distribution of slip from 11 mw 6 earthquakes in the northern chile subduction zone we use interferometric synthetic aperture radar gps and teleseismic data to constrain the relative location of coseismic slip from 11 earthquakes on the subduction interface in northern chile 23 25 s between the years 1993 and 2000 we invert body wave waveforms and geodetic data both jointly and separately for the four largest earthquakes during this time period 1993 m w 6 8 1995 m w 8 1 1996 m w 6 7 1998 m w 7 1 while the location of slip in the teleseismic only geodetic only and joint slip inversions is similar for the small earthquakes there are differences for the 1995 m w 8 1 event probably related to nonuniqueness of models that fit the teleseismic data there is a consistent mislocation of the harvard centroid moment tensor locations of many of the 6 6 earthquakes as well as three m w 7 events from the 1980s all of these earthquakes appear to rupture different portions of the fault interface and do not rerupture a limited number of asperities

two step deoxygenation at the end of the paleoproterozoic lomagundi event abstract the ca 2 1 ga francevillian group of gabon was deposited in the aftermath of the great oxidation event goe and records the lomagundi event le which is the most pronounced and long lived carbon isotope excursion in the geologic record moreover the sedimentary succession contains putative evidence for the earliest appearance of macro eukaryotes an emerging paradigm is that the end of the le was accompanied by a deoxygenation event that preceded the apparent stability of environmental and redox conditions as well as the carbon cycle characteristic of the mesoproterozoic however the processes that led to deoxygenation some 300 to 200 ma after the beginning of the goe are not well understood here we present a multi proxy stable isotope 34 s 33 s 36 s 98 mo 13 c org 13 c carb and 18 o carb study of the francevillian group we suggest that sedimentation of the lower part of the francevillian group took place during the le in oxygenated shallow waters with elevated sulfate concentrations two episodes of anoxic water shoaling during deposition of the upper francevillian group correspond with broader marine deoxygenation and a contraction of the seawater sulfate reservoir this shoaling of anoxic conditions may be linked to intense submarine hydrothermal and volcanic activity that led to sedimentary manganese deposits we propose that increased concentrations of aqueous hydrothermally sourced reductants drove oxygen consumption during the first deoxygenation event and established a sulfidic oxygen minimum zone at the margin of the shallow shelf carbonates with positive 13 c carb values characteristic of the le precipitated during this first stage of deoxygenation the second deoxygenation separated from the previous event by a period of well oxygenated conditions was marked by a stronger contraction of the seawater sulfate reservoir and coincided with the end of the le during this time widespread euxinic conditions were established in shallow above storm wave base marine environments the presence of a shallow water redoxcline points to a generally low oxygen atmosphere ocean system further the negative co variation between 34 s and 13 c values in sediments of the francevillian group and other sedimentary successions of similar age worldwide suggests that the inferred two step deoxygenation corresponding to the end of the le reflects global rather than local events that likely occurred between 2 1 and 2 05 ga ago

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

the mw 8 0 antofagasta northern chile earthquake of 30 july 1995 a precursor to the end of the large 1877 gap abstract an m w 8 0 earthquake occurred on 30 july 1995 in the antofagasta region northern chile the main rupture corresponding to thrust faulting developed from 10 to 50 km in depth along the subduction interface between the nazca and the south american plates the 1995 earthquake took place just south of the large seismic gap where a great earthquake m 9 had occurred in 1877 most of the 1995 rupture was located within a local network consisting of nine short period stations that had been previously installed at the southern end of the 1877 gap and the aftershock sequence could be accurately monitored little destruction resulted from the 1995 earthquake in spite of its large size ground acceleration in antofagasta reached 29 of gravity a tsunami wave 2 to 2 5 m high was observed along the coast from mejillones to taltal one strong foreshock m w 6 2 occurred in the 1995 hypocentral region 6 months before the main event body wave modeling of broadband seismograms from the global network along with the analysis of the aftershock distribution allows us to propose a well constrained model for the whole rupture process some additional details of the rupture were obtained from an accelerometer record at antofagasta the main rupture started as a double even with thrust mechanism below the southern part of the mejillones peninsula and it propagated southward in a n200 e direction with an average velocity of 2 8 km sec it ended near the trench in normal faulting the total rupture area and seismic moment are 185 90 km 2 and 1 2 10 28 dyne cm respectively the aftershock distribution delineates a well defined rupture surface along the subduction interface the distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the mejillones peninsula hence the 1995 main rupture did not propagate north of the mejillones peninsula into the 1877 gap aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north the mechanisms of the strongest aftershocks are similar to that of the mainshock the down dip termination of the main rupture corresponds to the maximum depth 50 km of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event a well constrained dislocation model is proposed for the 1995 main rupture which produces surface displacements in good agreement with available observations of coastal uplift and gps measurements the dislocation model as well as global positioning system gps measurements indicate that the 1995 earthquake generated e w extension in the coastal region of antofagasta the atacama fault located 40 to 50 km above the 1995 main rupture showed small fresh surface ruptures near sierra remiendos 70 km to the sse of antofagasta with a maximum vertical offset of 20 cm this offset corresponds to normal faulting which is in agreement with the e w co seismic extension the mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap modeling of static stress changes induced by the antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface thus the chances for the reactivation of the 1877 gap after this event are greater now

nature of the plate contact and subduction zones diversity abstract in recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision we investigated two fundamental states of the plate contact one based on a fault and the other based on a subduction channel using geodynamic modeling we determined the speci c signatures of both states of the subduction contact we established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate and is one of the controlling factors whether back arc extension develops or not in the present study we combine results of our previous numerical experiments with a re analysis of published observations overall our synthesis connects seismic moment release with back arc deformation and tectonic processes at the margin it leads us to identify four classes of subduction zones the rst two classes result directly from our numerical experiments in class 1 subduction zones are characterized by a plate contact that is largely fault like with an accretionary margin in class 2 the plate contacts are largely channel type and have an erosive margin class 3 where the plate contact is entirely channel like consists of accretionary margins with a high sediment supply subduction zones of class 4 mostly characterized by an erosive convergent margin northern chile peru honshu and kuril are more complicated they can be explained by incorporating regional observations

sulfur isotopic trends and pathways of iron sulfide formation in upper holocene sediments of the anoxic black sea down core trends for extents of iron sulfidation in upper holocene microlaminated deposits of the black sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and or very close to the sediment water interface sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core with a mean 34scdt value of 37 2 0 9 1 n 18 these isotopic trends support the hypothesis of early pyrite formation isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and to 34s values for water column dissolved sulfide within a narrow zone immediately below the o2 h2s interface these three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column 175 m with 34s values for the deep dissolved sulfide reservoir being consistently lower by 2 to 4 these isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow shallow region at the top of the sulfidic water mass isotopic data from a fine grained turbidite collected in the deep basin and from rapidly accumulating iron monosulfide rich muds on the anoxic upper slope are enriched in34s relative to the microlaminated deposits the enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial a broad suite of geochemical data including sulfur isotopic results are consistent with an upper slope sediment source for the ubiquitous muddy turbidites of the deep basin the collective effects of reworking transport and redeposition result in a complete conversion of avs to pyrite with little additional iron sulfidation this conclusion is supported by a detailed isotopic mass balance the present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin sediments of the upper slope and the genetically linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep basin floor suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin importantly isotopic patterns from the modern black sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record

paleoproterozoic high δ13c dolomites from the lastoursville and franceville basins se gabon stratigraphic and synsedimentary subsidence implications abstract new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern gabon franceville and lastoursville sub basins indicate that the seawater value of 13 c in these sediments underwent a large positive excursion around 2 1 ga these carbonate rocks belong to the fc formation of the francevillian group deposited along the margin of the archean chaillu craton in open marine shelf and evaporite settings and experienced locally lower greenschist facies metamorphism four microfacies mf1 mf4 are recognized from subtidal cyanobacterial dolobindstones to inter and supratidal collapse breccias mf4 the dolomites exhibit elevated 13 c values most values are above 0 50 the highest value is 9 74 and the average value is near 4 vs vpdb consistent with deposition during the large worldwide positive excursion between 2 22 and 2 06 ga these positive 13 c values are independent of the microfacies type such high 13 c values are also recorded in reworked carbonates dolomites embedded in coarse grained sandstones and conglomerates interstratified in the black shales of the underlying fb formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits interaction between early structural development and sediment fill lead to the formation of several sub basins with distinct differences in terms of amount and timing of subsidence the 13 c enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the fb sediments in this extensional geotectonic context as a consequence the cherty and dolomitic fc formation cannot longer be considered as a marker interval the end of the positive excursion is located near the top of the francevillian series i e fe and could be used in future studies as a potential chemostratigraphic tool in the paleoproterozoic

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

high isotope fractionations during sulfate reduction in a low sulfate euxinic ocean analog a detailed record of the early earth sulfur s cycle is chronicled by the s isotope values of sulfide and sulfate preserved in the rock record interpretation of this record rests on our understanding of sulfur cycling in modern systems experiments and the resulting isotopic signatures very large fractionations in 34s of 70 are commonly measured between sulfide and sulfate in modern systems and in ancient sediments theoretical calculations suggest that sulfate reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide although they have only been demonstrated to generate fractionations up to 48 here we report the first direct determination of 60 70 fractionations by natural populations of sulfate reducers these high fractionations occur under the relatively low sulfate conditions 1 1 2 mm of meromictic lago di cadagno in switzerland the major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction with little evidence for modification by further oxidative sulfur cycling these observations help us to constrain the evolution of seawater sulfate concentrations

lusitanisuchus a new generic name for lisboasaurus mitracostatus crocodylomorpha mesoeucrocodylia with a description of new remains from the upper jurassic kimmeridgian and lower cretaceous berriasian of portugal lusitanisuchus mitracostatus is a small mesoeucrocodilian crocodylomorph from the upper jurassic kimmeridgian of guimarota leiria in portugal which was originally described as an anguimorph lizard lisboasaurus mitracostatus seiffert 1970 based on jaw fragments and isolated teeth a recent reexamination of the crocodylomorph material of guimarota yielded new skull and dentary remains assignable to lusitanisuchus mitracostatus the new skull material not only enables for the first time a skull reconstruction of this taxon but also leads to renaming and reclassifying lisboasaurus mitracostatus teeth of lusitanisuchus are also reported from the locality porto dinheiro lourinha early cretaceous berriasian of portugal the presence of this crocodylomorph taxon in two portuguese localities suggests its occurence in the lusitanian basin during the kimmeridgian to the berriasian

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

two dimensional velocity models of the nazca plate subduction zone between 19 5 s and 25 s from wide angle seismic measurements during the cinca95 project the interpretation of seismic refraction wide angle reflection data from the 1995 crustal investigations off and on shore nazca central andes cinca95 project has resulted in the derivation of nine e w two dimensional 2 d velocity cross sections for the region between the peru chile trench and the coast between 19 5 s and 25 s with three of the cross sections extending a farther 100 200 km inland these sections define the major lithospheric structures of the upper south american plate and the lower nazca plate down to uppermost mantle depths of 30 60 km beneath this part of the present day forearc region in addition to showing the nazca plate subducting at an increasing angle of 9 25 down to 30 50 km depth near the coast these cross sections show a portion of the moho dipping eastward from 43 50 km near the coast to 55 64 km up to about 240 km inland owing to a gap of almost 50 km in the data coverage of the moho near the coast it is uncertain from this data set alone whether the moho defined east of the coast should be correlated with the lower oceanic nazca plate or the upper continental south american plate however a comparison with other seismological data suggests that the moho identified here east of the coast defines the base of the continental crust of the upper south american plate immediately behind the downgoing nazca plate thus the hypothesis of delouis et al 1996 that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported the cross sections also show between 20 s and 22 5 s a boundary extending from upper crustal levels down to 25 30 km depth subparallel to and 5 10 km above the top surface of the subducting nazca plate the discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate

evidence for oxygenic photosynthesis half a billion years before the great oxidation event the early earth was characterized by the absence of oxygen in the ocean atmosphere system in contrast to the well oxygenated conditions that prevail today atmospheric concentrations first rose to appreciable levels during the great oxidation event roughly 2 5 2 3 gyr ago the evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise but it has proved difficult to constrain the timing of this evolutionary innovation1 2 the oxidation of manganese in the water column requires substantial free oxygen concentrations and thus any indication that mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing mn oxides are not commonly preserved in ancient rocks but there is a large fractionation of molybdenum isotopes associated with the sorption of mo onto the mn oxides that would be retained here we report mo isotopes from rocks of the sinqeni formation pongola supergroup south africa these rocks formed no less than 2 95 gyr ago3 in a nearshore setting the mo isotopic signature is consistent with interaction with mn oxides we therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

velocity structure in northern chile evidence of subducted oceanic crust in the nazca plate summary 2 d p wave velocity models were determined for the subduction zones near iquique and antofagasta in northern chile simultaneously inverting p and s wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations a 2 d parametrization was used because of the paucity of data but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3 d inversions the crust and upper mantle are parameterized by constant velocity regions of irregular shape with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve p wave velocities beneath the antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting nazca plate this crust has a p wave velocity of 7 3 f 0 1 km s and is observed down to a depth of 60 f 10 km this depth also corresponds to the maximum depth of seismogenic coupling in the chilean subduction zone the subducted oceanic crust overlies an oceanic upper mantle with a p wave velocity of 8 0 f 0 1 km s apparently oceanic crust is being subducted beneath iquique as well however this feature is less constrained by the data available from this region

micro scale quadruple sulfur isotope analysis of pyrite from the 3480 ma dresser formation new insights into sulfur cycling on the early earth abstract we report in situ quadruple sulfur isotope analysis 32s 33s 34s and 36s of a pyritized microbial mat from the 3480 ma dresser formation pilbara craton western australia these data yield positive 34s and 33s indicative of sulfur sourced from a pool with similar character as the putative atmospheric elemental sulfur channel of pavlov and kasting 2002 contrary to previous data from the dresser formation however this pyrite is heavily depleted in 36s with a 36s 33s slope of c 3 6 much steeper than slopes typically seen in other early archean rock successions 36s 33s 1 which suggests either a different atmospheric signature for deposited s or a different pool altogether significant micro scale isotopic heterogeneity is observed within the microbial mat 34s 1 6 to 6 7 33s 0 4 to 2 6 36s 3 1 to 8 1 implying a role for microbial s metabolism while metabolic s cycling has been shown to shift 36s to lower values microbial metabolization of s does not appear sufficient to account for the full range of 36s we conclude that the isotopic composition of the pyrite was controlled by the relative proportions of mass independently fractionated mif s0 and sulfate derived sulfur incorporated into polysulfide pyrite precursors during reactions in the microbial mat the dominance of the mif s0 isotopic signature 34s 33s 36s indicates that contributions from the sulfate derived sulfur pool were relatively small consistent with low concentrations of sulfate in archean seawater and that contributions from a non sulfate pool were significant micro scale isotopic heterogeneity in the pyrite points to mixing between the two sulfur pools in selected micro environments within the microbial mat the particularly negative 36s observed here reveals a 3480 ma sulfur reservoir with novel 36s 33s chemistry whose significance now requires further investigation

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

determination of seismogenic interplate contact zone and crustal seismicity around antofagasta northern chile using local data summary during september and october 1988 a microseismic field experiment was carried out around the city of antofagasta northern chile with 29 portable analogue and digital stations a total of 197 reliable microearthquake locations and 19 focal mechanisms were determined it is proposed that it is possible to estimate the maximum depth of the coupled uncoupled transition of the subducting lithosphere using local data defined by the depth of the expected change of the stress field from compressional to tensional along the slab this change is observed at about 70 km depth in the antofagasta field work two estimations of the width of the seismogenic interplate contact are discussed 1 the maximum depth of the coupled zone defined by the observed maximum depth of the shallow dipping thrust events recorded during the experiment of 47 km corresponding to a width of the seismogenic contact zone of about 90 km and 2 the maximum depth of the coupled zone defined by the depth of the observed change from a compressional to tensional stress field which is 70 km and corresponds to a width of the seismogenic contact zone of about 130 km with both values the maximum magnitude ms estimated for the region varies between 8 6 and 8 7 no shallow event associated with the atacama fault system was observed during the experiment

fault plane structure of the antofagasta chile earthquake of 1995 from a large dataset of aftershocks in the rupture area of the mw 8 0 1995 antofagasta earthquake we derived the spatial distribution of hypocenters and a b value map along the fault plane we further examined various focal mechanisms of the aftershock sequence the b value distribution on the fault plane clearly indicates the heterogeneity of the rupture area we found a positive correlation between the b value variations and the main shock source time function determined by delouis 1996 where zones of high b value are related to high slip with these results we propose to have found evidence for the segmentation of the fault plane and suggest that the mapped inhomogeneities could have played the role of asperities prior to the main shock rupture

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

a statistical analysis of the carbon isotope record from the archean to phanerozoic and implications for the rise of oxygen organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales permanent burial of organic carbon in the crust or mantle oxidizes the surface environment atmosphere ocean and biosphere by removing reduced carbon it has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout earth9s history thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen schidlowski 1988 catling and others 2001 holland 2002 holland 2009 kump and others 2009 rothman 2015 however the opposite conclusion has been drawn from the same carbon isotope record des marais and others 1992 bjerrum and canfield 2004 to test these opposing claims we compiled an updated carbon isotope database and applied both parametric and non parametric statistical models to the data to quantify trends and mean level changes in fractional organic carbon burial with associated uncertainties and confidence levels we first consider a conventional mass balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon for this model statistical analysis implies fractional organic burial has increased over earth history by a factor of 1 5 relative to organic burial at 3 6 ga with the 95 percent confidence interval ranging from factors of 1 2 to 2 0 an increase in organic burial by a factor of 1 2 cannot explain the rise of oxygen whereas an increase by a factor of 2 could conceivably explain the rise of oxygen there is however a highly significant and well constrained increase in organic burial from the proterozoic to the phanerozoic we also analyze changes in the difference between carbonate and organic carbon isotopic ratios over earth history there is a statistically significant increase in this difference from the early to late archean possibly caused by increased biological fractionation due to methanotrophic recycling this transition is consistent with the evolution of oxygenic photosynthesis at 2 8 ga or earlier finally we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes specifically ocean crust carbonatization and authigenic carbonates because the size of these fluxes has a large poorly constrained range our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all however it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time in which case conclusions from the conventional model would be valid

coupled molybdenum iron and uranium stable isotopes as oceanic paleoredox proxies during the paleoproterozoic shunga event the paleoproterozoic era was a time of remarkable importance in the redox evolution of earth s atmosphere and oceans here we present a multi proxy study of mo u and fe isotopes together with fe speciation of black shales and siltstones from the upper zaonega formation of the onega basin in karelia we attempt to better understand oceanic redox conditions during the 2 05 ga shunga event as the next step following the great oxidation event goe and the lomagundi carbon isotope excursion event a cautious examination of the fe speciation data shows that the studied section was deposited under dominantly euxinic conditions anoxic and sulfidic and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite during this episode the system was closed with respect to fe but not sulfur the mo and u isotopic compositions corrected for detrital input were not affected by the metamorphism and loss of s and are fairly uniform throughout the entire section the fe isotope compositions are exceptionally heavy in the lower part of the section up to 56 fe irmm 14 0 83 and become lighter towards the upper intervals which also show significant mo and u enrichments we suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column the upper part was deposited within a locally euxinic portion of the basin where h 2s availability was highest removal of mo and u was more efficient and precipitated pyrite captured relatively non fractionated dissolved fe in other words quantitative uptake of fe was favored in contrast the lower interval was deposited on the lower margin of a euxinic wedge where h 2s availability was lower and removal of mo and u was less efficient pyrite precipitation in this part of the water column reflected a more fractionated dissolved fe reservoir due to more protracted non quantitative fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of fe the u isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co precipitation into carbonates was the main process of u removal at this time we estimate the mo isotope composition of the contemporaneous ocean to be 98 mo sw 0 85 0 21 this is the lowest value yet reported for the proterozoic ocean suggesting that the oceanic mo cycle was dominated by euxinic and anoxic sinks with negligible mo removal into oxic environments recent studies have proposed a sharp increase in ocean atmosphere oxygen levels during the lomagundi event followed by a dramatic crash our results from black shales of the 2 05 ga shunga event are consistent with a post lomagundi decrease in biospheric oxygen levels

recent crustal deformation in the antofagasta region northern chile and the subduction process new neotectonic observations along with a detailed aerial photograph analysis allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern chile between 22 5 s and 24 5 s antofagasta region both the coastal cordillera and the mejillones peninsula are under e w extension normal faults dipping east with an almost n s orientation are predominant large scale neogene to recent deformation is characterized by vertical uplift and subsidence related to normal faulting recent kinematics along the atacama fault system including the atacama fault itself are controlled by the regional extensional stress regime and the predominant component of recent displacement along the atacama fault is vertical normal however strike slip components are also observed but with moderate offsets no larger than one to two tens of metres the sense of shear and the ratio between vertical and horizontal offsets vary coherently with the fault azimuth compatible with e w extension oblique slip faulting with normal and left lateral offsets of similar amplitude is observed on the most northeasterly oriented segment of the atacama fault this small left lateral component of displacement and the strike slip motion predicted by models of strain partitioning of the convergence along the atacama fault are in opposite senses this occurs because the stress regime in the coastal region is not compressional but extensional very fresh recent ruptures are numerous especially between 23 s and 24 s and provide clear indications of the occurrence of moderate to large continental earthquakes during the late quaternary as pointed out by several authors indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern chile we propose that part of the long term extension is due to the broad scale flexure of the outer forearc this flexure may be controlled by offshore subsidence caused by subduction erosion near the trench and by onshore uplift related to the underplating of eroded low density material beneath the coastal cordillera neotectonic observations show that the state of stress in the outer forearc remains extensional in a deviatoric sense in the long term however it is proposed that the state of stress has a strong time dependent component which is intimately linked to the subduction seismic cycle the mw 8 0 antofagasta earthquake of 1995 july 30 showed that large subduction earthquakes produce e w extension in the coastal region the overall e w deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes this reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle in agreement with microseismic observations which indicate an absence of shallow crustal seismicity we assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction this occurs because the displacement field generated by subduction earthquakes is concentrated in the outer forearc the e w extension should grow over prepeated subduction seismic cycles and lead to large continental earthquakes occurring within the overriding block simultaneous with large subduction events we show new surface ruptures associated with normal faulting along the atacama fault which were most probably triggered by the 1995 antofagasta subduction earthquake

geochemistry of precambrian carbonates vi aphebian albanel formations quebec canada abstract the 1 85 0 15 ga old albanel formations of northern quebec canada an example of paleoproterozoic carbonate buildups are lithologically composed of cherty dolostones subordinate limestones are encountered only near the base of the sequence coincident with maximal transgression the chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases such as chert quartz plagioclase illite k spar pyrite and fe mn oxides all abundant particularly in the basal part of the section the chemistry of the carbonate portion of limestones is strongly reset by post depositional phenomena but the relict high sr contents suggest that aragonite may have been an original precursor phase geochemically the carbonate portion of dolostones can be subdivided into two populations reflecting the stratigraphie bottom and top of the sequence trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures but in a more open system the latter reflects the greater lithological variability and permeability of the younger strata the 13c varies up section from about 3 to 1 perhaps a reflection of progressively more open marine circulation the associated four generations of post depositional pyrites have overlapping 34s from 7 1 to 54 with a mean of 24 a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and or replenishment the best measured 87 sr 86 sr of 0 70600 although possibly not a direct reflection of the coeval seawater may still be a near original value as indicated by its coherency with the roughly co stratigraphic paleo proterozoic sequences evaluation of the existing data indicates that strontium isotopic composition of the precambrian seawater was characterized by oscillations similar to those of the phanerozoic the interval of 0 8 1 5 ga ago being dominated by relatively nonradiogenic values followed by a radiogenic period at 1 6 2 2 ga ago near mantle values during the late archean and possibly another radiogenic peak at 3 5 ga ago

pyrite multiple sulfur isotope evidence for rapid expansion and contraction of the early paleoproterozoic seawater sulfate reservoir abstract earth s oxygenation is often described in terms of two unidirectional steps at the beginning and end of the proterozoic eon separated by a long lived intermediate redox state recent work defines a more complicated path to oxygenation exemplified by an apparent drop in oxidation state following the early paleoproterozoic lomagundi carbon isotope excursion the timing of this proposed drop in oxidation state is not well constrained and it is not clear how it relates to redox conditions during the remainder of the proterozoic here we present a study of pyrite multiple sulfur isotopes supported by fe speciation and organic carbon isotopes from early paleoproterozoic black shales we find evidence for the rapid expansion of the seawater sulfate reservoir during the great oxidation event at ca 2 3 ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca 2 05 ga this scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early paleoproterozoic comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early paleoproterozoic did not return until the late neoproterozoic

local earthquake tomography of shallow subduction in north chile a combined onshore and offshore study selected travel time data from the aftershock series of the great antofagasta earthquake m w 8 0 have been inverted simultaneously for both hypocenter locations and three dimensional v p and v p v s struc ture the data were collected with a dense 44 station seismic network including ocean bottom hydrophones we performed a series of inversions with increasing complexity 1 d 2 d and 3 d this approach was cho sen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution special efforts were made to assess the solution quality including standard resolution esti mates and tests with synthetic travel times the subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high v p feature high v p v s ratios within the oceanic crust possibly indicate elevated fluid content underplating of material eroded close to the trench is found beneath the mejillones peninsula the lower crust of the overlying plate is characterized by an average v p of 6 8 6 9 km s and an average to low v p v s ratio large areas of anomalously high v p are found in the lower crust south of the city of antofagasta they are interpreted as remants of magmatic intrusions a zone of high v p v s ratios is found within the rup ture area of the antofagasta main shock just above the subducted slab its location within the region of high est stress release from the main shock suggests that the main shock rupture causes the high v p v s ratio the high v p v s ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overly ing lower crust

exceptional preservation of expandable clay minerals in the ca 2 1 ga black shales of the francevillian basin gabon and its implication for atmospheric oxygen accumulation abstract clay minerals are exceptionally well preserved in marine black shale of the ca 2 1 ga francevillian group in southeastern gabon the fb formation of the francevillian group is characterized by smectite rich clay minerals including randomly ordered r0 type and ordered r1 type mixed layer illite smectite i s the preservation of r0 type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis which is unique considering the paleoproterozoic age of the sedimentary rocks r0 and r1 type smectite rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures suggesting formation of clay organic matter complexes our data suggests that clay minerals may have enhanced organic matter preservation providing the oldest example where a link between clay minerals and organic matter sequestration can be established our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

seismogenic plane of the northern andean subduction zone from aftershocks of the antofagasta chile 1995 earthquake 1 a temporary seismological network which was located in the antofagasta northern chile region recorded a large part of the aftershocks from a mw 8 0 earthquake in 1995 results from a tomographic inversion show that the aftershocks were concentrated in a plane approximately 3 km thick spatial zone comparison of our results with earlier processed wide angle reflection data shows clearly that this zone is positioned at the top of the oceanic crust therefore it represents the shearing boundary between the subducting oceanic and the continental plate and gives an in situ estimation of the thickness of the boundary outside of this zone events were detected in the continental crust as well as within the oceanic mantle