a paleohydrologic model for early proterozoic dolomitization and silicification abstract primary depositional features observed in the intertidal and supratidal facies of the 2 25 ga malmani dolomite including tepees and finely laminated domal stromatolites have equivalents in holocene salinas on the southern coast of australia the hydrological regime so important in the formation of these recent primary features is still active and so it has been possible to interpret the palaeohydrological significance of analogous features in the malmani dolomite the palaeoclimate of the malmani dolomite shoreline was probably humid to semi arid and the supratidal zone is interpreted as a zone of groundwater resurgence tepees indurated fenestral limestone sheets algal mats small domal stromatolites and intraclast breccias were forming on the supratidal flats in areas of periodically outcropping groundwaters in the shallow subsurface the mixing zone between these resurging continental groundwaters and the marine derived phreatic waters was probably a zone of secondary coarse grained dolomite precipitation and or chert nodule formation this groundwater controlled mixing zone model explains why such fabrics are restricted to porous and immediately adjacent units which at the time of diagenesis were below the regional water table in the upper supratidal zones above the regional water table no secondary dolomites or chert nodules could form nor could diagenetic fabrics form in the subtidal zone for there the stromatolitic mounds contained no palaeoaquifer

f cl and s input via serpentinite in subduction zones implications for the nature of the fluid released at depth the abundances of f cl and s in arc magmas are systematically higher than in other mantle derived magmas suggesting that these elements are added from the slab along with h2o we present ion probe microanalyses of f cl and s in serpentine minerals that represent the p t evolution of the oceanic lithosphere from its serpentinization at the ridge to its dehydration at around 100 km depth during subduction f cl and s are incorporated early into serpentine during its formation at mid ocean ridges and serpentinized lithosphere then carries these elements to subduction zones more than 50 of the f cl and s are removed from serpentine during the prograde metamorphic lizardite antigorite transition due to the low solubility of f in water and to the low amount of water released during this phase transition the fluids mobilizing these elements must be dominated by sox rather than h2o

earthquake activity related to seismic cycles in a model for a heterogeneous strike slip fault we investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike slip fault in elastic solid the model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault static kinetic friction and dislocation creep along the fault and 3d elastic stress transfer the fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events and aseismic creep cells which are characterized by high ongoing creep motion this mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified omori law and scale with the mainshock size furthermore we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations the temporal occurrence however is magnitude dependent in particular the small events are clustered in time whereas the largest earthquakes occur quasiperiodically averaging the seismicity before several large earthquakes we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached these results are characteristics of a critical point behavior the presence of critical point dynamics is further supported by the evolution of the stress field in the model which is compatible with the observation of accelerating moment release in natural fault systems

long range earthquake forecasting with every earthquake a precursor according to scale scaling relations previously derived from examples of the precursory scale increase before major earthquakes show that the precursor is a long term predictor of the time magnitude and location of the major earthquake these relations are here taken as the basis of a stochastic forecasting model in which every earthquake is regarded as a precursor the problem of identifying those earthquakes that are actually precursory is thus set aside at the cost of limiting the strength of the resulting forecast the contribution of an individual earthquake to the future distribution of hazard in time magnitude and location is on a scale determined through the scaling relations by its magnitude provision is made for a contribution to be affected by other earthquakes close in time and location e g an aftershock may be given low weight using the new zealand catalogue the model has been fitted to the forecasting of shallow earthquakes exceeding magnitude 5 75 over the period 1965 2000 it fits the data much better than a baseline poisson model with a location distribution based on proximity to the epicenters of past earthquakes further the model has been applied with unchanged parameters to the california region over the period 1975 2001 there also it performs much better than the baseline model fitted to the same region over the period 1951 1974 the likelihood ratio is 1015 in favor of the present model these results lend credence to the precursory scale increase phenomenon and show that the scaling relations are pervasive in earthquake catalogues the forecasting model provides a new baseline model against which future refinements and other proposed models can be tested it may also prove to be useful in practice its applicability to other regions has still to be established

application of stress release models to historical earthquakes from north china univariate and multivariate stress release models are fitted to historical earthquake data from north china it is shown that a better fit is obtained by treating separately the eastern part of the region including the north china plain and bohai sea and the western part of the region including the ordos plateau and its eastern boundary further improvement is obtained by fitting the large events m 7 6 and smaller events in the western region by different stress release models the comparisons are made by computing the likelihoods of the fitted models and discounting the number of parameters used by akaike s aic criterion the models are used to develop long term risk scenarios for the east and west regions

importance of small earthquakes for stress transfers and earthquake triggering 1 we estimate the relative importance of small and large earthquakes for static stress changes and for earthquake triggering assuming that earthquakes are triggered by static stress changes and that earthquakes are located on a fractal network of dimension d this model predicts that both the number of events triggered by an earthquake of magnitude m and the stress change induced by this earthquake at the location of other earthquakes increase with m as similar to 10 dm 2 the stronger the spatial clustering the larger the influence of small earthquakes on stress changes at the location of a future event as well as earthquake triggering if earthquake magnitudes follow the gutenberg richter law with b d 2 small earthquakes collectively dominate stress transfer and earthquake triggering because their greater frequency overcomes their smaller individual triggering potential using a southern california catalog we observe that the rate of seismicity triggered by an earthquake of magnitude m increases with m as 10 alpha m where alpha 1 05 0 05 we also find that the magnitude distribution of triggered earthquakes is independent of the triggering earthquake s magnitude m when alpha approximate to b small earthquakes are roughly as important to earthquake triggering as larger ones we evaluate the fractal correlation dimension d of hypocenters using two relocated catalogs for southern california the value of d measured for distances 0 1 r 5 km is d 1 54 for the shearer et al catalog and d 1 73 for the hauksson et al catalog the value of d reflects both the structure of the fault network and the nature of earthquake interactions by considering only those earthquake pairs with interevent times larger than 1000 days we can largely remove the effects of short term clustering then d approximate to 2 close to the value d 2 alpha 2 1 predicted by assuming that earthquake triggering is due to static stress the value d approximate to 2b implies that small earthquakes are as important as larger ones for stress transfers between earthquakes and that considering stress changes induced by small earthquakes should improve models of earthquake interactions

the degree of predictability of earthquakes in several regions of china statistical analysis of historical data the stress release model a stochastic version of the elastic rebound theory is applied to the historical earthquake data from three strong earthquake prone regions of china including north china southwest china and the taiwan seismic regions the results show that the seismicity along a plate boundary taiwan is more active than in intraplate regions north and southwest china the degree of predictability or regularity of seismic events in these seismic regions based on both the akaike information criterion aic and fitted sensitivity parameters follows the order taiwan southwest china and north china which is further identified by numerical simulations c 2004 elsevier ltd all rights reserved

loss of trace elements from serpentinites during fluid assisted transformation of chrysotile to antigorite an example from guatemala abstract we studied a unique chrysotile antigorite serpentinite drilled on deep sea drilling project leg 84 site 566 in the guatemala forearc our in situ major and trace element data provide new constraints on possible reactions and associated trace element mobilisation during shallow serpentinite subduction chrysotile of the studied serpentinite formed by the hydration of an upper mantle peridotite precursor is partially replaced by antigorite alone which also occurs in 0 5 mm wide unoriented veins crosscutting the rock based on textural relationships and the p t x stability of the rock forming phases the replacement of chrysotile by antigorite occurred at t the fluid assisted chrysotile to antigorite transformation and associated trace element loss documented here can occur in the shallow

trace element behavior during serpentinization de serpentinization of an eclogitized oceanic lithosphere a la icpms study of the lanzo ultramafic massif western alps serpentinites are one of the major components of the oceanic lithosphere and are stable in the slab and the mantle wedge up to 100 150 km depth in subduction zones during oceanic mantle hydration and alteration they trap trace and fluid mobile fme b li as sb rb ba cs sr u and pb elements that participate to elemental transfer occurring between the dehydrating slab and the mantle wedge in subduction context the lanzo massif is an eclogitized oceanic lithosphere that preserved its oceanic structure and recorded different steps of serpentinization de serpentinization from oceanic lizardite to prograde antigorite in subduction context up to its dehydration and secondary olivine crystallization and finally retrograde antigorite during massif exhumation it constitutes a suitable place to study trace element behavior during serpentinization de serpentinization processes and associated chemical transfers between the different envelopes of the oceanic lithosphere and the mantle wedge geochemical analyses of serpentine and associated minerals show that the serpentinization de serpentinization of the lanzo massif took place in a relatively closed system without significant trace element transfer between the different parts of the oceanic lithosphere in the deeper part of the lithosphere from the slightly serpentinized mantle peridotites ssp 90 serpentinization in that zone the alpine deformation enhances the mobility of trace elements and permits their redistribution and the homogenization of antigorite composition at massif scale locally in the ssp and ms the crystallization of metamorphic veins of 1 2 m corresponds to channelized fluid flows that allowed fluid transfers and thereby trace elements to longer distance the successive crystallizations of antigorite and then olivine are accompanied by a diminution of some fme b li as sb ba rb and eu contents attesting that these elements are removed from slab to mantle wedge during subduction

accelerating seismic release from a self correcting stochastic model 1 we investigate the conditions under which the stress release model a stochastic version of the elastic rebound model produces synthetic earthquake sequences characterized by accelerating seismic release asr in this model the level or stress of the process accumulates linearly with time through tectonic input and decreases as the result of earthquakes these stress drops correspond to some power of the energy released in the earthquakes either e0 5 benioff strain or e seismic moment earthquakes occur in a point process with rate controlled by the level of the process we hypothesize that the critical factor in the appearance of asr is the manner in which the event sizes depend on the level of the process this is modeled by the square root of energy released following either a tapered pareto or truncated gutenberg richter distribution with maximum earthquake size controlled by a tail off or truncation point as the tail off point becomes large so does the average size corresponding to an acceleration to criticality of the system we found that those cases where the underlying level of the process corresponded to accumulated seismic moment produced numerous asr sequences whereas those cases using accumulated benioff strain as the level did not these results suggest that the occurrence of asr is strongly dependent on how large earthquakes affect the dynamics of the fault system in which they are embedded and hopefully provide some insight into the mechanics of acceleration to criticality i e on the possible causes of occurrence nonoccurrence of asr

high pressure metamorphism of the lanzo peridotite and its oceanic cover and some consequences for the sesia lanzo zone northwestern italian alps abstract this paper presents new data on the alpine high pressure evolution of the lanzo peridotite massif and its metasedimentary cover western alps field relations in the northwestern lanzo massif show that the serpentinized part is covered by a metasedimentary sequence including local occurrences of metaophicarbonate metabasalt calcschist metaquartzite and gneiss remnants of granulite facies mafic gneiss associated with metaophicarbonate are found within the serpentinite major element garnet zonation profiles in metaquartzite and mafic gneiss were used to explore the contrasting origin of metasediments bell shaped mn zoning in garnet from metaquartzite indicates a one stage alpine metamorphic evolution while inverse zoning in garnet of the mafic gneiss shows that the protolith equilibrated under high grade metamorphic conditions in pre alpine times similar to kinzigites within the neighbouring sesia zone phase equilibria in metaquartzite and mafic gneiss point to 500 50 c at pressures of 0 9 to 1 3 gpa for the alpine metamorphic equilibration that contrasts with the 550 620 c and pressures in excess of 2 0 gpa obtained for kyanite talc chloritoid and talc chloritoid garnet domains in a metagabbro dike from the internal part of the lanzo massif these different p t trajectories can be explained by a much stronger retrograde overprint at the basement cover contact than in the internal part of the lanzo massif

boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fluid mobile element reservoirs for arc magmatism though direct proof of their dominance in the subduction zone volatile cycles has been elusive boron isotopes are established markers of fluid mediated mass transfer during subduction altered oceanic crust and sediments have been shown to release in the subarc mantle 11 b depleted fluids which cannot explain 11 b enrichment of many arcs in contrast to these crustal reservoirs we document high 11 b values retained in subduction zone alpine serpentinites no 11 b fractionation occurs in these rocks with progressive burial the released 11 b rich fluids uniquely explain the elevated 11 b of arc magmas b o h and sr isotope systems indicate that serpentinization was driven by slab fluids that infiltrated the slab mantle interface early in the subduction history

transfer of olivine crystallographic orientation through a cycle of serpentinisation and dehydration our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from one stage to another within the leka ophiolite complex in the central norwegian caledonides prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures the outer part of the veins comprises coarse grained olivine that forms an unusual brick like microstructure the inner part of the veins surrounding a central fault is composed of fine grained olivine where the fault movement included a dilational component optically clear equant olivine occurs in the centre electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation cpo from the olivine in the porphyroclastic host rock however misorientation is weaker and associated to different rotation axes we propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its cpo growth towards pre existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick like microstructure in the veins when dehydration embrittlement induced slip a strong strain localisation on discrete fault planes prevented distortion of the cpo due to cataclastic deformation grain size reduction did not significantly modify the olivine cpo this illustrates how a cpo can be preserved though an entire metamorphic cycle including hydration dehydration and deformation processes and that the cpo and the microstructures e g grain shape of one phase do not necessarily record the same event

ascending freshwater mesohaline mixing a new scenario for dolomitization abstract the process of mixing zone dolomitization has received enough criticism in recent years to suggest that it no longer be considered viable for making large amounts of dolomite this study proposes an alternative model that still requires fluid mixing in this model ascending flow of freshwater mixes with evaporated seawater leading to extensive dolomitization the new model is constrained by data from upper miocene strata of southeast spain where dolomite yields fluid inclusion tm ice data that range from 0 2 to 2 3 c indicating precipitation from fluids of 4 to 43 ppt seawater salt equivalent thus confirming fluid mixing the 13c and 18o of the dolomite ranges from 4 5 to 3 0 vpdb and 0 9 to 6 0 vpdb respectively and displays positive covariance although such positive covariation may represent fluid mixing the mixing of two dolomites during sampling or an originally homogeneous dolomite that has been partially altered recrystallized fluid inclusion data and cathodoluminescence petrography disprove the mixture during sampling and partial alteration hypotheses modeling of the most positive dolomite 18o values indicates an evaporated seawater endmember of 43 ppt salinity identical to that of the most saline fluid inclusions because paleotopography is preserved in the study area the spatial variation in dolomite geochemistry can be mapped to evaluate if it is consistent with the hydrogeology of a typical mixing zone model or an alternative the data reveal that both downdip and updip areas were influenced by high salinity and low salinity fluid endmembers and that stratigraphically lower units have more depleted 13c and 18o than upper units these results are consistent with a model of upward flow of freshwater that provides the mechanism of mixing of freshwater and evaporated seawater upward flow and degassing of co2 from loss of pressure contributed to nearly platform wide dolomitization in this succession and may have analogs in other ancient successions

coupled stress release model for time dependent seismicity based on the original stress release model of seismicity proposed by vere jones 1978 this paper has developed a stochastic coupled stress release model of time dependent seismicity which considers the earthquake interaction and stress transfer between different seismic subregions as an example the model is applied to a statistical analysis of the historical earthquake catalog with magnitude m 6 0 during the period from 1480 to 1996 in north china according to the akaike information criterion aic the results show that the coupled stress release model is better than the original model which demonstrates the existence of long range correlations between different seismic subregions we also apply both the stochastic original and developed coupled models to analyze the synthetic catalog produced by a cellular automata model which is based on mechanics of a slide spring damper system to model the fault network the stress release model provides a good fit to the synthetic regional stress and the coupled stress release model provides an improvement in fit to the synthetic catalog over the original model

seismic quiescence as an indicator for large earthquakes in a system of self organized criticality seismically active fault systems may be in a state of self organized criticality soc investigations of simple soc models have suggested that earthquakes might be inherently unpredictable in this paper we analyze the question of predictability in a more complex and realistic soc model which consists of a spring block system with transient creep characteristics additionally to the power law distribution of earthquake sizes this model reproduces also foreshock and aftershock sequences aside from a short term increase of seismicity immediately prior to large model earthquakes these events are preceded on average by an intermediate term period of reduced seismicity the stronger and the longer the duration of this period the larger on average is the subsequent mainshock we find that the detection of seismic quiescence can improve the time independent hazard assessment the improvement is most significant for the largest target events

the stability of antigorite in the systems mgo sio2 h2o msh and mgo al2o3 sio2 h2o mash the effects of al3 substitution on high pressure stability the high pressure stability of antigorite in the systems msh and mash was investigated using two structurally and chemically well constrained natural samples careful sample selection and characterization ensured that the only significant difference between the samples was al content one sample being essentially al free and the other containing 3 06 2 wt al 2 o 3 in the system msh the reaction antigorite forsterite clinoenstatite water was bracketed under water saturated conditions between 630 and 650 c at 1 6 gpa between 620 and 660 c at 2 5 gpa between 620 and 660 c at 3 9 gpa and between 4 5 and 5 0 gpa at 520 c in the system mash the reaction antigorite forsterite clinoenstatite chlorite water was bracketed under water saturated conditions between 660 and 700 c at 2 0 gpa between 660 and 680 c at 2 9 gpa and between 5 0 and 5 5 gpa at 600 c at pressures above 5 8 gpa intersection of this reaction with the reaction chlorite clinoenstatite pyrope forsterite water leads to an additional reaction whereby the al component of the antigorite is transferred to pyrope upon antigorite breakdown the addition of a few weight percent al 2 o 3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets this effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure in addition antigorite breakdown in the system mash transfers a significant volume of water to chlorite bearing assemblages thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system msh

simulation based distributions of earthquake recurrence times on the san andreas fault system earthquakes on a specified fault or fault segment with magnitudes greater than a specified value have a statistical distribution of recurrence times the mean recurrence time can be related to the rate of strain accumulation and the strength of the fault very few faults have a recorded history of earthquakes that define a distribution well for hazard assessment in general a statistical distribution of recurrence times is assumed along with parameter values assumed distributions include the weibull stretched exponential distribution the lognormal distribution and the brownian passage time inverse gaussian distribution the distribution of earthquake waiting times is the conditional probability that an earthquake will occur at a time in the future if it has not occurred for a specified time in the past the distribution of waiting times is very sensitive to the distribution of recurrence times an exponential distribution of recurrence times is poissonian so there is no memory of the last event the distribution of recurrence times must be thinner than the exponential if the mean waiting time is to decrease as the time since the last earthquake increases neither the lognormal or the brownian passage time distribution satisfies this requirement we use the virtual california model for earthquake occurrence on the san andreas fault system to produce a synthetic distribution of earthquake recurrence times on various faults in the san andreas system we find that the synthetic data are well represented by weibull distributions we also show that the weibull distribution follows from both damage mechanics and statistical physics

petrogenesis of graphitic cherts in the armorican segment of the cadomian orogenic belt nw france graphitic cherts are interbedded within terrigenous sediments in the cadomian orogenic belt of end proterozoic age in the armorican massif nw france the graphitic cherts are of two types massive cherts essentially composed of quartz sio2 96 and with rare sedimentary structures laminated cherts containing up to 3ae4 al2o3 and 92 98 sio2 sedimentary structures observed in the laminated cherts are indicative of a restricted hypersaline tidal or supratidal environment the origins of both types of chert are to be found in the diagenetic processes of silification of terrigenous and mixed terrigenousevaporitic facies these processes which could be mediated by the presence of organic matter were controlled by the migration of the freshwater saltwater mixing zone during periods of relative sea level change the proposed diagenetic origin for the cherts places a number of constraints on their use in the establishment of stratigraphic correlations

decoupling of fluid and trace element release in subducting slab comment on redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies implications for deep subduction zone processes by c spandler et al contrib mineral petrol 146 205 222 2003

deep mantle serpentinization in subduction zones insight from in situ b isotopes in slab and mantle wedge serpentinites abstract serpentinization occurs both within the seafloor when seawater percolates through fractures to alter underlying peridotite and in subduction zones when aqueous fluids from the dehydration of the slab are released within the subduction channel the main difference between these two environments is therefore the source of serpentinizing fluid however distinguishing between the various types of serpentinites encountered in suture zones is difficult essentially because of the lack of significant differences in their mineralogy and major and trace element chemistry eight samples from corsica and the western alps representing slab serpentinites i e serpentinites embedded within the subducting slab which are fragments of former abyssal serpentinites and seven samples from guatemala and nicaragua representing mantle wedge serpentinites i e serpentinites that form in situ above the subducting slab by hydration of the overlying mantle wedge have been selected to study their fluid mobile elements ree concentrations and boron b isotopic signatures the trace element results do not allow a conclusion about the source of fluid s but the b isotopic signatures do permit identification of the different fluid sources responsible for the serpentinization the 11b values of slab serpentinites are significantly positive ranging from 12 to 34 in the same range as present day abyssal serpentinites 5 5 to 40 5 which are altered by seawater therefore these positive signatures indicate serpentinization by seawater derived fluid the b isotopic signature acquired during oceanic residency is therefore preserved even after subduction processes subjected the serpentinites to eclogite facies conditions by contrast 11b in samples from the mantle wedge range from 14 to 10 supporting the interpretation of interactions with fluids derived from the progressive dehydration of the slab which releases fluids with slightly positive and negative 11b it indicates that mantle wedge serpentinite isotopic signature is acquired via the infiltration of subducted crust derived metamorphic fluids during the subduction processes

application of the stress release model to the nankai earthquake sequence southwest japan abstract the stress release model was applied to the nankai earthquakes which occurred in an area off the kii peninsula and shikoku island in the southwest of japan these are considered to be inter plate earthquakes the result of relative movement between the eurasian and the philippine sea plates the dates and sizes of earlier events were determined from historical documents recurrence intervals of the earthquakes were distributed in a range from around 90 to 210 years the stress release model attempts to provide a general formulation of the idea of the elastic rebound theory the hazard rate of the model is given in the form of exp a0 b0t c0s t where a0 b0 and c0 are the parameters adjusted to the data the function of s t corresponds to the stress released by the events up to time t the adjusted parameters are estimated through the optimization of the likelihood function by using the akaike information criterion aic the stress release model is compared with models based on a renewal process several cases were studied using subsets of the sequence the best example of the stress release model in comparison with renewal models is observed in the case of the eight most recent events this model evaluates the timing of the next event in later years and in a narrower time range than those obtained from the renewal models

small earthquakes tectonic forces earthquake scaling and frequency of occurrence relations require that small earthquakes be just as important as larger ones in redistributing the forces that drive relative displacements across active faults of any dimension including plate boundaries

slab derived origin of tremolite antigorite veins in a supra subduction ophiolite the peridotite nappe new caledonia as a case study hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks reservoirs involved fluid rock ratios temperature of interaction highly depleted harzburgites and dunites of the peridotite nappe of new caledonia are crosscut by fractures which have been pervasively serpentinized producing lizardite brucite magnetite and minor chrysotile in a near static environment probably by sea water circulating in cooling joints this event is generally referred to as primary serpentinization in a next step already serpentinized joints were re opened to produce tension and shear cracks sealed by higher temperature synkinematic fibrous minerals locally tremolite bearing veins and pockets which do not display evidence for void infill were generated by metasomatic replacement of the wall rock peridotite most veins only contain fibrous antigorite but some display tremolite antigorite intergrowths or even pure tremolite the latter rocks yield high contents of ca and incompatible elements which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source whole rock geochemical and isotopic features 87sr 86sr 18o and h suggest that antigorite veins which bear highly radiogenic sr isotope signatures were strongly influenced by fluids emitted by the subducted slab and associated sediments in contrast the geochemical and isotopic signatures of tremolite bearing rocks suggest a genetic link with early eocene supra subduction dykes and fluids that leached them calcium strontium and ree bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation chromite alteration and high cr contents in tremolite the bulk of syn tectonic fluid rock interaction associated with shear and tensile fracture development probably occurred at the onset of eocene intra oceanic subduction when the buoyant lower plate the south loyalty basin obliquely forced its way beneath the nascent loyalty fore arc mobile elements extracted from supra subduction dykes temporarily enriched the circulating fluids and generated tremolite antigorite mg chlorite and magnetite instead of antigorite and magnetite solely tremolite crystallization probably ceased due to the exhaustion of ca rich fluids in a globally cooling fore arc environment

isotopic evidence for iron mobility during subduction subduction zones are one of the most important sites of chemical interchange between the earth s surface and interior one means of explaining the high fe3 fe ratios and oxidized nature of primary arc magmas is the transfer of sulfate sox carbonate co3 and or iron fe3 bearing fluids from the slab to the overlying mantle iron mobility and fe stable isotope fractionation in fluids are influenced by fe redox state and the presence of chlorine and or sulfur anions here we use fe stable isotopes 56fe as a tracer of iron mobility in serpentinites from western alps metaophiolites which represent remnants of oceanic lithosphere that have undergone subduction related metamorphism and devolatilization a negative correlation r2 0 72 is observed between serpentinite bulk 56fe and fe3 fe that provides the first direct evidence for the release of fe bearing fluids during serpentinite devolatilization in subduction zones the progressive loss of isotopically light fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate rich and or hypersaline fluids which preferentially complex isotopically light fe in the form of fe ii sox or fe ii cl2 species fe isotopes can therefore be used as a tracer of the nature of slab derived fluids

the composition of the precambrian ocean waters abstract the composition of the precambrian oceans was different from the present composition this is a direct consequence of a co2 atmosphere depleted in oxygen at the beginning the mineralization was lower the oceans waters were chloro dicarbonated with a lower tenor of chlorine richer in phosphates and they were depleted in sulphates sulphur was in the reduced state of sulphides the abundance of phosphates supported the expansion of life the cations na and mg2 coexisted with ca2 fe2 and ba2 whose levels were higher than they are today the main trends of the evolution are schematized in the figure

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

seismic gap hypothesis ten years after the seismic gap hypothesis states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries one of the earliest and clearest applications of the seismic gap theory to earthquake forecasting was by mccann et al 1979 who postulated zones of high medium and low seismic potential around the pacific rim in the 10 years since there have been over 40 large m 7 0 earthquakes enough to test statistically the earlier forecast we also analyze another forecast of long term earthquake risk that by kelleher et al 1973 the hypothesis of increased earthquake potential after a long quiet period can be rejected with a large confidence the data suggest that contrary to these forecasts places of recent earthquake activity have larger than usual seismic hazard whereas the segments of the circum pacific belt with no large earthquakes in recent decades have remained relatively quiet the clustering of earthquake times does not contradict the plate tectonic model which constrains only the long term average slip rate not the regularity of earthquakes

significance of stress transfer in time dependent earthquake probability calculations 1 a sudden change in stress is seen to modify earthquake rates but should it also revise earthquake probability data used to derive input parameters permit an array of forecasts so how large a static stress change is required to cause a statistically significant earthquake probability change to answer that question effects of parameter and philosophical choices are examined through all phases of sample calculations drawing at random from distributions of recurrence aperiodicity pairs identifies many that recreate long paleoseismic and historic earthquake catalogs probability density functions built from the recurrence aperiodicity pairs give the range of possible earthquake forecasts under a point process renewal model consequences of choices made in stress transfer calculations such as different slip models fault rake dip and friction are tracked for interactions among large faults calculated peak stress changes may be localized with most of the receiving fault area changed less than the mean thus to avoid overstating probability change on segments stress change values should be drawn from a distribution reflecting the spatial pattern rather than using the segment mean disparity resulting from interaction probability methodology is also examined for a fault with a well understood earthquake history a minimum stress change to stressing rate ratio of 10 1 to 20 1 is required to significantly skew probabilities with 80 85 confidence that ratio must be closer to 50 1 to exceed 90 95 confidence levels thus revision to earthquake probability is achievable when a perturbing event is very close to the fault in question or the tectonic stressing rate is low

field based evidence for devolatilization in subduction zones implications for arc magmatism metamorphic rocks on santa catalina island california afford examination of fluid related processes at depths of 15 to 45 kilometers in an early cretaceous subduction zone a combination of field stable isotope and volatile content data for the catalina schist indicates kilometer scale transport of large amounts of water rich fluid with uniform oxygen and hydrogen isotope compositions the fluids were liberated in devolatilizing relatively low temperature 400 degrees to 600 degrees c parts of the subduction zone primarily by chlorite breakdown reactions an evaluation of pertinent phase equilibria indicates that chlorite in mafic and sedimentary rocks and melange may stabilize a large volatile component to great depths perhaps 100 kilometers depending on the thermal structure of the subduction zone this evidence for deep volatile subduction and large scale flow of slab derived water rich fluids lends credence to models that invoke fluid addition to sites of arc magma genesis

antigorite high pressure stability in the system mgosio2h2o msh abstract the breakdown reactions of antigorite 1 forming talc forsterite water at low pressures and 2 forming forsterite clinoenstatite water at high pressures were determined in reversed equilibrium experiments results on reaction 1 were found to be in good agreement with former experimental determinations by both johannes johannes w 1975 zur synthese und thermischen stabilitat von antigorit fortschr mineral beih 53 36 and evans et al evans b w johannes w oterdoom h trommsdorff v 1976 stability of crysotile and antigorite in the serpentinite multisystem schweiz mineral petrogr mitt 56 79 93 from our experiments the invariant point i1 interconnecting the two reactions can be located at about 15 kbar 650 c this is consistent with the thermodynamic calculations using the dataset of berman berman r g 1988 internally consistent thermodynamic data for minerals in the system j petrol 29 445 522 however it is in contrast to recent experimental studies of ulmer and trommsdorff ulmer p trommsdorff v 1995a serpentine stability to mantle depths and subduction related magmatism science 268 858 861 who determined i1 at 21 kbar 730 c our pt conditions for i1 could be confirmed by equilibrium experiments on reaction 10 talc forsterite clinoenstatite water which is generated at i1 as well up to about 25 kbar the breakdown reaction 2 is nearly pressure independent towards still higher pressures the dp dt slope of reaction 2 bends and becomes negative schreinemakers analysis as well as thermodynamic calculations of the upper pressure stability of antigorite show that the possible antigorite breakdown reaction 3 antigorite clinoenstatite brucite water and reaction 4 brucite clinoenstatite forsterite water could originate at a new invariant point i2 provided that the reactions 2 and 11 brucite antigorite forsterite water intersect bracketing equilibrium 4 and combining these results with those on reaction 2 i2 was located at only about 51 kbar 490 c compared to 77 kbar 680 c according to berman s data however when taking into account the dense hydrous magnesium silicate dhms phase a mg7si2o8 oh 6 the phase relations of antigorite are changed resulting i in the metastability of i2 and reaction 3 and ii in a new invariant point i7 at about 44 kbar and 580 c generating the new antigorite breakdown reaction 16 antigorite phase a clinoenstatite water on the basis of these new data on the stability of antigorite earlier conclusions about dehydration depths in subducted serpentine bearing oceanic lithosphere have to be reconsidered the maximum pressure stability of antigorite according to reaction 16 extends between 44 and 55 kbar that is between about 130 and 160 km depths as opposed to about 75 kbar 220 km following ulmer and trommsdorff see above because many different thermal regimes are possible in subduction zones no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

pressure temperature estimates of the lizardite antigorite transition in high pressure serpentinites serpentine minerals in natural samples are dominated by lizardite and antigorite in spite of numerous petrological experiments the stability fields of these species remain poorly constrained this paper presents the petrological observations and the raman spectroscopy and xrd analyses of natural serpentinites from the alpine paleo accretionary wedge serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub greenschist p 20 kbar t 460 c along a subduction geothermal gradient we used the observed mineral assemblage in natural serpentinite along with the tmax estimated by raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures we show that below 300 c lizardite and locally chrysotile are the dominant species in the mesh texture between 320 and 390 c lizardite is progressively replaced by antigorite at the grain boundaries through dissolution precipitation processes in the presence of sio2 enriched fluids and through a solid state transition in the cores of the lizardite mesh above 390 c under high grade blueschist to eclogite facies conditions antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460 c

how can one test the seismic gap hypothesis the case of repeated ruptures in the aleutians the test that kagan and jackson 1991 1995 applied to the seismic gap hypothesis did not bring us closer to understanding the generation of large earthquakes on the contrary it led some to the conclusion that the rebound theory of earthquake generation should be rejected we disagree with this point of view and argue that a global test of the simplified gap hypothesis cannot be done because it cannot account for differences in the slip history of fault segments and tectonic differences between separate plate boundaries kagan and jackson did show however that the original gap hypothesis was oversimplified and should be refined we propose that consideration of all the facts including slip history and seismicity patterns in the andreanof islands show that the concept of seismic gaps and the elastic rebound theory are correct for that segment of the plate boundary the coseismic slip in the m w 8 7 earthquake that broke this plate boundary segment in 1957 was only 2 m as published before the repeat earthquake of 1986 m w 8 and thus using a plate convergence rate of 7 3 cm year the return time in this cycle was expected to be less than 30 years unless substantial aseismic creep occurs this supports the time predictable model of mainshock recurrence in addition kisslinger et al 1985 and kisslinger 1986 noticed a seismic quiescence in the subsequent source volume before the 1986 earthquake and attempted to predict it the specific parameters he estimated were not entirely correct although his interpretation of the observed quiescence as a precursor was we conclude that the 1986 m w 8 andreanof earthquake was not an example that disproves the seismic gap hypothesis on the contrary it shows that the hypothesis that plate motions reload plate boundaries after most of the elastic energy is released in great ruptures was correct in this case this suggests that great earthquakes occur preferably in mature gaps we believe the testing of the seismic gap hypothesis by algorithm on a global scale is an example that illustrates that overly simplified tests can lead to erroneous conclusions to make progress in the actual understanding of the physics of the process of great earthquake ruptures one must consider all the facts known for case histories

long term earthquake clustering summary we analyse statistically the long term properties of several instrumental earthquake catalogues complete catalogues exhibit both short and long term clustering for earthquakes of all depth ranges after accounting for the effect of short term clustering we find that in residual declustered catalogues long term clustering not periodicity characterizes the occurrence of all earthquakes shallow intermediate and deep the degree of clustering in residual catalogues is the same for earthquakes in different depth ranges circumstantial evidence indicates that the long term v ariation of seismicity is governed by a power law temporal distribution as in short term clustering it is scale invariant the fractal dimension of an earthquake set on the time axis is of the order of 0 8 0 9 therefore mainshock occurrence is closer to a stationary poisson process than standard aftershock sequences of shallow earthquakes

primary fluids entrapped at blueschist to eclogite transition evidence from the tianshan meta subduction complex in northwestern china the tianshan is the first locality worldwide where primary fluids at blueschist to eclogite transition have been documented veins containing high pressure minerals in massive host eclogites transitional to blueschists or eclogite boudins surrounded by blueschists indicate that a free fluid phase was present at the time of eclogitization the high pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite the transition from blueschist to eclogite facies parageneses occurs as dehydration halos around these veins clinozoisite paragonite glaucophane and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of blueschist to eclogite c axis parallel non planar populations of fluid inclusions were trapped during the growth of omphacite in high pressure veins and dehydrated wall rocks low salinity h2o nacl solid bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks none of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite facies metamorphic conditions suggesting that although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions their densities must have been modified during exhumation the fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions such as 13 gln 5 czo 9 prp 26 jd 12 di 19 qtz 15 h2o and gln pg prp 3 jd 2 otz 2 h2o the similarity of vein and wall rock mineral compositions fluid inclusion characteristics and o isotope data also favor an internal source for the fluids the major element composition of veins indicate that si na and ca rich aqueous fluids were released during dehydration at a depth of 50 10 km within a paleozoic subduction zone

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

evidence for boron incorporation into the serpentine crystal structure serpentinite mud volcanoes from the mariana forearc comprise b rich mantle wedge peridotites serpentinized by slab fluids the major component of these rocks are serpentine group minerals mg 3 si 2 o 5 oh 4 showing highly variable textural and geochemical features micro raman spectroscopy reveals that the serpentine minerals are well crystallized lizardite and chrysotile in situ sims spot analyses and element mapping via tof sims show that b is evenly distributed across serpentine grains suggesting that serpentine both lizardite and chrysotile in different textural regions can host significant amounts of b up to 200 g g into its crystal structure as such structurally bound b can only be released during recrystallization or serpentine breakdown our results have implications for modeling of the efficiency of cross arc fluid mobile element recycling in subduction zones and stress the importance of the hydrated forearc mantle as a reservoir for fluid mobile elements

a review of the origin and setting of tepees and their associated fabrics carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal lacustrine and subaerial carbonate shelf sediments these are commonly disrupted and brecciated when the surface area of these crusts increases in the subtidal environment megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand similar megapolygons form in the supratidal lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and or cement also causes the area of the hardgrounds to expand the arched up antiform margins of expansion megapolygons are known as tepees the types of tepees found in the geological record include 1 submarine tepees which form in shallow carbonate saturated waters where fractured and bedded marine grainstones are bound by isopachous marine phreatic acicular and micritic cements the surfaces of these brecciated crusts have undergone diagenesis and are bored unlike tepees listed below they contain no vadose pisolites or gravity cements 2 peritidal and lacustrine tepees are formed of crusts characterized by fenestral pisolitic and laminar algal fabrics this similarity in fabric makes these tepees of different origins difficult to separate peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical they are associated with fractured and bedded tidal flat carbonates their fracture fills contain geopetal asymmetric travertines of marine vadose origin and or marine phreatic travertines and or terra rossa sediments the senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common they include groundwater tepees which form over evaporitic boxwork carbonates and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas but the dominant sediment type is carbonate mud these latter tepee crusts are coated and crosscut by laminated micrite the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions in time intervals of days to years cement morphologies reflect this and the crusts often contain gravitational meniscus vadose cements as well as phreatic isopachous cement rinds 3 caliche tepees which are developed within soil profiles in a continental setting they are formed by laminar crusts which contain pisolites and fractures filled by micritic laminae microspar spar and terra rossa most of the cements are gravitational and or meniscoid in ancient carbonates when their cementation and diagenetic fabric can be interpreted tepee structures can be used as environmental indicators they can also be used to trace the evolution of the depositional and hydrological setting

seimological studies at parkfield v characteristic microearthquake sequences as fault zone drilling targets studies at very high resolution of microearthquakes at parkfield california since 1987 reveal a systematic organization in space and time dominated by clustering of nearly identical regularly occurring microearthquakes characteristic events on 10 to 20 m wide patches within the fault zone more than half of the 4000 events in our 1987 to 1996 catalog exhibit this trait in general recurrence intervals 0 5 to 2 yr scale with the magnitude of the repeating events for the on scale range mw 0 2 to 1 3 in this study the similar waveforms superimposed locations quasi periodic recurrence and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months clustered characteristic events occur at depths as shallow as about 3 km and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate boundary fault zone at parkfield the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault zone measurements

further applications of the stochastic stress release model to historical earthquake data abstract the stochastic stress release model provides a stochastic version of the elastic rebound model which may be used to fit simulate and forecast occurrence of large earthquakes within a seismic region this paper reviews its applications to historical earthquakes from china iran and japan thereby extending and supporting earlier work on the chinese historical data the model generally provides an improvement in fit over poisson models with either constant rate or linear trend a crucial factor is the identification of suitable subregions which can act as largely independent seismic units in northern china divisions into two and four such units are considered based on recognized geophysical subdivisions in iran the best results are obtained from a subdivision into three such units the situation in japan seems more complex a preliminary model with two units gives a significant improvement of fit in one out of the two cases in all regions treating the subregions separately gives marked improvements over treating the whole region as a single unit however the quality of fit within subregions differs sharply from case to case pointing to substantial differences in the seismic regimes as reflected in the historical data

application of linked stress release model to historical earthquake data comparison between two kinds of tectonic seismicity the linked stress release model incorporating a slow buildup of stress within a seismic region its stochastic release through earthquakes and transfer between seismic regions is applied to fit historical data from two typical kinds of seismicity earthquakes occurring in intraplate north china and plate boundary new zealand regions the best model among different modifications of the basic model which may reflect on a possible geophysical mechanism for earthquake occurrences is obtained in terms of akaike information criterion for both tectonic regions studied the linked stress release model fits the new zealand data better than a collection of independent simple models but is nearly indistinguishable from the simple stress release model in the case of north china the seismicity in a plate boundary region due to subduction is more active and complex than that in an intraplate region due to collision between tectonic plates the results highlight the major differences in tectonic seismicity especially the heterogeneities of tectonic stress fields and dynamic triggering mechanism with evidence that the crust may lie in a near critical state

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

evidence of clustering and nonstationarity in the time distribution of large worldwide earthquakes 1 the purpose of this paper is to characterize the statistical distribution of worldwide largest earthquakes we analyze the distribution of worldwide shallow events with ms 7 0 since 1900 by following the occam s razor philosophy we start from the simplest possible model stationary poisson process and we inspect more complicated models only if the data show significant departures from the simplest one the results show two important characteristics first worldwide ms 7 0 earthquakes tend to cluster in time and space with features similar to smaller events second for some seismic regions there is evidence in favor of long term fluctuations of the earthquake rate these results support the hypothesis of universality suggesting that an etas model with a background varying with time can be considered reliable to describe the seismicity distribution over a wide space time magnitude window from a practical point of view our findings suggest that the paradigm that seismic zones are stationary systems implicitly assumed in seismic hazard assessment should be regarded with caution

probabilistic seismic hazard estimation in low seismicity regions considering non poissonian seismic occurrence summary in low seismicity regions such as france or germany the estimation of probabilistic seismic hazard must cope with the difficult identification of active faults and with the low amount of seismic data available since the probabilistic hazard method was initiated most studies assume a poissonian occurrence of earthquakes here we propose a method that enables the inclusion of time and space dependences between earthquakes into the probabilistic estimation of hazard combining the seismicity model epidemic type aftershocks sequence etas with a monte carlo technique aftershocks are naturally accounted for in the hazard determination the method is applied to the pyrenees region in southern france the impact on hazard of declustering and of the usual assumption that earthquakes occur according to a poisson process is quantified showing that aftershocks contribute on average less than 5 per cent to the probabilistic hazard with an upper bound around 18 per cent

new seismic gap hypothesis five years after we use earthquake data from 1989 1994 to test a forecast by nishenko based on the seismic gap theory we refer to this forecast as the new seismic gap hypothesis because it is the first global forecast based on the seismic gap hypothesis that considers the recurrence time and characteristic earthquake magnitude specific to each plate boundary segment nishenko s forecasts gave probabilities that each of about 100 zones would be filled by characteristic earthquakes during periods of 5 10 and 20 years beginning on the first day of 1989 only the first of these can be tested now we used three tests based on 1 the total number of zones filled by characteristic earthquakes 2 the likelihood that the observed list of filled zones would result from a process with the probabilities specified in nishenko s hypothesis and 3 the likelihood ratio to that of a poissonian null hypothesis the null hypothesis uses a smoothed version of seismicity since 1977 and assumes a gutenberg richter magnitude distribution we used both the harvard centroid moment tensor and the national oceanic and atmospheric administration preliminary determination of epicenters catalogs in our test we also used several different magnitude cutoffs in our tests because nishenko s forecast did not specify a clear relationship between the characteristic earthquake magnitude and the threshold magnitude for a successful prediction using a strict interpretation that only earthquakes equal to or larger than the characteristic magnitude should be counted both catalogs show only two qualifying earthquakes in the entire area covered by the forecast the predicted number is 9 2 and the discrepancy is too large to result from chance at the 99 confidence level the new seismic gap hypothesis predicts too many characteristic earthquakes for three reasons first forecasts were made for some zones specifically because they had two or more earthquakes in the previous centuries biasing the estimated earthquake rate second open intervals before the first event and after the last event are excluded in calculation of recurrence rate third the forecast assumes that all slip in each zone is released in characteristic earthquakes of the same size while in fact considerable slip is released by both smaller and larger earthquakes the observed size distribution of earthquakes is inconsistent with the characteristic hypothesis instead of a deficit of earthquakes above the characteristic limit earthquake numbers are distributed according to the standard gutenberg richter relation by lowering the magnitude threshold for qualifying earthquakes it is possible to reduce the discrepancy between the observed and predicted number of earthquakes to an acceptable level however for every magnitude threshold we tried the new seismic gap model failed the test on the number of filled zones or the likelihood ratio test or both at least at the 95 confidence level

non poissonian earthquake occurrence in coupled stress release models and its effect on seismic hazard summary most seismic hazard estimations are based on the assumption of a poisson process for earthquake occurrence even though both observations and models indicate a departure of real seismic sequences from this simplistic assumption instrumental earthquake catalogues show earthquake clustering on regional scales while the elastic rebound theory predicts a periodic recurrence of characteristic earthquakes on longer timescales for individual events recent implementations of time dependent hazard calculations in california and japan are based on quasi periodic recurrences of fault ruptures according to renewal models such as the brownian passage time model however these renewal models neglect earthquake interactions and the dependence on the stressing history which might destroy any regularity of earthquake recurrences in reality to explore this we investigate the coupled stress release model a stochastic version of the elastic rebound hypothesis in particular we are interested in the time variability of the occurrence of large earthquakes and its sensitivity to the occurrence of gutenberg richter type earthquake activity and fault interactions our results show that in general large earthquakes occur quasi periodically in the model the occurrence probability of large earthquakes is strongly decreased shortly after a strong event and becomes constant on longer timescales although possible stress interaction between adjacent fault zones does not affect the recurrence time distributions in each zone significantly it leads to a temporal clustering of events on larger regional scales the non random characteristics especially the quasi periodic behaviour of large earthquakes are even more pronounced if stress changes due to small earthquakes are less important the recurrence time distribution for the largest events is characterized by a coefficient of variation from 0 6 to 0 84 depending on the relative importance of small earthquakes

cross arc geochemical variations in the kurile arc as a function of slab depth lavas from transects across the kurile islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab enrichment factors for boron cesium arsenic and antimony all elements with strong affinities for water decreased across the arc this decrease is presumably related to losses of water rich fluids during the dehydration of the subducting plate enrichments of potassium barium beryllium 10 and the light rare earth elements remained constant these species may move in silica rich fluids liberated from the slab at greater depths

paragenesis of box work geodes tampa bay florida abstract an unusual suite of silicified rocks was excavated during a recent harbour deepening project in tampa bay florida these rocks which we have termed box work geodes are composed of convoluted intersecting silica walls enclosing cavities which are either voids or filled with relatively pure monoclinic palygorskite the box work geodes are interpreted as having formed in shallow lagoonal environments similar to the coorong lagoon of south australia synaeresis of syngenetic palygorskite was followed by opal deposition and case hardening of the material subsequent chemical deposition of chalcedony megacrystalline quartz barite and calcite on the void facing walls indicates an open chemical system the existence of opal saturated lagoons as inferred from the box work geodes suggests that much of the replacement chert porcelanite and silicified fossils in the tertiary deposits of peninsular florida formed in the shallow subsurface subsequent weathering of carbonates and clays not encapsulated in the box works has resulted in formation of a green montmorillonite residual clay bed

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

bayesian inference of stress release models applied to some italian seismogenic zones summary in this paper we evaluate the seismic hazard of a region in southern italy by analysing stress release models from the bayesian viewpoint the data are drawn from the most recent version of the parametric catalogue of italian earthquakes for estimation we just use the events up to 1992 then we forecast the date of the next event through a stochastic simulation method and we compare the result with the really occurred shocks in the span 1993 2002 the original version of the stress release model proposed by vere jones in 1978 transposes reid s elastic rebound theory in the framework of stochastic point processes since the nineties enriched versions of this model have appeared in the literature applied to historical catalogues from china iran japan they envisage the identification of independent or interacting tectonic subunits constituting the region under exam it follows that the stress release models designed for regional analyses are evolving towards studies on fault segments realizing some degree of convergence to those models that start from an individual fault and considering the interactions with nearby segments are driven to studies on regional scale the optimal performance of the models we consider depends on a set of choices among which the seismogenic region and possible subzones the threshold magnitude the length of the time period in this paper we focus our attention on the influence of the subdivision of the region under exam into tectonic units in the light of the recent studies on the fault segmentation model of italy we propose a partition of sannio matese ofanto irpinia one of the most seismically active region in southern italy the results show that the performance of the stress release models improves in terms of both fitting and forecasting when the region is split up into parts including new information about potential seismogenic sources

the hydrological significance of holocene tepees stromatolites and boxwork limestones in coastal salinas in south australia abstract holocene groundwater lakes salinas are common in the coastal zone of south australia they usually contain an evaporitic carbonate sequence that surrounds a central gypsum unit to form a bull s eye pattern of sediment distribution in the salina the evaporitic carbonates can be subdivided into three units a boxwork boundstone unit a veneer boundstone unit and an algal boundstone unit these units contain many interesting structures including tepees stromatolites and mound springs all of which show a megapolygonal outcrop pattern the salina carbonates have formed under a hydrological regime that has changed little since its inception some 6000 years ago consequently it has been possible to interpret the hydrological significance of the salina carbonates the salinas are fil ed by schizohaline surface waters whose water levels and salinities fluctuate seasonally all of the salina carbonates form where marine derived groundwater resurges from a surrounding dune aquifer into the margin of the salina tepee structures form in the veneer boundstone as a response to groundwater induced seasonal changes in the pore pressures of the underlying boxwork sediments the recognition of such groundwater tepees or other groundwater controlled structures in ancient carbonate sequences could aid paleohydrological interpretation of the ancient basin

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

the linked stress release model for spatio temporal seismicity formulations procedures and applications summary the linked stress release model is based on the build up of stress through elastic rebound and its dissipation in the form of earthquakes in addition stress can be transferred between large scale geological or seismic features the model can be statistically fitted to both historical and synthetic seismicity catalogues and through simulation can be used to create probabilistic forecasts of earthquake risk we review the genesis of the model provide some observations on forecasting using the model and follow with a comprehensive review of applications to date a systematic procedure for identification of the best model is illustrated by data from the persian region we then consider the evaluation of fitted models using residual point processes and information gains implications of the use of benioff strain rather than seismic moment are discussed the sensitivity of the model to regionalization magnitude errors catalogue incompleteness catalogue size and declustering magnitude cut off is then considered in detail with reference to data from north china the latter data are also used to illustrate the model evaluation techniques introduced earlier some technical material on numerical fitting simulation and calculation of the information gain is given in an appendix

boron isotope fractionation in subducted serpentinites a modelling attempt abstract boron b and its isotopes are powerful geochemical tracers of subduction processes enabling to understand the role played by fluids in large scale mass transfer at convergent margins in such settings oceanic and forearc serpentinites are the most important reservoirs for b and heavy b isotopes contributing to the genesis of arc lavas literature data support the release of significant amount of b during the chrysotile lizardite to antigorite atg phase transition and during the partial to complete antigorite dehydration to form secondary peridotites however the information concerning the b isotope fractionation associated to these reactions is limited in this letter i attempt to model the b isotope fractionation affecting subducted serpentinites through the ab initio b isotope fractionation calculations provided for trigonal and tetrahedral coordinated b in both minerals and fluids the obtained results mainly confirm serpentinites as the main vector of positive 11b to depth as shown by natural high pressure serpentinites analysed so far however a lack of information on ph conditions hampers the proper estimate of the fractionation of b in serpentinites during their prograde evolution in subduction zones this may represent a limit in the approach proposed here since poor alkaline close to neutral ph conditions can result in low potentially negative 11batg signatures finally the letter try to address new research trends and major topics worth investigating to better unravel the b cycle in serpentinites and their dehydration products

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data