the initiation of subduction by crustal extension at a continental margin summary we investigate how subduction may be triggered by continental crust extension at a continental margin the large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement subduction requires the oceanic plate to get submerged in mantle so that negative buoyancy forces may take over and drive further descent this is promoted by two mechanisms loading by continental crust bends the oceanic plate downwards extension in the continental domain induces crustal thinning which acts to raise mantle above the oceanic plate in this model the width of the continental region undergoing extension is an important control parameter the main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading three governing dimensionless parameters are identified one involves the poorly constrained oceanic plate buoyancy we find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant enabling metamorphic reactions and density increase in the oceanic crust another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong the third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness such that a weak plate and a thick crust do not favour subduction thus the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere it is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed

corona structures driven by plume lithosphere interactions and evidence for ongoing plume activity on venus in the absence of global plate tectonics mantle convection and plume lithosphere interaction are the main drivers of surface deformation on venus among documented tectonic structures circular volcano tectonic features known as coronae may be the clearest surface manifestations of mantle plumes and hold clues to the global venusian tectonic regime yet the exact processes underlying coronae formation and the reasons for their diverse morphologies remain controversial here we use three dimensional thermomechanical numerical simulations of impingement of a thermal mantle plume on the venusian lithosphere to assess the origin and diversity of large venusian coronae the ability of the mantle plume to penetrate into the venusian lithosphere results in four main outcomes lithospheric dripping short lived subduction embedded plume and plume underplating during the first three scenarios plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy driven topographic inversion from circular trenches surrounding elevated interiors to raised rims surrounding inner depressions as observed on many venusian coronae different corona structures may represent not only different styles of plume lithosphere interactions but also different stages in evolution a morphological analysis of large existing coronae leads to the conclusion that at least 37 large coronae including the largest artemis corona are active providing evidence for widespread ongoing plume activity on venus thermomechanical modelling shows that the formation and diverse morphologies of coronae on venus can be explained by interactions between the lithosphere and impinging mantle plumes some corona structures are consistent with ongoing plume activity

carbonate recrystallization in shallow marine environments a widespread diagenetic process forming micritized grains abstract integrated morphologic petrographic and mineralogic analyses of sediment grains of the miliolid foraminifer archaias and the green alga halimeda from shallow marine environments in florida the bahamas and belize document pervasive syndepositional recrystallization in these grains scanning electron microscopy shows that original skeletal rods and needles recrystallize on the sea floor to a variety of equant micritic fabrics in thin section this textural alteration corresponds to progressive micritization with loss of the familiar golden brown color exhibited by fresh miliolids and halimeda and gradual development of gray cryptocrystalline carbonate x ray diffraction and electron microprobe studies show that this textural alteration may be accompanied by mineralogical changes from mg calcite to aragonite or from aragonite to mg calcite our findings support petrographic studies that reported syndepositional recrystallization in a wide variety of carbonate grains about thirty years ago but which have been largely ignored together these studies indicate that carbonate recrystallization is a widespread process of early diagenesis extensive recrystallization in shallow tropical seas challenges basic principles regarding the setting and timing of textural and mineralogic alteration of metastable carbonate grains in addition recognition of recrystallization as an important process of micritization contradicts prevailing theories that micritization results solely by infilling of microborings finally our findings raise fundamental questions concerning the nature of carbonate crystal structure mechanisms of crystal growth and driving forces for recrystallization answering these questions will require sedimentologists to step beyond traditional bounds into fields such as biomineralization and materials science

modification and preservation of environmental signals in speleothems speleothems are primarily studied in order to generate archives of climatic change and results have led to significant advances in identifying and dating major shifts in the climate system however the climatological meaning of many speleothem records cannot be interpreted unequivocally this is particularly so for more subtle shifts and shorter time periods but the use of multiple proxies and improving understanding of formation mechanisms offers a clear way forward

numerical analysis of subduction initiation risk along the atlantic american passive margins as the oceanic lithosphere ages and cools its density increases and at some stage exceeds the density of the underlying asthenosphere so that the plate can sink spontaneously under its own weight however despite the fact that some seafloors are 170 m y old an undeniable cenozoic example of a passive continental margin transforming into an active margin by subduction initiation is not yet known several workers have indicated the sources of difficulty of such a transformation however no evaluations of existing passive margins from the viewpoint of their future stability have been provided as suggested by recent numerical experiments with generalized passive margin structures spontaneous subduction initiation may have a hidden initial phase that is not expressed in diagnostic features such as trench and magmatic arc here we analyze numerically the probability of subduction initiation along the atlantic american passive margins based on their topography and lithospheric and crustal structure according to our experimental results proper subduction will likely start during the next 10 20 m y along the southern part of the brazilian margin while other atlantic margins of north and south america are stable under the present geodynamic conditions

aragonite calcite transformation based on pennsylvanian molluscs a mineralogical microstructural and geochemical approach was used to trace the postdepositional alteration of molluscs and to quantify the biogenic aragonite to calcite transformation in the boggy formation mid desmoinesian of oklahoma the results show that with progressive alteration primary biogenic aragonite was gradually replaced by diagenetic low mg calcite during the initial phase of alteration the nacreous tablets of the shell layers fused and with progressive postdepositional diagenesis they were replaced by either fine or coarsegrained calcite during this diagenetic transformation no significant changes in chemical concentrations were noted for ba cu ni pb and zn whereas mg mn and fe increased by factors of about 14 to 25 relative to their depositional concentrations concurrently na sr 18 o and 13 c concentrations decreased by factors ranging from 2 5 to 10 under similar diagenetic conditions matrix samples were more altered than were contemporaneous fossils and it is postulated that grain size surface area was a major control on the degree of alteration of carbonate grains the matrix of the boggy formation has retained sufficient geochemical information to suggest an original aragonitic limemud precursor of nonmolluscan affinity with the geochemical methods and quantified trends the least altered mollusc samples of the boggy formation consisting of only aragonite can be identified geochemical data of the most altered samples suggest that the diagenetic fluids which affected boggy sediments and fossils were similar in composition to meteoric water moreover the dissolving aragonite through diffusion controlled dissolution determined the elemental as well as isotopic chemistry of the diagenetic calcite in the altered shells it is postulated that similar diagenetic waters and processes affected sedimentary units of the breathitt pennsylvanian kentucky brush creek pennsylvanian pennsylvania ohio lukow jurassic poland bear paw lea park cretaceous western canada and nugssuaq cretaceous greenland formations which contain well preserved aragonitic molluscan faunas

rapid growth and strain rates inferred from synkinematic garnets penokean orogeny minnesota recent reinterpretations of the early proterozoic penokean orogeny in minnesota invoke a plate tectonic model with characteristics similar to phanerozoic collisional belts deformation during collision produced large flattening strains 89 and the development of a tectonic foliation and progressive metamorphism up to amphibolite facies as indicated by the presence of synkinematic s1 garnets biotite garnet thermometry and plagioclase garnet biotite muscovite barometry give southward increasing temperatures and pressures and indicate burial to depths of 21 25 km similar to medial and internal zones of collisional orogens worldwide gibbs method modeling on zoned synkinematic garnets indicates virtually no change in pressure p or temperature t during garnet growth growth and rotation of garnets at a point in p t space in an actively evolving deformational and thermal regime are surprising and suggest rapid garnet growth rates and high rates of strain during orogeny this is consistent with previous studies that indicate more rapid rates of tectonometamorphic processes in the precambrian relative to younger orogenic belts

lead isotopes as a provenance tool for quartz examples from plutons and quartzite northeastern minnesota usa abstract the u pb isotope system in plutonic quartz samples approximates the crystallization age of the host granitoid and thus provides a useful provenance tool for detrital quartz grains we present lead isotopic data and u and pb concentrations of quartz from three late archean granitoids and from the early proterozoic pokegama quartzite all in northeastern minnesota lead in granitic quartz is considerably more radiogenic than in its associated whole rock host but it is similar or only slightly more thorogenic than its host pb pb ages obtained from quartz whole rock leached feldspar from the late archean granitic plutons are consistent with existing geochronology suggesting that the u pb system in quartz remained closed since crystallization u pb isotopic data for detrital clear quartz populations in the pokegama quartzite record late archean ages 2550 2650 consistent with a source from the superior province lead isotopic compositions of detrital k feldspar populations and lead and neodymium isotopic compositions of whole rocks samples from the pokegama quartzite also support a superior province source this study highlights the potential of the lead isotope system in detrital quartz k feldspar and whole rocks in provenance studies of sandstones

backward modelling of the subsidence evolution of the colorado basin offshore argentina and its relation to the evolution of the conjugate orange basin offshore sw africa abstract in this study we focus on reconstructing the post rift subsidence evolution of the colorado basin offshore argentina we make use of detailed structural information about its present day configuration of the sedimentary infill and the crystalline crust this information is used as input in a backward modelling approach which relies on the assumption of local isostasy to reconstruct the amount of subsidence as induced by the sedimentary load through different time stages we also attempt a quantification of the thermal effects on the subsidence as induced by the rifting here included by following the uniform stretching model of lithosphere thinning and exponentially cooling through time based on the available information about the present day geological state of the system our modelling results indicate a rather continuous post rift subsidence for the colorado basin and give no significant evidence of any noticeable uplift phase in a second stage we compare the post rift evolution of the colorado basin with the subsidence evolution as constrained for its conjugate sw african passive margin the orange basin despite these two basins formed almost coevally and therefore in a similar large scale geodynamic context their post rift subsidence histories differ based on this result we discuss causative tectonic processes likely to provide an explanation to the observed differences we therefore conclude that it is most probable that additional tectonic components other than the ridge push from the spreading of the south atlantic ocean are required to explain the observed differences in the subsidence of the two basins along the conjugate passive margins such additional tectonic components might be related to a dynamic mantle component in the form of either plume activity africa or a subducting slab and the presence of an ongoing compressional stress system as revealed for different areas in south america

