methane fluxes and carbonate deposits at a cold seep area of the central nile deep sea fan eastern mediterranean sea abstract high acoustic seafloor backscatter signals characterize hundreds of patches of methane derived authigenic carbonates and chemosynthetic communities associated with hydrocarbon seepage on the nile deep sea fan ndsf in the eastern mediterranean sea during a high resolution ship based multibeam survey covering a 225 km2 large seafloor area in the central province of the ndsf we identified 163 high backscatter patches at water depths between 1500 and 1800 m and investigated the source composition turnover flux and fate of emitted hydrocarbons systematic parasound single beam echosounder surveys of the water column showed hydroacoustic anomalies flares indicative of gas bubble streams above 8 of the high backscatter patches in echosounder records flares disappeared in the water column close to the upper limit of the gas hydrate stability zone located at about 1350 m water depth due to decomposition of gas hydrate skins and subsequent gas dissolution visual inspection of three high backscatter patches demonstrated that sediment cementation has led to the formation of continuous flat pavements of authigenic carbonates typically 100 to 300 m in diameter volume estimates considering results from high resolution autonomous underwater vehicle auv based multibeam mapping were used to calculate the amount of carbonate bound carbon stored in these slabs additionally the flux of methane bubbles emitted at one high backscatter patch was estimated 0 23 to 2 3 106 mol a 1 by combined auv flare mapping with visual observations by remotely operated vehicle rov another high backscatter patch characterized by single carbonate pieces which were widely distributed and interspaced with sediments inhabited by thiotrophic chemosynthetic organisms was investigated using in situ measurements with a benthic chamber and ex situ sediment core incubation and allowed for estimates of the methane consumption 0 1 to 1 106 mol a 1 and dissolved methane flux 2 to 48 106 mol a 1 our comparison of dissolved and gaseous methane fluxes as well as methane derived carbonate reservoirs demonstrates the need for quantitative assessment of these different methane escape routes and their interaction with the geo bio and hydrosphere at cold seeps

metamorphism of the witwatersrand gold fields conditions during peak metamorphism pelitic assemblages from all major witwatersrand gold fields record metamorphic conditions of the greenschist facies with minimal regional grade changes over at least 200 km strike length diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites the actively exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons bulk rock composition has been a major control on assemblage development key metapelitic assemblages include pyrophyllite chloritoid chlorite and muscovite in each gold field with less common metamorphic biotite accessory minerals are pyrite tourmaline rutile and zircon the abundance of chloritoid and pyrophyllite in thin shaly units together with their minor but widespread distribution in quartzites and conglomerates indicate that metamorphic temperatures reached 350 c 50 c in all the gold fields pressures are less well constrained 1 2 kbar being inferred outside the gold fields higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions the temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to re mobilize gold and may explain its secondary textural features

xenon systematics of individual lunar zircons a new window on the history of the lunar surface abstract we demonstrate a new way of investigating the processing of the lunar surface and other planetary regoliths that combines xes xen ages based on uranium fission in individual zircons with their xenon isotopic record of solar wind and cosmic ray exposure we report the first xenon isotopic analyses of individual lunar zircons from apollo 14 soil and breccias samples parallel analyses of a suite of zircons from the vredefort impact structure in south africa revealed xes xen ages that agree well with u pb systematics suggesting that the diffusion kinetics of xenon and lead in zircon are similar in the pressure temperature environment of sub basin floors in contrast all apollo 14 zircons examined exhibit xes xen ages markedly younger than the associated u pb and 207pb 206pb ages and soil zircons with 207pb 206pb ages greater than 3900 ma produced an abundance of xes xen ages 2400 ma suggesting that these samples may be useful for investigating ancient events and regolith processing at an earlier epoch however none of the zircons contain xenon from now extinct 244pu implying that either the samples have completely degassed since 3900 ma or that the initial pu u ratio of the moon is lower than that on earth we also describe a methodology for conducting component deconvolution that can be applied to multi isotopes systems beyond xenon we have also determined new xenon isotopic yields from rare earth element spallation in the lunar environment and high precision yields for neutron induced fission of 235u in geologic samples

fission xenon dating of witwatersrand uraninites implications for geological activity in the central kaapvaal craton about 1 ga ago abstract four uraninite separates from different reefs from in and above the central rand group upper witwatersrand supergroup were obtained from gold mines in the east rand west rand and welkom goldfields of the witwatersrand basin in south africa these samples were dated by the xes xen spectrum technique all four age spectra have a typical stepup form with plateaux for high temperature xe release the plateau ages are 0 832 0 026 ga for the carbon leader reef sample from blyvooruitzicht gold mine 1 043 0 024 ga for the basal reef sample from free state geduld gold mine 1 115 0 036 ga for the main reef uraninite concentrate from sub nigel gold mine and 1 300 0 058 ga for the ventersdorp contact reef sample from kloof gold mine no apparent ages in excess of 1600 ma were observed it is obvious from the general witwatersrand chronological database that these fission xenon dating results do not correspond to primary crystallization ages for the uraninites instead they most likely represent resetting events that affected the u xe isotopic system in these samples comparison with other recent chronological results obtained on rocks from the region of and around the witwatersrand basin indicates that the region was affected by magmatotectonic activity at 1 3 to 1 ga ago which thus was not only localized along the southern margin of the kaapvaal craton the so called namaqua natal thrust front but also affected other parts of the craton

dating ultra deep mine waters with noble gases and 36cl witwatersrand basin south africa abstract concentrations and isotopic ratios of dissolved noble gases 36cl d and 18o in water samples from the ultra deep gold mines 0 718 to 3 3 km below the surface in the witwatersrand basin south africa were investigated to quantify the dynamics of these ultra deep crustal fluids the mining activity has a significant impact on the concentrations of dissolved gases as the associated pressure release causes the degassing of the fissure water the observed under saturation of the atmospheric noble gases in the fissure water samples 70 98 normalized to asw at 20 c and 1013 mbar is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature corrections for degassing result in 4he concentrations as high as 1 55 10 1cm3stp4he g 1 40ar 36ar ranging between 806 and 10331 and 134xe 132xe and 136xe 132xe ratios above 0 46 and 0 44 respectively corrected 134 136 xe 132xe and 134 136 xe 4he ratios are consistent with their production ratios whereas the nucleogenic 4he 40ar and 134 136 xe 40ar ratios generally indicate that these gases are produced in an environment with an average u th k content 2 3 times above that of crustal average in two scenarios one considering only accumulation of in situ produced noble gases the other additionally crustal flux components the model ages for 14 individual water samples range from 13 to 168 ma and from 1 to 23 ma respectively the low 36cl ratios of 4 37 10 15 and comparatively high 36cl concentrations of 8 350 10 15 atoms 36cl l 1 reflect subsurface production in secular equilibrium indicating an age in excess of 1 5 ma or 5 times the half life of 36cl in combination the results suggest residence times of the fluids in fissures in this region up to 3 3 km depth are of the order of 1 100 ma we cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

morphology of witwatersrand gold grains from the basal reef evidence for their detrital origin thousands of rounded particles of fine grained gold released from a crossbedded gold specimen of the basal reef placer in the welkom gold field after acid digestion of the host quartz arenite have revealed morphological features and abrasion textures that are characteristic of detrital gold the size range which is between 0 038 and 0 473 mm has a lognormal distribution with a mean of 2 88 phi units 0 136 mm and a standard deviation of 0 48 this is a characteristic of well sorted sediment the shapes are predominantly disclike with toroidal forms being most common a corey shape factor of 0 08 indicates extreme flattening the preponderance of these shapes is interpreted to be the result of selective sorting by entrainment of particular shapes during extensive transport a knobby surface texture similar to placer grains recovered from both recent and devonian placers and produced during laboratory controlled tumbling with sand and cobbles in water is considered distinctive of sediment abrasion secondary gold comprising 25 percent of the total gold content is finer grained has an interstitial jagged form and displays unabraded smooth to crystalline surfaces study of this remarkable specimen has produced unequivocal evidence of the detrital origin of gold in the extensive basal reef placer and led to research concerning its metamorphic overprints

a case study of the postdepositional alteration of the witwatersrand basal reef gold placer an investigation of the mineralogy quartz texture fluid inclusions and gold composition of a crossbedded auriferous specimen of the archcan basal reef placer from the welkom gold field witwatersrand south africa has established the degree of postdepositional alteration to which the detrital gold particles were subjected three different postdepositional events can be distinguished 1 regional static recrystallization at a peak metamorphic temperature between 290 degrees and 350 degrees c and a pressure between 2 and 3 kbars 2 local fluid infiltration along fractures and sedimentary interfaces at a temperature between 240 degrees and 300 degrees c and a pressure similar to the above and 3 local k super metasomatism during a later fluid infiltration event along the preexisting zones of weakness intracrystalline diffusion during these events led to homogenization of the gold on a hand specimen scale with respect to ag 8 9 wt and with respect to hg 1 0 wt furthermore during the second event 2 fluid infiltration caused not only homogenization but also partial mobilization of some of the detrital gold with transport over distances of not more than a few millimeters to centimeters these processes were accompanied by microscopic silicification calcitization and possibly hg metasomatism the ph value of this infiltrating fluid was around 4 primary fluid inclusions associated with secondary gold in authigenic quartz consist of cacl 2 rich aqueous solutions whereas secondary fluid inclusions within such authigenic quartz are dilute aqueous solutions having a very low salinity that is attributed to nacl and kcl event 3 it is believed that gold mobilization was coeval with a fracturing and shearing event after deposition of at least parts of the transvaal supergroup on top of the witwatersrand and ventersdorp supergroups the last event 3 the infiltration of k super enriched fluid after the partial mobilization of the gold is tentatively correlated with the bushveld or vredefort events

deep water archaeological discoveries on eratosthenes seamount abstract a team of earth scientists conducted a series of exploratory transects across the summit of eratosthenes seamount in 2010 and 2012 to better understand the geologic structure of this unique flat topped feature in the east mediterranean sea the team encountered 219 isolated amphorae and 3 shipwrecks using remotely operated vehicles equiped with both acoustic and optical image systems the researchers could instantaneously communicate with archaeologists on shore by utilizing a high bandwidth satellite system on the e v nautilus this system allowed researchers and archaeologists to better take advantage of this series of discoveries the data collected during these field seasons presented and discussed in this article contributes to existing literature on the maritime traffic patterns of the ancient eastern mediterranean and of open sea travel over the underwater plateau from 800 bc 800 ce

integrated geophysical modelling of a giant complex impact structure anatomy of the vredefort structure south africa abstract only three very large confirmed impact structures are known on earth the chicxulub crater mexico 65 ma ca 180 km wide the sudbury structure canada 1 85 ga 200 km in diameter and the vredefort structure in south africa 2 02 ga while extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on earth integrated modelling of gravity and magnetic data constrained by geological as well as refraction and reflection seismic data accomplished the reconstruction of the vredefort impact structure in south africa approximately 250 km wide the original vredefort impact structure covered the whole extent of the archaean witwatersrand basin distinguished by enormous gold resources as it is structurally preserved today in fact it is clear that the preservation of vast volumes of economically important witwatersrand strata is the direct result of the formation of the ring basin around the central uplift the vredefort dome of the impact structure this study is the first attempt to create an integrated and geophysically well constrained model of this very large complex impact structure

south african crustal fracture fluids preserve paleometeoric water signatures for up to tens of millions of years abstract fracture fluids in earth s crust may remain isolated for millions to billions of years and contain information on paleohydrogeology subsurface microbial life and conservative components that help elucidate the atmospheric evolution of the early earth examples include fluids in the south african kaapvaal craton which host chemolithoautotrophic microbial communities that survive independent of the photosphere and billion year old fluids in the canadian shield which preserve the xe isotopic signature of an evolving early atmosphere stable isotope analyses of the aqueous phase combined with isotopic analyses of the dissolved noble gases provide unrivalled insight into the time alteration history of aqueous fracture fluids here we report stable isotope and noble gas data for fracture fluids in the witwatersrand basin and bushveld igneous province systems south africa we determine closed system radiogenic noble gas residence times of 0 77 97 million years myr open system residence times range between 6 0 kyr and 10 8 myr one sample from masimong mine has a mean closed system residence time of 85 myr making it one of oldest paleometeoric waters ever recorded the 2h and 18o of water in this sample and in previously reported samples from the same mining district that are shown to have similar ages require an isotopically depleted source of groundwater recharge this could reflect a recharge regime at a higher paleolatitude elevation or with higher rainfall established up to tens of myr ago and perhaps similar to the recharge regime in the modern lesotho highlands these data suggest that groundwater isotopes can provide useful paleoclimatic information for many myr

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

multi disciplinary investigation of fluid seepage on an unstable margin the case of the central nile deep sea fan we report on a multidisciplinary study of cold seeps explored in the central nile deep sea fan of the egyptian margin our approach combines in situ seafloor observation geophysics sedimentological data measurement of bottom water methane anomalies pore water and sediment geochemistry and 230 th u dating of authigenic carbonates two areas were investigated which correspond to different sedimentary provinces the lower slope at 2100 m water depth indicates deformation of sediments by gravitational processes exhibiting slope parallel elongated ridges and seafloor depressions in contrast the middle slope at not vert 1650 m water depth exhibits a series of debris flow deposits not remobilized by post depositional gravity processes significant differences exist between fluid escape structures from the two studied areas at the lower slope methane anomalies were detected in bottom waters above the depressions whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities carbonate u th age dates 8 kyr bp pore water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past in contrast large 1 km2 carbonate paved areas were discovered in the middle slope with u th isotope evidence for ongoing carbonate precipitation during the late holocene since 5 kyr bp at least our results suggest that fluid venting is closely related to sediment deformation in the central nile margin it is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of fossil carbonate ridges whereas pervasive diffuse flow prevails at the unfailed middle slope

present day stress field in egypt based on a comprehensive and updated earthquake focal mechanisms catalog the present day stress field in egypt has been investigated on the basis of updated earthquake focal mechanism catalog covering the period from 1951 to 2017 our catalog contains 234 focal mechanisms compiled from previous studies in addition to 22 new source mechanism solutions achieved in this study according to the distribution of the recent earthquake epicentres egypt is divided into nine seismotectonic regions the available fault plane solutions in egypt demonstrate a spatial variability of source mechanisms which categorize the study area into three groups the first group includes dahshour beni suef cairo suez district northern central gulf of suez and southern gulf of suez which characterized by pure normal faulting mechanism to normal faulting with strike slip component pure strike slip faulting has clearly characterised aswan and gulf of aqaba regions in the second group however the third group which contains abu dabbab and the northern egyptian continental margin is characterized by thrust and strike slip faulting to calculate the orientation of the principle stress axes and the shape ratio we have applied the stress inversion technique the present day stress regime shows a variability of the tectonic stresses including extensional tectonic transtensional and strike slip the transtensional stress regime with a maximum horizontal extensional nne stress axis represents the dominant stress field pattern in egypt the results exhibit a good agreement with the tectonic settings and recent deformations in egypt

some aspects of the uranium minerals in witwatersrand sediments of the early proterozoic abstract uraninite was introduced into the witwatersrand sediments as detrital grains and hydraulic sorting processes were responsible for the selective concentration of the minerals when the sedimentary trap was favourable for the growth of primitive plants they entrapped uraninite grains at the same time breaking them up into fragments owing to the continuous multiplication of the micro organisms the uraninite fragments gradually disintegrated to increasingly smaller sizes several textural features of the intergrowth of kerogen with uraninite and with phyllosilicates strongly suggest that the multiplication of organisms was responsible for the fragmentation of the uraninite grains but that chemical interaction was negligible before consolidation of the sediment uraninite and leucoxene reacted with post depositional intrastratal solutions to form uranous titanates of variable composition these compounds also precipitated at grain boundaries and in available interstitial voids and could have formed from very fine grained uraninite and leucoxene that during the transport cycle were abraded from the detrital grains and became entrapped in the organic matter or matrix

implementation of magnetic gravity and resistivity data in identifying groundwater occurrences in el qaa plain area southern sinai egypt abstract el qaa plain is one of the areas that have been proved to be promising as to its soil and groundwater resources in the southwestern part of sinai this study was carried out to study the lateral and vertical variations in the subsurface lithologic properties in el qaa plain area and delineating the subsurface structure that affecting el qaa plain depression gravity magnetic data fifty one 51 vertical electrical sounding ves s and digital elevation model dem map were used two dimensional density modeling analytical signal tilt derivatives euler deconvolution techniques and interpretation of the resistivity data were applied on potential data this study concluded that el qaa plain is a ne sw depression confined by two normal faults and subdivided into five sedimentary units rock fragments and boulders derived to the plain from the surrounding highlands are the main components of the first surface unit that characterized by high resistivity values this layer is overly another unit composed of gravelly sand with thickness ranging between 10 and 140 m the third unit is composed mainly of sand with intercalations of clay with thickness ranging from 2 to 152 m a reefal limestone unit is a fourth unit interpreted from this study the fifth unit is composed of clay the water bearing formations in el qaa plain area are located principally in the second and the third layers finally the depth to the basement surface ranges from 400 m to more than 1 5 km below sea level also there are many minor structural trends interpreted from this study and affecting the el qaa plain are striking in n s e w and ne sw directions

inland jump of the arabian northwest plate boundary from the levant continental margin to the dead sea transform 1 while the breakup of arabia and africa began in the early oligocene the northwestern boundary of the arabian plate along the dead sea transform dst formed 10 15 myr later early middle miocene during the early stage of breakup the red sea rift continued propagating northwestward forming the suez rift the present 45 northward twist of the plate boundary from the red sea toward the gulf of eilat aqaba still did not exist what happened at the northern tip of the suez rift at that time how was strain distributed to its surroundings and where did the plate boundary continue from there here we describe an abandoned segment of the arabian northwestern plate boundary that extended from the northern tip of the suez rift northeastward along the levant margin seismic data collected offshore israel support a subsurface deep rooted fault zone running along the base of the continental slope these faults indicate oligocene transpressional lateral shearing we propose that during the early stage of continental breakup a left lateral strike slip motion of 10 km took place along this embryonic plate boundary such deep rooted tectonism implies that unlike the passive situation of the israel sinai continental margin witnessed presently and before the oligocene during the early stage of the africa arabia breakup this part of the continental margin was reactivated we further suggest that the inland jump of the plate boundary toward the dst was not immediate and that during the transitional period the israel sinai triangular block was an independent subplate with deformation all around it

40ar39ar dating of pseudotachylite from the vredefort dome south africa a progress report abstract to date six pseudotachylite samples from the vredefort dome have been analysed by the stepheating 40ar 39ar technique the resulting ages ga 2 25 0 04 k ar 2 01 0 06 1 39 0 08 1 44 0 08 1 33 0 04 and 1 09 0 10 may be understood as suggesting several pseudotachylite forming events at vredefort this and other chronological and field evidence may be interpreted as supporting the possibility that vredefort pseudotachylite is not derived from a single catastrophic central shock event but was generated in several local high strain possibly seismic shock events diffusion calculations have shown that it is possible that several thermal spikes could have effectively degassed pseudotachylite at relatively low temperatures and during short periods of time e g at 340 c in 10000 yrs there is however no geological or isotopic evidence directly supporting the existence of these proposed temperature spikes at 2 0 1 3 1 4 and 1 1 ga in the region of the vredefort structure on the contrary field and laboratory observations can be invoked that seem to favour the formation of multiple generations of pseudotachylite in the vredefort dome and in the surrounding witwatersrand basin however it must be noted that likewise no direct geological evidence for local tectonic activity in the region of the witwatersrand basin at those times can be presented with the exception of emplacement of pilanesberg age dykes further tests on the validity of the 40ar 39ar ages on vredefort pseudotachylite are in progress and are discussed

ages of altered granites adjoining the witwatersrand basin with implications for the origin of gold and uranium one of the main problems in understanding the origin of the witwatersrand basin mineralization concerns the source of the prodigious concentrations of gold and uranium in the basin another problem is the age and nature of the rocks in the source area recent data1 2 indicate that the source area was largely 3 200 2 900 myr old becoming progressively younger as the basin evolved granites adjoining the basin are commonly overprinted by pervasive and vein related hydrothermal alteration which resulted in an enrichment of both gold and uranium3 4 here we report uranium lead data demonstrating that these hydrothermally altered granites were intruded sporadically both before and during the 360 myr period5 constraining witwatersrand sediment deposition and that hydrothermal alteration was associated with granitoid intrusion and subsequent cooling thus fertilization of the crust occurred during basin evolution a feature that supports the idea19 that gold and uranium mineralization occurred simultaneously with sediment deposition

dramatic pliocene quaternary subsidence of the southern rhodes basin and concomitant north tilting and uplift of the anaximander mountains the junction of hellenic and cyprus arcs eastern mediterranean sea abstract the anaximander mountains and the rhodes basin at the junction of cyprus and hellenic arcs are one of the tectonically most complex regions in the eastern mediterranean harmonically tilted messinian and the uppermost messinian pliocene successions across the northern and eastern margins of the rhodes basin showed that these regions experienced dramatic subsidence exceeding 1500 2500 ms 1125 1875 m 1500 m s 1 during the upper pliocene quaternary tilted upper pliocene quaternary growth strata wedges showing apparent north directed downlap along the northern foothills of the anaximander mountain suggested that this region experienced 1000 2500 ms 750 1125 m concomitant uplift the absence of faults showing extensional stratigraphic separations and the relative subsidence rates calculated using amount of tilting collectively suggest that the rhodes basin experienced sagging of its deeper basinal regions which is attributable to the loading effect of the building of the tauride mountains modified by the vertical axis rotations of the beydaglari block and the island of rhodes and the horizontal axis rotation of the anaximander mountain messinian and uppermost messinian pliocene were tectonically relatively quiet as shown by the harmonically structured and isopachous successions across the rhodes basin and anaximander mountain this interval coincides with the paleomagnetically determined 9 counterclockwise vertical axis rotation of the island of rhodes the dramatic subsidence observed within the rhodes basin and the concommitant uplift of the anaximander mountain and its western extension occurred broadly coincident with southeast tilting of the island of rhodes and its associated 500 600 m subsidence between 2 5 and 1 8 ma the northwest tilting between 1 5 and 1 1 ma and the associated re emergence of the southeastern coast the large counterclockwise rotation of the island of rhodes since 0 8 ma corresponds with the enormous subsidence observed across the northeastern sector of the rhodes basin which brought the shelf edge deltas from the near sea level to their present day depth of 1200 1500 m

constraints on the post 25 ka slip rate of the yammouneh fault lebanon using in situ cosmogenic 36cl dating of offset limestone clast fans the most active seismogenic structure along the eastern shore of the mediterranean is the n s trending left lateral levant fault system lfs the plate boundary between arabia and africa in lebanon it forms a 160 km long restraining bend responsible for the uplift of mount lebanon the resulting transpression is partitioned between the offshore tripoli roum thrust and the yammouneh strike slip fault there are few quantitative constraints on the quaternary slip rate along the lfs here we present a direct estimate of the 25 ka mean slip rate on the yammouneh fault mapped offsets of alluvial fans at two sites 50 km apart on the eastern flank of mount lebanon range between 24 2 and 80 8 m about 30 limestone cobbles sampled on these fans yield in situ cosmogenic 36cl exposure ages mostly between 6 and 27 ka a statistical assessment of offsets versus ages provides bounds on the late pleistocene holocene slip rate on the fault 3 8 6 4 mm yr these results are consistent with long term geological inferences confirming that the yammouneh fault is the main strike slip branch of the lfs in lebanon they illustrate both the potential and the difficulties of using in situ cosmogenic 36cl dating of limestone clast fan deposits for deciphering tectonic and geomorphic processes in the mediterranean

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers