loess like qrc type b mainly distributed between 25 n and 28 n shows alluvial features with many coarse grains and large scale vertical variation of grain size and chemical compositions type ab distributed between type a and type b approximately 28 n 29 n shows double unit structure the upper part is similar to type a the lower however shows alluvial features like type b the loess like qrc is mainly distributed along the yangtze river but gradually declines southwards and is rarely seen south of 28 n the existence of the loess like qrc is strong evidence for the occurrence of heavy dustfalls in the modern red soil areas in subtropical china during the quaternary glacial periods the region distributed with the loess like qrc is at least 3 latitude south of the xiashu loess implying that the loess red clay boundary in the yangtze valley had once oscillated with the alternations between glacial and interglacial climates during the quaternary period the loess like qrc actually includes both glacial and interglacial paleoclimatic information we suggest that the yellow brown earth ybe in the upper qrc was formed during the last glacial period the underlying red clay with the aeolian dust characteristics but being strongly weathered was initially deposited during the second last glacial or earlier glacial stages but heavily weathered and rubefied in the subsequent interglacial events no multi cyclical paleoclimatic records were found in the qrc profiles due to the overlapped information but the weathering degree of the profiles clearly show a trend of decreasing upwards which suggests that the climate in the yangtze valley has become colder and drier since the late quaternary period

geochemical constraints on provenance of the mid pleistocene red earth sediments in subtropical china abstract the source of mid pleistocene red earth sediments in the middle to lower reaches of the yangtze changjiang river was investigated based on their geochemical characteristics the xuancheng and jiujiang red earth sediments have similar major and trace element distribution patterns compared to the loess and paleosol deposits of the chinese loess plateau the upper continental crust ucc and the post archean australian average shale paas the sediments display notable depletion of cao mgo na2o and accumulation of tio2 al2o3 and fe2o3 t the trace element distribution patterns of the red earth sediments are also different from those of loess and the paas but are similar to those of the loess deposits except for lower values of mobile trace elements sr ba and ni and higher values of zr and y the red earth samples have uniform la th ratios of 2 8 compatible with those of the ucc loess and paleosol they also have similar chondrite normalized ree patterns characterized by enriched lree and relatively flat hree profiles and consistent negative eu anomalies similar to those of the ucc the loess and paleosol and the yangtze deposits these results suggest that the red earth sediments have been subject to considerable mixing prior to deposition and strong subsequent chemical weathering the sediments have very uniform 143nd 144nd and 147sm 144nd ratios this points to well mixed and multi recycled sediments the 143nd 144nd and 87sr 86sr values of the red earth sediments match well with those of the deposits in the middle to lower reaches of the yangtze river but are different from those of the loess and paleosols this suggests that the red earth sediments are derived from the drainage basins of the middle to lower yangtze river and might have experienced more intense chemical weathering relative to the yangtze deposits as reflected by their higher rb sr ratios intense depletion of mobile elements and accumulation of immobile elements as well as their well developed net like structure

magnetic susceptibility of the quaternary red clay in subtropical china and its paleoenvironmental implications abstract magnetic susceptibility lf and other magnetic parameters of 11 typical profiles of quaternary red clay qrc in subtropical china were measured lf curves of the qrc profiles show similar vertical variations lf of the uppermost yellow brown earth ybe and uniform red clay urc of the profiles is high mostly ranging from 50 to 150 10 8 m 3 kg 1 that of the reticulate red clay rrc sharply decreases downwards especially in the lower rrc lrrc with dense reticulate pattern lf is often less than 10 10 8 m 3 kg 1 grain size characteristics and geochemical properties of the qrc suggest that the sharp decrease of lf in the rrc is not related to sudden change of parent materials but to the formation of the reticulate pattern further study indicates that pedogenic fine ferrimagnetic minerals fm superparamagnetic particles sp and or stable single domain ssd are the main magnetic carriers of the qrc the ybe and urc with higher lf show low coercivity and magnetic soft behavior the rrc especially the lrrc with weak magnetism shows high coercivity and magnetic hard behavior the temperature dependent susceptibility tds of the qrc suggests that the dominant type of fm in the ybe and urc is pedogenic maghemite the rrc especially the lrrc however contains little maghemite but a trace amount of lithogenic multi domain md magnetite inherited from the parent rock during post depositional hydromorphic processes pedogenic maghemite was more easily dissolved than hematite which caused the sharp decrease of lf in the rrc as lf of the qrc was mainly controlled by the intensity of hydromorphic processes rather than the weathering degree lf of the qrc in subtropical china cannot accurately indicate the paleoclimate during the quaternary period when the red clay was initially formed

deep tow studies of the vema fracture zone 2 evidence for tectonism and bottom currents in the sediments of the transform valley floor the deep tow geophysical instrument package has been used to survey a 65 km segment along the principal transform displacement zone ptdz of the vema fracture zone away from either ridge transform intersection the surface trace of the ptdz is a small groove 5 80 m deep and 20 200 m wide incised in the otherwise flat lying turbidites of the transform valley floor the fault trace cuts across the transform valley diagonally from ridge crest to ridge crest hugging the southern flank of the median ridge the groove is straight or gently sinuous relatively free of the splays and bifurcations which characterize the transform fault zone near the ridge transform intersection macdonald et al this issue a pull apart basin was observed at the location of a slight clockwise bend in the fault trace vertical offsets of turbidites suggest that the west end of the median ridge and the east end of the south wall are rising relative to the transform valley floor at rates of the order of millimeters per year sediment waves were observed buried under 10 20 m of sediment suggesting that antarctic bottom water may have flowed through the transform valley more vigorously several tens of thousands years ago than at present

vema fracture zone central atlantic tectonic and magmatic evolution of the median ridge and the eastern ridge transform intersection domain the eastern vema fracture zone intersection with the mid atlantic ridge mar axis was surveyed with the french submersible nautile during the vemanaute cruise in 1989 at this ridge transform intersection rti an elongated e w ridge more than 50 km long is present in the transform valley this median ridge rises up to 1000 m above the surrounding seafloor the crest of the median ridge is lower and presents an arcuate shape at the tip of the mar axis dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation this detrital cover consists of polymictic sedimentary breccias sandstones and siltstones composed of basaltic doleritic and gabbroic clasts with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation disaggregation mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls this event could be related to the uplift of the southern wall of the fracture zone the transverse ridge which took place probably between 10 and 3 ma ago since this uplift episode the transverse ridge which is now undergoing subsidence and the detritus covered transform valley floor separated by the transform fault zone have migrated westward and eastward respectively vertical tectonics of the median ridge at the approach of the rti can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors a possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western rti is more than 1 km out of isostatic equilibrium recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the mar axis have been recorded on the southern flank of the median ridge several stages in the very recent tectonic volcanic history of the eastern rti that is roughly during the last 300 000 years can thus be defined the lower elevation and narrow arcuate shape of the median ridge east of 40 57 w are inferred to have resulted from tectonic extension during the creation of the nodal basin