gas hydrates clathrates causing pore water freshening and oxygen isotope fractionation in deep water sedimentary sections of terrigenous continental margins the occurrence of gas hydrates in deep water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep sea drilling results on the pacific continental slope off guatemala gas hydrates were brought up for the first time from two holes 497 498a drilled during leg 67 of the dsdp in water depths of 2360 and 5500 m respectively the hydrates occur in organic matter rich pleistocene to miocene terrigenous sediments in the hydrate bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10 20 m subbottom depth pore waters at the bottom of the holes near 400 m subbottom have as little as half the chlorinity of seawater i e 9 similar but less pronounced trends were observed during previous legs of the dsdp in other hydrate prone segments of the continental margins where recharge of fresh water from the continent can be excluded e g leg 11 the crystallization of hydrates like ice excludes salt ions from the crystal structure during burial the dissolved salts are separated from the solids subsidence results in a downward motion of the solids including hydrates relative to the pore fluids thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation the volume of the latter decreases downhole thus causing downward decreasing salinity chlorinity hydrate formation is responsible for oxygen isotope fractionation with18o enrichment in the hydrate explaining increasingly more positive 18o values in the pore fluids recovered after hydrate dissociation with depth

seismic evidence for anomalous crustal structure beneath mesozoic fracture zones in the gambia basin eastern equatorial atlantic abstract five sonobuoy refraction profiles shot in the eastern equatorial atlantic reveal the presence of thin and heterogenous crust in a zone at least 200 km wide near the southern margin of the gambia basin oceanic layer 2 v p 4 1 4 4 km s is detected on all profiles on some sections it is underlain by crustal material with apparent seismic velocities between 5 7 km s and 7 0 km s on others layer 2 directly overlies a zone with apparent velocities characteristic of normal upper mantle velocities 8 km s the formation of anomalous crust is attributed to a modification of accretion processes resulting from a close spacing of fracture zones during a period of slow spreading in the early cretaceous

along axis variations in oceanic crustal density and their contribution to gravity anomalies at slow spreading ridges gravity data from mid ocean ridges are routinely used to infer crustal thickness variations commonly the contribution to the gravity anomaly from mantle density variations are taken into account but variations in crustal density are rarely considered wide angle seismic experiments conducted in the same locations as detailed gravity surveys on the mid atlantic ridge show that the crustal thickness inferred from gravity data can deviate substantially from that derived from seismic data particularly in the vicinity of ridge offsets this discrepancy may be partly attributed to unmodeled crustal density variations at slow spreading ridges crust formed near ridge offsets has consistently lower velocity than crust formed near segment centers and a corresponding density reduction of up to 20 may be inferred

crystallographic and structural transformations of sedimentary chalcedony in flint upon heat treatment the early occurrence of intentional heat treatment of silica rocks has recently become a key element in the discussion about the cultural modernity of prehistoric populations lithic vestiges are the only sources that remain of this process and the understanding of the material s properties and transformations are essential for reconstructing the conditions and parameters applied during heat treatment several models of the structural transformations upon heating have been proposed in the current literature these models are often contradictory and do not account for the most recent structural and mineralogical data on chalcedony in order to propose a new model we elaborated an experimental procedure and applied different techniques involving infrared spectroscopy solid state nmr x ray diffraction and electron microscopy the results show that the major transformation to happen is the loss of silanole sioh and the creation of new si o si bonds according to the reaction si oh ho si si o si h2o this reaction starts between 200 degrees c and 300 degrees c and causes an increase in the hardness of the rocks the maximal annealing temperature and the ramp rate are the functions of the ability of the structure to evacuate newly created h2o and depend on the size of the specimen and the volume of its porosity these results also show that the annealing duration at maximum temperature can be relatively short 50 min for a sufficiently large amount of transformation to be accomplished

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

seismic tomography in boreholes summary synthetic seismic tomographic data sets for cross hole and hole to surface arrays are computed and inverted inversion imaging is performed through an iterative projection solution to a matrix equation the orientation as well as the position of an anomaly relative to the source and recorder arrays affects both the rate of convergence of the solution and the resolution obtained data density and distribution determine the physical validity of the image obtained at convergence borehole tomography is apparently a viable method for imaging inter hole structure

heat treatment in the south african middle stone age temperature induced transformations of silcrete and their technological implications abstract it was recently found that silcrete raw material was heat treated during the south african middle stone age msa for altering its flaking properties this finding led to hypotheses about the implications for the msa hunter gatherers such as the cost of thermal treatment in terms of investment and firewood to date these hypotheses lack a solid basis for data on the thermal transformations of south african silcrete and hence the necessary heating procedure and heating environment is missing in order to produce such data we conducted an experimental study within the framework of the diepkloof project this work is based on the petrographic mineralogical and structural analysis of south african silcrete from the west coast and its thermal transformations our results shed light on the nature of these transformations the ideal heating temperatures and the tolerated heating speed the processes occurring in silcrete are comparable to flint i e the loss of chemically bound water and the formation of new si o si bonds but their intensity is less pronounced effective heating temperatures are significantly higher than for flint and the heating speed tolerated by south african silcrete is relatively fast these findings imply that silcrete heat treatment cannot be directly compared with flint heat treatment unlike flint heating silcrete does not require the setup of a dedicated heating environment and may have been performed in the same time as other fire related activities this would represent only a minor supplementary investment in time and firewood these results have broad implications for the discussion about technological evolution and the acquisition of specialised knowledge in the msa

