implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

platinum in earth surface environments platinum pt is a rare precious metal that is a strategic commodity for industries in many countries the demand for pt has more than doubled in the last 30years due to its role in the catalytic conversion of co hydrocarbons and nox in modern automobiles to explore for new pt deposits process ores and deal with ecotoxicological effects of pt mining and usage the fundamental processes and pathways of pt dispersion and re concentration in surface environments need to be understood hence the aim of this review is to develop a synergistic model for the cycling of pt in earth surface environments this is achieved by integrating the geological biogeo chemical literature which focuses on naturally occurring pt mobility around ore deposits with the environmental ecotoxicological literature dealing with anthropogenic pt dispersion in pt deposits pt occurs as sulfide telluride and arsenide native metal and alloyed to other pges and iron fe increased mining and utilization of pt combined with the burning of fossil fuels have led to the dispersion of pt containing nano and micro particles hence soils and sediments in industrialized areas urban environments and along major roads are now commonly pt enriched platinum minerals nuggets and anthropogenic particles are transformed by physical and bio geochemical processes complexation of pt ions with chloride thiosulfate ammonium cyanide low and high molecular weight organic acids lmwoas and hmwoas and siderophores can facilitate pt mobilization iron oxides clays organic matter and micro biota are known to sequester pt complexes and particles microbes and plants are capable of bioaccumulating and reductively precipitating mobile pt complexes bioaccumulation can lead to toxic effects on plants and animals including humans bio mineralization in organic matter rich sediments can lead to the formation of secondary pt particles and grains ultimately pt is enriched in oceanic sediments where pt is commonly concentrated in manganese mn oxides when these sediments are subducted pt re enters the magmatic cycle in conclusion this review demonstrates that geological geochemical as well as biological and most recently anthropological processes are strongly interlinked in driving the cycling of pt in surface environments

effect of soil properties on gold and platinum nanoparticle mobility abstract soils overlying buried gold au and platinum pt deposits are often au pt anomalous widening exploration footprints this may be a result of the dispersion of bio geochemically formed au pt nanoparticles and their sorption deposition by solid soil phases however the influence of soil properties on nanoparticle dispersion sorption and partitioning are little understood to assess the partitioning and sorption behaviour of au and pt nanoparticles soils were spiked with spherical citrate stabilized au and pt nanoparticles diameter 10 12 nm final concentration 100 g kg 1 soil slurries 1 10 w w soil deionized water were incubated at 22 c on an end over end shaker for 28 days gold pt concentrations were measured in 0 45 m and 1 kda filtered solutions using inductively coupled plasma mass spectrometry neither pt nor au were detected in 1 kda filtrates between 10 3 and 98 7 wt of au pt was sorbed to solid soil phases clay corg mg mn fe and ca contents of soils were significant predictors of au pt nanoparticle sorption with clay content explaining 80 of variation within the au sorption dataset partitioning of sorbed nanoparticles between solid phases was assessed using selective sequential extractions in all soils 1 20 wt of au pt was detected in watery extracts a further 1 25 wt were associated with the easily remobilizable fraction the majority of sorbed au pt nanoparticles were sorbed to the organic matter om 53 77 wt for pt and 14 80 wt for au and the fe mn oxides up to 42 wt these results show that i au and pt nanoparticles are stable and can remain highly mobile dispersible in soil solutions and ii elevated clay corg and fe mn oxide contents decrease whereas elevated sand contents and ph increase their mobility across all soils om and clays were the most important sorbent of au pt nanoparticles suggesting that the separation of clays and om and analyses of their au pt contents may enhance the contrast of geochemical footprints

finding fault in fault zones what controls the location and size of seismic rupture along the 43 500 km of the world s subduction faults two reports in this issue based on seismological observations emphasize the key role of the structural evolution of faults on page 207 dean et al 1 propose that a structural flaw of the sedimentary formation brought into the sumatra subduction zone may facilitate rupture propagation on page 210 kimura et al 2 propose that fracturing of the downward going plate beneath the kanto area in central japan causes the subduction interface to be diverted to a deeper position and change its seismogenic behavior

shedding light on ore deposits a review of synchrotron x ray radiation use in ore geology research abstract geology researchers commonly confront critical research questions that require microscopic and sub microscopic level observations e g distribution of chemical elements of economic interest nature of mineralising fluid in order to explain the macroscale phenomena related to the characterisation of and controls on ore mineralisation the emergence and technological evolution of fourth generation synchrotron radiation light sources render these facilities as important but still largely under utilised tools for advancing the frontiers of ore related research to this end the present contribution seeks to critically review and highlight the power and affordances of synchrotron x ray techniques specifically to the fields of fundamental and applied ore geology the review introduces a range of commonly applied synchrotron x ray techniques before comprehensively highlighting past applications of synchrotron x rays to a range of commodities and ore deposits types a synthesis of this information highlights the historical importance of synchrotron x ray techniques towards 1 understanding ore textures and chemical relationships at high m spatial resolutions 2 understanding coordination chemistry of metals in aqueous fluids at geologically relevant p t conditions 3 in situ evaluation of the chemistry and speciation of fluid inclusions 4 evaluating the coordination environment of deleterious and economic trace metal substitutions in major mineral structures and 5 understanding the biogeochemical behaviour and transformations of metals in the low temperature supergene mineralising environment these applications as well as recommendations for future work are described in the hope that a greater diversity of ore geology researchers will incorporate synchrotron x ray tools into their existing suite of analytical and experimental techniques

discriminating between tectonic and sedimentary burial in a foredeep succession northern apennines an extensive apatite fission track survey has been carried out on the marnoso arenacea foredeep succession in the northern apennines the data show a general decrease of the maximum paleotemperature undergone by the sediments toward the foreland areas the maximum burial calculated by using a geothermal gradient of 20 c km 1 spans from more than 5 km to less than 2 5 km and indicates that the previously assessed total thickness of the marnoso arenacea succession is not enough to justify the determined values it is concluded that a now eroded ligurian wedge up to 5 km thick was present on top of the marnoso arenacea sediments

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

biogenic caliches in texas the role of organisms and effect of climate abstract biogenic constituents are ubiquitous and abundant in the caliches of texas investigation of 51 caliche profiles on various host strata alluvium limestone igneous rocks etc across approximately 900 km of texas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenic constituents these profiles exhibit significant differences in thickness varying from centimeters to meters and maturity varying from i to vi all of the different caliche facies are composed of low mg calcite biogenic features generally occur in the upper part of the profiles including the uppermost portion of massive caliche horizons platy horizons laminar crusts and pisoids the main biogenic caliche facies include rhizoliths calcified root structures stromatolite like laminar crusts and coated grains compared to the abiogenic massive micritic to microsparitic calcite groundmass biogenic constituents are morphologically distinct these biogenic constituents are composed of several microscopic mineral components including calcified filaments needle fiber calcite e g single crystalline needles and needle pairs triangular crystals and polycrystalline chains of rhombohedrons spherulites micro rods and nano spheres a large number of calcified root cellular structures and micro organisms e g fungal filaments actinomycetes and rod like bacteria are also present plant roots as well as soil biota produce distinctive structures and also enhance lithification by inducing calcite precipitation in the caliches i e biologically controlled or influenced processes host strata did not significantly influence the abundance nor type of biogenic features in the caliches in contrast climate had an evident effect on the development of biogenic constituents in these caliches in terms of the amount as well as type the thickness of laminar crusts and grain coatings and the abundance of biotic constituents within those facies decrease as the climate shifts from subhumid and subarid in southeast and central areas to subarid and arid in west and northwest texas in addition root structures and micro rods diminish significantly from the subhumid east to the arid west

can biological toxicity drive the contrasting behavior of platinum and gold in surface environments abstract in spite of the similar chemical properties of gold au and platinum pt au is highly mobile in surface environments whereas pt appears to be far less so in this study we assess if geomicrobial processes are likely to cause these differences as the mobility of pt and au should differ little based on thermodynamic solubility alone to achieve an accurate comparison it is important that both metals occur in the same environment mineral and groundwater samples were obtained from the fifield pt au field in new south wales australia where pt and au nuggets occur in a series of tertiary eluvial and alluvial paleo placers in particular we studied the m scale dispersion of au and pt within an extraordinary 10 mm sized fragment of ferruginous paleochannel material which contained abundant native au and isoferroplatinum grains gold grains displayed complex secondary morphologies indicative of biogeochemical transformations whereas isoferroplatinum grains appeared smooth and well rounded and showed no signs of supergene transformation gold grains were surrounded by a dusting of highly pure metallic au particles 10 m in diameter whereas no metallic pt particles were detected a search for ionic pt was also unsuccessful a series of biotic and abiotic incubation experiments was conducted to investigate the hypothesis that these differences in mobility are driven by interactions with microbiota biofilms consisting of metallophilic bacteria formed on ultraflat au surfaces and caused significant surface transformations in contrast only subtle changes were observed on pt surfaces incubated under similar conditions minimal inhibitory concentrations for au complexes are more than an order of magnitude lower than those measured for pt complexes in cupriavidus metallidurans cells this higher cell toxicity of mobile au compared to pt complexes can lead to toxic levels of mobile au in the vicinity of au grain surfaces the elevated toxicity drives the formation of au detoxifying biofilms that catalyze the biomineralization of spheroidal nano particulate and bacteriomorphic au in contrast this does not occur on isoferroplatinum grains due to the lower toxicity of pt in conclusion these results are consistent with microbial adaptation to element toxicity driving the cycling of precious metals in surface environments

a revised subduction inception model to explain the late cretaceous double vergent orogen in the precollisional western tethys evidence from the northern apennines the meso cenozoic alpine belts of the mediterranean area are characterized by complex architectures result of a complex subduction and collision evolution that preserve also a legacy of the rifting related configuration of the continental margins the northern apennines is a segment of these belts originated during closure of the ligure piemontese ocean and collision between the europe and adria plates the different configuration of the adria and europe margins inherited from asymmetric rifting is recorded in the ligurian units that preserve incorporation into the subduction factory of fragments of the oceanic domain internal ligurian units and portions of the ocean continent transition zone octz toward adria external ligurian units we provide here unpublished data on the stratigraphy and sedimentology of these units together with a review of what is already established in literature both datasets combined testify that at 80 m a an accretionary prism was growing between the deposition basins of the two groups of units and fed both basins with clasts from the ocean realm the continental crust and the subcontinental mantle we propose that closure of the ligure piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned adria margin and developed a double vergent prism by accreting oceanic material and continental extensional allochthons from the octz we believe the revised site of subduction initiation and the pre collisional architecture inherited from the rifting and spreading phases allow reconciling most of the discrepancies between the various interpretation proposed in literature for the pre collisional evolution of the apennines

the in vitro growth of gold grains gold colloids octahedral platelets and foils directly and indirectly formed from the reduction of soluble au i thiosulfate and au iii chloride complexes by iron oxidizing bacteria cyanobacteria and sulfate reducing bacteria were combined in an experimental system this system represented simplified biogeochemical conditions occurring within a fluvial environment in which placer au could occur in this study biofilm formation and physical aggregation i e sedimentation processes were critical for the accumulation of nanometer to micrometer sized au particles into grains 4 5 mm in size characterization of grain surface textures by scanning electron microscopy in association with monitoring soluble au concentrations over time suggested that dissolution and reprecipitation processes were occurring at the au grain fluid interface this laboratory model demonstrates that the biogeochemical cycling of au can contribute to the formation of anomalous enrichments such as placer au deposits

exhumation and reshaping of far travelled allochthonous tectonic units in mountain belts new insights for the relationships between shortening and coeval extension in the western northern apennines italy abstract coeval extensional and contractional tectonics are a common feature in orogenic belts especially in post collisional phases the attempt to constrain their relative timing and the responsible mechanisms led to several models taking into account different geodynamic contexts of various mountain chains in peri mediterranean collisional orogens extension and compression are commonly associated and coeval to the emplacement of far travelled allochthonous units therefore their internal deformation acquired during the translation across collisional belts is likely imprinted by the extensional and contractional tectonics affecting the orogen in the western portion of the northern apennines of italy one of the northernmost peri mediterranean tertiary orogens the far travelled allochthonous units i e the ligurian and subligurian units are widely exposed the present study integrates published and newly acquired surface geological data geological maps cross sections subsurface boreholes data and paleothermal vitrinite reflectance mixed layer illite smectite and kubler index and thermochronological data apatite fission tracks in order to investigate the deformation history and consequently the record of the extension compression relationships within these far travelled allochthonous units the study of modes and timing of reshaping of the far travelled allochthonous units allowed us to strengthen the hypothesis that extensional and contractional tectonics were coeval in the northern apennines and tightly connected by a cause effect relationship in particular the late orogenic low and high angle extensional tectonics represent the response to an overall contractional regime which induced episodes of gravitational instability in the orogenic wedge

from soft sediment deformation to fluid assisted faulting in the shallow part of a subduction megathrust analogue the sestola vidiciatico tectonic unit northern apennines italy

stable isotope study of karstic related dolomitization jurassic rocks from the coastal plain israel the middle upper jurassic section in israel contains scattered areas of dolomitization within predominantly limestone formations one of these dolomitization sites in the subsurface in the ashdod gan yavne area is associated with karst features as well as mississippi valley type mineralization saddle dolomite fluorite quartz bituminous clay calcite and hydrocarbons four distinct carbonate phases limestone coarsely crystalline brown host rock dolomite white sparry saddle dolomite and white vein calcite were analyzed for stable isotopes of carbon and oxygen these phases are believed to represent successive stages of diagenesis epigenesis of the ashdod gan yavne deposits water taken from fluid inclusions from saddle dolomite and calcite pairs was analyzed for stable isotopes of hydrogen as well in all but a few samples the carbon isotope ratio 1 or 1 per thousand was identical for all the carbonate phases studied the oxygen isotope ratio for the limestones in all but a few cases was much heavier 4 5 to 7 0 per thousand than that of the host rock dolomites 5 7 to 11 3 per thousand and is interpreted as reflecting dolomitization following limestone deposition the oxygen isotopes in the host rock dolomites show geographic as well as stratigraphic variations the host rock dolomites in the zohar formation in the ashdod area have characteristically less negative delta 18 o values 6 7 to 9 0 per thousand than the host rock dolomites in the shderot formations in the ashdod area and the dolomites from both the zohar and shderot formations in the ashqelon area delta 18 o 9 2 per thousand the oxygen isotope results can be interpreted as evidence for either two different events of dolomitization one regional and the other restricted to the ashdod area or a single event of dolomitization affecting two separate aquifers which differ in their oxygen isotope composition saddle dolomites which were earlier thought to be a separate phase of epigenesis representing the earliest stage of mineralization postdating dolomitization showed isotopic values identical to the host rock dolomites and showed the same formational and geographic variations as the host rock dolomites isotopic signatures of the vein calcites believed to be a much later stage of mineralization showed delta 18 o values of 5 6 to 10 8 per thousand similar to the trend expected for dolomite calcite pairs in the zohar formation and the opposite trend in the shderot formation a large range of delta d values 38 to 58 per thousand was measured from fluid inclusions from saddle dolomites and vein calcites this range is significantly different from the hydrogen isotope composition presently found in formation waters as well as local meteoric water

the thickness of subduction plate boundary faults from the seafloor into the seismogenic zone the thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior we have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges we describe a general geometric model for subduction thrust decollements which includes multiple simultaneously active anastomosing fault strands tens of meters thick the total thickness encompassing all simultaneously active strands increases to 100 350 m at 1 2 km below seafl oor and this thickness is maintained down to a depth of 15 km thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens ofmeters thick fault strands although fl attening primary inherited chaotic fabrics and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker to several kilometers this thickness does not describe the active fault at any depth these observations suggest that models should treat the subduction thrust plate boundary fault as 1 20 cm thick during earthquakes with a concentration of postseismic and interseismic creep in single to several strands 5 35 m thick with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands

ore petrography using megapixel x ray imaging rapid insights into element distribution and mobilization in complex pt and u ge cu ores geologic samples are extremely diverse and share a tendency for both heterogeneity and complexity this is especially true for ores which commonly result from a complex interplay of processes in highly dynamic environments in recent years a number of tools allowing the chemical mapping of major e g mineral liberation analysis mla minor e g electron microprobe epma and trace e g laser ablation inductively coupled plasma mass spectrometry la icp ms elements in geologic samples at 1 to 50 m resolution and over mm2 areas have seen rapid development and have become readily available to date the application of synchrotron based x ray fluorescence sxrf mapping has been limited to addressing key questions because of low availability and high cost this paper demonstrates how recent advances in x ray fluorescence detector technology are bringing new possibilities to ore petrology millisecond dwell times allow collection of thin section size maps at resolutions of a few m in hours while improvements in data analysis software simplify the production of quantitative elemental maps based on the imaging of six samples representative of different commodities pt u cu ge and different geologic contexts pge deposit sandstone hosted u deposit vein type polymetallic hydrothermal deposit iron oxide copper gold iocg deposit we demonstrate that megapixel sxrf msxrf can efficiently provide the information necessary to understand metal speciation in the context of thin section scale textural complexity image analysis revealed a number of new results for the studied deposits for example 1 the distribution of micrometer sized pt rich grains and ti mobility during the formation of schistosity at the fifield point prospect new south wales australia 2 the presence of ge contained in organic matter and of hg minerals associated within quartzite clasts in the lake frome u ores south australia 3 confirmation of the two stage ge enrichment in the barrigao deposit with demonstration of the presence of ge in solid solution in the early chalcopyrite portuguese iberian pyrite belt and 4 the enrichment of u during late dissolution reprecipitation reactions in the bornite ores of the moonta and wallaroo iocg deposits south australia these results illustrate that msxrf is a powerful technique for locating nano to microparticles of precious metals pt and trace contaminants e g hg that form distinct micro minerals in addition it is a powerful tool for understanding commodities with relatively low ore grades and complex distribution 100 1 000 ppm e g u ge

thermal models of the middle america trench at the nicoya peninsula costa rica 1 we present thermal models of the 24 ma cocos plate subducting under the caribbean plate at the nicoya peninsula costa rica our modeling incorporates the effect of hydrothermal cooling of the seafloor and the absence of an accretionary prism initial geotherms of the incoming plate representing different effective cooling depths of hydrothermal circulation affect the thermal structure and temperatures along the main subduction thrust excluding the deforming prism results in a higher heat flow immediately landward of the trench than previous models and therefore significant frictional heating is not required conductive plate cooling over predicts heat flow across the frontal forearc with no or very low frictional heating models with hydrothermal cooling to depths of 2 km under predicts the heat flow comparisons with recently determined hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100 150 c but there are large along strike variations in both the seismicity and thermal regime

from magmatic generation to uhp metamorphic overprint and subsequent exhumation a rapid cycle of plate movement recorded by the supra subduction zone ophiolite from the north qaidam orogen abstract the rapid subduction and exhumation of high pressure hp and ultrahigh pressure uhp rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments herein ophiolitic fragments eclogites and amphibolites from a subduction channel exposed in the dulan terrane north qaidam orogen were investigated garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation new uranium lead u pb zircon ages of the current study combined with previous rutile u pb ages from eclogites indicate a rapid cycle of events starting with the generation of the magmatic protolith at ca 446 ma its subsequent subduction reaching a depth of ca 100 110 km at ca 435 ma and its exhumation into shallow portions of the crust which occurred at ca 416 ma thus a rapid 30 million years myr protolith generation subduction exhumation cycle could be recorded according to an ophiolitic rock association with island arc basalt iab like and mid ocean ridge basalt morb to ocean island basalt oib like trace element compositions the slab derived components determined by sr nd hf isotopes and the short time span of only about 11 myr between photolith generation and uhp metamorphism in eclogites the protolith of eclogite is best explained as a supra subduction zone ophiolite this finding likely records the closure of the south qilian ocean and the initiation of a continent arc collision at ca 446 ma furthermore starting with subduction erosion during continent arc collision in the early palaeozoic ophiolitic and arc related materials become subducted in the north qaidam it was inferred that such a rapid succession of events from protolith formation to uhp overprint and final exhumation could be explained by movements of rock material in a subduction channel thus this study provides a new view in understanding the mechanics of different episodes of subduction systems in north qaidam uhp metamorphic belt

material transfer and subduction channel segmentation at erosive continental margins insights from scaled analogue experiments abstract whereas tectonically erosive convergent margins make up nearly 60 of all present day convergent margins processes and amounts of material transfer and recycling remain enigmatic removal of material from the frontal forearc leaves no features which could be imaged seismically or probed through drilling only a few scaled laboratory analogue experiments have analyzed material transfer at convergent margins characterized by long term subduction erosion and none has focused on processes and amounts of material transfer and recycling therefore in this study we attempt to identify potentially relevant parameters controlling material transfer in brittle tectonically erosive forearcs this is addressed by a series of sandbox experiments performed with granular materials we here mainly focus on the amount of sediment that can be transported downdip through the subduction channel sc we analyzed our experiments with regard to the evolution of internal and basal material transfer material transfer mode patterns wedge geometry and sc evolution to achieve a truly erosive scenario we built a large initial wedge representing the brittle forearc featuring a moderately high friction basal detachment and composed of strong granular material there was no incoming material the slope of the wedge initially had a critical taper through opening a subduction gap sg the wedge was subject to rear material loss our observations point to a close interaction between sc segmentation and wedge deformation basal erosion mainly took place beneath the middle upper slope our experiments confirmed the strong influence of the sc on the wedge evolution a larger amount of material loss led to the development of a sc with an inhomogeneous distribution of velocities along the base of the wedge segmenting the sc and slope geometry the latter was comparable to observations in natural forearcs which are segmented into lower middle and upper slopes

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

frictional strength and healing behavior of phyllosilicate rich faults 1 we study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals we report laboratory and microstructural work from fault rocks associated with a regional low angle normal fault in central italy experiments were conducted in a biaxial deformation apparatus at room temperature and humidity nominally dry under constant normal stresses of 20 and 50 mpa and at a sliding velocity of 10 mm s our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure the phyllosilicate contents of our samples which include clay talc and chlorite range from 0 to 52 weight we study both intact rock samples sheared in their in situ geometry and powders made from the same rocks to address the role of fabric in fault healing we measured frictional healing dm using slide hold slide tests with hold periods ranging from 3 to 3000 s phyllosilicate free materials show friction values of m 0 6 and healing rates that are larger in powdered samples b 0 006 dm per decade in time s compared to intact wafers of fault rock b 0 004 for phyllosilicate bearing materials healing rates are low b 0 002 and independent of fabric phyllosilicate content and normal stress we observe that frictional strength decreases systematically with increasing phyllosilicate content intact phyllosilicate bearing fault rock is consistently weaker than its powdered equivalent 0 2 m 0 3 versus 0 4 m 0 5 respectively we compare our data to results from experiments conducted on a wide range of materials and conditions deformation microstructures show localized slipping along sub parallel shear planes we suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate bearing faults and promote stable aseismic creep

mobility and microbially mediated mobilization of gold and arsenic in soils from two gold mines in semi arid and tropical australia the mobility and microbially mediated solubilization of au and as in regolith materials from two au mines in australia i e the peak hill gold mine in semi arid new south wales and the hit or miss gold mine in tropical northern queensland was studied using a combination of geochemical and microbiological techniques gold is highly mobile in both environments the mobility of au increases with increasing degree of weathering of host materials and the resident microbiota are capable of mediating its solubilization the results of the microcosm experiments demonstrate that the activity of microorganisms needs to be taken into account when studying the mobility and solubilization of au in the australian regolith in primary unweathered mineralization material from the hit or miss mine 99 wt of au was extracted only in the strongest final step of the sequential extractions in concentrated aqua regia in alteration zone material from the peak hill gold mine 80 wt of au was associated with the operationally defined mn and fe oxides in contrast in auriferous soils overlying mineralization at both sites 90 95 wt of au was associated with the operationally defined exchangeable claybound and organic fractions microcosm experiments were incubated biologically active and inactive sterilized in 1 4 w v aqueous slurries at 25 c in the dark for up to 95 days in biologically active microcosms with soils from the peak hill and the hit or miss gold mines approximately 55 wt 907 ng g 1 d w soil and 20 wt 233 ng g 1 d w soil of the total au respectively was solubilized during the incubation in contrast no or significantly lower au concentrations were observed in biologically inactive microcosms the mobility and microbially mediated release of as was limited at both sites and appears to be mostly controlled by abiotic adsorption and desorption on mn and fe oxides arsenic has a low solubility in the more mobile fractions and is mostly associated with mn and fe oxides and the residual fraction the release of as was not elevated in biologically active compared to inactive microcosms from the peak hill gold mine in contrast in biologically active microcosms with samples from the hit or miss mine elevated concentrations of as were detected in solution compared to the biologically inactive experiments crown copyright 2006 published by elsevier inc all rights reserved

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

petrologic analysis and geochemistry of the late neogene early quaternary hardpan calcretes of western rajasthan india abstract calcretes formed over bedrock such as precambrian hornblende chlorite schist rhyolite and carbonaceous phyllites in western rajasthan india were studied for their mineralogical composition and geochemistry they form a distinct element of the quaternary landscape of western rajasthan india the study area is located within the nagaur churu jaipur tract of western rajasthan the purpose of this study was to determine the processes governing the development of hardpan calcretes and to evaluate the local and regional controls on their formation micromorphology included pedogenetic and groundwater features within the hardpan calcretes thickening of calcite laminae downward and tapering at the sideward edges around the unweathered minerals of quartz and feldspars indicated cumulative and compound pedogenesis which probably occurred locally and downward movement of carbonate solution and pore water carbonate solutions were probably derived from the upper horizons or surfaces bringing about the process of dissolution and recementation of individual laminae occurrence of fibrous palygorskite as coatings around the detrital grains and siderite oolitic and pisolitic in shape points to a subalkaline subacidic process of pedogenesis in a semi arid to arid climate stable 13c and 18o isotope data of the hardpan calcrete laminae vary between 0 3 to 1 5 and 5 9 and 1 5 respectively indicating their formation at or near surface capillary fringe probably supporting a thin column of soil the source of most of the calcite is groundwater however calcite nodule formation was largely dependent on pedogenic processes associated with evaporation evapotranspiration and or microenvironmental changes in ph and co2 partial pressure dust is also a major source for carbonate precipitation although it is commonly assumed that the powdery calcretes are younger in age compared to the more complex forms the occurrence of middle palaeolithic tools below the hardpan calcrete at roopangarh and dayalpura and above the hardpan calcretes at mitri genana and rol indicates that the morphology of calcretes is not a reliable indicator of age

dolomitization of permian reef carbonates in a jurassic subduction complex central japan abstract a 100 m thick sequence of dolomite occurs within reef carbonates of the permian nabeyama formation central japan analyses of sediments fossils minerals and geochemical ingredients of a dolomitized sequence reveal that dolomitization was the consequence of active seawater circulation through the carbonate column stable carbon and oxygen isotopic data precluding dolomitizing fluids of freshwater or hybridized seawater origin provide arguments for atoll dolomitization by seawater thermal gradients between the atoll and the underlying volcanic structure caused the convection flow of interstitial water upwelling oceanic water supersaturated with dolomite but undersaturated with respect to calcite was subjected to thermal convection through the porous and permeable sediment column leading to massive dolomitization of the oceanic atoll

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

fluid history related to the early eocene middle miocene convergent system of the northern apennines italy constraints from structural and isotopic studies 1 the late eocene middle miocene erosive plate boundary between the european and the adriatic plates is exhumed in the northern apennines of italy the fossil fault zone is 500 m thick and the outcropping portion exposes the first 5 km of its depth at this plate boundary basal and frontal tectonic erosion incorporated unlithified fluid rich sediments into the fault zone the deformation and nature of the material along the plate boundary define a fossil subduction channel here we couple a detailed structural analysis of the apennine subduction channel focusing in particular on calcite veins with a stable isotope analysis to characterize the fluid regime along an active subduction channel the 13c and 18o composition of calcite vein and host rock samples within the fault zone indicates that there is a deep metamorphic source of fluids migrating upward along the subduction channel in addition to locally derived fluid components dewatering of subducting turbidites contributes significantly only in the shallowest part of the channel structural observations indicate fluid flow along and across the subduction channel at deep levels fluid flow is associated with discrete deformation events on shear faults offset by dilational jogs filled with implosion breccias at intermediate levels deformation is still cyclic and associated with repeated crack and seal events at the shallowest levels deformation occurred while portions of the subducting material were still unlithified here the deformation was quasicontinuous without associated vein development both isotope and structural analyses indicate that this erosive subduction channel behaved as a weak fault with a vertical maximum principal stress

interplate patchiness and subduction erosion mechanisms evidence from depth migrated seismic images at the central ecuador convergent margin prestack depth migrated seismic lines provide accurate images of the convergent ecuadorian margin where the southern flank of the carnegie ridge subducts the margin is fronted by a 5 7 km wide compressional sediment prism beneath the slope margin thinning is accomplished by gradual thinning of the lower part of the basement accompanied by pervasive seaward dipping normal faulting indicating basal subduction erosion at a low friction plate interface along the plate boundary a thin subduction channel locally thickens to form low velocity 600 m thick lenses of underthrusted fluid rich sediment supplied by slope mass wasting the contrast between the fluid rich sediment and the surrounding thinner and drier sections of the subduction channel creates a three dimensional patchiness across the plate boundary implying variations in mechanical interplate coupling the subduction channel patchiness modulates subduction erosion processes by alternately favoring margin basement weakening and material removal basement weakening would occur 1 at the base of the upper plate where enhanced reflectivity indicates basement damage by overpressured fluids from the subduction channel and 2 at the basement apex where basement breakup is caused by superposition of compressional and extensional fault systems resulting from time variable interplate mechanical coupling the detachment of rock debris in the higher coupled sections and the subsequent dragging of the fragments into the subduction channel cause tectonic erosion deeper in the subduction the subduction channel patchiness may influence processes like earthquake nucleation and rupture propagation and material recycling in the mantle

calcium in regolith carbonates of central and southern australia its source and implications for the global carbon cycle abstract the widespread regolith carbonates of the australian continent are a potential sink for co 2 we have used sr isotopes to investigate the source of the ca in regolith carbonates that cover approximately 1 6 10 6 km 2 of inland australia 87 sr 86 sr ratios for nearly all the carbonates were in the range 0 7094 to 0 7211 the results show that only about 10 of the ca in regolith carbonates is derived from weathered bedrock with the remaining component being derived from an external marine source the exceptions are the most northerly samples with a range of 0 7270 to 0 7374 in which bedrock content may be as high as 30 the likely ca source areas are sedimentary carbonates located on the continental shelf and calcareous aeolianites that surround much of the southern and western australian coastline winds passing over these carbonates continually rework and transport the material over the australian continent where it has settled and formed the regolith carbonates we see today the impact of this process is that despite the immense area covered australian regolith carbonates do not capture any additional co 2 instead the carbonate is simply being remobilised from one pool marine to another terrestrial

subduction erosion crustal structure and an evolutionary model of the northern yap subduction zone new observations from the latest geophysical survey the yap subduction zone is a distinctive erosive margin with an extremely slow convergence rate the high relief of the subducting plate generated by horst and graben structures and seamounts leads to attenuation of the crust in this study we present the latest geophysical data collected by the chinese research vessel kexue to investigate subduction erosion at the yap subduction zone and develop subduction models for yap subduction zone structures we show an anomalous distance between the yap trench and the adjacent volcanic arc the steep slope of the trench arcward a frontal prism and rare sediment in the trench all typical features of erosive margins we propose that the high relief subducting plate has led to erosion of the overlying plate and that different subduction processes controlled locally by the topography of the subducting plate have modified the yap subduction zone numerous normal faults increase the relief of the seafloor entering the trench and an anomalously large slope angle along the trench reflects uplift of the fore arc high via seamount subduction in addition the thin crust of the subducted horst and graben structures along with the normal faults might have eroded the fore arc crust and subsequently eroded the yap arc crust during subduction these subduction erosional processes at the yap trench provide one of the best cases of an erosive margin in the world particularly for a subduction zone with an extremely slow convergence rate

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

a new source of water in seismogenic subduction zones 1 seismogenic plate boundary faults at accretionary margins e g the nankai margin southwest japan may occur where the uppermost part of subducting oceanic crust composed of basaltic rocks is in contact with the overriding plate of a lithified accretionary prism the plate boundary faults in ancient accretionary complexes typically record high velocity slip under fluid rich conditions although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip weakening the source of fluid in seismogenic subduction zones remains poorly constrained in this work we focus on the hydrous smectite in the uppermost oceanic crust an alteration product of intact basalt before arrival at the trench axis a comparison between 1 new mineralogical data on basalt drillcore recovered by integrated ocean drilling program iodp expedition 322 at site c0012 a reference site for subduction input to the nankai trough and 2 mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the shimanto belt southwest japan suggests that progressive smectite chlorite conversion would liberate bound fluids at a rate of 0 34 to 0 65 10 14 s 1 along the plate interface this rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources including from overlying sediments via smectite illite conversion and the expulsion of pore fluids and may facilitate seismic slip along plate boundary faults

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

quantifying arc migration and the role of forearc subduction erosion in the central aleutians abstract the active aleutian arc is currently classified as an accreting margin as most of the arc has a well developed frontal accretionary prism that formed in the plio pleistocene we focus on the central aleutian island arc where the arc massif was built to near its current size by the late eocene and has since seen limited growth and endured episodic erosion prior to the development of the accretionary prism as a result the volcanic front has migrated northwards tens of kilometers since inception along with previous mapping dating and geochemical data sixteen new 40ar 39ar dates and new geochemical data for volcanic rocks on several central aleutian islands supplement recent geochronologic data from six other islands in this region and facilitate an attempt to understand how and where aleutian arc volcanism has migrated with time long term average arc migration rates range from 0 6 to 5 km ma in three distinct locations the arc migration rate increases to the west and is greatest over the last five million years as the arc migrated from a 7 5 ma volcanic front to its current location arc migration and erosion during this most recent period may have been expedited by either the subduction of the kula ridge which ceased spreading in the early eocene or clockwise rotation of the pacific plate in the late miocene some central aleutian lavas and plutonic rocks have elevated la yb sm yb and sr yb most of which were emplaced during times of arc migration or in the waning stages of a peak in magmatic activity the strongly fractionated trace element patterns are similar to the basaltic andesites to dacites from the western aleutian seafloor which are interpreted to be the result of residual garnet during partial melting of eclogite in the subducting oceanic crust however the existing subduction parameters coupled with the timing of the central aleutian adakite like lavas favors a model in which significant subduction erosion of the mafic forearc including erosion and subsequent melting of the trapped kula plate i e morb like oceanic crust as well as early aleutian arc basalts in the mid crust may be partly responsible for the eclogitic component observed in central aleutian magmas

microseismicity at the seaward updip limit of the western nankai trough seismogenic zone 1 the seismogenic zone at subduction zones does not normally extend to the trench axis the shallowest part of the plate interface is considered to be seismically decoupled the location of the seaward updip limit of the seismogenic zone and its relation to the crustal structure are important for understanding the transition process from aseismic slip to the seismic rupture at the plate interface the nankai trough seismogenic zone southwestern japan is one of the most well studied subduction seismogenic zones in the world however the offshore seismicity around the nankai trough is very low and hypocenters are not determined accurately by the on land seismic network we performed microseismicity observations using ocean bottom seismographs off cape muroto hypocenters were determined by using a three dimensional vp and vs structure based on seismic survey results the observed microseismicity seems to be classifiable into two types the first is earthquakes that occur around the plate interface the second is a group of earthquakes that occur in the uppermost mantle of the subducting oceanic plate the seismicity around the plate interface forms several clusters the seaward limit of the clusters is characterized by earthquakes with a similar waveform these similar waveform earthquakes are considered to occur at small asperities in the aseismic seismogenic transition zone of the plate interface the seismicity in the uppermost mantle may be related to dehydration embrittlement of subducting serpentinized mantle

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

thickening of fault zones a mechanism of melange formation in accreting sediments sediments accreted at subduction zones undergo stratal disruption and form a type of melange the thickness of the disrupted zones grows with progressive deformation this suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment factors causing the abandonment of fault surfaces during continuing deformation include 1 strengthening owing to porosity loss during consolidation 2 localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits and 3 reorientation of fault surfaces the disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass

origin of dolomitization on the barbwire terrace canning basin western australia a thick areally extensive subsurface sequence of upper devonian carbonates occurs on the barbwire terrace in the canning basin of western australia it is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized slightly deeper water mark have remained as limestones the major regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite up to 600 m with no primary calcite a small volume of evaporitic supratidal dolomite is present at one location this dolomite is derived from highly saline fluids developed in an arid supratidal environment replacement dolomite of the regional dolomite type has a xenotopic form with undulose extinction and irregular crystal boundaries in addition saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite this suggests the regional dolomite type was precipitated at slightly elevated temperatures dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution geochemically the synsedimentary supratidal and regional dolomite types are quite distinctive supratidal dolomites have 6 o values which are significantly higher 6180 2 to i pdb than the regional dolomite type 6l80 9 to 2 pdb assuming the lowest 6 o values for the sabkha dolomite represent replacement in marine waters the oxygen isotopic composition for upper devonian canning basin marine dolomite would be around 6l80 2 pdb the petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin however sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the barbwire terrace therefore it is suggested that dolomitization may have taken place in a near surface environment with a major recrystallization event superimposed during burial diagenesis

reflux stratabound dolostone and hydrothermal volcanism associated dolostone a two stage dolomitization model jurassic lebanon the kesrouane formation which is characterized by pervasive dolomitization has a stratigraphic thickness that exceeds 1000 m it is part of a broad carbonate platform deposited in the levant region and represents 60 of the lebanese jurassic rocks two genetically distinct dolostones are recognized within this unit 1 fine to medium crystalline non planar grey dolostone and 2 coarse crystalline planar beige dolostone the former is stratabound and of early jurassic age 87 sr 86 sr 0ae707455 this dolostone locally exhibits pseudomorphs of evaporite nodules pointing towards seepage reflux dolomitization by hypersaline to marine related fluids exposures of the coarse crystalline dolostone are associated with regional pre cretaceous faults along which late jurassic volcanics also occur sedimentological and diagenetic considerations coupled with microthermometry support a hydrothermal origin for this dolostone with th values of primary inclusions between 50 and 80 c the related dolomitizing fluids are mesosaline 3ae5 12ae0 eq wt nacl and are believed to result from the mixing of evaporative brines and sea water dolomitization is thus believed to have occurred in two stages whereby fluids invaded the host rocks first by seepage reflux explaining the resulting early jurassic stratabound dolostone and later through fracture flow along the faults associated with the late jurassic volcanism explaining the coarse crystalline hydrothemal dolostone

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

on the tectonic evolution of the ligurian accretionary complex in southern italy unraveling the deformation pattern characterizing the transition from final oceanic subduction stages to early stages of deformation of a foreland continental margin is crucial for a better understanding of the geodynamic processes taking place at convergent plate boundaries in particular the combined role of internal wedge dynamics and continental margin architecture in controlling the tectonic evolution of an accretionary complex during its final emplacement onto the foreland continent is discussed in this study to this purpose we conducted integrated structural stratigraphic and low temperature thermochronometric analyses on the ligurian accretionary complex units exposed in the campania region italy and on continental margin successions located in their footwall as well as on related foredeep and wedge top basin deposits our results point out a series of late early miocene burdigalian shortening events also involving buttressing of the accretionary wedge against the crustal ramp of the foreland continental margin emplacement of the overthickened accretionary complex onto the distal part of the continental margin was followed by horizontal extension and wedge thinning aiding the development of wedge top depocenters extension may have been either related to reduced subduction rates during the middle miocene or to a period of subduction erosion known to have occurred in the northern apennines in the same time frame early miocene nw se shortening recorded by ligurian accretionary complex units was completely unrelated to later late miocene to pleistocene ne directed thrusting in the apennines which was coeval with backarc extension in the tyrrhenian sea therefore our results emphasize the occurrence of a major discontinuity in the neogene geodynamic evolution of the southern apennines the tectonic history of which may be clearly subdivided from a kinematic point of view into pre and syn tyrrhenian backarc extension stages

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

correlating shelf carbonate evolutive phases with fluid expulsion episodes in the foredeep miocene northern apennines italy abstract in the miocene of the northern apennines of italy evolutionary phases of shallow water carbonates in wedge top basins are coeval with methane rich fluid expulsion episodes in the inner foredeep we identified four main correlative events primarily controlled by synsedimentary tectonics sediment input and climate cooling 1 a burdigalian tectonic phase determined the onset of shelfal sedimentation in wedge top areas and is correlated with an important episode of fluid expulsion in marly sediments of the inner foredeep croce di moggiona seep carbonates 2 the decrease of shelfal carbonate production and the increase of detrital input in the early langhian correspond with a second important episode of seep carbonate precipitation in the inner foredeep castagno d andrea 3 the demise of the carbonate shelf and the coeval expulsion of methane rich fluids in the inner foredeep vicchio and m citerna seep carbonates at the langhian serravallian boundary approximate the mi3b cooling event the lower temperatures associated with increased siliciclastic weathering led to the crisis of shallow water carbonate production whereas the pressure drop due to the eustatic fall may have triggered the gas charged fluid expulsion 4 the tectonic related uplift of frontal sectors of the wedge top basin led to the wide erosion of the burdigalian langhian carbonates and caused the precipitation of authigenic seep carbonates deruta seep both in the slope and the adjacent foredeep

shocks in a box an analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution 1 we introduce and test an experimental approach to simulate elastoplastic megathrust earthquake cycles using an analogue model and apply it to study the seismotectonic evolution of subduction zones the quasi two dimensional analogue model features rate and state dependent elastic frictional plastic and viscoelastic material properties and is scaled for gravity inertia elasticity friction and viscosity the experiments are monitored with a high resolution strain analysis tool based on digital image correlation particle imaging velocimetry piv providing deformation time series comparable to seismologic geodetic and geologic observations in order to separate elastic and nonelastic effects inherent the experimental deformation patterns we integrate elastic dislocation modeling edm into a hybrid approach we use the analogue earthquake slip and interseismic locking distribution as edm dislocation input and forward model the coseismic and interseismic elastic response the residual which remains when the edm prediction is subtracted from the experimental deformation pattern highlights the accumulation of permanent deformation in the model the setup generates analogue earthquake sequences with realistic source mechanisms and elastic forearc response and recurrence patterns and reproduces principal earthquake scaling relations by applying the model to an accretionary type plate margin we demonstrate how strain localization at the rupture peripheries may lead to a seismotectonically segmented forearc including a tectonically stable shelf and coastal high 20 plate convergence accommodated by internal shortening overlying the area of large megathrust earthquake slip fifty to 75 of plate convergence is accommodated by internal shortening in the slope region where earthquake slip tapers out toward the trench the inner forearc region remains undeformed and represents a basin

the source of pedogenic carbonate associated with gold calcrete anomalies in the western gawler craton south australia abstract elevated au concentrations above bedrock mineralization are commonly closely associated with calcrete in arid residual and semi residual soils in australia the origin of australian calcrete has been argued for many years but there have been very few published studies calcrete from au deposits and prospects and elsewhere in the gawler craton south australia were studied for their geochemical composition and sr and c isotope ratios by comparing carbonate 87 sr 86 sr ratios with underlying weathered rock and bedrock it is demonstrated that many samples have overwhelmingly atmospheric sr 94 and by inference ca 98 rather than a local bedrock or soil mineral origin the isotopic composition of calcretes at the challenger gold deposit lie on a mixing trend of decreasingly marine sr from the nullarbor plain to tarcoola 300 km to the east the 13 c pdb values 7 5 to 0 1 suggest that the carbonate c in calcrete has a mixed origin derived from c3 and c4 plants a model is presented to explain how the au ca association may form in calcrete at challenger based on these new sr and c isotope data and in which the role of plants and atmospherically deposited sr and ca combine with pre existing au in the soil the implication for mineral exploration is that since most of the ca is derived from marine sources it is not a pre requisite that autochthonous soil minerals derived from underlying weathered bedrock supply ca for calcrete to form and for the au ca association to occur thus the independence of the ca with respect to underlying lithology reaffirms the usefulness of calcrete as a sampling medium for au exploration

geology of the seismogenic subduction thrust interface a microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces the exhumed chrystalls beach complex in the otago schist new zealand may be analogous to an actively deforming underthrust rock assemblage the complex contains asymmetric competent lenses of sandstone chert and basalt enclosed in a cleaved mudstone matrix continuous fabrics such as folds boudins and asymmetric phacoids formed by distributed cataclasis and dissolution precipitation creep discontinuous deformation is evident in an extensive fault fracture mesh involving mutually cross cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces the hikurangi margin provides an example of along strike variations in seismic style possibly related to heterogeneous fluid pressure state and interface geology in both the ancient and active subduction related shear zone fluid pressure state appears to be a critical control on frictional failure which primarily occurs on weak fluid overpressured discontinuities continuous aseismic deformation occurs where other mineral deformation mechanisms such as dissolution precipitation creep are preferred the geometry and composition of the underthrust rock assemblage appear to be first order controls on megathrust fluid pressure distribution bulk rheology and dominant deformation mechanism and thus may be significant controls on megathrust seismic style

bacterially induced mineralization of calcium carbonate in terrestrial environments the role of exopolysaccharides and amino acids this study stresses the role of specific bacterial outer structures such as glycocalix and parietal polymers on calcium carbonate crystallization in terrestrial environments the aim is to compare calcium carbonate crystals obtained in bacterial cultures with those obtained during abiotically mediated synthesis to show implications of exopolysaccharides and amino acids in the mineralogy and morphology of calcium carbonate crystals produced by living bacteria this is done using various amounts of purified exopolysaccharide xanthan eps and l amino acids with a range of acidities amino acids and increasing xanthan content enhance sphere formation in calcite and vaterite regarding calcite the morphology of crystals evolves from rhombohedral to needle shape this evolution is characterized by stretching along the c axis as the amino acid changes from glutamine to aspartic acid and as the medium is progressively enriched in eps regarding vaterite the spherulitic habit is preserved throughout the morphological sequence and starts with spheres formed by the agglomeration of short needles which are produced in a xanthan free medium with glutamine monocrystals forming spheres increase in size as xanthan is added and the acidity of amino acids glutamic and aspartic acids is increased at high xanthan concentrations amino acids and mainly aspartic and glutamic acids induce vaterite precipitation the role of the carboxyl group is also probably critical because bacterial outer structures associated with peptidoglycan commonly contain carboxyl groups this role combined with the results presented here clearly demonstrate the influence of bacterial outer structure composition on the morphology and mineralogy of bacterially induced calcium carbonate this point should not be neglected in the interpretation of calcite cements and carbonate accumulations in terrestrial environments

development of arc continent collision melanges linking onshore geological and offshore geophysical observations of the pliocene lichi melange southern taiwan and northern luzon arc western pacific abstract although the lichi melange has been studied intensively over the last decade its enigmatic nature has generated debates regarding its origin and evolution two prominent models have been proposed to explain the lichi melange the olistostrome model with slumping sediments and the tectonic collision model with intensive shearing neither model can explain what causes the interpreted slumping and complex faulting processes for the sediments here we study the lichi melange using a time space equivalence approach in which the tectonics of the offshore continuation of the lichi melange should represent an earlier stage in its evolution our study of marine multichannel seismic data suggested that the backthrusts in the accretionary prism propagate arcward above and within the deforming forearc and arc basement to incorporate the sediment and basement materials into the rear of the accretionary prism therefore we proposed the retrowedge evolution model whose novel key feature is that the irregular topography of the arc basement affects the taper angle of the retrowedge the retrowedge has a greater taper than the minimum taper of the prowedge so the slope can be steeper and thus favors gravitational failure such processes would generate complex faulting and slumping processes in the backthrusted forearc ridge and basin in sum this retrowedge model reconciles the mixture of slumping and faulting processes found in the lichi melange

ca carbonates precipitation and limestone genesis the microbiogeologist point of view abstract experiments show that the production of carbonate particles by heterotrophic bacteria follows different ways in heterotrophy the passive carbonatogenesis is generated by modifications of the medium that lead to the accumulation of carbonate and bicarbonate ions and to the precipitation of solid particles it is induced by several metabolic pathways of the nitrogen cycle ammonification of amino acids degradation of urea and uric acid dissimilatory reduction of nitrates and of the sulphur cycle dissimilatory reduction of sulphates the active carbonatogenesis is independent of the mentioned metabolic pathways the carbonate particles are produced by ionic exchanges through the cell membrane following still poorly known mechanisms in autotrophy non methylotrophic methanogenesis and cyanobacterial photosynthesis also may contribute to the precipitation of carbonates autotrophic carbonates as carbonatogenesis is neither restricted to particular taxonomic groups of bacteria nor to specific environments it has been an ubiquitous phenomenon since precambrian times carbonatogenesis is the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter after a phase of latency there is an exponential increase of bacterial numbers together with the accumulation of metabolic end products these induce a ph increase and an accumulation of carbonate and hydrogenocarbonate ions in the medium this phase ends into a steady state when most part of the initial enrichment is consumed and there is a balance between death and growth in bacterial populations particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state the active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth of initially produced particles in eutrophic conditions the first solid products are patches that appear on the surface of the bacterial bodies and coalesce until forming a rigid coating and or particles excreted from the cell all these tiny particles assemble into biomineral aggregates which often display precrystalline structures these aggregates grow and form biocrystalline build ups which progressively display more crystalline structures with growth in oligotrophic conditions the primary solid products are rapidly smoothed in the crystalline structure and leave no trace in present aqueous environments apart from deep ocean the potential efficiency of heterotrophic bacterial carbonatogenesis in ca carbonate sedimentation is much higher than autotrophic or abiotic processes it much more likely accounts for extensive apparently abiotic limestone formation than any of the latter as far as biodetrital particles are concerned it may be observed that the shells and tests of organisms are built from the activity of cellular organites which are nowadays considered by a number of biologists as endosymbiotic bacteria thus apart from probably mythical purely evaporitic and autotrophic ones most limestones must be considered as principally of heterotrophic bacterial origin as the carbon of limestones is issued from organic matter bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the earth

a possible transoceanic tsunami directed toward the u s west coast from the semidi segment alaska convergent margin the semidi segment of the alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby unimak segment in 1946 reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami a 200 km long ridge or escarpment with crests 1 km high is the surface expression of an active out of sequence fault zone recently referred to as a splay fault such faults are potentially tsunamigenic this type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone the greater slip may derive from quaternary subduction of the patton murray hot spot ridge that extends 200 km toward the east across the north pacific estimates of tsunami repeat times from paleotsunami studies indicate that the semidi segment could be near the end of its current inter seismic cycle gps records from chirikof island at the shelf edge indicate 90 locking of plate interface faults an earthquake in the shallow semidi subduction zone could generate a tsunami that will inundate the us west coast more than the 1946 and 1964 earthquakes because the semidi continental slope azimuth directs a tsunami southeastward this article is protected by copyright all rights reserved

finding a realistic velocity distribution based on iterating forward modelling and tomographic inversion tomography is like a photograph that was taken by a camera with blurred and defective lenses that deform the shapes and colours of objects reporting quantitative parameters derived from tomographic inversion is not always adequate because tomographic results are often strongly biased to quantify the results of tomographic inversion we propose a forward modelling and tomographic inversion fm ti approach that aims to find a more realistic solution than conventional tomographic inversion the fm ti scheme is based on the assumption that if two tomograms derived from the inversion of observed and synthetic data are identical the synthetic structure may appear to be closer to the real unknown structure in the ground than the inversion result however the manual design of the synthetic velocity distribution is usually time consuming and ambiguous in this study we propose an approach that automatically searches for a probabilistic model in this approach a synthetic model is iteratively updated while taking into account the bias of the model in previous stages of the fm ti performance here we present an example of synthetic modelling and real data processing for an active source refraction data set corresponding to a marine profile across the subduction zone in chile at about 32 s latitude a key feature of the model is a low velocity channel above the subducted oceanic crust which was defined in the synthetic model and expected in the real case the conventional first arrival traveltime tomography was barely able to resolve this channel however after several iterations of the fm ti modelling we succeeded in reconstructing this channel clearly in the paper we briefly discuss the nature of this low velocity subduction channel and we compare the results with other studies

tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion the case of the ligurian wedge of the northern apennines italy subduction can be associated either with accretion or with removal of material from the overriding plate these two processes can either coexist or alternate in time along the same margin their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out of sequence thrusts normal and strike slip faults or large submarine landslides in the frontal part of the subduction zone in this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern apennines ligurian subduction complex new structural data suggest that in the early neogene aquitanian the removal and underthrusting of the toe of the wedge formed by both the accreted oceanic sediments and the overlying wedge top basin fill i e the subligurian units implied a process of frontal tectonic erosion the presence on top of the subduction complex of a complete succession of mid late eocene to late miocene slope apron sediments i e the epiligurian succession facilitates a reconstruction of the sedimentary response to this event in the aquitanian large areas of the wedge were denuded from the lower slope sedimentary cover through extensive gravitational mass movements the subsequent deposition of a thick body of submarine debris flows is documented the mass wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the ligurian subduction complex formed by the subligurian units

nanoparticle factories biofilms hold the key to gold dispersion and nugget formation biofilms living on gold au grains play a key role in the biogeochemical cycle of au by promoting the dispersion of au via the formation of au nanoparticles as well as the formation of secondary biomorphic au gold grains from queensland australia are covered by a polymorphic organic inorganic layer that is up to 40 m thick it consists of a bacterial biofilm containing au nanoparticles associated with extracellular polymeric substances as well as bacterioform au focused ion beam fib sectioning through the biofilm revealed that aggregates of nanoparticulate au line open spaces beneath the active biofilm layer these aggregates bacterioform au type 1 resulted from the reprecipitation of dissolved au and their internal growth structures provide direct evidence for coarsening of the au grains at the contact between the polymorphic layer and the primary au bacterioform au type 2 is present it consists of solid rounded forms into which crystal boundaries of underlying primary au extend and is the result of dealloying and ag dissolution from the primary au this study demonstrates that 1 microbially driven dissolution precipitation and aggregation lead to the formation of bacterioform au and contribute to the growth of au grains under supergene conditions and 2 the microbially driven mobilization of coarse au into nanoparticles plays a key role in mediating the mobility of au in surface environments because the release of nanoparticulate au upon biofilm disintegration greatly enhances environmental mobility compared to au complexes only

cambrian plutonism in northeast japan and its significance for the earliest arc trench system of proto japan new u pb zircon ages of the oldest granitoids in the kitakami and ou mountains in order to clarify the early history of japan particularly during the early paleozoic pre devonian granitoids in the south kitakami belt skb ne japan were dated by u pb zircon age by laser ablation inductively coupled plasma mass spectrometry la icp ms two samples of diorite tonalite from the nagasaka area shoboji diorite and two of mylonitic tonalite from the isawa area isawagawa tonalite yielded late cambrian ages 500 490 ma for the primary magmatism these ages newly identify a ca 500 ma late cambrian arc plutonism in central ne japan which has not been recognized previously and has the following geological significance the cambrian granitoids are the oldest felsic plutonic rocks in ne japan which are independent of the previously known ca 450 ma latest ordovician hikami granite in skb the cambrian granitoids are extremely small in size at present but likely had a much larger distribution primarily at least 30 km wide and potentially up to 80 km wide in a cross arc direction their southerly extension was recognized in the hitachi area ca 200 km to the south and in central kyushu ca 1500 km to sw they likely represent remnants of the same mature arc plutonic belt in early paleozoic japan which developed in western panthalassan paleo pacific margin as well as the khanka block in far east russia the extremely small size of the cambrian granitoids at present can be best explained by intermittent severe tectonic erosion since the paleozoic this cambrian arc granitoid belt likely developed from primorye possibly to eastern cathaysia south china via japan

distribution and speciation of gold in biogenic and abiogenic calcium carbonates implications for the formation of gold anomalous calcrete abstract calcrete pedogenic ca carbonate is an important sampling medium for geochemical gold au exploration in semi arid and arid regions of australia because it is widespread easy to sample and calcium ca shows a strong positive correlation with au but not with base metals in calcrete overlying buried au mineralization in this study we show that the formation of au anomalous calcrete can be biomediated through the activity of resident microorganisms and may not simply be the result of passive nucleation on inactive cells or evapotransporative processes calcified microfossils are highly abundant in calcrete from the barns au prospect in south australia these microfossils are morphological analogues of calcified cells and biofilms formed in laboratory experiments conducted with active bacterial cultures enriched from au anomalous calcareous sand from the barns prospect calcium carbonates precipitated by these cultures consisted mostly of calcite which is the main carbonate mineral in calcrete synchrotron micro x ray fluorescence s xrf mapping was used to assess the distribution of au zn ca and other metals in ca carbonates precipitated by active bacterial cultures on a m scale the distribution of au was heterogeneous in these ca carbonates and differed from base metal distribution thus mimicking the spatial separation of these metals observed in calcrete the speciation of au in ca carbonates precipitated by active bacteria was measured using micro x ray absorption near edge structure spectroscopy xanes and resembled that observed in au anomalous calcrete closely while metallic au was observed in au hotpots ionic au was detected in the halo surrounding the hotspot in contrast the precipitates produced in the presence of dead bacterial cells or by raising solution ph or pco 2 i e hydroxylapatite portlandite and vaterite respectively did not reflect the mineralogy of calcrete gold distribution and speciation in vaterite formed by raising pco 2 were homogenous and did not reproduce the variation observed in calcrete and ca carbonates precipitated by active cells increasing the supersaturation with respect to ca in solution by incremental drying of the medium produced only x ray amorphous precipitates or hydroxylapatite in the presence heat killed cells in conclusion this study shows that active microbial processes that combine biogenic ca carbonatogenesis with au precipitation are likely to drive the formation of au anomalous calcrete

progressive development of block in matrix fabric in a shale dominated shear zone insights from the bobbio tectonic window northern apennines italy 1 block in matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary tectonic and mud diapiric processes in many exposed orogenic belts a true melange originates when this fabric is associated with mixing of rocks of different ages and provenance as that characterizing the main decollement shear zone developed between the highly allochthonous ligurian nappe and its substratum of foredeep deposits at the margins of the bobbio tectonic window trebbia valley northern apennines mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement evidence of polyphased deformations spanning from liquefaction related and hydroplastic structures to pseudo hydrofracturing features and mineralized veining have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure a novel evolutionary scheme for block in matrix fabric development is here proposed involving the progressive dismembering of already highly deformed units originally developed in non to poorly lithified conditions through a generalized simple shearing achieved during the evolution of the shear zone coupled with strain hardening due to tectonic compaction synkinematic diagenesis and fluid expulsion

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

distributed deformation along the subduction plate interface the role of tectonic melanges recent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically however the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified tectonic melange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate in this work we examine several tectonic melange zones hyuga okitsu mugi from the shimanto belt an accretionary prism in southwest japan connecting to the active nankai subduction zone these tectonic melange zones have a block in matrix structure with lenses of sandstones and basalts within a metapelitic matrix and their deformation is distributed over zones of hundreds of meters in thickness in addition the examples of melange considered here are bounded by sharp faults some of them bearing pseudotachylyte layers so that distributed deformation within the melange and localized deformation on its boundary are juxtaposed distributed deformation involves the development of a foliation as well as of a pervasive network of macroscopic and microscopic shear zones along with slip on this network strain proceeds by fracturing and precipitation of quartz in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins the analysis of shear band kinematics shows in all three examples a dominant top to the trench sense of shear consistent with deformation along the plate boundary during subduction moreover most shear zones when foliation is restored back to syn subduction position are extensional structures finally the geometry and kinematics of the melange bounding faults as well as radiometric constraints show that in most cases the faults localized structures were formed during a later stage than melange internal deformation these findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths first there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation second rather than proposed models of underplating where all deformation is localized into the thrusts bounding the tectonic sheets we suggest that underplating was to a large extent accommodated by distributed deformation within the melange sheets this underplating model accounts for i the large amount of strain within the melange ii the absence of contractional structures during underplating iii thinning of the melange required by the network of extensional shear bands and stretched boudins third melanges appear as likely candidates for portions of the plate interface deforming by aseismic slip the seismic vs aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain which in turn depends on the efficiency of pressure solution to operate

the association of gold with calcrete abstract the discovery that au accumulates in calcrete pedogenic carbonate or caliche was made in 1987 by csiro calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi arid terrains around the world the principal constituent of calcrete is calcite while au is a noble metal calcrete has been a significant tool in a number of au deposit discoveries so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery numerous laboratory experiments case histories and exploration models have been published most from australia yet we do not fully understand the mechanisms involved it is timely therefore twenty five years on since the first publication of this phenomenon to review this highly unusual but economically important association critical to any review on au in calcrete is to first consider calcretes themselves the nature of a particular calcrete where it has formed and mode of formation is relevant to how where and why au accumulates within it this review commences with a background nomenclature history classification and some examples of calcrete types found near au deposits how calcretes form their origins and the role of biota is considered their locations in the regolith and landscape as well as exploration models for au in calcrete are discussed a section on the chemistry of au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us following on is a summary of practical aspects of identifying collecting and analysing samples for exploration purposes selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings concluding sections include a summary and implications of this accumulated knowledge to discovering au deposits

the immobilization of gold from gold iii chloride by a halophilic sulphate reducing bacterial consortium a consortium containing halophilic dissimilatory sulphate reducing bacteria was enriched from basque lake 1 located near ashcroft british columbia canada to evaluate the role these bacteria have on the immobilization of soluble gold the consortium immobilized increasing amounts of gold from gold iii chloride solutions under saline to hypersaline conditions over time gold iii chloride was reduced to elemental gold in all experimental systems salinity did not affect gold immobilization scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold iii chloride was immobilized as c 3 10 nm gold colloids and c 100 nm colloidal aggregates at the fluid biofilm interface the precipitation of gold at this organic interface protected cells within the biofilm from the toxic effect of ionic gold analysis of these experimental systems using x ray absorption near edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes the immobilization of gold by halophilic sulphate reducing bacteria highlights a possible role for the biosphere in intercepting mobile gold complexes within natural hydraulic flow paths based on the limited toxicity demonstrated in this experimental model significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably non toxic to the biosphere