interference of lithospheric folding in western central asia by simultaneous indian and arabian plate indentation abstract large scale intraplate deformation of the crust and the lithosphere in central asia as a result of the indentation of india has been extensively documented in contrast the impact of continental collision between arabia and eurasia on lithosphere tectonics in front of the main suture zone has received much less attention the resulting neogene shortening and uplift of the external zagros alborz kopeh dagh and caucasus mountain belts in iran and surrounding areas is characterised by a simultaneous onset of major topography growth at ca 5 ma at the same time subsidence accelerated in the adjacent caspian turan and amu darya basins we present evidence for interference of lithospheric folding patterns induced by the arabian and indian collision with eurasia wavelengths and spatial patterns are inferred from satellite derived topography and gravity models the observed interference of the patterns of folding appears to be primarily the result of spatial orientation of the two indenters differences in their convergence velocities and the thermo mechanical structure of the lithosphere west and east of the kugitang tunka line

environmental controls of speleothem mineralogy in a karstic dolomitic terrain soreq cave israel pleistocene speleothems of the soreq cave beth shemesh israel situated within karstic cretaceous dolomitic rocks consist of two major mineralogical types 1 monomineralic low magnesium calcite lmc 0 5 and 7 5 mol mgco 3 straw stalactites stalagmites helictites and pool deposits 2 mineralogically heterogeneous deposits in the form of conical stalactites flowstones and cave corals produced from thin flowing solution films or sprays in which lmc alternates with high magnesium calcite hmc 13 to 33 mol mgco 3 dolomite 33 50 mol mgco 3 and lmc laminations a unique feature of the type 2 is the continuous 0 50 mol range of mgco 3 sem and delta 18 o studies show that all calcites and dolomites grew by direct precipitation from local groundwater solutions lmc formation represents equilibrium precipitation from dripping water solution with mg ca 2 whereas hmc and dolomite precipitation occurred from solution films enriched in mg as the result of prior clos

calcite and aragonite precipitation under controlled instantaneous supersaturation elucidating the role of caco3 saturation state and mg ca ratio on calcium carbonate polymorphism abstract for many decades researchers have attempted to clarify the factors controlling the precipitation of calcium carbonate polymorphs and the incorporation of mg into calcite precipitating in natural systems in this study a novel experimental approach delineates the role of instantaneous supersaturation and solution mg ca ratio in calcium carbonate mineralogy and composition the results of this study show that in solutions with different mg ca ratios the transition between the aragonite and the calcite aragonite precipitation fields is controlled by a combination of the saturation state of the solution with respect to caco3 and the mg ca ratio in solution the data show that for increasing solution mg ca ratios progressively higher supersaturation level is required for calcite precipitation the predominance of aragonite at high solution mg ca and low supersaturations is attributed to a relative decrease in calcite growth rates as a result of increasing incorporation of mg2 in the calcite lattice at higher solution mg ca ratios as suggested by abundant experimental work increasing solution mg ca decreases the growth rate of calcite while aragonite growth rates stay unaffected our experiments do demonstrate that despite decreased growth rate the percentage of mgco3 in calcite increases with increasing solution mg ca ratio as calcite growth rates decrease aragonite growth rates stay constant and it becomes the dominant mineral phase in solutions with high mg ca ratio and low supersaturations these findings are relevant to present day mineralogical distributions in carbonate systems and might be the key to elucidate the environmental conditions responsible for mineralogic changes through geologic time the observed dependence of calcium carbonate mineralogy on solution composition and supersaturation suggests that carbonate mineralogy is controlled by kinetics of growth

understanding the kinematic evolution and genesis of a back arc continental sag basin the neogene evolution of the transylvanian basin abstract formation and evolution of back arc basins is related to the interplay between subduction and convergence velocities during the formation of an orogenic chain sedimentary basins lacking fault systems that are genetically associated can be locally observed in the hinterland of the main orogenic belt in the usual location of a back arc basin these are interesting features potentially driven by a number of contrasting mechanisms such as lithospheric traction forces driven by plate boundary processes lateral migration of lower crustal material during orogenesis or sub lithospheric processes such as the rapid retreat of subducted slabs the transylvanian basin is a miocene basin that evolved at the rear of the main carpathians arc during the subduction of a slab attached to stable europe the pannonian basin is a typical continental extensional back arc domain to the east transylvania accommodated up to 3 5 km of middle upper miocene sediments deposited in a period coeval with the shortening in the neighbouring carpathians the basin fill was subsequently exhumed and partly eroded when the carpathians collided with the european craton in late miocene times this was associated with contractional deformation and widespread salt diapirism that forms the bulk of miocene deformation observed inside the basin our study shows that no significant normal faults are present inside the basin that could have accommodated 3 5 km subsidence in terms of upper crustal extension furthermore the basin kinematics cannot be linked directly with any other mechanical process explaining the observed miocene subsidence therefore we discuss several mechanisms that may explain regional scale observations and we conclude that the miocene transylvanian basin most likely formed in response to an asymmetric simple shear extension assisted by erosion and or lower crustal flow the upper crustal effects were localized in the neighbouring pannonian basin

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

re os depositional age for archean carbonaceous slates from the southwestern superior province challenges and insights abstract rhenium osmium re os dating of carbonaceous rocks using h 2 so 4 cro 3 dissolution provides a powerful new geochronologic tool for archean terranes here we document the oldest re os depositional ages yet reported for carbonaceous sedimentary rocks carbonaceous slates are intercalated with graywackes and mafic tuffs of the joy lake sequence in the western wawa subprovince of the superior province well developed d 2 cleavage is sub parallel to bedding pyrite occurs in nodules disseminations and bedding parallel stringers samples from two drill holes dh26503 and dh26506 yield isochron ages of 2695 14 ma and 2684 16 ma and initial 187 os 188 os ratios os i of 0 15 0 16 and 0 29 0 20 respectively the ages identical within uncertainty provide the first temporal constraints for the westernmost wawa subprovince greenstone sequence the ages are consistent with cessation of mafic ultramafic volcanism before ca 2695 ma in the western wawa subprovince and d 2 deformation before 2685 ma farther east the joy lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east both sourced from arc volcanoes during collision of the wawa and wabigoon arcs along the southern margin of the superior province a chondritic os i 0 109 at 2 69 ga is expected for archean shales but the negative os i for dh26506 requires explanation re which is significantly more mobile than os may have been released from intercalated high re os mafic tuffs by local oxidation during recent or cretaceous exposure to meteoric waters the adjacent carbonaceous rocks are a ready sink for mobile re a small re addition can shift isochron data points to higher 187 re 188 os ratios with minimal slope change this increases scatter and uncertainty for the os i shifts the isochron to the right and produces a spuriously low os i intercept if re addition is recent the age is not significantly affected notably the slates in dh26506 which yield the negative os i are intercalated with relatively permeable and reactive lithic wackes and mafic tuffs whereas the slates in dh26503 are in contact with less permeable massive intrusive rocks this process can be detected only if the true os i is approximately chondritic and is more likely where reactive mafic rocks are prevalent both the case for archean greenstone belts

stable isotope geochemistry of organic matter alteration in animikie basin sediments within the thermal aureole of the duluth ccomplex abstract we have determined total organic carbon toc total carbonate carbon tcc and 13 c values for toc in samples from the early proterozoic animikie basin sediments virginia and biwabik iron formations that have been variably metamorphosed by the duluth complex toc values for the variably metamorphosed virginia formation show no systematic variation with distance from the intrusion toc from 0 06 to 2 54 in the biwabik iron formation samples outside the thermal aureole of the intrusion have toc generally below 1 compared to 1 7 6 2 for samples within the thermal aureole this enrichment may reflect extensive remobilization of organic carbon within the thermal aureole however 13 c org values measured for both the virginia and biwabik iron formations within the thermal aureole are not significantly different from those outside the thermal aureole it thus appears that in the iron formation the thermal shock caused by the intrusion induced migration of labile organic carbon that was isotopically similar to the residual kerogen the relatively low tcc in samples within the thermal aureole of the intrusion appear to indicate decarbonation of the iron formation also occurred

stable isotope geochemistry of early protezozoic carbonate concretions in the animikie group of the lake superior region evidence for anaerobic bacterial processes abstract thirteen carbonate concretions from the rove thomson and gunflint formations of the early proterozoic 2 0 ga animikie group in the lake superior region were analyzed for variations in carbon and oxygen isotopes the isotope data of ferroan dolomite concretions from the rove formation yield trends comparable to the isotope systematics displayed by authigenic carbonate in modern organic rich marine burial environments the isotope systematics of the ferroan dolomite concretions suggest that carbonate initially precipitated in a zone of bacterial methane production and later at greater burial depths in a zone of thermal decarboxylation recrystallization of calcite concretions in the rove and thomson formations has homogenized the oxygen isotopic composition however because the carbon system was apparently rock dominated the primary 13c signateres have been preserved calcite concretions from the rove formation display a strong correlation between 13c weight percent carbonate and position in the concretion the data suggest that these concretions formed progressively during burial in the zone of bacterial sulfate reduction sulfur isotopic analysis of pyrite disseminated in the concretions yield high positive 34s values isotopic data for the pyrite and the host concretion are consistent with models that suggest that high positive 34s values of early proterozic pyrite result from bacterial reduction of a limited supply of seawater sulfate the isotopic composition of concretions in the gunflint formation suggest early formation in a system open with respect to seawater bicarbonate the detailed isotopic study of precambrian carbonate concretions is a useful approach for investigating precambrian organic matter diagenesis and the evolution of bacteria

calcite fabrics growth mechanisms and environments of formation in speleothems from the italian alps and southwestern ireland five fabrics were identified in alpine and irish caves on the basis of morphological and microstructural characteristics and re lated to growth mechanisms and growth environment columnar and fibrous fabrics grow when speleothems are continuously wet and from fluids at near equilibrium conditions low supersaturation sicc 0 35 through the screw dislocation mechanism the highly defective microcrystalline fabrics form at the same supersaturation range as co lumnar fabric but under variable discharge and the presence of growth inhibitors dendritic fabrics which have the highest density of crystal defects develop in disequilibrium conditions high supersaturation un der periodic very low flow regime periods that result in prolonged out gassing cave calcareous tufa forms in disequilibrium conditions only the calcite crystals of fabrics formed at low supersaturation seem to precipitate near isotopic equilibrium conditions

loss of primary texture and geochemical signatures in speleothems due to diagenesis evidences from castanar cave spain geochemical signals from speleothems are commonly used in the investigation of palaeoenvironments in most cases however little attention is paid to whether or not these signals are primary or altered by diagenesis the speleothems of the castanar cave caceres spain which are initially formed of calcite or aragonite have undergone a variety of meteoric diagenetic processes such as micritization and neomorphism inversion that collectively modify their primary features textures mineralogy geochemical signals the mean 13c and 18o values of the aragonites in the cave are 8 66 and 4 64 respectively whereas the primary calcites have mean 13c and 18o values of 9 99 and 5 77 respectively following the diagenetic process of micritization the aragonite isotopic signals averaged 7 63 13c and 4 74 18o and the calcite micrite signals 9 53 13c and 5 21 18o where inversion took place some secondary calcites after the aragonite show preserved aragonite whereas others do not the secondary calcites without aragonite relics show isotopic values slightly higher than those of the primary calcite due to the inheritance of the aragonite signal where aragonite relics are preserved the isotopic signatures are very similar to those of the aragonite micrite in addition the stable isotopic values and sr and mg contents of the speleothems became also modified by micritization and or inversion these diagenetic processes were driven by the changes in composition of the cave waters over time and space but also in the case of aragonite by its initial unstable mineralogy the present results highlight how important diagenesis is in caves and how the initial features of cave minerals may be lost these changes alter the geochemical signals shown by speleothems which may have an impact on the interpretation of the results obtained in palaeoenvironmental studies

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

the alteration of sparry calcite crystals in a vadose setting grand cayman island calcite crystals that cement the dolostone clasts in a karst breccia from grand cayman island have been substantially altered as a result of infestation by algae and or fungi as well as etching that may or may not be related to organic activity the features produced include 1 borings 2 constructive micrite envelopes 3 spiky calcite 4 almond shaped etch pits 5 crystal surfaces with a blocky topography and 6 etched crystal surfaces that suggest some of the crystals may have grown by spiral growth spar calcite crystals in a vadose setting can be subjected to complex chemical and physical processes that are commonly mediated by algae and or fungi these microorganisms are capable of removing caco3 from substrates via dissolution transporting that caco3 elsewhere and then directly or indirectly causing its reprecipitation during the calcification of their filaments and or the formation of the constructive micrite envelopes the extensive alteration of calcite cements in the karst brecci

replacement of aragonite by quasi rhombohedral dolomite in a late pleistocene tufa mound great salt lake utah u s a abstract fabric preserving quasi rhombohedral dolomite forms as cement and replacement of aragonite in a tufa mound 2 m thick on the western shoreline of the great salt lake the rounded crystals range in size from 5 to 20 m in diameter crystal faces are not pitted and have numerous sharp edged steps and kinks the quasi rhombohedral forms are the result of crystal growth phenomena and not of post crystallization dissolution microprobe analyses n 92 of the dolomite yield an average 46 mole mgco3 in agreement with the 44 to 45 mole mgco3 content determined from x ray diffraction 30 8 2 position of the 104 peak the great salt lake dolomite is poorly ordered with relative ordering of 28 5 and a reflection ratio of 0 9503 compared to a well ordered standard dolomite the great salt lake dolomite has the poorest ordering of any natural dolomite recorded to date more than half of the electron microprobe analyses show elevated sr abundance indicating that dolomite formed in a rock buffered diagenetic system where much of the bicarbonate and ca were supplied by dissolution of aragonite and mg from the lake water preservation of the tufa fabric by dolomite was accomplished by the closely related dissolution of aragonite and precipitation of dolomite in a fluid undersaturated with respect to aragonite and supersaturated with respect to ca rich dolomite formation of spheroids might reflect a high growth rate from a high alkalinity fluid that was undersaturated with respect to other carbonate minerals maintenance of this growth rate might have been aided by the ca enrichment and cation disordering of the precipitating dolomite phase and the large surface area of a sphere compared to that of an inscribed rhomb

dolomite in caves recent dolomite formation in oxic non sulfate environments castanar cave spain abstract dolomite is a common mineral in the rock record but rare in recent superficial environments where it does occur it is related to anoxic sulfate rich environments and microbial activity the occurrence of some dolomite deposits in caves however indicates that dolomite formation is also possible in oxic non sulfate settings dolomite is forming at 17 c and in oxic vadose conditions on the host rocks and aragonite speleothems of the castanar cave caceres spain it appears as spheroids and dumbbells 50 300 m in diameter that internally consist of micron sized rhombic to rounded crystals initially this dolomite is ca rich non stoichiometric and poorly ordered mg rich solutions allow the precipitation of metastable mg rich carbonates such as huntite this soon transforms into this ca rich dolomite which later ages to form a more stoichiometric dolomite these dolomites show similarities to those grown under anoxic sulfate rich conditions and their presence in caves provides a different setting that may contribute to the understanding of the dolomite problem including their initial formation and later recrystallization processes

the plume head continental lithosphere interaction using a tectonically realistic formulation for the lithosphere summary current debates on the existence of mantle plumes largely originate from interpretations of supposed signatures of plume induced surface topography that are compared with predictions of geodynamic models of plume lithosphere interactions these models often inaccurately predict surface evolution in general they assume a fixed upper surface and consider the lithosphere as a single viscous layer in nature the surface evolution is affected by the elastic brittle ductile deformation by a free upper surface and by the layered structure of the lithosphere we make a step towards reconciling mantle and tectonic scale studies by introducing a tectonically realistic continental plate model in large scale plume lithosphere interaction this model includes i a natural free surface boundary condition ii an explicit elastic viscous ductile plastic brittle rheology and iii a stratified structure of continental lithosphere the numerical experiments demonstrate a number of important differences from predictions of conventional models in particular this relates to plate bending mechanical decoupling of crustal and mantle layers and tension compression instabilities which produce transient topographic signatures such as uplift and subsidence at large 500 km and small scale 300 400 200 300 and 50 100 km the mantle plumes do not necessarily produce detectable large scale topographic highs but often generate only alternating small scale surface features that could otherwise be attributed to regional tectonics a single large wavelength deformation predicted by conventional models develops only for a very cold and thick lithosphere distinct topographic wavelengths or temporarily spaced events observed in the east african rift system as well as over french massif central can be explained by a single plume impinging at the base of the continental lithosphere without evoking complex asthenospheric upwelling

aragonite calcite relationships in speleothems grotte de clamouse france environment fabrics and carbonate geochemistry abstract in grotte de clamouse france aragonite forms in a variety of crystal habits whose properties reflect the conditions of formation prolonged degassing and evaporation yield needle aragonite which is more enriched in 18o and 13c than aragonite ray crystals which form near isotopic equilibrium at present aragonite ray crystals form at the tops of stalagmites at very low discharge 0 00035 ml min and when fluid mg ca ratio is 1 1 temperature and evaporation do not seem to have a significant role in their formation the presence of aragonite in stalagmites should be indicative of a decrease in drip rate related to either dry climate conditions or local hydrology fossil aragonite was in part replaced by calcite in a time frame 13c signal and u content and preserved aragonite relicts up to 16 weight the isotope signal of different aragonite habits may reflect conditions of formation rather than climate parameters the real extent of aragonite to calcite transformation may be underestimated when replacement calcite inherits both textural and chemical properties of the precursor

thermo mechanical controls on the mode of continental collision in the se carpathians romania abstract the carpathians orogenic system with its along arc variations in topography developed in the aftermath of continental collision is associated with unusual foredeep basins large scale strain and seismicity concentration and high velocity mantle bodies the east carpathians continental collision was non cylindrical leading to large scale variations in thrust nappe kinematics orogenic uplift patterns and foredeep subsidence controlled by the mechanics and geometry of the lower plate thermo mechanical modelling demonstrates that in this low rate convergence regime the subducted lithosphere had enough time to interact with the mantle to advance towards a thermal resettlement this is favored by the low degree of metamorphism mechanical weakness of the lower plate and the lack of active surface processes at the contact with and in the upper plate in contrast low buoyant thick lower crust and active surface processes keep the continuity of the slab intact and promote the development of typical foredeep basins the model explains in a self consistent manner the unusual geometry of the vrancea seismogenic slab in the bend zone of the romanian carpathians the model is also consistent with the presence of two high velocity bodies inferred from seismic tomography studies and explains the depth zonation of seismicity in the vrancea area differences between the northern part of east carpathians and the southeastern bend of the carpathians arc are largely controlled by lateral variations in crustal structure topography emplacement and surface processes along the arc mechanical heterogeneity of the carpathians subduction leads to the development of two end member modes of collision allowing a study of these states and their transition lithospheric configuration and tectonic topography appear to be prime factors controlling variations in slab behavior in the se carpathians at the terminal phase of continental convergence slab delamination roll back and depocenter migration appear to play a more limited role at shallow and lithospheric levels

experimental and observational evidence for plume induced subduction on venus venus lacks plate tectonics but some trenches on venus resemble subduction zones laboratory experiments suggest that upwelling plumes can initiate localized subduction of a thin lithosphere such as the one on venus

kinetic isotopic fractionation during carbonate dissolution in laboratory experiments implications for detection of microbial co2 signatures using δ13c dic laboratory experiments on reagent grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric co2 there is preferential dissolution of ca 12 co3 during hydrolysis resulting in 13 c dic values that are significantly lighter isotopically than the bulk carbonate the fractionation factor for this kinetic isotopic effect is defined as carb carb is greater on average for glacial sediments 17 4 than for calcium carbonate 7 8 for the 63 m size fraction a sediment concentration o f5gl 1 and closed system conditions at 5 c this difference is most likely due to the preferential dissolution of highly reactive ultra fine particles with damaged surfaces that are common in subglacial sediments the kinetic isotopic fractionation has a greater impact on 13 c dic at higher caco3 water ratios and is significant during at least the firs t6ho fcarbonate dissolution driven by atmospheric co2 at sediment concentrations o f5gl 1 atmospheric co2 dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least 6 h after the start of the experiment at 5 c this is considerably longer than previously reported in the literature thus kinetic fractionation processes will likely dominate the 13 c dic signal in natural environments where rock water contact times are short 6 24 h e g glacial systems headwaters in fluvial catchments and there is an excess of carbonate in the sediments it will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial co2 signatures in dic from 13 c dic data copyright 2004

subduction initiation by thermal chemical plumes numerical studies abstract prior suggestions for the initiation of subduction have included sediment loading compression and plate reconfiguration as potential triggers here we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume numerical testing of this hypothesis has been conducted in 2d with the i2vis thermo mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal chemical or purely thermal plume we demonstrate that a mantle plume can break the lithosphere and initiate self sustaining subduction provided the plume causes a critical local weakening of the lithospheric material above it the intensity of the required weakening depends on the plume volume plume buoyancy and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes another necessary condition is the presence of high pressure fluids at the slab upper interface reducing the effective friction coefficient there to very low values based on our results we suggest that sheet like instabilities of the archean mantle convection could have initiated subduction on earth where ocean was already present in less stable tectonic settings provided that mantle plumes sheets at that time were rich in water and melt which could drastically reduce the effective friction coefficient in the lithosphere above the plume our numerical models are also in good agreement with suggested concepts for corona formation on venus

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

stable isotope geochemistry of cherts and carbonates from the 2 0 ga gunflint iron formation implications for the depositional setting and the effects of diagenesis and metamorphism abstract 18 o values for all but three of 97 samples of chert along a 220 km transect through the 2 0 ga gunflint iron formation of the animikie basin north america range from 21 3 to 24 7 o there is no correlation between 18 o and the type of chert i e deeper water lutitic chert shallower water arenitic chert and peritidal algal chert the fact that the gunflint iron formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the values of the chert have not been significantly affected by metamorphism field and petrographic observations and the small 18 o range of all the chert types suggests that silicification lithification and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment water interface the uniform 18 o values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval 20 c from a common parent water having a relatively uniform 18 o the isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water or the result of evaporative processes siderite in the deep water banded facies of the gunflint displays a 18 o range of 6 0 to 2 6 o whereas siderite in the more shoreward organic rich shale facies is significantly enriched in 13 c 13 c 2 5 to 0 5 o 18 c of siderite does not appear to have been altered as a result of neomorphic recrystallization and authigenic microsparitic siderite along stylolites displays the same 13 c range as primary and neomorphosed siderite however neomorphic and authigenic siderite are depleted in 18 o by up to 7 o relative to primary unaltered microspheroidal siderite the lower 13 c of banded facies siderite relative to the shale facies siderite does not seem to be the result of thermal decarboxylation or anaerobic oxidation abiotic or bacterial of organic matter but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes the more basinward banded facies siderite precipitated from a 13 c depleted fe 2 rich hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic volcanic hydrothermal activity onto the continental shelf the siderite of the shale facies precipitated from a more near shore shallow seawater mass that was comparatively enriched in 13 c 13 c 0 2 o ankerite in the gunflint iron formation occurs as a replacement of primary siderite in the banded facies and as a replacement of greenalite and chert in the arenite facies many ankerite samples have 13 c values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 o depleted waters resulted in a wide range of 18 o values 13 8 22 5 o the unique mineralogy of the upper most gunflint iron formation calcite serpentine and magnetite compared to the majority of the formation the very low values and overlying diabase sills suggest that the non ferroan nature of the carbonate in the upper gunflint is the result of contact metamorphism

continental plate collision unstable vs stable slab dynamics we study possible scenarios for the evolution of continental collision zones by using a dynamic thermomechanical model that includes brittle elastic ductile rheology surface erosion and explicit metamorphic changes this paper focuses primarily on the influence of four key parameters 1 geotherm or thermotectonic age which controls the rheological profile 2 lower crustal composition weak or strong rheology 3 convergence rate and 4 metamorphic changes in the downgoing crust the experiments suggest that de pending on these parameters plate convergence is accommodated by four distinct mech anisms stable subduction shortening by pure shear thickening or folding and rayleigh taylor instabilities it appears that stable oceanic type subduction can only occur in the case of cold lithospheres moho temperature tmoho 550 8c and basically needs high convergence rates 4 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction it appears that in the case of weak metamorphic rheologies phase changes only slightly improve chances for stable subduction lithospheric shortening becomes a dominant mechanism when tmoho 550 8c or convergence rates are 4 5 cm yr pure shear thickening becomes important in all cases of hot lithospheres tmoho 650 8c large scale folding is favored in the case of tmoho 5 500 650 8c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust grav itational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho 800 8c and may lead to development of subvertical cold spots

testing the influence of far field topographic forcing on subduction initiation at a passive margin abstract despite favourable gravitational instability and ridge push elastic and frictional forces prevent subduction initiation from arising spontaneously at passive margins here we argue that forces arising from large continental topographic gradients are required to initiate subduction at passive margins in order to test this hypothesis we use 2d numerical models to assess the influence of the andean plateau on stress magnitudes and deformation patterns at the brazilian passive margin the numerical results indicate that plateau push in this region is a necessary additional force to initiate subduction as the se brazilian margin currently shows no signs of self sustained subduction we examined geological and geophysical data to determine if the margin is in the preliminary stages of subduction initiation the compiled data indicate that the margin is presently undergoing tectonic inversion which we infer as part of the continental oceanic overthrusting stage of subduction initiation we refer to this early subduction stage as the brazilian stage which is characterized by 10 km deep reverse fault seismicity at the margin recent topographic uplift on the continental side thick continental crust at the margin and bulging on the oceanic side due to loading by the overthrusting continent the combined results of the numerical simulations and passive margin analysis indicate that the se brazilian margin is a prototype candidate for subduction initiation

formation of metamorphic core complexes in non over thickened continental crust a case study of liaodong peninsula east asia pre thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes mccs however the discovery of mccs in the liaodong peninsula where the crust has a normal thickness 35 km challenges the universality of this scenario therefore we implement a series of 2 d numerical thermo mechanical modeling experiments in which we investigate the conditions of mcc formation in normal crusts as well as the relationships between the underlying mechanisms and the syn rift basin evolution in these experiments we explore the impact of the lithostratigraphic and thermo rheological structure of the crust we also examine the lithosphere thickness strain softening extension rate and surface erosion sedimentation processes the experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome which is geometrically incompatible with the observations of the mccs in the liaodong peninsula according to our further findings the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift while synchronous surface erosion controls the polarity of the syn rift basin the synthetic model data are compatible with the geological observations and cooling history based on the thermo chronology for the eastern part of the east asia during the late mesozoic to the early cenozoic the model predicted p t t paths are essentially different from those inferred for the other known mccs confirming the exceptional character of the mcc formation in the wide rift system of the east asia

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

revised crism spectral parameters and summary products based on the currently detected mineral diversity on mars the investigation of hyperspectral data from the mars reconnaissance orbiter compact reconnaissance imaging spectrometer for mars crism and the observatoire pour la mineralogie l eau les glaces et l activitie omega on mars express has revealed an increasingly diverse suite of minerals present on the martian surface a revised set of 60 spectral parameters derived from corrected spectral reflectance at key wavelengths in crism targeted observations and designed to capture the known diversity of surface mineralogy on mars is presented here as summary products some of the summary products have strong heritage to omega spectral parameter calculations this paper also presents newly derived parameters that highlight locations with more recently discovered spectral signatures type locations for the diversity of currently identified mineral spectral signatures have been compiled into a library presented in this work our analysis indicates that the revised set of summary products captures the known spectral diversity of the surface and successfully highlights and differentiates between locations with differing spectral signatures the revised spectral parameter calculations and related products provide a useful tool for scientific interpretation and for future mission landing site selection and operations

compositional control of magnetic anisotropy in the thomson formation east central minnesota abstract anisotropy of magnetic low field susceptibility ams and anisotropy of anhysteretic susceptibility aas of the middle precambrian thomson formation east central minnesota generally record structural features but orientations and axial ratios of the susceptibility ellipsoids depend on local magnetic mineralogy the northern structural zone of the thomson formation is characterized by an upright east west slaty cleavage the susceptibility ellipsoids plot in the flattening field of a flinn diagram with minimum axes normal to the cleavage aas ellipsoids are coaxial with ams ellipsoids but are generally more prolate anisotropy increases with mean susceptibility at one outcrop interpreted as mixing of weakly anisotropic paramagnetic chlorite with more strongly anisotropic magnetite a second outcrop shows the opposite pattern anisotropy decreases with increasing mean susceptibility suggesting that here the magnetite may be relatively isotropic or possibly post deformational the southern structural zone of the thomson formation is characterized by a subhorizontal foliation s 1 and an upright crenulation cleavage s 2 here the magnetic susceptibility minima are coincident with poles to s 1 and maxima are parallel to the s 1 s 2 intersection correlation between directional susceptibilities and mean susceptibility technique of henry and daly 1983 suggests that paramagnetic minima are poles to the early foliation while possibly interdeformational magnetite minima record the second deformation this interpretation however is not supported by the aas fabric by thin section observations or by high field measurements of paramagnetic susceptibility alternatively both directional and mean susceptibilities may be controlled by syndeformational chlorite

genesis and dispersal of carbonate mud relative to late quaternary sea level change along a distally steepened carbonate ramp northwestern shelf western australia abstract accumulation of carbonate mud is minor on the northwest shelf western australia a broad 200 300 km wide distally steepened tropical carbonate ramp it is negligible along most of the inner ramp 0 to 50 m water depth owing to siliciclastic riverine input it is also negligible along the mid from 50 to 120 m ramp and where the entire ramp narrows to 100 km owing to constant reworking by storm waves and ocean swell in these areas carbonate mud is derived mostly from mechanical degradation of skeletal fragments but this does not include calcified remains of calcareous green algae e g halimeda although these plants grow locally only along the outer 120 to 200 m water depth ramp and slope is there any relatively abundant accumulation of carbonate mud here the sediment consists of modern pelagic foraminifer pteropod and nannoplantkon ooze and aragonitic needle rich 2 m micrite the latter having an age based on ams 14c of 19 ka distribution of aragonite micrite defines an extensive 5 0 104 km2 yet relatively narrow 40 km tract running 600 km along the platform margin crystal morphology mineralogy and isotope c o composition identify the micrite to be a seawater precipitate its production and seaward export having occurred during the last glacial maximum when lowered sea level transformed the otherwise broad platform into a narrow epicontinental shelf margin now the modern distal slope today this sediment body lies stranded in deep water about 200 300 km seaward of the shallow inner ramp the volume of micrite produced appears equal to the present day production potential of the great bahama bank rapid postglacial rise in sea level and change in platform geometry climate and oceanography were very influential factors in addition to water depth and area of submergence in the production and distribution of aragonite micrite along the northwest shelf in general 1 epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded across a platform and coastal clastics and absence of shoal barriers prevent development of protective lagoons and carbonate tidal flats 2 not all highstand carbonate systems are associated with significant amounts of micrite in shallow water environments nor do they necessarily export large volumes of micrite to the periplatform realm and 3 some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems

control of lithosphere rheology on subduction polarity at initiation insights from 3d analogue modelling abstract a series of analogue experiments was used to explore how lateral variations in lithosphere rheology may result in a subduction polarity reversal lithosphere models made of sand and silicone putty were designed to represent a plate convergence setting varying laterally from continent continent to ocean continent several experiments were performed to test the effects of strength profile variations in the converging plates the modelling outcomes categorized into models that develop with and without a subduction polarity reversal while a subduction polarity reversal was the most prevalent experiment outcome the results suggest that upper mantle strength is the dominant factor in determining whether a subduction polarity shift occurs or not a weak upper mantle layer promotes obduction of the oceanic lithosphere in the western segment of the model in model iterations where the upper mantle of the continental lithosphere was stronger than the oceanic lithosphere a reversal in subduction polarity from north dipping in the east to south dipping in the west occurred the experiment design was inspired by the iberian eurasian plate convergence and may provide a simple regional scale explanation for the deformation patterns along the pyrenean and cantabrian orogen

state of stress and age offsets at oceanic fracture zones and implications for the initiation of subduction abstract the recycling of oceanic lithosphere back into the earth s interior through subduction is a central component of plate tectonics the process by which new subduction zones initiate however remains poorly understood several different mechanisms for subduction initiation have been proposed including passive margin collapse aided by sediment loading and or rheological weakening due to the presence of volatiles and forced convergence across a zone of preexisting lithospheric weakness in this paper we focus on the latter type of model which identifies three conditions necessary for subduction initiation a zone of weakness such as a fracture zone an age and therefore density offset along the fracture zone and significant normal compressive stress which leads to shortening we identify regions on the present day earth which meet these conditions and which may correspond to regions of relatively likely subduction initiation in the near future using a digital seafloor age model we have created a database of oceanic fractures and quantified the associated age offsets we have evaluated the state of stress on these lithospheric weak zones using two different global stress models we find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present day earth and that there is little indication of incipient subduction at regions identified as relatively likely for subduction initiation using the same technique we have evaluated the state of stress and seafloor age offset at regions of inferred present day incipient subduction and find that most of these regions are not associated with both high far field compressive stresses and large age and thus density offsets subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past but our results indicate that the conditions needed for this type of subduction initiation are rare on the present day earth

plume like upper mantle instabilities drive subduction initiation 1 the control of upper mantle lithosphere interactions mli on compressional tectonics is not well resolved this applies to the role of mli in triggering of subduction initiation or lithosphere mantle lm downwellings we present results of thermo mechanically thermo dynamically coupled numerical experiments that are consistent with an array of recent geophysical constraints on lithosphere and upper mantle rheology and structure we demonstrate that mli can lead to initiation of continental lithosphere subduction inducing its spontaneous downthrusting to depths of 300 500 km upon plume impingement of the lithosphere this downthrusting is pre conditioned by rheological stratification of visco elasto plastic lithosphere and its free surface the subsequent evolution of the slab is governed by phase changes and its interactions with the surrounding mantle we demonstrate that the mode of mli is strongly affected by the lateral heterogeneities and the presence of suture zones in the lithosphere

age of volcanic rocks and syndepositional iron formations marquette range supergroup implications for the tectonic setting of paleoproterozoic iron formations of the lake superior region a rhyolite in the hemlock formation a mostly bimodal submarine volcanic deposit that is laterally correlative with the negaunee iron formation yields a sensitive high resolution ion microprobe shrimp u pb zircon age of 1874 9 ma but also contains inherited archean zircons as old as 3 8 ga this precise age determination for the classic paleoproterozoic stratigraphic sequence of northern michigan the marquette range supergroup mrs necessitates modification of previous depositional and tectonic models our new data indicate that the menominee group previously ascribed to continental rifting during early pre collision phases of the penokean orogenic cycle is coeval with arc related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit we interpret these to be second order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin along with a recently reported age for the gunflint formation in ontario of 1878 2 ma our data suggest that an extensive foredeep in the western lake superior region was the locus of iron formation deposition during arc accretion from the south further we interpret the lower mrs chocolay group a glaciogenic and shallow marine succession that lies atop archean basement to be equivalent to the upper part of the huronian supergroup of ontario and to represent the original continental rifting and passive margin phase of the penokean cycle the upper mrs baraga group represents deeper marine basins dominated by turbidites and lesser volcanic rocks resulting from increased subsidence and continued collision a stitching pluton which cuts correlatives of the hemlock formation in a thrust sheet yielded a u pb zircon age of 1833 6 ma consistent with other post tectonic plutons in wisconsin and northern michigan indicating that penokean convergence lasted no longer than 40 million years

bundled slaty cleavage in laminated argillite north central minnesota abstract exceptional bundled slaty cleavage defined herein has been found in drill cores of laminated folded weakly metamorphosed argillite at several localities in the early proterozoic animikie basin of north central minnesota the cleavage domains are more closely spaced within the cleavage bundles than outside them the mean tectosilicate grain size of siltstone layers measured normal to cleavage is less in the cleavage bundles than outside them and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra bundle segments these facts suggest that pressure solution was a major factor in bundle development if it is assumed that opaque phases have been conserved during pressure solution the modal differences in composition between intra bundle and extra bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage argillite samples from the central part of the animikie basin have been shortened a minimum of about 22 as estimated by this method these estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite and are also consistent with the regional pattern of deformation

origin of endogenetic micrite in karst terrains a case study from the cayman islands abstract cavities in the dolostones of the cayman formation miocene on grand cayman and cayman brae commonly contain spar calcite cements and or a variety of exogenetic derived from sources external to the bedrock and endogenetic derived from sources in the bedrock internal sediments micrite is a common component in many of these internal sediments the exogenetic micrite which is typically laminated and commonly contains fragments of marine biota originated from the nearby shallow lagoons the endogenetic micrite formed as a residue from the breakdown of spar calcite crystals by etching as constructive and destructive envelopes developed around spar calcite crystals by calcification of microbes by breakdown of calcified filamentous microbes and by precipitation from pore waters nce produced the endogenetic micrite may be transported from its place of origin by water flowing through the cavities endogenetic micrite can become mixed with the exogenetic micrite subsequently it is impossible to recognize the origin of individual particles because the particles in endogenetic micrite are morphologically like the particles in exogenetic micrite formation of endogenetic micrite is controlled by numerous extrinsic and intrinsic parameters in the cayman formation for example must endogenetic micrite is produced by etching of meteoric calcite crystals that formed as a cement in the cavities or by microbial calcification as a result the distribution of the endogenetic micrite is ultimately controlled by the distribution of the calcite cement and or the microbes factors controlled by numerous other extrinsic variables irrespective of the factors involved in its formation it is apparent that endogenetic micrite can be produced by a variety of processes that are operating in the confines of cavities in karst terrains

stratigraphy of the southern norfolk ridge and the reinga basin a record of initiation of tonga kermadec northland subduction in the southwest pacific abstract seismic stratigraphic interpretations of a large new 2d seismic reflection dataset from the reinga basin region northwest of new zealand constrain the history of cretaceous fragmentation of gondwana and cenozoic initiation of tonga kermadec northland subduction the southern norfolk ridge system lay in a proximal location to plate boundaries that were active during both cretaceous and cenozoic events and persistent marine conditions led to a relatively complete record of sedimentation cretaceous extension was followed by regional subsidence and transgression two cenozoic contractional events are separated by substantial 1 km subsidence late eocene contraction led to reverse faulting and folding between new caledonia and the southern norfolk ridge system and topographic highs created in the north western sector were locally eroded by wave abrasion reinga basin was located at the southern tip of this event which had only limited impact in adjacent northern new zealand regional oligocene and miocene subsidence was contemporaneous with late oligocene and early miocene emplacement of nappes in northern new zealand and the onset of arc volcanism early miocene contraction in the reinga basin led to formation of the wanganella ridge these events can be related to calculated plate motions and are shown to be consistent with models of induced subduction nucleation that require c 150 km of convergence the first contraction is associated with regional inception of tonga kermadec subduction whereas oligocene to miocene events and the onset of hikurangi northland subduction were in response to a local change in plate boundary displacement rate and consequent linkage between subduction north of new zealand with alpine fault formation farther south

lithospheric folding based understanding on the origin of the back arc basaltic magmatism beneath jeju volcanic island 1 compression near collisional plate boundaries causes lithospheric folding which results in the decrease of the pressure beneath the ridge of the fold while the pressure beneath the trough increases the decompression beneath the lithosphere is likely to bring on basaltic magmatism along and below the ridge we investigate the subsurface structure beneath jeju cheju volcanic island and its vicinity and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by episodic lithospheric folding unlike the prevailing hypothesis of the intraplate basaltic magmatism that requires extending lithosphere our hypothesis can explain how the basaltic magma could be generated at the back arc regions without the extension

controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics differences and similarities summary mantle plume activity and lithospheric folding by far field stresses exerted from plate boundaries are two important end members as mechanisms for continental intraplate deformation the topographic expression of mantle plume impingement on continental lithosphere and lithospheric folding has some striking similarities observations from a number of areas in europe s intraplate lithosphere demonstrate that these mechanisms commonly interact in space and time we present the results of thermomechanical modelling addressing the role of factors such as the presence of a hot upper mantle the spatial dimensions of the plume and the time constants involved in the temporal succession of plume activity and lithospheric folding by stress accumulation in intraplate continental lithosphere the results demonstrate that both the processes plume lithosphere interactions and folding may interact resulting either in strong amplification attenuation or modification of their surface expression these inferences are compatible with a number of key observations on the nature and the temporal succession of topography evolution in the alpine foreland the pannonian basin the scandinavian continental margin and the iberian peninsula

evidence for anomalous mantle upwelling beneath the arabian platform from travel time tomography inversion abstract we present a new model of p velocity anomalies in the upper mantle beneath the arabian peninsula red sea and surrounding regions this model was computed with the use of travel time data from the global catalogue of the international seismological center isc for the years of 1980 2011 the reliability of the model was tested with several synthetic tests in the resulting seismic model the red sea is clearly associated with a higher p velocity anomaly in the upper mantle at least down to 300 km depth this anomaly might be caused by upward deviation of the main mantle interfaces caused by extension and thinning of the lithosphere due to passive rifting thick lithosphere of the arabian platform is imaged as a high velocity anomaly down to 200 250 km depth below this plate we observe a low velocity structure that is interpreted as a hot mantle upwelling based on the tomography results we propose that this upper mantle anomaly may represent hot material that migrates westward and play a major role in the formation of cenozoic basaltic lava fields in western arabia on the northeastern side of the arabian plate we clearly observe a dipping high velocity zone beneath zagros and makran which is interpreted as a trace of subduction or delamination of the arabian plate lithosphere

the future of earth s oceans consequences of subduction initiation in the atlantic and implications for supercontinent formation subduction initiation is a cornerstone in the edifice of plate tectonics it marks the turning point of the earth s wilson cycles and ultimately the supercycles as well in this paper we explore the consequences of subduction zone invasion in the atlantic ocean following recent discoveries at the sw iberia margin we discuss a buoyancy argument based on the premise that old oceanic lithosphere is unstable for supporting large basins implying that it must be removed in subduction zones as a consequence we propose a new conceptual model in which both the pacific and the atlantic oceans close simultaneously leading to the termination of the present earth s supercycle and to the formation of a new supercontinent which we name aurica our new conceptual model also provides insights into supercontinent formation and destruction supercycles proposed for past geological times e g pangaea rodinia columbia kenorland

an early cambrian plume induced subduction initiation event within the junggar ocean insights from ophiolitic melanges arc magmatism and metamorphic rocks abstract the central asian orogenic belt caob is one of the largest accretionary orogens in the world the mechanism of continental growth and tectonic evolution of the caob remain debated here we present an overview of early paleozoic ophiolitic melanges calc alkaline intrusions and metamorphic rocks in west junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the junggar ocean a branch of the southern paleo asian ocean pao the early paleozoic ophiolitic melanges are composed of ultramafic mafic rocks cherts pelagic limestones basaltic breccias and tuffs the mafic rocks from these ophiolitic melanges are divided into morb like and oib like types the morb like rocks were formed in a fore arc setting but the oib like mafic rocks were formed by the intra plate magmatism related to mantle plume activities the early paleozoic intrusions are occurred as small stocks with a dominant composition of diorite trondhjemite and granodiorite these granitoids display high k calc alkaline affinities and have high and positive end t and ehf t values formed in an arc related setting the metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from 500 ma to 460 ma their protoliths are calc alkaline andesite and tholeiitic and alkaline basalts formed in an arc related and seamount setting respectively it is clearly show that the west junggar was under an intra oceanic subduction regime during the early paleozoic and the initial subduction of the southern pao might have occurred in the early cambrian based on our observations and in combination with previous work we propose the plume induced subduction initiation model for the early middle cambrian tectonic evolutionary of the junggar ocean our new model not only shed light on subduction initiation dynamics of the southern pao but also contribute to tectonic evolution of the caob

subduction initiation at relic arcs although plate tectonics is well established how a new subduction zone initiates remains controversial based on plate reconstruction and recent ocean drilling within the izu bonin mariana we advance a new geodynamic model of subduction initiation si we argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone the combination of thermal and compositional density contrasts between the overriding relic arc and the adjacent old pacific oceanic plate promoted spontaneous si we suggest that thermal rejuvenation of the overriding plate just before 50 ma caused a reduction in overriding plate strength and an increase in the age contrast hence buoyancy between the two plates leading to si the computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation while transform faults and passive margins are not

subduction initiation along the inherited weakness zone at the edge of a slab insights from numerical models summary a preexisting weakness zone in the lithosphere is required to initiate subduction here we focus on a new type of weakness zone a subduction transform edge propagator step fault which is inherited from a tear along the edge of a slab using coupled thermal mechanical models we show that step fault perpendicular convergence results in a dipping shear zone in any tectonic setting at a continental margin this shear zone dips towards the continent which is an excellent starting condition for ocean continent subduction if far field convergence persists step faults become new subduction boundaries the trench moves landward during the earliest stages of convergence when slab pull becomes a dominant driving force after 80 km convergence trench roll back commences the initial geometry and mechanical properties of the sub crustal step fault zone affect the results subduction initiation is facilitated by a wide 100 km and low viscosity weakness zone incipient subduction is easier for young oceanic lithosphere due to its lower flexural rigidity and is insensitive to the far field convergence rate as step faults are commonly associated with young oceanic lithosphere subduction initiation is thus relatively easy along them of particular interest are continent ocean margins where step faulting has occurred

palaeo climate reconstruction from stable isotope variations in speleothems a review abstract speleothems are now regarded as valuable archives of climatic conditions on the continents offering a number of advantages relative to other continental climate proxy recorders such as lake sediments and peat cores they are ideal materials for precise u series dating yielding ages in calendar years thereby circumventing the radiocarbon calibration problems associated with most other continental records stable isotope studies in speleothems have shifted away from attempting to provide palaeo temperature reconstructions to the attainable goal of providing precise estimates for the timing and duration of major o isotope defined climatic events characterised by high signal to noise ratios e g glacial interglacial transitions dansgaard oeschger oscillations the 8200 year event unlike the marine records speleothem data sets are not tuned and their independent chronology offers opportunities to critically assess leads and lags in the climate system that in turn can provide important insights into forcing and feedback mechanisms improved procedures for the extraction and measurement of stable isotope ratios in fluid inclusions trapped in speleothems are likely to provide in the near future a much enhanced basis for the quantitative interpretation of o isotope ratios in speleothem calcite the latter developments open up once again the tantalising prospect of palaeo temperature estimates but more importantly perhaps provide a direct test for a new generation of general circulation models whose hydrological cycles will incorporate the water isotopes the literature is reviewed briefly to provide for the reader a sense of the current state of the art and to provide some pointers for future research directions

subduction initiates at straight passive margins subduction initiation at straight passive margins can be investigated with two dimensional 2 d numerical models because the geometry is purely cylindrical however on earth straight margins rarely occur the construction of 3 d models is therefore critical in the modeling of spontaneous subduction initiation at realistic curved passive margins here we report on the results obtained from gravitationally driven 3 d thermomechanical numerical models using a visco plastic rheology and a passive margin with a single curved section in the middle the models show that the curvature angle can control subduction initiation the greater is the more difficult subduction initiation becomes the 3 d thermomechanical models provide an in depth physical understanding of the processes specifically we find that pressure gradients arising from density differences between oceanic and continental rocks drive subduction initiation and strongly influence the timing the main difference between straight cylindrical and curved margins is that the orientation of the pressure gradient in 3 d is no longer constant thus producing a horizontal along margin component of flow we thus conclude that the reason for the impedance of subduction initiation is the result of partitioning of the vertical velocity component into a horizontal component which therefore decreases the effective slab pull we infer that although favorable for subduction initiation in a 2 d model because the estimated force balance is adequate the pronounced curvature in the southeast brazilian margin is a likely explanation why subduction initiation is hampered there

large scale deformation in a locked collisional boundary interplay between subsidence and uplift intraplate stress and inherited lithospheric structure in the late stage of the se carpathians evolution 1 the interplay between slab dynamics and intraplate stresses in postcollisional times creates large near surface deformation particularly in highly bent orogens with significant lateral variations in mechanical properties this deformation is expressed through abnormal foredeep geometries and contrasting patterns of vertical movements intraplate folding is often the controlling mechanism particularly when the orogenic belt is locked the study of these tectonic processes in the se carpathians indicates a generalized subsidence period during latest miocene pliocene times driven by the slab pull and an intraplate folding due to an overall quaternary inversion the latter accommodates 5 km ese ward movement of this area with respect to the neighboring units which creates complicated three dimensional deformation patterns potentially driven at a larger scale by the interaction between the adriatic indentor and the entire carpathians system the lithospheric anisotropy inherited from the subduction times concentrates strain and induces large scale deformation far away from the active plate margins this anisotropy is dynamic because of deep mantle processes related to the subducted slab during postcollisional times such as thermal reequilibration or increase in slab dip

topo europe from iberia to the carpathians and analogues abstract earth sciences are rapidly changing from largely descriptive to process oriented disciplines that aim at quantitative models for the reconstruction and forecasting of the complex processes in the solid earth prediction is highly relevant for the basic needs of humanity supply of water and resources protection against natural hazards and control on the environmental degradation of the earth intensive utilization of the human habitat carries largely unknown risks of and makes us increasingly vulnerable there is an urgent need for scientifically advanced geo prediction systems that can accurately locate subsurface resources and forecast timing and magnitude of natural hazards the design of such systems is a major multidisciplinary scientific challenge the quantitative understanding of the earth has made significant progress in the last few decades important ingredients in this process have been the advances made in seismological methods to obtain information on the 3d structure of the mantle and the lithosphere in the quantitative understanding of the lithospheric processes as well as the recognition of the key role of quantitative sedimentary basin analysis in connecting temporal and spatial scales quantitative understanding of the mass transfer by erosion and deposition as well as their feed back with crustal and subcrustal dynamics presents a new frontier in modern earth sciences this research bridges investigations on high resolution near surface time scales and the large scale long term approaches characteristic for the lithosphere and basin wide studies the essential step towards a 4d approach in space and time is a direct response to the need for a full incorporation of geological and geophysical constraints in the modelling of solid earth processes topo europe is a multidisciplinary international research program that addresses the interaction of processes inherent to the deep earth lithosphere and mantle with surface processes erosion climate and sea level which together shaped the topography of europe the objective of this programme is to quantify natural hazards by incorporating data interactive modelling focused on the lithosphere memory and neotectonics with special attention on the thermo mechanical structure of the lithosphere mechanisms controlling large scale plate boundaries and intraplate deformations anomalous subsidence and uplift and links with surface processes and topography evolution

skeletal aragonite neomorphism in plio pleistocene sandy limestones and sandstones hollywood florida usa abstract the basic mechanics of the neomorphism of aragonitic shells to calcite are generally understood but fundamental questions remain concerning the details of the process completely and partially neomorphosed aragonitic skeletal fragments recovered in cores of plio pleistocene limestones and sandstones from hollywood florida provide some insights into the process skeletal aragonite neomorphism occurred in a volume for volume manner across solution films which resulted in the preservation of ghosts of fine scale microstructures neomorphic calcite crystals that replaced skeletal aragonite in at least some instances inherited their crystallographic orientation from adjoining calcite cement crystals rather than from the host shell microstructure some neomorphic calcite crystals inherited the optical orientation of nearby echinoderm ossicles however host shell microstructure commonly controlled the direction of migration of the neomorphic replacement front anisotropies in shell solubility influenced the directional rates of growth of neomorphic calcite crystals as evidenced by the strong tendency for the boundaries between neomorphic calcite and aragonite to parallel or coinicide with shell microstructural features no examples were observed where neomorphic calcite replaced aragonite along euhedral crystal faces such as commonly occurs during dolomitization and the replacement of limestone by megaquartz

microboring versus recrystallization further insight into the micritization process sem observations of lightly etched thin sections of bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity a coccoid cyanobacterium tentatively identified as solentia sp bores tunnels which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain these tunnels are filled by radial fibrous aragonite which is precipitated as the microorganism advances extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes the rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring this type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization

episodic formation of neotethyan ophiolites tibetan plateau snapshots of abrupt global plate reorganizations during major episodes of supercontinent breakup abstract neotethyan ophiolites along the bangong nujiang and yarlung tsangpo sutures tibetan plateau consist of mantle peridotite cumulates isotropic gabbro diabase plagiogranite and basaltic lava they comprise morb iab oib and boninitic rocks and mostly belong to supra subduction zone ssz type compositionally different rocks crystallized synchronously over a distance of 2000 km along the bangong nujiang and yarlung tsangpo sutures during the short periods of 190 182 and 132 120 ma respectively resembling the izu bonin mariana ibm proto arc the early jurassic bangong nujiang ophiolites formed in forearc settings during subduction initiation while the middle jurassic early cretaceous bangong nujiang ophiolites and the early cretaceous yarlung tsangpo ophiolites formed in forearc settings at pre existing subduction zones crystallization of the neotethyan ssz ophiolites from the eastern mediterranean middle east tibetan plateau to southeast asia and formation of the associated metamorphic rocks temporally coincided with major supercontinent breakup events 1 initial breakup of pangea in central atlantic and westernmost tethys 190 ma 2 separation of west and east gondwana 170 160 ma and initial opening of piemont ligurian tethys 175 164 ma 3 initial breakups of east and west gondwana 135 125 ma and 4 final breakup of gondwana 104 96 ma far field forces associated with abrupt global plate reorganizations may have triggered subduction initiation or transient interplate coupling decoupling at pre existing subduction zones leading to episodic formation of the neotethyan ssz ophiolites and rapid exhumation of metamorphic rocks in subduction zones the formation events of neotethyan ssz ophiolites and associated metamorphic rocks represent snapshots of global plate reorganizations during the major episodes of supercontinent breakup

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

re os isotopic composition and pge contents of proterozoic carbonaceous argillites virginia formation northeastern minnesota abstract carbonaceous sulfidic argillites from the proterozoic virginia formation in minnesota have been investigated in terms of platinum group elements pge distribution and re os isotopic signatures kerogen from protoliths and slightly metamorphosed equivalents is characterized by re and os enrichment factors relative to whole rock concentrations that range from 4 to 272 re os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1 85 ga reference isochron this age is consistent with that estimated for the virginia formation and suggests that dating of organic rich precambrian sedimentary rocks and the retrieval of data pertinent to the os isotopic evolution of seawater may be possible using the re os isotopic system

geodynamics and intermediate depth seismicity in vrancea the south eastern carpathians current state of the art abstract the vrancea region of the south eastern carpathians is a remarkable site of intra continental intermediate depth seismicity a large set of geological geophysical and geodetic observations has been accumulated for the last few decades and utilised to improve our knowledge of the shallow and deep structures beneath vrancea the crustal and mantle dynamics and the linkage between deep and surface processes in the region in this article we review geology and tectonics of the vrancea region including post collisional to recent deformations syn to post collisional magmatism and orogenic exhumation along the east and south carpathians the regional seismicity is analysed and the recent seismic studies including reflection refraction body and surface wave tomography are reviewed we discuss new geodetic measurements of horizontal and vertical movements in the region geoelectric studies density gravity and thermal modelling qualitative and quantitative including retrospective geodynamic models developed for vrancea are analysed the knowledge of regional tectonics geodynamics seismicity lithospheric deformation and stress regime in the vrancea earthquake prone region assists in an assessment of strong ground motion seismic hazard and risk the earthquake simulation seismic hazard and earthquake forecasting models have also been reviewed providing a link between deep geodynamic processes and their manifestation on the surface finally we discuss unresolved problems in vrancea in order to improve our understanding of the regional evolution present tectonics mantle dynamics intermediate depth seismicity and surface manifestations of the lithosphere dynamics and to enhance our ability to forecast strong earthquakes in the vrancea region the problems to be solved include i the origin of the high velocity body revealed by seismic tomography studies oceanic versus continental ii the lithospheric scale mechanism driving the miocene subsidence of the transylvania basin iii sub crustal structure between 40 and 70 km iv contemporary regional horizontal and vertical movements and v a comprehensive seismic hazard assessment in the region