a note on the correction of ocean floor depths for sediment loading the measured physical properties of sediments recovered in a number of deep sea drill holes in the north atlantic have been used to derive expressions for the correction to basement depths required to account for loading by sediments the average density and porosity over a given thickness of sediment show an approximate linear dependence on sediment thickness this behavior can be explained as primarily the result of compaction if the compaction of the sediments is neglected in correcting basement depths for sediment loading errors of a few hundred meters in the corrected depths can be made a sediment loading correction based on the observed variation of density with depth can give corrected depths to better than 100 m with remaining uncertainties due to variablity in sediment properties and rebound effects on the measured densities a relation between sediment thickness and vertical travel time was obtained from sonic velocity measurements on samples from drill holes sonobuoy wide angle reflection experiments and direct correlation of seismic reflectors with downhole lithology the sediment loading correction can then be expressed directly in terms of the vertical travel time measured along seismic reflection profiles the loading correction obtained here for the north atlantic may not be applicablemore in areas with different sedimentary enviroments when direct measurements of physical properties in drill holes are not available in such areas it is shown that it is preferable to assume a compaction curve for the appropriate lithology rather than to use observed seismic velocities together with a velocity density relationship less

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

methane rich plumes on the carolina continental rise associations with gas hydrates seafloor venting of microbial gases occurs at 2167 m water depth over the blake ridge diapir gas rich plumes were identified acoustically in the water column up to 320 m above a pockmarked sea floor associated with active chemosynthetic biological communities plumes and venting fluids emanate from near a small fault that extends downward toward a dome in the bottom simulating reflector indicating that fluid and or gas migration is associated with gas hydrate bearing sediment below these plumes might be caused by gas bubbles or buoyant dumps of gas hydrate that float upward from the seafloor 18 refs 3 figs

methane driven oceanic eruptions and mass extinctions comment and reply reply dickens raises an important question phase diagrams do not tell the whole story however if they did diamonds would have turned into graphite long ago instead they seem to last forever for methane in seawater handa 1990 1 calculated the metastable gas liquid solubility saturation

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

sedimentological control on saturation distribution in arctic gas hydrate bearing sands abstract a mechanistic model is proposed to predict explain hydrate saturation distribution in converted free gas hydrate reservoirs in sub permafrost formations in the arctic this 1 d model assumes that a gas column accumulates and subsequently is converted to hydrate the processes considered are the volume change during hydrate formation and consequent fluid phase transport within the column the descent of the base of gas hydrate stability zone through the column and sedimentological variations with depth crucially the latter enable disconnection of the gas column during hydrate formation which leads to substantial variation in hydrate saturation distribution one form of variation observed in arctic hydrate reservoirs is that zones of very low hydrate saturations are interspersed abruptly between zones of large hydrate saturations the model was applied to data from mount elbert well a gas hydrate stratigraphic test well drilled in the milne point area of the alaska north slope the model is consistent with observations from the well log and interpretations of seismic anomalies in the area the model also predicts that a considerable amount of fluid of order one pore volume of gaseous and or aqueous phases must migrate within or into the gas column during hydrate formation this paper offers the first explanatory model of its kind that addresses converted free gas reservoirs from a new angle the effect of volume change during hydrate formation combined with capillary entry pressure variation versus depth

effect of heat treatment on siliceous rocks used in prehistoric lithic technology stone tool manufacture by many prehistoric and recent societies was characterized by deliberate heating of fine grained siliceous rocks to improve their flaking properties extensive mechanical testing of heated and unheated cryptocrystalline and macrocrystalline quartz lithologies has shown that thermal treatment causes a consistent marked reduction in fracture toughness this mechanical property can be used as an objective measurement of the flaking qualities of stone materials and a reliable criterion for the recognition of intentionally heated artefacts in the archaeological record x ray diffraction studies and scanning electron microscopy have demonstrated that the change in fracture toughness with heating is the result of recrystallization the poorly ordered strongly interlocking cryptocrystalline fabric of the unheated samples becomes more equigranular and better crystallized with thermal treatment as a consequence fractures propagate more readily in heated samples accounting for their better flaking properties

first molecular identification of a hafting adhesive in the late howiesons poort at diepkloof rock shelter western cape south africa abstract the hafting of tools using adhesive is one of the innovative features that characterizes the southern african middle stone age this technology has mainstream implications but remains insufficiently documented largely due to unequal organic preservation and non adapted analytical procedures a notable exception is provided by the recent results from the site of sibudu lombard 2006 wadley et al 2009 the excavation at diepkloof rock shelter has revealed several lithic artifacts with a black residue distributed over their surface their stratigraphic distribution reveals a strict association with the howiesons poort hp and suggests a close relationship between the appearance of hafting adhesive and the appearance of blades and geometric backed tools macroscopic and microscopic observations attest to a hafting that was exclusively lateralized and preliminary use wear analysis igreja and porraz in this issue supports the hypothesis that hafted tools were mostly integrated within daily domestic activities in this study we focused specifically on a chemical study of a thick black residue found on a quartz flake attributable to the late phase of the hp we determine for the first time in a msa context the nature of the compound adhesive and reconstruct a picture of the multilevel operations and interactions that comprise the process of hafting the molecular analysis attests to the exploitation of podocarpus elongatus yellowwood collected in the form of a resin that was naturally dried or heated at a low temperature and possibly mixed with fragmented bone and quartz grains compared to sibudu where ochre additive is documented the hafting technology at diepkloof introduces another level of variability within the hp tradition and suggests the existence of regional expressions and adaptations

water segregation in the course of gas hydrate formation and accumulation in submarine gas seepage fields abstract gas hydrate accumulations associated with submarine gas seepage fields have been detected in the okhotsk sea and the gulf of mexico it is possible that gas hydrates known from the continental margin offshore northern california oregon and nigeria are also connected with gas seepage fields gas hydrated sediments in the accumulations 1 occur at very low subbottom depths sometimes directly at the sea bottom 2 have specific structures resembling cryogenic structures caused by hydrates subhorizontal lenticular bedded and porphyraceous structures 3 the sediments can be distinguished from overlying non hydrated sediments by a higher water content 4 an association of gas hydrates and calcium carbonate concretions occurs the behaviour of gas and water during gas hydrate formation under bottom gas seepage conditions is considered gas hydrate formation occurs at the upper part of the sedimentary cover within the limits of the diffusion halo around the up going flow of gas which saturates the pore water water migrating to the gas hydrate accumulation sites from adjacent sediments is involved in hydrate formation as well the mechanism of gas transportation to the gas hydrate accumulation sites is diffusion the way of water transport during hydrate accumulation is similar to ice segregation during permafrost formation the water segregation is responsible for the formation of specific structure caused by hydrates and for higher than usual water content of hydrate bearing sediments in submarine gas seepage fields

crustal structure of atlantic fracture zones iii the tydeman fracture zone summary an extensive seismic survey was performed in march 1982 on two different parts of the tydeman fracture zone this survey consisted of two separate experiments a reversed seismic refraction experiment centred on 36 n 26 w using ocean bottom seismometers and free floating sonobuoys which was shot with explosives and a two ship multichannel experiment of the expanding spread type also shot with explosives at approximately 36 n 23 30 w these experiments are the first and to date only wide angle seismic surveys of fracture zone crust older than 25 myr slope intercept and tau p inversion of the obs sonobuoy travel time data indicates low crustal and mantle seismic velocities within the fracture zone these results are confirmed and refined by a more detailed analysis using raytracing and synthetic seismogram modelling for laterally varying structures which implies velocities for the upper mantle beneath the fracture zone of 7 3 7 5 km s 1 similar analysis of a line perpendicular to the fracture zone reveals the transition from normal mantle velocities to the low values found beneath the fracture zone and suggests that the width of the anomalous region is approximately 12 km the geometry of the obs experiment permitted a delay time analysis and a further simple tomographic inversion of the traveltime data which produced an approximate image of the low velocity mantle region associated with the fracture zone the width of the region of anomalous crust is the same as the width of the sediment filled trough which marks the fracture zone location at the line intersection analysis of the two ship seismic data from a site some 240 km to the east also by synthetic seismogram modelling gave similar results to the obs sonobuoy experiment with a mantle velocity of 7 2 km s 1 detected beneath the crust which possessed lower than normal seismic velocities the close seismogram spacing of less than 0 1 km allowed us to transform the dataset into the tau p domain and enabled us to detect arrivals obscured in the x t domain by the reflected water wave both experiments indicate low crustal seismic velocities within the fracture zone together with unusually low velocities of 7 2 7 5 km s 1 in the upper mantle comparison with other fracture zone surveys reveals that the seismic velocity in the upper mantle beneath fracture zones decreases with increasing age we attribute this to serpentinization of mantle peridotite brought about by hydrothermal circulation to the base of the fractured and faulted crust beneath the fracture zone

the oceanographer fracture zone i crustal structure from seismic refraction studies summary this paper presents the results of four lookm long seismic refraction lines carried out in and around the active transform of the oceanographer fracture zone which offsets the mid atlantic ridge rightlaterally by 130 km near 35 n the seismic data are interpreted using standard travel time inversions the time term method and two dimensional ray tracing techniques the results show that beneath the oceanographer fracture zone the crust is typically only 4 5 km thick it consists of a thin upper crustal layer in which the p wave velocity increases rapidly with depth and a main crustal layer with velocity increasing from 6 4 km s l at the top to 6 8 km s at the base of the crust some degree of crustal thinning is found to extend for distances of 30 40 km on either side of the fracture zone axis in addition to this a small area 10 15 km long of crust less than 2 5 km thick is found beneath the centre of the transform valley crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the oceanographer but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it we conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault however we attribute the occurrence locally of extremely thin crust to a different process in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault this may happen if the accretion rate at the end of the spreading centre becomes so slow that the usual processes which give rise to normal oceanic crust can no longer operate it therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone this might account for the extreme variability in crustal thickness found in different fracture zones by previous studies

middle stone age bone tools from the howiesons poort layers sibudu cave south africa abstract recently discovered bone implements from middle stone age msa deposits at sibudu cave south africa confirm the existence of a bone tool industry for the howiesons poort hp technocomplex previously an isolated bone point from klasies river provided inconclusive evidence this paper describes three bone tools two points and the end of a polished spatula shaped piece from unequivocal hp layers at sibudu cave with ages greater than 61 ka comparative microscopic and morphometric analysis of the sibudu specimens together with bone tools from southern african middle and later stone age lsa deposits an iron age occupation nineteenth century bushman hunter gatherer toolkits and bone tools used experimentally in a variety of tasks reveals that the sibudu polished piece has use wear reminiscent of that on bones experimentally used to work animal hides a slender point is consistent with a pin or needle like implement while a larger point reminiscent of the single specimen from peers cave parallels large un poisoned bone arrow points from lsa iron age and historical bushman sites additional support for the sibudu point having served as an arrow tip comes from backed lithics in the hp compatible with this use and the recovery of older larger bone and lithic points from blombos cave interpreted as spear heads if the bone point from the hp layers at sibudu cave is substantiated by future discoveries this will push back the origin of bow and bone arrow technology by at least 20 000 years and corroborate arguments in favour of the hypothesis that crucial technological innovations took place during the msa in africa

a middle stone age paleoscape near the pinnacle point caves vleesbaai south africa abstract open air middle stone age msa contexts in southern africa have received relatively little research attention compared to caves rock shelters msa caves rock shelters can provide long sequences of msa behaviors dominated by residential activities in circumscribed contexts but most procurement activities occurred on the landscape in uncircumscribed space we have a limited understanding of these activities at present making studies of open air sites crucial to alleviate this bias the south african coast paleoclimate paleoenvironment paleoecology paleoanthropology sacp4 project expanded its research scope to include msa archaeology from open air contexts we report on a series of msa open air assemblages that are exposed on ancient land surfaces suggestive of intact paleosols at vleesbaai and visbaai south africa importantly these sites occur in close proximity to the long cave rock shelter sequences at pinnacle point this presents the novel potential to study evidence of msa behavior in closed and open settings where their proximity to each other approximates the typical hunter gatherer daily foraging radius documented in ethnography we present a fabric and technological analysis of msa stone tool assemblages from three areas analysis of total station piece plotting of artifact bearing plunge suggests that the lithic assemblages have undergone limited post depositional disturbance the technological analysis and exploratory comparisons between these open air assemblages and msa cave and rock shelter contexts at pinnacle point cave 13b and 9 and cape st blaize cave suggest that the quartzite artifacts from vleesbaai were procured from locally available sources and may have been field processed there before being transported elsewhere perhaps to the caves rock shelters further the analysis suggests a dichotomous pattern of retouched tool discard where quartzite tools are discarded similarly across the landscape in contrast non quartzite tools may have been made primarily at the cave sites and discarded or lost more frequently on the landscape

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

in the footsteps of pliny tracing the sources of garamantian carnelian from fazzan south west libya abstract references in the ancient sources indicate that the libyan desert was a source of carbunculi semi precious red stones and gemstones variously interpreted as ruby garnet and spinel amongst others while gemstones are not attested in the geological strata of fazzan south west libya a range of silica based stones including chert chalcedony agate and carnelian are known to originate in this area linked to an early civilisation known as the garamantes it has been long proposed that the geochemical signature and the variations in the relative proportions of quartz moganite phases can be used to distinguish between groups of stones of different origin the proposed methodology was tested on a number of archaeological samples from the garamantian sites of jarma ancient garama and saniat jibril in fazzan fragments of chert carnelian and amazonite found at the two sites have been identified as raw materials associated with beadmaking trace elemental data obtained by la icp ms were combined with mineralogical data obtained by x ray powder diffraction and raman spectroscopy on the same samples and a group of reference samples the dataset has been compared with the available literature and data from other localities around the world to this purpose a preliminary database of silica based materials was established for provenance work based on the scarce data available in the literature the importation of these stones from eastern localities such as india may be ruled out the measured data on archaeological samples and debitage allow us to define a reliable reference group of parameters for materials from fazzan which are likely to be derived from a unique geological source the methodology should be extended and compared with cherts and carnelians from a range of mediterranean and sub saharan sites this characterisation work is a tool of high potential utility for a new investigation of ancient contact and trade across the trans saharan zone

crosshole seismology and seismic imaging in crystalline rocks a borehole to borehole audiofrequency seismic system using piezoelectric transducers as active elements has been developed and used to investigate crystalline rocks in canada high quality seismograms have been recorded for transmitter receiver distances of up to 230 meters it has been found that both travel times and signal amplitudes provide diagnostic information about rock quality two dimensional images of seismic velocity and transparency distributions between two boreholes derived from observed arrival times and amplitudes by simple back projection tomography reflect lithology and geological structure the results indicate the potential usefulness of crosshole seismic imaging as a tool for rock mass characterization

geoarchaeological investigations at diepkloof rock shelter western cape south africa abstract the sedimentary sequence at diepkloof rock shelter formed through a complex interaction of depositional and post depositional processes and was variously influenced by biogenic geogenic and anthropogenic agents here we present the results of a micromorphological study of the sediments at diepkloof focusing in particular on the numerous anthropogenic inputs and modifications recorded within the sequence by applying the microfacies concept we were able to identify hearth burning and maintenance bedding construction and burning and surface modification by trampling as major processes that contributed to the formation of the site the human activities recorded within the sediments show a marked change throughout the sequence implying a shift in the use of the site over time the present study also provides a context for evaluating other classes of artifacts and data collected from the site

early use of pressure flaking on lithic artifacts at blombos cave south africa pressure flaking has been considered to be an upper paleolithic innovation dating to 20 000 years ago 20 ka replication experiments show that pressure flaking best explains the morphology of lithic artifacts recovered from the 75 ka middle stone age levels at blombos cave south africa the technique was used during the final shaping of still bay bifacial points made on heat treated silcrete application of this innovative technique allowed for a high degree of control during the detachment of individual flakes resulting in thinner narrower and sharper tips on bifacial points this technology may have been first invented and used sporadically in africa before its later widespread adoption

