comment on formation of chenier plain of the donana marshland sw spain observations and geomorphic model by a rodriguez ramirez and c m yanez camacho marine geology 254 2008 187 196 abstract the application of the regional marine 14 c reservoir effect r to obtain an accurate calibration of conventional marine shell 14 c dates requires prior research concerning the oceanographic conditions and the r of that region rodriguez ramirez and yanez camacho a rodriguez ramirez c m yanez camacho formation of chenier plain of the donana marshland sw spain observations and geomorphic model marine geology 254 2008 187 196 and afterwards a rodriguez ramirez j a morales j borrego and e g san miguel in a reply a rodriguez ramirez j a morales j borrego e g san miguel reply to the comment on formation of chenier plain of the donana marshland sw spain observations and geomorphic model by a rodriguez ramirez and c m yanez camacho marine geology 254 2008 187 196 marine geology 263 2009 123 125 to the comment by j rodriguez vidal f ruiz and l m caceres on the paper mentioned above j rodriguez vidal f ruiz l m caceres comment on formation of chenier plain of the donana marshland sw spain observations and geomorphic model by a rodriguez ramirez c m yanez camacho marine geology 254 2008 187 196 marine geology 263 2009 120 122 use in both cases not only incorrect r values for the calibration of 14 c dates obtained from shell samples collected from the east coast of the gulf of cadiz but they also confuse the r concept with that of r t the 14 c reservoir age because the misunderstanding of these concepts by these authors and in the first case also their failure to take into account the r values obtained by us and already published from the east coast of gulf of cadiz all the calibrated dates presented either in the first paper or in the reply cannot be accepted here i present a new calibrated dataset using accurate r values and i recommend that the conclusions obtained by rodriguez ramirez and their collaborators be revised as j rodriguez vidal f ruiz and l m caceres proposed in their comment

sedimentary record of recent tsunamis in the huelva estuary southwestern spain abstract the coast of the gulf of cadiz has been repeatedly struck by tsunamis during the late holocene and the historic period historical documents record at least seven catastrophic tsunamis affecting this coast while others have been detected by oceanic instruments nevertheless very few studies have documented the sedimentary record of these tsunamis in the low energy coastal environments of the gulf of cadiz the huelva estuary is located at the central sector of the gulf of cadiz and is a mesotidal estuary fed by the tinto and odiel rivers the mean tidal range of the huelva estuary is ca 2 m and the mean significant wave height is ca 0 5 m the outer part of the estuary is characterized by extensive sand rich sedimentary bodies of marine origin whereas the central and inner parts are infilled by muddy tidally deposited sediment bodies recorded within these estuarine sediment bodies are five laterally continuous shelly units these units display many of the typical characteristics of tsunami deposits each typically comprises an erosional base followed by a fining upwards sequence that begins with the shell accumulation and ends with bioturbated muddy sand the results of 14 c ages and recent radionuclides accumulation rates in this study illustrate that these five tsunami deposits correspond with known events that occurred in ad 1755 1531 949 1033 881 and 395 381 all of which are documented in historical sources

experiments on phase transformations and chemical reactions of mechanically activated minerals by grinding petrogenetic implications abstract prolonged grinding increases the energy of solids by the production of stored energy in the form of new surfaces and internal defects moreover grinding also generates quasi hydrostatic pressures which can result in polymorphic transformations and mineral decomposition here we demonstrate the solid state transformation of metastable to stable polymorphs aragonite calcite anatase rutile the transformation of low pressure to high pressure phases calcite aragonite and the lowering of the dehydration and decarbonation temperatures of minerals siderite magnetite or hematite diaspore corundum in the presence of a fluid phase stored energy from grinding can be released resulting in accelerated reaction rates and more importantly phase transformations in this paper we demonstrate the following transformations ground calcite magnesian calcite at low mg2 concentration in solution ground calcite aragonite at high mg2 concentration ground magnesite hydromagnesite and ground dolomite aragonite mg2 assuming an analogy between laboratory and natural grinding tectonic activity may have important consequences on the release of hydrothermal fluids the solubilization of minerals and on solid state transformations as examples the possible role of deformation on the formation of metamorphic aragonite and diaspore bauxites is discussed

obducted nappe sequence in the san juan islands northwest cascades thrust system washington and british columbia the san juan islands northwest cascades thrust system in washington and british columbia is composed of previously accreted terranes now assembled as four broadly defined composite nappes stacked on a continental footwall of wrangellia and the coast plutonic complex emplacement ages of the nappe sequence are interpreted from zircon ages field relations and lithlogies to young upward the basal nappe was emplaced prior to early turonian time 93 ma indicated by the occurrence of age distinctive zircons from this nappe in the sidney island formation of the nanaimo group the emplacement age of the highest nappe in the thrust system postdates 87 ma detrital zircons within the nappe the nappes bear high pressure low temperature hp lt mineral assemblages indicative of deep burial in a thrust wedge however several features indicate that metamorphism occurred prior to nappe assembly metamorphic discontinuities at nappe boundaries absence of hp lt assemblages in the footwall to the nappe pile and absence of significant unroofing detritus in the nanaimo group a synorogenic relationship of the thrust system to the nanaimo group is evident from mutually overlapping ages and by conglomerates of nanaimo affinity that lie within the nappe pile from the foregoing relations and broader cordilleran geology the tectonic history of the nappe terranes is interpreted to involve initial accretion and subduction zone metamorphism south of the present locality uplift and exhumation orogen parallel northward transport of the nappes as part of a forearc sliver and finally obduction at the present site over the truncated south end of wrangellia and the coast plutonic complex

brittle frictional mountain building 2 thermal structure and heat budget this paper describes a simple thermal model of an actively deforming critically tapered fold and thrust belt the model determines the steady state temperature distribution and heat flow as well as the pressure temperature time histories of rocks that outcrop at the surface the main parameters controlling the thermal structure are the accretion and erosion rates the undisturbed geothermal gradient at the toe and the amount of frictional heating both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner and they dominate the effect of radiogenic heating except in fold and thrust belts with significantly overpressured pore fluids the mean stresses temperatures and surface heat flow all increase with an increase in the basal and internal coefficients of friction and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold and thrust belt in taiwan rocks outcropping in the core of the central mountain range of taiwan experience maximum theoretical temperatures in excess of 400 c and maximum mean pressures in excess of 500 mpa if the coefficient of basal friction is b 0 5 qualitatively these conditions are in good agreement with the observed high greenschist facies metamorphism the theoretical surface heat flow which increases from 95 mw m2 at the front of the fold and thrust belt to 240 mw m2 at the rear is in excellent agreement with the results of a recent geothermal survey of taiwan and theoretical cooling histories are in good agreement with fission track and other geochronologic studies taken together these results provide strong evidence that sliding on the basal decollement fault beneath taiwan is governed by a coefficient of friction in the range of typical laboratory measurements b 0 5 0 2 approximately 35 of the total surface heat flux of 3 gw is heat conducted into the base of the wedge from the top of the basal decollement fault and somewhat more than 30 is heat advected into the toe by accretion the remaining heat is generated internally about 25 by internal strain heating and about 10 by radiogenic heating either an increase in the coefficient of basal friction b or a reduction in the pore fluid pressure ratio b leads to an increase in the surface heat flow because of the increased frictional heating within the wedge and on the basal decollement fault the overall balance of energy in a steady state fold and thrust belt is described by the equation e w g q where is the rate at which both mechanical and heat energy are added from external sources w g is the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate and q is the rate at which waste heat flows out of both the upper and lower boundaries the total power input into the taiwan fold and thrust belt is approximately 4 2 gw the mechanical work being done on the base and front of the fold and thrust belt accounts for 3 of these 4 2 gw in addition 0 9 gw of heat are being advected from the subducting plate into the toe by accretion the remaining 0 3 gw are being supplied by in situ radiogenic heating the outgoing energy is dominated by the 3 gw of heat conducted out the top in the surface heat flow however another 0 8 gw are conducted down beneath the rear portion of the basal decollement fault to heat the underlying subducting slab only 0 4 of the incoming 4 2 gw do useful mechanical work against gravity within the wedge the efficiency of brittle frictional mountain building in taiwan is therefore 10

ikaite pseudomorphs in neoproterozoic dalradian slates record earth s coldest metamorphism calcite pseudomorphs have replaced euhedral ikaite caco 3 6h 2 o porphyroblasts in dalradian calcareous slates and metadolostones of western scotland with a volume decrease of at least 47 porphyroblast fabric relationships indicate that the initial growth of ikaite post dates a penetrative tectonic fabric developed during upright folding this is the first reported occurrence of metamorphic ikaite porphyroblasts and points towards growth within the slates during an ultra low temperature metamorphism with an exceptionally low geothermal gradient this event is associated with the penetration of long lived and extreme permafrost deep into subaerially exposed bedrock during neoproterozoic glaciation the presence of the well preserved pseudomorphs within the easdale slates of the argyll group implies that a neoproterozoic orogenic unconformity exists above the stratigraphic position of these rocks

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

transformation kinetics of polycrystalline aragonite to calcite new experimental data modelling and implications experimental data are used to model the transformation rate of polycrystalline aragonite grain diameter 80 m to calcite optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280 to 380 c and 0 10 mpa to an expression for a grain boundary nucleated and interface controlled transformation the nucleation rate is 4 5 orders of magnitude faster than for calcite nucleated within aragonite grains and the growing in rate is slower below 300 c than for calcite growing in aragonite single crystals the activation enthalpy for growth in polycrystalline aggregate is 247kj mol compared to 163 kj mol for growth in single crystals permanent deformation of the phases limits the elastic strain energy due to the 7 volume change and reduces the coherency of the calcite aragonite interace theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures and data from 0 10 to 400 mpa are used to evaluate the effect of pressure on the grain boundary nucleation rate because of permanent deformation of the phases the effective strain energy for nucleation is 0 55 kj mol which is less than a quarter of the value for purely elastic deformation these data are used to predict the percent transformation for various p t t paths without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235 c and the kinetic data are also consistent with published p t t paths which include heating during uplift the predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite but strongly dependent on the effective strain energy

partial high pressure aragonitization of micritic limestones in an accretionary complex tavsanli zone nw turkey pelagic micritic limestones within an upper cretaceous accretionary complex in the tavsanli zone nw turkey preserve textures indicating incomplete prograde transformation of micritic calcite to aragonite representing the only known example of this type aragonitization starts at the central parts of the micritic limestone beds and advances towards the lower and upper parts of the layers at the expense of micrite micrite is very fine grained 0 003 mm and contains radiolaria foraminifera and thin shell fragments aragonite forms large crystals up to 3 cm across with straight grain boundaries and c axis mostly subparallel to the carbonate beds relict micritic portions are devoid of any aragonite grain stylolites characterized by the accumulation of clay minerals fe mn hydroxides and quartz are concentrated in the upper and lower parts of the beds stylolite formation precedes aragonitization conditions of aragonitization are estimated as 200 50 c and 0 45 0 65 gpa based on metamorphic mineral assemblages observed in associated basalts several features such as i constant composition of micritic calcite 98 99 mol caco3 throughout individual beds ii enormous grain size difference between micritic calcite and aragonite up to 1200 times and iii absence of any aragonite grains within the relict micritic portions suggest that kinetic rather than thermodynamic factors controlled selective aragonite formation in the central portions of carbonate layers

the significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization the spatial disposition compositional zoning profiles and size distributions of garnet crystals in 11 specimens of pelitic schist from the picuris range of new mexico usa demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks an ordered disposition of garnet centers and a significant correlation between crystal radius and near neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre existing crystals compositional zoning profiles require diffusionally controlled growth the rate of which increases exponentially as temperature increases with time an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process but decline in the later stages because of suppression effects in the vicinity of earlier formed nuclei simulations of porphyroblast crystallization based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics quantitatively replicate textural relations in the rocks the simulations employ only two variable parameters which are evaluated by fitting of crystal size distributions both have physical significance the first is an acceleration factor for nucleation with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock the second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it crystal size distributions for the picuris range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals the same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet phlogopite sphene and pyroxene in rocks from both regional and contact metamorphic occurrences these commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments

kinetics of the coesite quartz transition application to the exhumation of ultrahigh pressure rocks the kinetics of the quartz coesite phase transition has been studied in situ by x ray diffraction in the 2 1 3 2 gpa 5001010 c pressure temperature range analysis of the data within cahn s model of nucleation and growth at grain boundaries reveals that the prograde and retrograde reactions have different kinetics the quartz coesite transformation is one order of magnitude faster than coesite quartz both reactions are characterized by high nucleation rates so that the overall reaction kinetics is controlled by crystal growth processes for the coesite quartz transformation growth rates are extrapolated using turnbull s equation with an activation energy for the transition of 163 23 kf mol this kinetic law is combined with an inclusion in a host elastic model to study the contribution of kinetics in coesite preservation this numerical modelling shows that above 400degreesc retrograde transformation of coesite to quartz is mainly controlled by the pressure vessel effect of the host phase whereas reaction kinetics is the controlling factor at lower temperatures the influence of the shape of the p t path and the exhumation rate upon the retrogression of coesite to quartz are investigated to use the percentage of unretrogressed coesite inclusions to constrain p t t paths

mechanisms of transformation and deformation in mg1 8fe0 2sio4 olivine and wadsleyite under non hydrostatic stress abstract we have studied the effect of non hydrostatic stress on the mechanisms of the olivine wadsleyite ringwoodite phase transformations and deformation mechanisms of olivine and wadsleyite at high pressure experiments were performed at 900 c in the stability field 15 gpa for 0 5 h and in the stability field 16 gpa for 11 h using a multianvil apparatus with san carlos olivine as the starting material a sample assembly designed to produce non hydrostatic stress was used the deformed samples have been characterised using optical and transmission electron microscopy remnant olivine contains high densities of mostly c dislocations and deformation occurs by dislocation glide involving the slip systems 010 001 and 100 001 in wadsleyite dislocations are in a climb configuration which suggests that self diffusion of si and or o is much faster in wadsleyite than in olivine at 900 c wadsleyite also contains 010 stacking faults which are interpreted to be growth defects that anneal out with time during the olivine wadsleyite transformation non hydrostatic stress results in anisotropic reaction textures wadsleyite nucleates preferentially on olivine grain boundaries that are oriented at a high angle to the direction of principal compressive stress and or the growth of wadsleyite occurs preferentially in this direction wadsleyite transforms to ringwoodite by two competing mechanisms i coherent intracrystalline nucleation on a dislocations probably where they intersect 010 stacking faults and ii partially coherent nucleation at wadsleyite grain boundaries non hydrostatic stress enhances transformation rates by increasing the density of dislocations which act as nucleation sites for ringwoodite although the samples were partially reacted under non hydrostatic stress there is no evidence for transformational faulting

zagros blueschists episodic underplating and long lived cooling of a subduction zone pressure temperature time p t tp t t trajectories of high pressure rocks provide important constraints to understand the tectonic evolution of convergent margins new field observations and p t tp t t constraints for the evolution of the only known blueschist facies exposure along the zagros suture zone in southern iran are reported here these blueschists now exposed in tectonic windows under the sanandaj sirjan zone upper plate constitute accreted fragments of the tethyan domain during n directed cretaceous subduction three units were identified in the field from top to bottom the ashin unit mafic and felsic gneisses the seghin complex mafic tuffs and ultramafics and the siah kuh massif coherent volcanic edifice microstructural observations p tp t estimates and rb sr deformation ages indicate that the ashin unit possibly underwent burial down to 30 35 km and 550 c along a relatively warm p tp t gradient c 17 km and was ultimately deformed between 85 and 100 ma the seghin complex exhibits remarkably well preserved hp lt assemblages comprising lawsonite glaucophane aragonite omphacite and garnet p t tp t t reconstruction indicates that this slice was subducted down to c 50 km at temperatures of c 500 c along a very cold subduction gradient c 7 km deformation in the seghin complex stopped at around 65 ma close to peak metamorphic conditions field relationships and estimates of the p tp t trajectory followed by the siah kuh volcanic edifice indicate that this massif was lately subducted down to 15 km depth along the same very cold gradient this slice stack represents a well preserved field example i highlighting the existence of transient underplating processes juxtaposing pluri kilometric tectonic slices along the subduction channel and ii imaging the discontinuous down stepping of the active main subduction thrust with ongoing accretion the zagros blueschists also record an apparent cooling of the zagros subduction zone between 90 and 65 ma most likely as a thermal response of a geodynamic perturbation toward the end of the cretaceous

optical determination of topotactic aragonite calcite growth kinetics metamorphic implications textures produced by the partial recrystallization of aragonite to calcite in the blueschist facies show that heterogeneous topotactic nucleation is rapid enough to permit growth kinetics to dominate the overall geologic rate of transformation both grain boundary velocities measured microscopically between 375 c and 455 c and analysis of topotactic relations predict a growth mechanism involving only small atomic displacements near a highly coherent boundary between the two phases the kinetic equation of turnbull for interface controlled growth based on that mechanism accurately describes the experimental results when calculated rates are integrated over geologically plausible pressure temperature time paths for convergent plate margins even the most extreme values lead to the conclusion that aragonite bearing rocks enroute to the surface must enter the calcite stability field between 125 c and 175 c the widespread occurrence of aragonite along the pacific margin of north america and by contrast

detachments in high pressure mountain belts tethyan examples abstract using the case examples of the phyllite quartzite nappe of crete the bundnerschiefer nappe of the engadine window in the alps and the infraophiolitic units of the saih hatat window in the oman mountains we describe the tectono metamorphic signature of large scale extensional detachments in high pressure belts in these three cases retrograde p t paths were accompanied by the formation of large scale extensional shear zones that contributed to the exhumation and extreme thinning of the metamorphic pile the direction of transport along later detachments was parallel to the direction of convergence detachments being first localized within or near the brittle ductile transition zone below the major shear zone or between the shear zones in oman the deformation regime was more coaxial and involved a significant component of vertical shortening metamorphic data mostly derived from the petrology of metapelites show that domains characterized by coaxial strain have nearly isothermal decompression paths type ii while those immediately below the detachments were subjected to more efficient cooling during exhumation type i the detachments thus affect both the deformation history and the p t evolution of the lower plate the existence of the two types of cooling paths is a good test of the extensional nature of a tectonic contact

on the significance of aragonite occurrences in the western alps aragonite occurrences from two areas of the western alps are described it is shown that aragonite has been formed under blueschist metamorphic conditions in the western vanoise while it has been precipitated under sub surface conditions in the queyras region an uplift p t path of the aragonite bearing rocks of the western vanoise is constructed using two independent methods 1 temperature estimates using the sr content of aragonite in the successive veins and 2 the kinetics of the aragonitecalcite solid state transition the uplift p t path has an unusual shape with an important temperature decrease 100 to 150 c for a pressure decrease of 0 2 gpa following blueschists metamorphism p 0 7 gpa t 300 c thermal models show that this unusual p t path of the western vanoise can be explained if one maintains a low temperature between 125 and 175 c at the base of the tectonic unit containing the aragonite bearing rocks during part of its burial history followed by the whole of its exhumation a tectonic scenario is proposed to account for the observed and modelled p t path

kinetics and fractionation of carbon and oxygen isotopes during the solid phase transformation of biogenic aragonite to calcite the effect of organic matter abstract the phase transformation of biogenic shells from aragonite to calcite can modify the primary carbon and oxygen isotopic values 13c and 18o and thus affect their significance as paleoenvironmental proxies while the biasing of 13c and 18o by dissolution and reprecipitation phase transformation dr has been extensively studied thermal solid phase transformation spt in archaeological shell middens is rarely considered in this study the effects of intra crystalline organic matter on the kinetics and carbon and oxygen isotopic fractionation during thermal spt are evaluated by comparing two sets of experiments on powdered aragonite snail shells heated in air and pure nitrogen atmospheres the results show that during heating temperature below 400 c the intra crystalline organic matter could inhibit the phase transformation due to the shielding effect of organic matter while during heating temperature above 400 c the decomposition of the intra crystalline organic matter increases the kinetic rate of the phase transformation the activation energy for the phase transformation was determined to be 150 5 kj mol 1 lower than that of 247 kj mol 1 obtained for abiogenic polycrystalline aragonite and close to that of 158 to 163 kj mol 1 for abiogenic single crystal aragonite this is attributed to the larger lattice parameter c in biogenic aragonite and the increasing porosity of calcium carbonate with the decomposition of organic matter in a nitrogen atmosphere the deviation of 13c and 18o between the heated sample the mixture of aragonite and calcite and the original shell aragonite 13cm a and 18om a is negligible during heating temperature below 400 c however 13cm a from 0 43 to 0 00 and 18om a from 1 54 to 0 02 are significant during heating temperature above 400 c the temperature dependence of 13cm a in biogenic carbonates is much more significant than that in abiogenic carbonates which clearly reflects the occurrence of isotopic exchange between the intra crystalline organic matter and the carbonates in air the heating induced calcium carbonate is depleted in 13c by up to 0 48 and in 18o by up to 3 46 compared with the original samples which is mainly caused by isotopic exchange with the external carbon dioxide in air and the intra crystalline organic matter with the latter being the dominant source our results demonstrate that the decomposition of the intra crystalline organic matter increases the kinetic rate and induces carbon and oxygen isotope fractionation during spt from aragonite to calcite this study provides direct evidence for evaluating the preservation of carbon and oxygen isotopes of biogenic calcium carbonate shells which have undergone the spt process in archaeological shell middens

coherent french range blueschist subduction to exhumation in 2 5 m y coherent oceanic strata in the french range belong to the exotic cache creek terrane of the canadian cordillera they were metamorphosed to blueschist grade tectonically extruded eroded and intruded by plutons perhaps in 2 5 m y sodic amphibole overprint chert as young as late pliensbachian to toarcian age ca 191 to ca 177 ma blueschist mineral assemblages defi ne the early metamorphic fabric along with phengite dated by 40 ar 39 ar as 173 7

stable isotope composition alteration produced by the aragonite to calcite transformation in speleothems and implications for paleoclimate reconstructions abstract aragonite a mineralogical constituent of speleothems in cave environments is unstable and susceptible to inversion to calcite a diagenetic process that involves changes in the mineralogy texture and geochemistry of speleothems however the exact alterations of stable isotope compositions during such diagenesis have not been fully investigated in this study two aragonite stalagmites sn3 and sn15 from the shennong cave southeast china were found partially inverted to calcite as determined by x ray diffraction xrd and scanning electron microscopy sem analyses and thin section inspections under microscope the fiber relics and textural ghosts of aragonite preserved in coarse and equant mosaic calcite crystals clearly indicate that the calcite in these two stalagmites were inverted from aragonite the stable isotope compositions 13 c and 18 o given in per mil versus vpdb standard of primary aragonite and secondary calcite were analyzed and compared along both growth layers and growth axes the results show that along growth layers differences of 13 c values between aragonite and calcite are negligible 0 1 0 2 whereas differences of 18 o values between aragonite and calcite are significant 0 63 0 87 comparisons along growth axes show similar results i e differences of 13 c values are negligible 0 06 0 22 whereas differences of 18 o values are significant 0 85 0 29 most likely the aragonite in sn3 and sn15 were internally inverted by interactions of trace calcite crystallites and pore water within intercrystalline pore spaces by a dissolution reprecipitation process occurring in trapped pore water in the case of the inversion of aragonite to calcite in speleothems such as that observed in sn3 and sn15 the 13 c values could be used in paleoclimate and paleoenvironment reconstructions because they are inherited from those of primary aragonite although the 18 o values might be cross calibrated to those of primary aragonite if the aragonite calcite fractionation offset is known e g 0 85 0 29 in this study however the 18 o values of secondary calcite should be used with caution in such reconstructions as the 18 o offset value is not consistently invariable

late holocene evolution of the southwestern donana national park guadalquivir estuary sw spain a multivariate approach four phases are distinguished in the late holocene evolution of the southwestern donana national park sw spain based on a multidisciplinary analysis of the sediments present in drill cores in the oldest phase 2400 2500 cal yr bp a coastal lagoon the roman lacus ligustinus was recognized in the central part of this area partly closed by the donana spit and limited by fluvial levees the following phase 2400 2200 cal yr bp is characterized by high energy events which caused the breakthrough of the donana spit and the creation of new littoral strands in the inner areas in the third phase 2200 2050 cal yr bp this new outlet was closed coinciding with the progradation of the donana spit the last phase 2050 cal yr bp recent comprises three periods a an unstable period 2050 1950 cal yr bp with the deposit of cheniers over the previous levees b an infilling period with a diminution of the marine influence and c the appearance of temporary ponds between the emerged levees and cheniers

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

matrix rheology effects on reaction rim growth i evidence from orthopyroxene rim growth experiments the rim forming reaction quartz olivine orthopyroxene is used to investigate the effect of matrix rheology on rim growth rates orthopyroxene rim growth around olivine grains in quartz matrix is compared to rim growth around quartz grains in an olivine matrix at constant p t within one single capsule orthopyroxene rims grow faster around quartz clasts in olivine matrix than around olivine clasts in quartz matrix fourier transform infra red spectra indicate that the entire samples are water saturated because of water adsorption on the reactant grain surfaces the increased orthopyroxene growth rates in olivine matrix as opposed to quartz matrix are interpreted in terms of matrix rheology where in the two different matrix inclusion arrangements the olivine matrix behaves softer and the quartz matrix more rigid the strain energy associated with accommodation of the negative reaction volume is higher for the quartz than the olivine matrix and reduces the free energy that drives orthopyroxene rim growth growth textures in both kinds of orthopyroxene rims indicate that the diffusivity of mgo slightly exceeds the diffusivity of sio2 the relative mobility of mgo and sio2 at given p t fh2o seems to be controlled by energy minimization during orthopyroxene growth at the compressive ol opx interface our experiments provide evidence for two previously overlooked effects relevant to rim growth reactions in metamorphic rocks i diffusivity along chemical potential gradients to reaction sites is a function of rheology and ii the relative diffusivity of components during reaction rim or corona growth is a function of local volume changes at the rim s interfaces

relative geobarometry from high pressure rocks of quartzofeldspathic composition from the sesia zone western alps italy in the sesia zone of the western alps italy early alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high pressure assemblage of quartz na pyroxene na amphibole paragonite phengite zoisite garnet magnetite sphene and fe sulphide the qfs assemblage over an area 150 km2 relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions mineralogical discontinuities do not occur in the qfs assemblage of this region increases in the jadeite content of the pyroxenes xjd 0 48 to xjd 0 93 and in the glaucophane content of the amphiboles xgl 0 89 to xgl 0 96 occur from the southwest to the northeast of the region studied analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet zoned pyroxenes and garnets together with the regional trends in mineral chemistries suggest that the evolution of the qfs assemblage with increasing pressure may be modelled by pressure sensitive continuous reactions in which amphibole zoisite and the more jadeitic pyroxene constitute the high pressure assemblage chemographic constraints permit the positioning in pressure temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages the low dp dt slope 20 bars c of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz in addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the naalca 1mg 1 exchange or the mgca 1 exchange are calculated comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied with these barometric observations it is possible to show that the p t path which the sesia body travelled towards the final recorded state was one of increasing pressure other blueschist and eclogite occurrences from syros and sifnos which contain rocks of quartzofeldspathic composition are also examined

geochronology of blueschists from west central baja california and the timing of uplift in subduction complexes a geochronologic study of blueschists from west central baja california has provided constraints on the timing of high pressure low temperature metamorphism and the subsequent uplift of this subduction complex mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by 40 ar 39 ar fission track and rb sr methods 40 ar 39 ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite matrix melange show slow cooling gradients between 115 95 ma rb sr apparent ages for one block are concordant with these results 40 ar 39 ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below 100 c at 22 ma and 32 ma respectively subduction related metamorphism of the coherent blueschists occurred in la

ultra high pressure uhp terrains lessons from thermal modeling the thermal history of subducted exhumed crustal fragments is explored to depths of 125 km using a forward modeling approach the finite element code uses an adaptive gridding technique that allows deformation and differential movement of subgrids to simulate tectonic mass flow the geometry and velocity of plate convergence and the large scale strain partitioning for example subduction channel mobile tectonic fragments are described explicitly as functions of time the evolution of metamorphic conditions during descent and subsequent exhumation of a terrane within the subduction channel is computed for different kinematic scenarios these include a range of velocities relative to a stable continental block for the subducting slab the exhuming uhp fragment and a subduction channel of given width though essentially generic in emphasis our models are designed to help understand real uhp occurrences most especially their exhumation history boundary conditions and the geometric set up were chosen according to three case studies of known uhp terranes eastern china western norway western alps simulations indicate that the temperature reached at a given depth during the subduction stage is crucially dependent on the convergence rate width of the subduction channel location of the fragment within the subduction channel and on rates of shear heating and convection of the subcontinental mantle the p t t path experienced by a tectonic fragment during its exhumation from pmax is determined chiefly by its size thickness and exhumation velocity the rate of ongoing subduction is less relevant given the rapid exhumation required to preserve uhp assemblages the depth at which tmax is reached by an exhuming uhp fragment and the dp dt slope of its exhumation path may critically affect the extent of irreversible phase transitions during decompression comparing data for the three uhp case studies with corresponding simulation results indicates general agreement of the p t paths predicted t t evolution paths are also compatible with well constrained mineral chronometry from the study areas this agreement between field data and simulation results leads to a number of observables that may be useful in relating uhp occurrences to their evolution for example no metamorphic field gradients are expected for fragments of less than 5 km thickness and the spread in mineral ages reflecting the high pressure stage is predicted to be less than 1 ma therefore observed variations in metamorphic conditions for the stage near tmax and p tmax may reflect real differences of evolution in very large fragments only and the same is valid for a substantial spread in metamorphic age data pertaining to stages near pmax or tmax tectonic extrusion of fragments along a subduction channel is not the only mechanism proposed to explain uhp occurrences however the broad agreement between several test cases and the present simulation results lend support to the viability of the kinematic scenario adopted where field evidence indicates a similar emplacement mechanism and allows the initial size of the uhp fragment to be inferred the melonpit model provides a powerful integrative tool to produce a quantitative picture of the dynamics of uhp terranes

the influence of oh in coesite on the kinetics of the coesite quartz phase transition metastable coesite is an important pressure indicator for ultrahigh pressure rocks however in many cases coesite does not survive exhumation but reacts back to quartz although it was shown experimentally that incorporation of h in coesite increases with increasing pressure most coesite relics found in nature are essentially dry i e oh concentrations are below detection limit 1 wt ppm h2o thus does the incorporation of h promote the back reaction of coesite to quartz during exhumation the influence of intrinsic oh on the kinetics of the coesite quartz phase transition was determined using synthetic dry coesite with approximate to10 wt ppm h2o and wet coesite with approximate to105 wt ppm h2o tem analysis of the quenched samples proved the presence and absence of water in the wet and dry samples respectively the kinetics of the coesite quartz transition was investigated in situ using the multi anvil apparatus max 80 at the hamburger synchrotron radiation laboratory hasylab the transition rates were measured by observing changes in selected diffraction line intensities as a function of time the transformation and growth rates were derived using cahn s model of nucleation and growth at grain boundaries under the same experimental conditions the transformation rate of the wet coesite is more than ten times higher than that of the dry coesite this difference may explain why oh bearing natural coesite is rare this study reveals the importance of structurally bound oh for the kinetics of phase transitions of nominally anhydrous minerals

density changes of fluid inclusions in high pressure low temperature metamorphic rocks from crete a thermobarometric approach based on the creep strength of the host minerals abstract the densities of fluid inclusions in quartz from high pressure low temperature metamorphic rocks are generally inconsistent with the p t conditions derived from solid phase equilibria therefore some process of change must regularly have occurred during exhumation however evidence of decrepitation is frequently lacking consideration of the rates of change of p and t in metamorphic rocks shows that very low strain rates are sufficient for volume adaptation dislocation creep appears to be a feasible deformation mechanism allowing for continuous ballooning driven by a small differential pressure during decompression down to temperatures of about 300 c in quartz decrepitation requires a higher differential pressure in natural rocks it is therefore restricted to relatively low temperatures this concept was tested on inclusions in the phyllite quartzite unit of crete in these rocks cooling to below ca 300 c at confining pressures of 3 to 4 kbar is indicated corresponding to depths of 10 to 13 km it is anticipated that in metamorphic rocks the isochores of all fluid inclusions in quartz that have been formed at higher temperatures and remain unaffected by decrepitation should intersect close to the 300 c isotherm thereby defining a point on the p t paths rarely accessible by other methods extension of this concept to other minerals with known mechanical properties is straightforward and can under favourable circumstances be used to fix several points on p t paths

strength of mg fe 2sio4 wadsleyite determined by relaxation of transformation stress abstract the growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change the strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth stress relaxation enables growth to continue at a rate controlled primarily by the mechanical properties of the rim we studied this process in high pressure experiments in order to constrain the rheology of wadsleyite one of the high pressure polymorphs of olivine single crystal spheres of san carlos olivine surrounded by a quasi hydrostatic pressure medium of nacl or au were partially transformed to wadsleyite at 1100 c and 16 17 gpa to determine the rate of growth as a function of time microstructural observations indicate that the relict olivine remains largely undeformed while the wadsleyite rims deform by dislocation creep application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 gpa a decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples kubo t ohtani e kato t shinmei t fujino k 1998a experimental investigation of the transformation of san carlos olivine single crystal phys chem miner 26 1 6 kubo t ohtani e shinmei t fujino k 1998b effects of water on the transformation kinetics in san carlos olivine science 281 85 87 modeling these results gives values for the yield strength of wadsleyite between 2 and 5 gpa under various conditions a strength difference of 1 gpa is indicated by experiments at 1030 c conducted under nominally dry 200 ppm h2o and water added 500 ppm h2o conditions supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite the calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions based on an extrapolation of the low temperature plasticity flow law for olivine these results imply a strong increase in the strength of subducting slabs in the mantle transition zone provided that the deformation mechanism is the same as in the experiments furthermore inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high pressure polymorphs in subducting slabs

metamorphic veining and mass transfer in a chemically closed system a case study in alpine metabauxites western vanoise in western vanoise french alps karstic pockets of triassic to jurassic metabauxites embedded in carbonate rocks and containing several generations of metamorphic veins were studied during blueschist facies metamorphism a cumulative amount of 13 vol of water is inferred to have been produced locally by successive dehydration reactions and part of this fluid remained in the bauxitic lenses during most of the metamorphic cycle field and geochemical evidence show that these rocks have been isolated from large scale fluid flow closed system behaviour it is proposed that the internally derived fluid has promoted the opening of fluid filled open spaces as attested by the euhedral habits of vein minerals and served as medium for mass transfer from rock to vein indeed the vein infill is obviously the result of chemical interactions at the millimetre to centimetre scale between the rock minerals and the locally produced aqueous fluid two vein types can be distinguished based on mineralogical and textural features i some veins are filled with newly formed products of either prograde chloritoid or retrograde chlorite metamorphic reactions in this case fluid filled open spaces seem to offer energetically favourable nucleation growth sites ii the second vein type is infilled with cookeite or pyrophyllite that were present in the host rock prior to the vein formation in this closed chemical system the components for the vein infill minerals have been transferred from rock to vein through the fluid in a dissolution transport precipitation process possibly stress assisted these different vein generations all contain al rich mineral infills suggesting that al was a mobile element cm scale during metamorphism in these hp rocks fluid flow may have been restricted and if so mass transfer occurred by diffusion in an almost stagnant fluid metamorphic veins can be seen as witnesses of fluid and mass redistribution that partly accommodate the rock deformation lateral segregation

miocene detachment in crete and exhumation p t t paths of high pressure metamorphic rocks a major problem posed by the geology of crete is the horizontal contact of an upper unit without miocene metamorphism onto a metamorphosed lower one with early miocene high pressure low temperature hp lt parageneses this very sharp contact is roughly parallel to the major oligo miocene thrust planes which were reactivated as a large scale detachment which allowed exhumation of high pressure units we describe the extensional deformation and the metamorphic evolution of the lower plate most first order deformation features relate to the retrogression from high pressure to low pressure conditions a n s pervasive stretching is observed everywhere often associated with a top to the north sense of shear the extreme variation of thickness of the phyllite quartzite nappe upper part of the lower plate is probably the result of large scale boudinage similar to the one seen in large outcrops the most important observation is the systematic occurrence of fresh carpholite immediately below the base of the tripolitza nappe except in northwestern crete where a late extensional shear zone is present deeper in the nappe pile carpholite is systematically retrograded this observation reveals a drastically different pt history for the upper part of the phyllite quartzite nappe it also suggests that the late extensional shear zone found along the northern side of crete cuts inside the metamorphic structure and brings the nonmetamorphosed tripolitza nappe directly in contact with the deeper parts of the phyllite quartzite nappe pt t paths suggest a fast temperature decrease in the top of the phyllite quartzite during retrogression and hence during the top to the north shear the deeper part of the phyllite quartzite nappe shows a low temperature regime throughout but its pt path includes an isothermal decompression in the first stage we produce a tentative map of domains having experienced similar pt trajectories during decompression the overall cool regime is related to the continuous underthrusting of cola continental units during exhumation isothermal decompression observed in the core of the phyllite quartzite nappe implies fast exhumation during extension and the faster cooling of the upper part is related to a continuous displacement toward the north of a cooler unit during exhumation single grain 39ar 40ar ages obtained on phengites 15 25 ma in various structural sites are in good agreement with these conclusions and with the geological context suggesting that underthrusting of cold units at the front the accretionnary complex occurred contemporaneously with unroofing below a north dipping detachment near the top of the wedge the age of this detachment is bracketed between the end of the high pressure event 20 ma and the deposition of the breccia early to middle miocene in the neogene basins

comment on formation of chenier plain of the donana marshland sw spain observations and geomorphic model by a rodriguez ramirez and c m yanez camacho marine geology 254 2008 187 196 abstract rodriguez ramirez and yanez camacho 2008 rodriguez vidal et al 2009 and rodriguez ramirez et al 2009 have kept an ongoing discussion about the validity of radiocarbon ages in reconstructing holocene palaeogeographical models in the gulf of cadiz the discussion considered the validity and correctness of the r value to be used in the area to calibrate radiocarbon ages of marine samples these papers suggest that the r value proposed by lario 1996 and dabrio et al 1999 2000 is erroneous and consequently that all the evolutionary models based on radiocarbon ages from marine samples that have been proposed in this area since 1996 must be revised however the papers commented here use erroneously the r regional r and r values after reviewing the published data it is apparent that the most reliable values of r in the gulf of cadiz for middle late holocene samples range between r 35 85 yr and r 95 15 yr this means that the values used in lario 1996 and dabrio et al 1999 2000 fall in this range and also that the calibrated ages used by several authors adopting these values are fully reliable calibrations using r values inside this range do not yield significant differences in terms of geological age owing to the magnitude of errors resulting from the methods employed

incorporation of stratification effects on the oceanic roughness length in the derivation of the neutral drag coefficient abstract based on the assumption that over the sea the roughness length of the wind profile scales with the wind stress a new formulation that describes the drag coefficient as a function of the given neutral drag coefficient and stability is derived the new formulation is compared to an earlier formulation where roughness changes with stability were ignored the two are then illustrated with data collected from both the marine remote sensing project 1979 and the tower ocean wave and radar dependence experiment 1984 it was found that when the surface roughness was allowed to depend on wind stress and therefore stability the stratification correction to the neutral drag coefficient was larger than for the case when the roughness length was not allowed to vary

the southern ocean waves experiment part i overview and mean results the southern ocean waves experiment sowex was an international collaborative air sea interaction experiment in which a specially instrumented meteorological research aircraft simultaneously gathered atmospheric turbulence data in the marine boundary layer and sea surface topography data over the southern ocean for a wide range of wind speeds the aim was to increase present knowledge of severe sea state air sea interaction this first paper presents an overview of the experiment and a detailed discussion of the methodology and mean results a companion paper describes the findings on variability of the wind speed and wind stress and their relationship to variations in the underlying sea surface roughness

depositional history of estuarine infill during the last postglacial transgression gulf of cadiz southern spain abstract the late pleistocene and holocene evolution of the estuaries in the gulf of cadiz is interpreted for the first time using drill cores logs trenches and 38 new radiocarbon data and the results compared with the shelf the odiel tinto and guadalete rivers deposited conglomerates during a highstand that did not reach the present sea level dated at ca 25 30 ka isotopic stage is 3 corresponding to a relatively humid period in the area rivers incised these coarse grained deposits during the last main lowstand at ca 18 ka when sea level dropped to 120 m and the coastline lay 14 km seawards from the present the erosional surface is a sequence boundary and the flooding surface of the postglacial eustatic rise overlain by the valley fill deposits of the transgressive and highstand phases of the last fourth and fifth order depositional sequences recognised in the shelf the first marine influence in the estuaries during the transgression occurs at 25 30 m at ca 10 000 years bp according to fossil assemblages the transgressed basins changed from brackish to more open marine as the sea rose until ca 6500 years bp when it reached the maximum flooding and the sandy estuarine barriers ceased to retrograde toward the muddy central basins then the rate of eustatic rise decreased drastically and the estuarine filling followed a two fold pattern governed by the progressive change from vertical accretion to lateral centripetal progradation at ca 4000 years bp the fluvial input surpassed the already negligible rate of rise causing partial emergence of tidal flats and spit barriers in the largely filled estuarine basins prevalence of coastal progradation upon vertical accretion at ca 2400 years bp caused accelerated expansion of tidal flats and rapid growth of the sandy barriers further changes since the 16th century reflect widespread anthropic impacts

microstructures of synthetic polycrystalline coesite aggregates the effect of pressure temperature and time the preservation of coesite bearing ultrahigh pressure metamorphic rocks in orogenic belts indicates that continental crust can be buried to a depth exceeding its present thickness the exhumation of these rocks requires large scale flow patterns not yet understood by analogy to the use of flow laws for quartz for the higher crustal levels rheological data and a flow law for coesite are required for the reconstruction and modelling of these important deep processes the first aim of the present study was the synthesis of coesitite samples appropriate for deformation experiments synthesis of coesite from silica glass follows the ostwald step rule with an interface controlled slow second step from metastable quartz tocoesite normal grain growth of coesite crystals is not possible at temperatures up to 1100 c because of the dependence of interfacial energy on crystallographic orientation this energy becomes independent of orientation at 1170 c and a foam microstructure tends to develop allowing for normal grain growth this temperature dependence should also hold true for natural rocks however normal grain growth was not measurably observed in an experiment with 1080 min duration at a temperature of 1170 c consequently the desired grain size of synthetic coesitite samples had to be achieved by controlling the nucleation rate which was successfully done by choosing appropriate p t t paths in the synthesis experiments

arc parallel extension and fluid flow in an ancient accretionary wedge the san juan islands washington structural analysis of the lopez structural complex a major late cretaceous terrane bounding fault zone in the san juan thrust system reveals a sequence of events that provides insight into accretionary wedge mechanics and regional tectonics after formation of regional ductile flattening and shear related fabrics the area was crosscut by brittle structures including 1 southwest vergent thrusts 2 extension veins and normal faults related to northwest southeast extension and 3 conjugate strike slip structures that record northwest southeast extension and northeast southwest shortening aragonite bearing veins are associated with thrust and normal faults but only rarely with strike slip faults high pressure low temperature hp lt minerals constrain the conditions for brittle deformation to 20 km and

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

transmission electron microscope observation of α β and γ mg fe 2sio4 in shocked meteorites planar defects and polymorphic transitions abstract high pressure polymorphs of olivine mg fe 2sio4 have been observed by transmission electron microscopy in the shocked tenham and catherwood meteorites planar defects are characterized in and polymorphs and their possible role in polymorphic transitions is assessed and discussed in relation with the published literature the stacking fault 1 2 1 01 010 observed in the phase can produce a shear transformation to the phase if it occurs on every other 010 plane conversely the stacking fault 1 2 1 1 2 110 observed in the phase can be transformed to the phase if it is repeated on every other 110 plane and to an intermediate yet unobserved e phase if it is repeated on every 110 plane shear transformation between olivine and spinel could involve the e phase as an intermediate stage

cheniers and chenier plains a general introduction abstract cheniers and chenier plains have been studied in many coastal regions throughout the world an inventory of the investigations currently being carried out is presented in this and accompanying papers cheniers and chenier plains are azonal features their distribution ranges from tropical to the subarctic regions and they show a variety of features progradational shorelines where mud accretion is temporarily interrupted favour the development of these features in addition to the concept of otvos and price who define cheniers and chenier plains as being formed exclusively in relation to littoral progradational environments it is suggested here that chenier formation in deltaic progradational environments should also be included genetically two types of cheniers can be distinguished depending on the origin of the related sediments cheniers developing due to wave winnowing of the muddy sediments and the subsequent concentration of the coarser particles to form a longshore bar or ridge which is driven shoreward and cheniers built by littoral drift in both cases they can be recognised by their sedimentary structures when cheniers arrive onshore they will migrate further landward due to washover processes under extreme hydrodynamical conditions e g spring tides storm surges etc to summarise the information concerning the episodic development of chenier plains in order to produce a or a few universal model s appears to be very difficult owing to the numerous factors that influence their formation these include river mouth shifting sealevel changes climatic changes storm frequency mortality of shellfish and bay geometry owing to this sensitivity to environmental changes during their formation cheniers are considered to be very important geomorphological features they provide a relatively accurate record of sealevel and environmental changes which can be dated with high resolution

formation of chenier plain of the donana marshland sw spain observations and geomorphic model abstract the guadalquivir estuary is one of the most important in europe it is isolated from the sea by two important littoral spits donana and la algaida its sedimentary filling occurred as a result of deltaic contributions of the guadalquivir river giving rise to marshes primarily composed of mud deposits among which a succession of sandy and shelly ridges can be found the aim of this paper is to explain the genesis and evolution of these sandy and shelly ridges clarifying what types of ridges are found and their spatial distribution identifying the processes responsible for their formation these ridges extend in the form of long strands along the estuary influencing its evolution during the late holocene the geomorphologic and lithostratigraphic features of these ridges allow them to be defined as transgressive cheniers regressive cheniers and laterally accreted ridges spits as components of the guadalquivir estuary chenier plain complex this chenier plain has the peculiarity of developing on a semi enclosed shoreline contrary to other chenier plains in the world which develop on open coasts in the guadalquivir estuary the chenier plain formation is related to two relatively higher sea level 4200 2800 yr bp and 1400 1000 yr bp during which high energy events occur storms and tsunamis that transgressed the large littoral spits the carrizosa 4000 yr bp and vetalengua 1400 1300 yr bp spits have been formed across the inlet and prograded landwards of the estuary from here several sequences of cheniers mainly transgressive cheniers associated with these events were formed these cheniers form two groups las nuevas 1100 1000 yr bp and marilopez 3000 yr bp composed mainly of shells and shell fragments these cheniers were constructed by waves which reworked the shells above estuarine levees and led to their intense landward migration thus explaining the peculiar high inland position of the cheniers

the stacking of thrust slices in collision zones and its thermal consequences the thermal consequences of stacking up thrust slices in collision zones are investigated using simple one dimensional thermal models the metamorphic evolution geotherms and p t time paths of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects cooling and pressure decrease associated with erosion cooling by the lowest units screen effect heating by the upper units cover effect it is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone the metamorphic evolutions are very sensitive to the following tectonic parameters the number of thrusted units involved in thickening the time delay between each thrust and finally the mode of stacking of the different units over and understacking it appears that for a given depth of burial the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units the screen effect during understacking is more efficient for a short time delay 10 ma between each thrust overstacking leads to higher temperatures before uplift when compared to understacking it is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small 10 km it is shown that for an erosion controlled uplift the shape of the p t t path depends on the position of the rocks within a given unit and on the unit position within the pile finally the general metamorphic evolutions during crustal thickening hp lt metamorphism and subsequent overprint are discussed in terms of the previously mentioned parameters in the case of the western alps it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events

regional progressive high pressure metamorphism seward peninsula alaska blueschist facies rocks on the seward peninsula constitute a structurally coherent terrane measuring at least 100 150 km radiometric age data indicate that high pressure metamorphism probably occurred in jurassic rather than in palaeozoic or precambrian time as previously suggested protolith sediments nome group are of intracontinental basin or continental margin type and of lower palaeozoic and possibly late precambrian age thus predating the high pressure metamorphism by more than 200 m y blueschist facies mineral assemblages were developed in almost all lithologies of the nome group and are best preserved in feti rich metabasites glaucophane almandine epidote and pelites glaucophane chloritoid phengite a lawsonite crossite subfacies was developed in possible nome group rocks on the east flank of the darby mountains albite epidote amphibolite facies assemblages characterize nome group rocks in the southwestern part of the peninsula metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote almandine glaucophane schist subfacies with the local development east of the nome river of eclogitic assemblages the high pressure metamorphic minerals were synkinematic with the development of mesoscopic scale intrafolial isoclinal folds and a flattening foliation of consistent orientation initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles and partial post kinematic greenschist facies replacements record later stages of decompression ophiolites and melange are not associated with the seward peninsula blueschists the high pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin the pt conditions of high pressure metamorphism were approximately 9 11 kbar 400 450 c thus falling between the pt paths of the shuksan and franciscan terranes

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

identification of typical scenarios for the surface lagrangian residual circulation in the iroise sea 1 this paper describes the surface lagrangian residual circulation lrc over 2 5 day intervals in the iroise sea west of france and evaluates for operational purposes the influence of the different physical mechanisms that govern it the method consists of the calculation of water displacements with a diagnostic lagrangian tool that computes the trajectories of numerical particles in a given velocity field the lrc is inferred from trajectories integrated over five m2 tidal cycles the analysis is applied to both gridded genuine current measurements and ocean model outputs the sea surface currents are derived from high frequency hf radar measurements and from mars a 3 d regional ocean model used here in idealized configurations to substantiate the analysis the lagrangian residual currents are also compared to genuine movements of drifters released in the iroise sea in 2005 and 2007 the lrc is mapped for typical scenarios identified from the lagrangian analysis of hf radar surface currents measured in winter and summer under weak 10 m s wind conditions and in neap tide and spring tide seasons idealized numerical simulations that switch on and off each individual physical process are used to isolate in the lrc the patterns induced by the atmospheric forcing tides and density driven currents

the franciscan complex california usa the model case for return flow in a subduction channel put to the test the model of return flow in a subduction channel rfsc is put to the test by opposing key assumptions of the model to an integrated review of field data petrology geochronology and structures of high p melanges of the franciscan subduction complex fsc to which rfsc has been first applied key assumptions are i the subduction channel sc is a low viscosity zone of finite width restrained by the rigid subducted and overriding plates ii forced return flow in the sc is associated with formation and km scale displacements of small tectonic blocks 1 km in a highly deformable matrix iii this results in apparently random distributions and close associations of blocks exhumed from different depths hence the block in matrix structures and exotic blocks in high p melanges would reflect the structural state at depth of the sc on the basis of the predictions ensuing from the key assumptions of the rfsc model on the rock record it is detailed why the actual record of the high p melanges of the fsc does not comply with this perception the record rather reflects recurring subduction underplating associated decollement erosion and mingling of high p rocks by sedimentary processes in the trench all occurring in an 80 m y lasting period distributed deformation of the melange matrix is unrelated to formation of apparently chaotic block in matrix structures since i creation subduction ascent and cooling of exotic blocks on counterclockwise p t paths predates deposition of the bulk of the metasediments constituting the matrix by 20 40 m y ii deformation of the matrix occurs during a second loop of high p metamorphism postdating creation and ascent of the exotic blocks by at least 20 40 m y iii the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in p t paths against each other from km scale vertical distances instead narrow few tens of metres wide fault shear and decollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high p metamorphism they bound several 100s of metres wide tabular tectonic bodies that are coherent sedimentary processes at the active continental margin formed the bulk of the melanges tectonically formed block in matrix structures within the shear and decollement zones are unrelated to juxtaposition of exotic blocks universal diagnostic criteria are developed to identify return rfsc as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology

can sharp seismic discontinuities be caused by non equilibrium phase transformations a long posed problem of the seismic discontinuities at the depths of 410 and 660 km in earth s mantle is whether these discontinuities are caused by phase transitions alone or together with chemical layering among several tests the sharpness of the boundary is one of the most crucial recent data suggest that the transition region thickness is less than 4 km for both the 410 and the 660 km discontinuities this may be smaller than predicted by phase equilibria we suggest that if there is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convective flow the transformation in rising or descending material does not take place until the metastable overshoot becomes equal to the nucleation barrier an avalanche like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity in this case the topography of the phase boundary is also substantially modified

wind stress and surface waves observed on lake washington abstract new results from turbulent flux measurements made over lake washington include the following 1 the only direct measure of the vertical transport of the horizontal momentum heat and matter in the surface boundary layer is the so called eddy correlation method however even if the measurement errors are negligible the results obtained from point observations may show large scatter due to lack of stationarity and horizontal homogeneity in the turbulent field and to the sampling variability scatter may be greatly reduced by spatial averaging in this study such an effect is achieved by determining the surface roughness length hence the neutral drag coefficient from the measured wave height spectrum which reflects the atmospheric input integrated over the fetch applicability and usefulness of the approach for general field measurements and remote sensing is discussed 2 the evolution of the wave field observed on lake washington agrees in peak frequency and the slope of the equilibrium ra

evidence of high energy events in the geological record mid holocene evolution of the southwestern donana national park sw spain abstract four phases are distinguished in the mid holocene evolution of the southwestern donana national park sw spain based on the multidisciplinary study of sediments in drill cores in the oldest phase 5500 5400 cal years bp a coastal brackish lagoon occupied the central part of the study area partly enclosed by the donana spit and limited by fluvial levees an evolution from subtidal to intertidal conditions characterized the areas located close to the inner side of the donana spit during the same phase the following phase 5400 5200 cal years bp is characterized by a high energy event tsunami which caused the breakthrough of the donana spit and the creation of new littoral strands in the inner areas in the first period of the third phase 5200 4200 cal years bp estuarine infilling was probably the dominant process with the accumulation of phyllosilicate rich clays in the lagoon bed this was followed by a renewed phase of instability 4200 4100 cal years bp indicated by the presence of fine storm lain deposits and thicker probably tsunami induced shelly deposits the last phase 4100 3700 cal years bp comprises a an infilling period with a diminution of the marine influence and b two new high energy events which caused the erosion of the donana spit and the creation of new cheniers in the last 7000 years the comparison of these results with a revision of both geological and historical records has revealed the occurrence of twenty tsunamis at least during this period in portugal and southern spain evidences of these high energy events are washover fans bioclastic coarse sized layers within salt marsh deposits or the presence of giant boulders and pebbles the determination of the recurrence period is very difficult owing to the very scarce data although our data and other authors do not discard a possible periodicity of 2000 years for a cycle of two events separated by 300 400 years

coupled dissolution and precipitation at mineral fluid interfaces reactions occurring at mineral fluid interfaces are important in all geochemical processes and essential for the cycling of elements within the earth understanding the mechanism of the transformation of one solid phase to another and the role of fluids is fundamental to many natural and industrial processes problems such as the interaction of minerals with co2 saturated water the durability of nuclear waste materials the remediation of polluted water and mineral reactions that can destroy our stone based cultural heritage are related by the common feature that a mineral assemblage in contact with a fluid may be replaced by a more stable assemblage

pressure temperature and tectonic evolution of triassic lawsonite aragonite blueschists from pinchi lake british columbia a blueschist and eclogite terrane is associated with one of the largest faults in the canadian cordilleran orogen the pinchi fault blueschists in situ and retrogressed eclogite blocks occur along the pinchi fault zone near 54 30 n and 124 w critical blueschist facies mineral assemblages include lawsonite glaucophane jadeite lawsonite glaucophane quartz and aragonite white mica 40ar 39ar spectra on blueschist and eclogite yield ages in the range 221 8 1 9 to 223 5 1 7 ma establishing a direct link between the blueschists and eclogites preservation of aragonite sets rigid constraints on the pressure temperature fluid time conditions of unroofing k ar dates indicate that this is some of the oldest documented metamorphic aragonite comparison with computed petrogenetic grids suggests that metamorphic temperatures were in the range 200 300 c with pressures greater than 8 10 kbar 1 kbar 100 mpa unroofing likely occurred during collision of the cache creek terrane with quesnellia in the lat

polycrystalline calcite to aragonite transformation kinetics experiments in synthetic systems the kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates with and without additional minerals present the reaction progresses as a function of time were measured at four temperature pressure conditions 1 550 c 1 86 gpa 2 600 c 2 11 gpa 3 650 c 2 11 gpa and 4 700 c 2 29 gpa experiments reveal that mg calcite and fe calcite transforms to aragonite at considerably slower rates than pure calcite and that sr bearing calcite and calcite quartz aggregates transform at significantly higher rates than pure calcite the reaction progresses vs time data for pure calcite were fitted to cahn s grain boundary nucleation and interface controlled growth model evidence for interface controlled growth is provided by petrographic observations of grain boundaries the activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble the discrepancy in rates and activation energy may be attributed to the nature of grain boundaries to deformational strain or the presence of impurities in the studied samples and likely to uncertainties in experimental conditions the results of this study imply that the variation of local petrologic conditions in addition to temperature pressure and grain size may play an important role in determining the rates for the calcite to aragonite transformation in nature

the effect of ductile deformation on the kinetics and mechanisms of the aragonite calcite transformation abstract the effect of ductile deformation dislocation creep on the kinetics of the aragonite calcite transformation has been studied at 1 atm 330 c and 360 c and 900 1500 mpa 500 c using undeformed and either previously or simultaneously deformed samples 500 c and a strain rate of 10 6 s deformation enhances the rate of the transformation of calcite to aragonite but decreases the rate of transformation of aragonite to calcite the difference results from a dependence of transformation rate on grain size coupled with a difference in the accommodation mechanisms climb versus recry stallization of these minerals during dislocation creep dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction the rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries in aragonite on the other hand dislocation climb is difficult and thus plastic strain produces extensive dynamic recry stallization resulting in a substantial grain size reduction the transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism

reply to the comment on formation of chenier plain of the donana marshland sw spain observations and geomorphic model by a rodriguez ramirez and c m yanez camacho marine geology 254 2008 187 196 abstract the application of the regional reservoir effect r on the south atlantic coast of iberia requires adjustments as a result of the variations it has undergone throughout the holocene since 1996 a constant value of 440 84 14c yr has been used dabrio c j zazo c lario j goy j l sierro f j borja f gonzalez j a flores j a 1999 sequence stratigraphy of holocene incised valley fills and coastal evolution in the gulf of cadiz southern spain geologie en mijnbouw 77 23 281 dabrio c j zazo c lario j goy j l sierro f j borja f gonzalez j a flores j a 2000 depositional history of estuarine infill during the last postglacial transgression gulf of cadiz southern spain marine geology 162 381 404 however a constant value for the whole of the holocene cannot be expected which has led to time intervals with different r being established morales j a borrego j san miguel e g lopez gonzalez n carro b 2008 sedimentary record of recent tsunamis in the huelva estuary southwestern spain quaternary science reviews 27 734 746 deduced from the marine reservoir correction database from calib radiocarbon calibration version 5 0 2html by m stuiver p j reimer and r reimer later soares soares a m m 2008 radiocarbon dating of marine samples from gulf of cadiz abstracts annual conference igcp 495 faro portugal 6 7 established a constant r value of 135 20 14c yr which can be applied to samples younger than 4000 14c yr bp rodriguez vidal et al comment on formation of chenier plain of the donana marshland sw spain observations and geomorphic model by a rodriguez ramirez and c m yanez camacho marine geology 254 2008 187 196 marine geology 2009 in press use these disquisitions about the correct r value to question the geodynamic model proposed by rodriguez ramirez and yanez camacho rodriguez ramirez a yanez camacho c m 2008 formation of chenier plain of the donana marshland sw spain observations and geomorphic model marine geology 254 187 196 without providing any data of their own these authors base their particular critique on the chronological difference caused by using a different r value from the one they use which in this case is the soares soares a m m 2008 radiocarbon dating of marine samples from gulf of cadiz abstracts annual conference igcp 495 faro portugal 6 7 value however if we take into account new progress in the r value all studies since 1996 have some kind of chronological error of this kind and therefore this critique should extend to all these works including their own a critique on geodynamic models based solely on chronological differences of this kind is not justifiable at the same time the ages determined by these authors by re calibrating the samples in rodriguez ramirez and yanez camacho rodriguez ramirez a yanez camacho c m 2008 formation of chenier plain of the donana marshland sw spain observations and geomorphic model marine geology 254 187 196 do contain serious errors due to the incorrect use of the calib 5 0 2 programme

cataclastic deformation and metasomatism in the subduction zone of mafic blocks in melange san simeon california abstract the primary origin of shale matrix melange forming the central belt of the franciscan complex is still debated the seacliff exposures near san simeon display numerous lensoidal blocks of graywacke greenstone lesser layered chert and blueschist that are encased in a matrix of scaly shale the boudinaging of mafic greenstone and blueschist blocks was accommodated by cataclasis and varied amounts of metamorphic reconstitution that is particularly intense along block margins zones of necking and intra block shear zones cataclastic zones in blueschists are greenish where chlorite and pumpellyite have formed the mineralogic assemblage produced during cataclasis in greenstones is the same as that replacing the interior of the block albite chlorite pumpellyite the cataclastic margins of many 7 of 12 greenstones gained k rb cs and ba in contrast the cataclastic margins of all blueschist blocks lost na several 3 of 9 with substantial decreases and some 3 of 9 had significant increases in k rb cs and ba this metasomatic alteration is indicative of fluid rock interaction between mafic blocks and sediments alteration of the mafic rocks with the growth of pumpellyite and chlorite occurred when blocks were encased in the shale matrix of the melange which was dynamically deforming and heated to temperatures of 150 c as synsubduction temperature depth gradients were 10 c km burial of mafic blocks while encased in the melange matrix to depths approaching 15 km is apparent surficial olistostromal movements alone do not account for the incorporation deformation and retrograde metasomatic alteration of the exotic blueschist blocks and the metamorphism of the shale matrix at such inferred depths these attributes indicate chemical exchange with shale and smearing displacement of the margins of mafic blocks occurred in the subduction zone as the shale matrix flowed and exerted shear tractions

a theoretical approach to the kinetics of calcite aragonite transition application to laboratory experiments a theoretical approach to the calcite aragonite transformation is presented for the case of a dislocation glide mechanism obeying first order kinetics the kinetic parameters are determined from recent experimental data from the literature the sensitivity of the model to these and other parameters like the grain size and the deviatoric stresses are discussed the model results in the definition of what could be the actual transformation domains of the caco3 polymorphs in the p t plane these domains are different from the classical ones i e different from the stability fields defined with respect to clapeyron s curve they can evolve in the p t plane when a deviatoric stress is applied to the system in this case we can define a domain in which the two senses of the calcite aragonite transition are activated simultaneously the application of the model to laboratory experiments needs the integration of the kinetic differential equation k varying withp andt using this method we demonstrate the role of p t t path on reaction rate and the role of experimental uncertainties on the reaction parametersk andn finally we discuss the problem of the synthesis of polymorphs out of their stability field and of their formation by grinding these problems difficult to tackle in terms of diffusion can be approached with the proposed dislocation transformation kinetic model

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

recent coastal evolution of the donana national park sw spain abstract since the last holocene sea level rise about 6900 bp a series of depositional littoral landforms has been generated at the outlet of the guadalquivir river with progradation along the predominant longshore drift towards the east the first coastal progradation occurred between 6900 and 4500 bp the donana and perhaps la algaida spits both associated with the oldest and highest marshland in the donana national park are assumed to have been developed at an early stage originally the guadalquivir estuary was wider and deeper than now and its environment was mainly marine the oldest littoral formations have been dated as ca 4735 bp they show erosional events and indicate the breaking up of earlier spit barriers to form inlets the marine environment became increasingly dominant with heavy erosion of cliffs and a retreating coastline this period was followed by another sedimentary cycle 4200 2600 bp that surrounded the earlier eroded barriers the size of the estuary decreased due to the increasing marsh deposits and a fluvial environment was born about 2600 years ago progradation gave way to a new period of intense erosion the resulting morphology of littoral strands and erosional surfaces permitted the return to a marine environment the shoreline again retreated from 2300 bp coastal progradation has prevailed with an erosional interruption at 1000 bp the present day outlet of the guadalquivir is an estuarine delta of inactive marshland the donana national park the dominant environment is fluvial

geochemical gradients within modern and fossil shells of concholepas concholepas from northern chile an insight into u th systematics and diagenetic authigenic isotopic imprints in mollusk shells abstract seriate geochemical measurements through shells of one modern one holocene and two sangamonian concholepas concholepas from marine terraces of northern chile were performed to document diagenetic vs authigenic geochemical signatures and to better interpret u series ages on such material subsamples were recovered by drilling from the outer calcitic layer to the inner aragonitic layer of each of the studied shells unfortunately this sampling procedure induces artifacts notably the convertion of up to 20 of calcite into aragonite and of up to 6 of aragonite into calcite as well as in the epimerization of a few percent of isoleucine into d alloisoleucine l isoleucine negligible sampling artifacts were noticed for stable isotope and total amino acid contents diagenetic effects on the geochemical properties of the shells are particularly pronounced in the inner aragonitic layer and more discrete in the outer calcitic layer the time dependent decay of the organic matrix of the shell is illustrated by a one order of magnitude lower total amino acid content in the sangamonian specimens by comparison with the modern shell conversely the sangamonian shells u contents increase by a similar factor and 13c 18o enrichments as high as 2 to 3 seem also to occur through the same time interval possibly due to partial replacement of aragonite by gypsum the decay of the organic matrix of the aragonitic layer of the shell is thought to play a major role with respect to u uptake processes and stable isotope shifts nevertheless asymptotic 230th ages 100 ka in the inner u rich layers of the sangamonian shells and 234u 238u ratios compatible with a marine origin for u suggest u uptake within a short diagenetic interval when marine waters were still bathing the embedding sediment thus u series ages on fossil mollusks from such a hyper arid environment should not differ much from the age of the corresponding marine unit deposition however the diagenetic enrichments in stable isotopes raise concerns about their use for paleoenvironmental reconstructions under such climate conditions

variation of the drag coefficient and its dependence on sea state using a gill propeller vane anemometer and resistance wave wires over a water column depth of 15 m simultaneous measurements of the momentum flux and sea surface wave spectra were acquired from the pisa mast 28 km offshore in the german bight during autumn and winter 1979 these data were analyzed to identify the relationship between wind stress and surface waves it was found that wind stresses for wind speeds above 15 m s were regularly higher than open ocean wind stresses as reported by smith 1980 and by large and pond 1981 for the same mean wind speed these results when described in terms of the drag coefficient compared closely with the results of sheppard et al 1972 who collected surface layer statistics over lough neagh northern ireland after modeling the surface waves of the north sea as a function of wave saturation or wave age it became evident that variations in the magnitude of the drag coefficient could be explained by coincident variations in the surface wave energy spectrum by applying the wave dependent roughness length model described by kitaigorodskii 1973 the north sea drag coefficient was predicted to be larger than drag coefficients reported from the open sea

microbially influenced formation of 2 724 million year old stromatolites nanoscale evidence suggests that the tumbiana formation stromatolites in australia were influenced by microbial activity in the stromatolites clusters of organic globules are closely associated with 2 724 million year old aragonite crystals