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

pb and nd isotope and trace element constraints on the origin of basic rocks in an early proterozoic igneous complex minnesota abstract a suite of early proterozoic basic to granitic rocks exposed near st cloud minnesota are cut by northeast trending basaltic dikes pb isotope data for all of these rocks overlap and plot about an 1800 ma pb pb correlation line the basic rocks which are light ree enriched have nd values between 0 4 and 4 8 petrogenetic considerations suggest that the basic rocks were derived from a light ree enriched source which had an fe mg ratio greater than that for pyrolite the enrichment in fe mg is probably a result of the addition of basic melts to the source the light ree enrichment may have a mantle origin by the addition of mantle derived light ree enriched basic melts or fluids or a sedimentary origin by the addition of light ree enriched fluids or melts derived from subducted sediments with an early proterozoic provenance in either case the nd isotopes suggest that the light ree enriched component existed since c 2300 ma the igneous complex may have formed at a convergent margin an anorthositic gabbro near mora has an nd of 5 indicating that it was derived from a mantle source with a history of light ree depletion this gabbro may have been part of an early proterozoic ocean crust

the dissolution kinetics of major sedimentary carbonate minerals abstract among the most important set of chemical reactions occurring under near earth surface conditions are those involved in the dissolution of sedimentary carbonate minerals these minerals comprise about 20 of phanerozoic sedimentary rocks calcite and to a significantly lesser extent dolomite are the major carbonate minerals in sedimentary rocks in modern sediments aragonite and high magnesian calcites dominate in shallow water environments however calcite is by far the most abundant carbonate mineral in deep sea sediments an understanding of the factors that control their dissolution rates is important for modeling of geochemical cycles and the impact of fossil fuel co 2 on climate diagenesis of sediments and sedimentary rocks it also has practical application for areas such as the behavior of carbonates in petroleum and natural gas reservoirs and the preservation of buildings and monuments constructed from limestone and marble in this paper we summarize important findings from the hundreds of papers constituting the large literature on this topic that has steadily evolved over the last half century our primary focus is the chemical kinetics controlling the rates of reaction between sedimentary carbonate minerals and solutions we will not attempt to address the many applications of these results to such topics as mass transport of carbonate components in the subsurface or the accumulation of calcium carbonate in deep sea sediments such complex topics are clearly worthy of review papers on their own merits calcite has been by far the most studied mineral over a wide range of conditions and solution compositions in recent years there has been a substantial shift in emphasis from measuring changes in solution composition to determine batch reaction rates to the direct observation of processes occurring on mineral surfaces using techniques such as atomic force microscopy afm however there remain major challenges in integrating these two very different approaches a general theory of surface dissolution mechanisms currently lacking although see lasaga and luttge science 291 2001 2400 is required to satisfactorily relate observations of mineral surfaces and the concentration of dissolved components studies of aragonite high magnesian calcites magnesite and dolomite dissolution kinetics are much more limited in number and scope than those for calcite and provide at best a rather rudimentary understanding of how these minerals are likely to behave in natural systems although the influences of a limited number of reaction inhibitors have been studied probably the greatest weakness in application of experimental results to natural systems is understanding the often profound influences of foreign ions and organic matter on the near equilibrium dissolution kinetics of carbonate minerals

thermo mechanical controls on intra plate deformation and the role of plume folding interactions in continental topography thermo tectonic age and inherited structure exert the main controls on the bulk strength of the lithosphere in intraplate settings mechanical decoupling within the lithosphere strongly affects the interaction between deep earth and surface processes thermo mechanical models demonstrate the particular importance of the rheological stratification of the lithosphere in the preservation of ancient cratonic blocks in the surface expression of plume and mantle lithosphere interactions and their impact on the dynamic topography in general the same is true for the effect of large scale lithospheric folding on intraplate basin formation and associated differential vertical motions initiation of continental lithosphere subduction crucial for linking orogenic deformation to intraplate deformation appears to be facilitated by plume lithosphere interactions we present a discussion of these aspects focusing on better process understanding of continental deformation in the context of a number of well documented cases of intraplate deformation

skeletal aragonite neomorphism quantitative modelling of a two water diagenetic system abstract the neomorphism of skeletal aragonite occurs by the simultaneous dissolution of aragonite and precipitation of calcite along thin solution films which may have an associated zone of partly dissolved aragonite chalky zone it has been proposed that pore waters at neomorphic calcite aragonite contact zones neomorphic fronts may be partly isolated from bulk pore waters and thus retain primary geochemical signatures to varying degrees iterative computer programs were developed that simulate aragonite neomorphism in two water diagenetic systems using stable isotope and trace element data neomorphism of aragonitic mollusks from the purbeck group of southern england late jurassic to early cretaceous was associated with a decrease in strontium concentration from approximately 5100 ppm to 500 ppm modelling results indicate a high bulk water rock ratio and minimal if any isolation of the neomorphic front waters for the purbeck group mollusks and also for some pleistocene corals partial isolation of neomorphic front waters from bulk pore waters is uncommon based on published data on the geochemistry of neomorphic calcites however partial isolation does occur in some thick low porosity shells such as the gastropod strombus gigas pleistocene process based modelling of fluid rock interaction provides a means for better interpreting trace element and stable isotope data in diagenetic carbonates sensitivity analysis results indicate that water rock ratios cannot be determined at much better than order of magnitude accuracies because of uncertainties over initial pore water composition fractionation factors and distribution coefficients

upwelling mantle plume and lithospheric delamination beneath the north china craton abstract various models have been proposed for the destruction of the eastern north china craton ncc however it is still a highly controversial scientific issue in this study abundant teleseismic data recorded by permanent and temporary stations in the ncc were collected and p wave tomography and common conversion point ccp stacking of receiver functions were performed the results from this study reveal an upwelling mantle plume and a shallow lehmann discontinuity or the lower boundary of a low velocity zone beneath the eastern ncc additionally a large scale plate like high velocity anomaly is defined under the eastern ncc that might be related to the delamination of a lithospheric keel based on the above results and assessment ncc destruction might include two causes lithospheric keel removal or delamination and mantle plume upwelling

3 d thermo mechanical modeling of plume induced subduction initiation abstract here we study the 3 d subduction initiation process induced by the interaction between a hot thermo chemical mantle plume and oceanic lithosphere using thermo mechanical viscoplastic finite difference marker in cell models our numerical modeling results show that self sustaining subduction is induced by plume lithosphere interaction when the plume is sufficiently buoyant the oceanic lithosphere is sufficiently old and the plate is weak enough to allow the buoyant plume to pass through it subduction initiation occurs following penetration of the lithosphere by the hot plume and the downward displacement of broken nearly circular segments of lithosphere proto slabs as a result of partially molten plume rocks overriding the proto slabs our experiments show four different deformation regimes in response to plume lithosphere interaction a self sustaining subduction initiation in which subduction becomes self sustaining b frozen subduction initiation in which subduction stops at shallow depths c slab break off in which the subducting circular slab breaks off soon after formation and d plume underplating in which the plume does not pass through the lithosphere and instead spreads beneath it i e failed subduction initiation these regimes depend on several parameters such as the size composition and temperature of the plume the brittle plastic strength and age of the oceanic lithosphere and the presence absence of lithospheric heterogeneities the results show that subduction initiates and becomes self sustaining when the lithosphere is older than 10 myr and the non dimensional ratio of the plume buoyancy force and lithospheric strength above the plume is higher than approximately 2 the outcomes of our numerical experiments are applicable for subduction initiation in the modern and precambrian earth and for the origin of plume related corona structures on venus

p and s velocity anomalies in the upper mantle beneath europe from tomographic inversion of isc data we present a new tomographic model for p and s velocity anomalies beneath europe 30 n 55 n 5 w 40 e extending in depth up to 700 km and constrained by inversion of data from the international seismological center isc catalogue the algorithm uses the traveltimes from events located in the study area recorded by all available worldwide stations as well as times from teleseismic events recorded by european stations the events from the isc catalogue have been relocated and combined into composite events all the traveltimes were corrected for crustal structure using the reference model eucrust 07 the resulting velocity anomalies show similar large scale patterns as observed in previous studies but have a higher resolution which allows detection of some features in more detail for example it is now possible to assess the depth extension of the small slow velocity body beneath the eifel region and eger graben the p and the s model show a good consistency in the uppermost 200 km below most of the european area and in some parts even in the deeper layers e g beneath the apennines and the hellenic arc the new model provides clear images of some principal features which were previously detected in a limited number of studies while the comparison between p and s velocity anomalies provides novel constraints to address on their nature e g the gap in the adriatic plate subducted below the central southern apennines in this paper we pay special attention to testing the reliability of the results the random noise effect is evaluated using a test with independent inversion of two data subsets with odd even events the spatial resolution is estimated using different checkerboard tests furthermore we present a synthetic model with realistic patterns which reproduces after performing forward and inverse modelling the same shape and amplitudes of the anomalies as in the case of the real data inversion in this case the parameters of the model can be used to assess the amplitudes of p and s anomalies that is critical for evaluation of other petrophysical parameters temperature density composition etc in the upper mantle 2009 the authors journal compilation 2009 ras