methane migration within the submarine gas hydrate stability zone under deep water conditions abstract possible pathways of methane migration and associated gas hydrate accumulation are considered gas hydrates precipitate in water filtration routes when the methane concentration in water exceeds its solubility which significantly decreases within the hydrate stability zone towards the sea floor in the vicinity of the ascending free gas flows gas hydrates are formed within a diffusion aureole resulting from the difference between chemical potentials of gaseous and water dissolved methane the role of diffusion is also essential in the vicinity of gas saturated water flows as well as within and above the sections where biochemical methane is intensively generated a chemical potential gradient of dissolved methane which is a necessary condition for gas hydrate accumulation is created due to inhomogeneity of the geological medium manifested as temperature and water salinity gradients or changes of permeability the thermobaric gas hydrate stability zone is a geochemical barrier for hydrocarbon gas emigrating from sediments into the water column however methane is not completely preserved in a hydrate state it is unknown yet which fraction of migrating methane is detained on this barrier

seismic indicators of gas hydrate and associated gas in the ulleung basin east sea japan sea and implications of heat flows derived from depths of the bottom simulating reflector abstract analysis of multi channel seismic reflection data from the ulleung basin east sea japan sea reveals various seismic indicators of gas hydrate and associated gas including the bottom simulating reflector bsr enhanced reflections below the bsr and seismic chimneys the recent recovery of massive gas hydrate by drilling together with these seismic indicators strongly suggests favorable conditions for the formation of gas hydrate in the area the bsr is most common and of a wide range of amplitude and continuity seismic chimneys characterized by columnar zones of amplitude reduction terminate mostly within the gas hydrate stability zone ghsz but a few extend to pockmarks or mounds on the seafloor seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the ghsz the increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate filled coated fractures in the ghsz bsr derived heat flows range from about 65 to over 115 mw m2 comparable to those from direct measurements heat flows in the northern part of the study area are very high for the age of the basin suggesting hotter than normal mantle temperature the area of high 105 mw m2 heat flows also coincides largely with the distribution of the presumed incipient oceanic crust formed during the earliest stage of seafloor spreading

axial intrusion zone beneath the median valley of the mid atlantic ridge at 37 n detected by explosion seismology summary four seismic refraction lines three of which had shots every 250 m were shot across along and parallel to the median valley of the mid atlantic ridge at 37 n a method has been developed for calculating the effect on the travel times of the rough sea floor relief beneath the profiles and has been used to correct all the travel times for this effect most arrivals were from a main refractor of apparent velocity 5 4 to 6 3 km s 1 only beyond 35 km were faster arrivals observed from an 8 09 36 km s 1 refractor the main refractor corresponds in depth at least approximately to the top of layer 3 of the ocean basins but its velocity is significantly less than normal for layer 3 perhaps due to dip a study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone about 3 2 km s 1 beneath the median valley floor this zone extends over the upper 2 5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor

geophysical evidence of gas hydrates in shallow submarine mud volcanoes on the moroccan margin 1 gas hydrates inside mud volcanoes have been observed in several locations but are generally found at water depths of 1000 m and deeper we present the first observation of the base of a gas hydrate stability zone within a shallow mud volcano in the el arraiche mud volcano field on the moroccan atlantic margin the mud volcano base is located at about 475 m and is over 125 m high on high resolution seismics we observed an anomalous but coherent reflection under the slopes of the mud volcano the event was interpreted as the base of a gas hydrate stability zone because of its inverse polarity and its morphology far from the crater the event is nearly parallel to the seafloor closer toward the crater the event shallows inside the mud volcano crater no event is observed a stability model using thermogenic gas compositions is applied to local p t conditions indicating that thermogenic gas hydrates can be stable at this depth the high modeled heat flow in the crater of the mud volcano indicates a focused flow of warm fluids below the slopes of the mud volcano the inferred heat flow is also elevated but less high in areas of thermogenic gas production gas hydrates can occur at shallow water depths even in areas with high heat flow this also suggests that dewatering of the accretionary wedge complex is mainly focused along fault surfaces and through seafloor structures such as mud volcanoes

first evidence of heat treatment during the early neolithic in northeastern italy the site of la vela sector vii trentino alto adige italy provides one of the few complete stratigraphic sequences ranging from the mesolithic to the middle neolithic in northwestern italy the site is located in the adige valley a way of communication between the po plan and the alps this research focuses on the early neolithic lithics ascribable to the gaban facies dated to 5000 4700 cal bc different rocks coming from local exposures have been exploited the goal of the reduction sequence was the production of blades by pressure knapping technique the technological study of the assemblage raised a particular interest for the presence of flint artifacts suggesting the usage of controlled heat treatment the presence of this technique has been tested by an experimental approach based on fourier transform infrared ft ir spectroscopy complementary microstructural and analytical characterizations with low vacuum scanning electron microscopy lvsem observations combined with energy dispersive x ray spectroscopy edxs have been performed the analyses confirm the presence of artifacts intentionally heat treated the thermal treatment achieved in order to increase the quality of the blade production is the first evidence of this technique in northeastern italy during the early neolithic

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

joint inversion of local regional and teleseismic data for crustal thickness in the eurasia africa plate boundary region summary a new map for the moho discontinuity eam02 in the eurasia africa plate boundary region is presented reliable results have also been obtained for the southern and eastern mediterranean basin the northern african coasts and the eastern atlantic ocean regions only occasionally considered in studies on the mediterranean region the moho topography model is derived from two independent sets of constraints information contained in the fundamental and highermode rayleigh waves obtained from waveform modelling is used to constrain the moho depth between estimates of crustal thickness taken from published reflection and refraction surveys gravity studies and receiver function analysis strong lateral variations in the moho topography have been observed in the mediterranean sea confirming the complex evolution of this plate boundary region in the west the moho discontinuity has been found at 15 20 km depth suggesting extended and at least in some locations oceanic crust while in the east the crust is on average 25 30 km thick there it is interpreted either as mesozoic oceanic or thinned precambrian continental crust covered by thick sedimentary deposits standard continental crust 30 35 km is observed along the eastern part of the northern african coast while to the west a rapid change from a relatively deep moho down to 42 km below the atlas mountain range to the thin crust of the southwestern mediterranean sea has been found the crust beneath the eastern north atlantic ocean can be up to 5 km thicker compared with standard oceanic crust 6 km the crust has been interpreted to be heterogeneous as a consequence of irregular magma supply at the mid atlantic ridge in addition serpentinization of the submoho mantle could contribute to the imaging of apparently anomalous thick oceanic crust in europe the presence of crustal roots 45 km beneath the major mountain belts has been confirmed while thin crust 25 km has been found beneath extensional basins comparing the obtained moho topography and moho depth computed assuming isostatic compensation at 60 km depth shows that most of the mediterranean and eastern atlantic region appears to be in isostatic equilibrium the large positive residuals observed for the eastern mediterranean are likely to be due to overestimating crustal thickness owing to the thick sediment deposits present

crustal velocity structure on the flanks of the mid atlantic ridge at 24 n summary a detailed seismic refraction survey was made over a 70 35 km area of young 9 ma crust at 24 n 45 w to investigate the detailed velocity structure of a portion of typical oceanic crust an array of eight ocean bottom hydrophones was used to monitor 358 explosive charges and over 750 airgun shots the crust exhibits normal oceanic structure with a steep vertical velocity gradient through layer 2 and almost 3 km of layer 3 material with a mean velocity of 6 65 km s 1 overlying probably anisotropic mantle ray tracing shows that the major amplitude variations observed in the upper crustal returns are governed more strongly by the seafloor topography than by the seismic velocity structure within the crust a delay time function method was used to invert the 1613 well determined first arrival travel times from the explosives we find that the delay time vanes by only 0 05 0 10s over most of the survey area but that there are rather larger delay times in the north west corner which may be associated with crust formed in the vicinity of a ridge jump we also find a systematic correlation between variations in delay time and in the seafloor depth at the ray entry point with smallest delay times where the seafloor is deepest this is consistent with a model of the crust in which the topography of the base of layer 2 and the moho is a subdued copy of the upper basement surface

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

putslaagte 1 pl1 the doring river and the later middle stone age in southern africa s winter rainfall zone abstract existing data suggest weak human occupation of southern africa s winter rainfall zone wrz during later marine isotope stage mis 3 the causes of which are unknown here we report briefly on the results of recent surveys of alluvial terrace sites of the doring river in the wrz which document occupation over a broad expanse of the later middle stone age msa and pleistocene later stone age we then report on test excavations at one terrace site denoted putslaagte site 1 pl1 describe in detail the assemblage of flaked stone artefacts produced from that excavation and present two osl ages obtained from 0 8 m to 1 5 m below surface the results suggest that a artefact accumulations at pl1 are dense b the technological systems documented are characteristically msa but differ in form from the range of systems known from other excavated sites in the region and c that the assemblages accumulated in mis 3 taken together with the survey data the results introduce new variation into the later msa in southern africa and imply reorganisation of land use in the wrz in late mis 3 rather than abandonment we suggest that a research emphasis on rock shelter deposits may have produced misleading depictions of regional occupation

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

multichannel seismic evidence for anomalously thin crust at blake spur fracture zone a two ship wide aperture multichannel seismic reflection profile obtained across a small western north atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones reflections from the crust mantle boundary typically observed at about 2 s below the basement reflection rise to only 0 8 s beneath the fracture zone trough this corresponds to crustal thicknesses of perhaps as little as 2 to 2 4 km in the fracture zone less than half the thickness of normal oceanic crust this is the first time that moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset the existence of thin crust at both large and small offset fracture zones suggests that the magnitude of the thermal edge effect at ridge transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones

thermal curved fragments a method for identifying anthropogenic fire in the archaeological record abstract the archaeology of fire is a developing field one challenge centers on equifinality distinguishing the affects of wildfire versus anthropogenic fire especially where evidence for control of fire by humans in the early pleistocene remains debated there is little consensus regarding what constitutes clear evidence of human control another concern is preservation bias which reduces clearly identifiable archaeological signatures of fire in this paper we argue that a peculiar lithic angular fragment termed thermal curved fragment tcf exhibits statistically distinct quantitative evidence of the confluence of human knapped stone tools and exposure to a high energy long duration ground level fire i e campfires experimental tcfs are described and compared to unfired knapping debitage natural exfoliations and suspected archaeological tcfs the provenance of these archaeological specimens has previously been argued to infer hominin controlled fire at 1 5 1 6 ma at the koobi fora formation we also present data from experiments exposing stone flakes arranged as scatters of similar raw materials to those found in the koobi fora formation to usda prescribed burns in a variety of conditions approximating landscape fires results indicate that tcfs are formed specifically where previously knapped stone is exposed to high energy ground level fires similar to those seen in ethnographic campfires our analysis indicates that tcfs can be used in conjunction with other lines of evidence indicating the presence of anthropogenic fire in the archaeological record

wide angle seismic imaging of two modes of crustal accretion in mature atlantic ocean crust we present a high resolution 2 d p wave velocity model from a 225 km long active seismic profile collected over 60 75 ma central atlantic crust the profile crosses five ridge segments separated by a transform and three nontransform offsets all ridge discontinuities share similar primary characteristics independent of the offset we identify two types of crustal segment the first displays a classic two layer velocity structure with a high gradient layer 2 0 9 s 1 above a lower gradient layer 3 0 2 s 1 here pmp coincides with the 7 5 km s 1 contour and velocity increases to 7 8 km s 1 within 1 km below we interpret these segments as magmatically robust with pmp representing a petrological boundary between crust and mantle the second has a reduced contrast in velocity gradient between the upper and lower crust and pmp shallower than the 7 5 km s 1 contour we interpret these segments as tectonically dominated with pmp representing a serpentinized alteration front while velocity depth profiles fit within previous envelopes for slow spreading crust our results suggest that such generalizations give a misleading impression of uniformity we estimate that the two crustal styles are present in equal proportions on the floor of the atlantic within two tectonically dominated segments we make the first wide angle seismic identifications of buried oceanic core complexes in mature 20 ma atlantic ocean crust they have a 20 km wide domal morphology with shallow basement and increased upper crustal velocities we interpret their midcrustal seismic velocity inversions as alteration and rock type assemblage contrasts across crustal scale detachment faults

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

petrography and diagenesis of silcrete from the kalahari basin and cape coastal zone southern africa abstract silcrete is widely distributed in southern africa and occurs both in genetic association with deeply weathered bedrock weathering profile type and as a significant component of the cenozoic kalahari beds where it has formed through silicification of a variety of continental deposits nonweathering profile type silcrete fabrics are classified into four major types gs grain supported skeletal quartz grains in contact f floating skeletal grains floating in matrix m matrix skeletal grains 5 of volume and c conglomeratic detrital component containing pebbles f and m fabric types are subdivided on the presence absence of glaebules replacement by silica of a precursor matrix can account for the development of all four fabric types but at least some gs and c fabr c silcretes have formed by passive silica precipitation in packing voids glaebules and colloform features predominantly composed of concentrations of anatase and iron oxide are common in weathering profile silcretes but their precise origin is uncertain authigenic silica is present as megaquartz microquartz length slow and length fast chalcedony predominantly in void fills cryptocrystalline silica and opal ct depending on the silica and ionic concentrations of pore waters ph and host sediment characteristics diagnostic petrographic features including glaebules colloform structures and chalcedonic vugh fills may be used in conjunction with geochemical data and sedimentary associations to interpret silcretes in the stratigraphic record with respect to weathering profile types formed in humid low ph environments or nonweathering profile types formed in high ph semiarid to arid environments

technological successions in the middle stone age sequence of diepkloof rock shelter western cape south africa the southern african msa comprises a series of phases of unique technological innovations that fuel current models on the evolutionary processes of anatomically modern humans however the nature and variability of these technological phases remain poorly documented this study reports on the previously unpublished sequence of diepkloof rock shelter south africa and investigates the main technological changes therein particularly emphasizing the emergence succession and disappearance of the still bay sb and the howiesons poort hp we argue for technological change that occurred at different rates and under the influence of variable driving factors our model implies that the sb and the hp are not related to the influx of new populations but have regionally specific origins unlike the other techno complexes the hp at diepkloof is subdivided into different phases early intermediate and late but only the upper phases resemble the so called classic hp finally the technological sequence of diepkloof questions the homogeneous picture which has been so far assumed for the southern african msa and places emphasis on the importance of exploring long regional sequences

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

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

osl and tl dating of the middle stone age sequence at diepkloof rock shelter south africa a clarification diepkloof rock shelter offers an exceptional opportunity to study the onset and evolution of both still bay sb and howiesons poort hp techno complexes however previous age estimates based on luminescence dating of burnt quartzites tribolo et al 2009 and of sediments jacobs et al 2008 were not in agreement here we present new luminescence ages for 17 rock samples equivalent dose estimated with a sar itl protocol instead of classical maad tl as well as for 5 sediment samples equivalent dose estimated with sar single grain osl protocol and an update of the 22 previous age estimates for burnt lithics modified calibration and beta dose estimates while a good agreement between the rock and sediment ages is obtained these estimates are still significantly older than those reported by jacobs et al 2008 after our own analyses of the sediment from diepkloof it is suspected that these authors did not correctly chose the parameters for the equivalent dose determination leading to an underestimate of the equivalent doses and thus of the ages from bottom to top the mean ages are 100 10 ka for stratigraphic unit su noel and 107 11 ka for su mark uncharacterized lower msa 100 10 ka for su lynn leo pre sb type lynn 109 10 ka for sus kim larry sb 105 10 ka for sus kerry kate and 109 10 ka for su jess early hp 89 8 ka for su jude msa type jack 77 8 ka for su john 85 9 ka for su fox 83 8 ka for su fred and 65 8 ka for su ob5 intermediate hp 52 5 ka for sus ob2 4 late hp this chronology together with the technological analyses greatly modifies the current chrono cultural model regarding the sb and the hp and has important archaeological implications indeed sb and hp no longer appear as short lived techno complexes with synchronous appearances for each and restricted to oxygen isotopic stage ois 4 across south africa as suggested by jacobs et al 2008 2012 rather the sequence of diepkloof supports a long chronology model with an early appearance of both sb and hp in the first half of ois 5 and a long duration of the hp into ois 3 these new dates imply that different technological traditions coexisted during ois 5 and 4 in southern africa and that sb and hp can no longer be considered as horizon markers

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

the tydeman morphology and seismic structure of old fracture zone crust the tydeman fracture zone was identified 160 km se of the azores in 1977 from a detailed bathymetry and magnetic survey between 22 309 and 23 409w twigt et al 1979 the fracture zone forms a linear trough here with a width of 20 km water depth of 4 7 km and orientation of 085 this study extends the path of this fracture zone between 18 and 30 w though not yet to the ridge crest using a larger dataset of bathymetric seismic reflection and magnetic profiles a discontinuity in the fracture zone valley indicates an interruption in its formation possibly during the formation of the azores platform two separate wide angle seismic experiments carried out over old crust in the active section of the tydeman fracture zone show low crustal velocities compared with normal oceanic crust of comparable age an absence of an identifiable layer 3 and a shallower than normal moho overlying a region of anomalously low upper mantle velocities estimates of poisson9s ratio suggest that hydrothermal circulation may continue for as long as 70 ma after crustal formation

heating of flint debitage from upper palaeolithic contexts at manot cave israel changes in atomic organization due to heating using infrared spectroscopy abstract the heat treatment of flint is known to change its mechanical properties and improve its fracture behaviour during knapping here we examine 20 flint artifacts from upper paleolithic contexts from manot cave israel using fourier transform infrared spectroscopy and compare them to geogenic flint beds from the walls inside the cave and from outcrops just above the cave we show that the 512 and 467 cm 1 peaks are broader in most of the flint debitage pieces as compared to the geogenic flint and that broadening of these peaks occurs when geogenic flint from the cave wall is heated we also present an empirical simple method to monitor these changes

multiproxy record of late quaternary climate change and middle stone age human occupation at wonderkrater south africa here we provide a multiproxy record of climate change and human occupation at wonderkrater a spring and peat mound site situated in the interior of southern africa recently extracted sediment cores yielded a number of middle stone age msa artefacts prompting exploratory excavation of the sediments to understand better the geomorphology of the site age of the sediments cultural lithic sequence vegetation and faunal remains and to try to establish whether human use of the site was to some extent climatically driven excavations yielded late pleistocene mammal fauna and flora and three small msa lithic assemblages with age estimates of 30 ka 45 ka and 138 01 7 7 ka the upper layers comprise peat that preserves macrobotanical and faunal remains implying local fen conditions in acacia savanna woodland at 12 ka below the upper peat layers a 1 m thick layer of white sand yielded two msa lithic assemblages in association with faunal remains dated to between 30 8 0 7 ka and 45 ka clay underlying the sand has an osl age of 63 1 5 8 ka and sandy peat below it has an infrared stimulated luminescence irsl age of 70 10 ka faunal remains in the lower sand levels and dental stable carbon isotope analysis of herbivores indicate a substantial grassland component in the landscape during late mis 3 45 ka charcoal phytolith and pollen data show a change from moderately warm and dry grassy savanna woodland in the lower sand levels to cooler and wetter grassland with woody shrubs in the uppermost levels by 30 ka the conditions that resulted in the deposition of the sand also attracted people to the site but whether it served as an oasis in an arid landscape or was occupied during wet phases is unclear the composition of the lithic assemblages which include many tools suitable for cutting suggest that the peat mound may have been used as a place to harvest reeds process plant materials and butcher animals that were either deliberately or accidentally trapped in mud or peat

crustal structure of atlantic fracture zones i the charlie gibbs fracture zone summary a seismic refraction experiment was carried out in 1982 over and in the vicinity of the southern transform valley of the charlie gibbs double fracture zone to measure the rate of crustal thinning normal to the southern transform valley axis three reversed lines were shot with obs two were east west one parallel to the fault on normal crust and one along the fault the third line was north south normal to and intersecting the first two lines travel times were modelled by ray tracing thin 4 km thick anomalously low velocity crust was detected in the fracture zone along the n s line normal crust thinned over a distance of 40 km from 8 to 5 km towards the fracture zone where the belt of anomalous crust was 12 km wide several publications demonstrate that such thinning is not due primarily to the effect of cold lithosphere on accretion at the ridge transform intersection the thinning may be related to the supply and distribution of magma from an injection zone situated roughly at the centre of each spreading segment a gravity profile along the n s line was also analysed the bouguer anomaly was modelled subject to the constraints of the seismic ray tracing after applying corrections for the three dimensional bathymetry near the profile and for the lithospheric anomaly due to the age contrast across the northern fracture zone the gravity models supported the seismic interpretation and also indicated low densities within the fracture zones two alternative models were obtained for the structure under hecate bank an east west ridge which lies within the crust generated at the short central spreading segment of the cgfz one model features dense material within the crust the other normal but uplifted crust the evidence favours the latter mode but the cause of the uplift is uncertain the uplift may have

active hydrate destabilization in lake baikal siberia abstract in this paper we present new seismic and heat flow data that show the base of the hydrate stability zone bhsz in lake baikal to be locally characterized by abnormal variations in depth with distinct regions of deeper than normal and regions of shallower than normal bhsz these variations are related to strong lateral variations in heat flow and occur in close association with important rift basin faults areas of shallow bhsz are also characterized by the presence of several methane seeps and mud volcanoes at the lake floor we infer that the seeps are the surface expression of escape pathways for overpressured fluids generated by the dissociation of pre existing hydrates in response to a thermal pulse caused by an upward flow of hydrothermal fluids towards the bhsz it thus seems that present day hydrate dissociation in lake baikal is modulated by the tectonic activity in the rift rather than by climatically controlled changes in lake level or water temperature

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments