active tectonics of the algerian atlas mountains evidence from aftershocks of the 1980 el asnam earthquake summary the el asnam algeria earthquake of 1980 october is the largest earthquake to have occurred in north africa since instrumental records began ms 7 3 it was caused by movement on a segmented reverse fault which is part of the fault zone that separates the chelif alluvial plains from the coastal range of the atlas mountains the coastal range is the actively deforming plate boundary between africa and eurasia in this paper the results of an aftershock study carried out using a 28 station portable network in the epicentral area in the 5 weeks after the main shock are presented a total of 4517 aftershocks have been located using both p and s phases of these 4517 events 1279 pass a set of quality criteria and are used to discuss the detailed tectonics of the aftershock zone the southwestern half of the aftershock zone has a relatively restricted band of seismicity running parallel to the main fault in cross section this seismicity is observed to lie almost exclusively in the footwall of the main fault and indicates both antithetic reverse faulting and extension parallel to the fault zone the only exception to this pattern occurs at a junction of the main fault segments where a distinct cluster of events with very variable focal mechanisms is located in the hangingwall this cluster may be equated with a barrier that impeded the rupture of the main shock beyond the southwest end of the main fault a mixture of strike slip and reverse faulting occurred and suggests a mechanism whereby this end of the fault can act as an asperity for earthquake generation aftershocks in the northeastern half of the study area indicate movement on a stack of listric reverse faults the focal mechanisms of these events show that the n dipping nodal planes assumed to be the fault planes flatten with depth we suggest that the faults sole into a low angle 20 decollement at about 8 10 km depth at the base of the aftershock zone we argue that the northeastern part of the 1980 aftershock zone is typical of the actively deforming coastal belt of northwest algeria subduction of the western mediterranean basin beneath the north african margin does not seem to occur instead the margin is absorbing the motion with old normal faults being reactivated as thrusts

zemmouri earthquake rupture zone mw 6 8 algeria aftershocks sequence relocation and 3d velocity model 1 we analyze the aftershocks sequence of the zemmouri thrust faulting earthquake 21 may 2003 mw 6 8 located east of algiers in the tell atlas the seismic sequence located during 2 months following the mainshock is made of more than 1500 earthquakes and extends ne sw along a 60 km fault rupture zone crossing the coastline the earthquake relocation was performed using handpicked p and s phases located with the tomodd in a detailed 3d velocity structure of the epicentral area contrasts between velocity patches seem to correlate with contacts between granitic volcanic basement rocks and the sedimentary formation of the eastern mitidja basin the aftershock sequence exhibits at least three seismic clouds and a well defined se dipping main fault geometry that reflects the complex rupture the distribution of seismic events presents a clear contrast between a dense sw zone and a ne zone with scattered aftershocks we observe that the mainshock locates between the sw and ne seismic zones it also lies at the nns sse contact that separates a basement block to the east and sedimentary formations to the west the aftershock distribution also suggests fault bifurcation at the sw end of the fault rupture with a 20 km long n 100 trending seismic cluster with a vertical fault geometry parallel to the coastline juxtaposed another aftershock cloud may correspond to 75 se dipping fault the fault geometry and related sw branches may illustrate the interference between pre existing fault structures and the sw rupture propagation the rupture zone related kinematics and velocity contrasts obtained from the aftershocks distribution are in agreement with the coastal uplift and reflect the characteristics of an active zone controlled by convergent movements at a plate boundary

permian palynostratigraphy and palaeoclimate of lingala koyagudem coalbelt godavari graben andhra pradesh india abstract godavari graben one of the biggest basins among several gondwana basins of india holds a unique position not only because of its geographical location in south india but also due to the presence of almost complete succession from permian to cretaceous sediments the graben is traversed by many small faults due to which the dating and correlation of coal bearing horizons is difficult in this area as palynology is one of the most reliable parameter for dating correlation and characterization of continental deposits palynological studies have been carried out in order to date and correlate the coal bearing horizons of gundala mamakannu and kachinapalli areas from lingala koyagudem coalbelt of godavari graben the distributional pattern of various palynotaxa has suggested the occurrence of eight distinct palynozones in five borecores of gundala mlg 23 mlg 24 and mlg 28 mamakannu mmk 19 and kachinapalli mgk 6 areas these palynozones are comparable to talchir lower karharbari upper karharbari barakar and raniganj palynoflora of indian lower gondwana palynozone 1 is equivalent to parasaccites assemblage of talchir palynoflora palynozone 2 is correlatable with callumispora parasaccites assemblage of lower karharbari palynoflora palynozone 3 corresponds well with parasaccites scheuringipollenites assemblage of upper karharbari palynoflora palynozone 4 is comparable to scheuringipollenites assemblage of barakar palynoflora palynozone 5 is correlatable with faunipollenites striatopodocarpites assemblage of raniganj palynoflora palynozone 6 is comparable with striasulcites assemblage of raniganj palynoflora palynozone 7 shows its equivalence with parasaccites assemblage of raniganj palynoflora and palynozone 8 corresponds well with the crescentipollenites assemblage of raniganj palynoflora on the basis of palynofloral evidences it has been suggested that palaeoclimate during palynozone 1 3 was cool but humidity was more in palynozone 3 in comparison to palynozone 2 warmer phase started during palynozone 4 with thick coal deposits while palynozones 5 8 representing raniganj palynoflora indicate warm climate with very high humidity

organomineralization in mesoproterozoic giant ooids abstract ooids are common carbonate particles that are traditionally considered as abiogenically formed by physical and chemical processes in highly agitated environments recent studies point to the importance of microbial activities in ooid formation but more case studies are required to confirm and clarify the roles of microbes and their organomineralization processes here we report an integrated petrographic element geochemical and isotopic study of mesoproterozoic giant ooids from the wumishan formation ca 1 50 1 45 ga of north china the wumishan giant ooids 2 0 14 4 mm are composed of small micritic nuclei and thick radial fibrous cortices abundant organic relics including putative bacterial filaments and mucus like extracellular polymeric substances eps are present in ooid cortices organominerals e g nanoparticles and polyhedrons are concentrated and lined with the axes of radial fibers suggesting in situ mineralization of bacterial filaments the abundance of organic relics and fiber aligned organominerals confirms the constructive roles of microbes in the formation of the wumishan giant ooids the preservation of bacterial filaments filament bundles in their growth orientation requires fast mineralization in caco 3 supersaturated environments which may have been controlled by the shallow chemocline in redox stratified mesoproterozoic basins

slip distribution of the 2003 boumerdes zemmouri earthquake algeria from teleseismic gps and coastal uplift data 1 using a joint inversion of seismological waveforms and ground displacement observations we estimate several parameters of the fault geometry and rupture process of the mw 6 9 may 21 2003 boumerdes zemmouri earthquake the relocated epicenter is considered as a known parameter total rupture length rupture duration and maximum slip are 55 km from 3 4 e to 4 0 e 12 s and 3 m the modeled south dipping reverse fault oriented ene wsw outcrops a few km offshore which is consistent with the absence of observed surface rupture inland two shallow and relatively localized slip zones are found on both sides of the hypocenter to the sw between boumerdes and zemmouri slip is concentrated between 11 and 2 km depth to the ne between zemmouri and dellys slip is concentrated between 6 km depth and the sea floor various resolution tests indicate that our model is well constrained by the available data and help understanding which data constrains each parameter of the model

microbe mineral interactions early carbonate precipitation in a hypersaline lake eleuthera island bahamas microbialites benthic microbial carbonate deposits were discovered in a hypersaline alkaline lake on eleuthera island bahamas from the edge towards the centre of the lake four main zones of precipitation could be distinguished 1 millimetre sized clumps of mg calcite on a thin microbial mat 2 thicker and continuous carbonate crusts with columnar morphologies 3 isolated patches of carbonate crust separated by a dark non calcified gelatinous mat and 4 a dark microbial mat without precipitation in thin section the precipitate displayed a micropeloidal structure characterized by micritic micropeloids strong autofluorescence surrounded by microspar and spar cement no fluorescence observations using scanning electron microscopy sem equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities these extracellular polymeric substances eps are progressively replaced with high mg calcite discontinuous eps calcification generates a micropeloidal structure of the micrite possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria at high magnification the microstructure of the initial precipitate consists of 200 500 nm spheres no precipitation is observed in or on the sheaths of cyanobacteria and only a negligible amount of precipitation is directly associated with the well organized and active filamentous cyanobacteria in deeper layers of the mat indicating that carbonate precipitation is not associated with co2 uptake during photosynthesis instead the precipitation occurs at the uppermost layer of the mat which is composed of eps empty filamentous bacteria and coccoids gloeocapsa spp two dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat in combination these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for caco3 precipitation and dissolution theoretically partial degradation of eps by aerobic heterotrophs or uv fuels sulphate reducing activity which increases alkalinity in microdomains inducing caco3 precipitation this degradation could also be responsible for eps decarboxylation which eliminates ca 2 binding capacity of the eps and releases ca 2 ions that were originally bound by carboxyl groups at the end of these processes the eps biofilm is calcified and exhibits a micritic micropeloidal structure the eps free precipitate subsequently serves as a substrate for physico chemical precipitation of spar cement from the alkaline water of the lake the micropeloidal structure has an intimate mixture of

a pulse of oxygen increase in the early mesoproterozoic ocean at ca 1 57 1 56 ga abstract the relationship between oxygen and evolution of early eukaryotes including algae and primitive animals in geological history has been debated partly due to the varying estimates of oxygen levels in the mid proterozoic ca 1 8 0 8 ga ocean and atmosphere the upper part of the gaoyuzhuang formation ca 1 60 1 54 ga in north china hosts decimeter scale multicellular eukaryotic fossils and is documented with a decrease in cerium anomaly indicative of ocean oxygenation however the atmospheric oxygen level across this interval and its subsequent oxidation state require further investigation using additional redox proxies here we report i ca mg ratios carbonate organic carbon isotopes 13 c carb and 13 c org and phosphorous p contents across the ca 1 57 1 56 ga fossil bearing interval in the north china platform high i ca mg ratios 2 6 mol mol up to 3 8 mol mol from shallow water carbonates of the gaoyuzhuang formation suggest an episode of significant oxygen increase up to 4 pal present atmospheric level the i ca mg ratios return back to 0 5 mol mol shortly after the peak values without evidence for increasing water depth or diagenetic alteration implying a short lived oxidation event the increase of i ca mg ratios is associated with a 3 5 negative 13 c carb and 13 c org anomaly and an increase in p al ratios that are best explained by oxidation of dissolved organic carbon doc in the ocean oxygen consumption through oxidation of doc may have quickly lowered marine and atmospheric o2 levels to the early mid proterozoic 1 8 1 4 ga background oxygen concentration of 0 1 1 pal short lived oxidation events in an overall anoxic mid proterozoic ocean and atmosphere best explain the existing geochemical data and evolutionary stasis of eukaryotes during the boring billion

the impact of static stress change dynamic stress change and the background stress on aftershock focal mechanisms the focal mechanisms of earthquakes in southern california before and after four m 6 7 main shocks provide insight into how fault systems respond to stress and changes in stress the main shock static stress changes have two observed impacts on the seismicity changing the focal mechanisms in a given location to favor those aligned with the static stress change and changing the spatial distribution of seismicity to favor locations where the static stress change aligns with the background stress the aftershock focal mechanisms are significantly aligned with the static stress changes for absolute stress changes of 0 02 mpa for up to 20 years following the main shock the dynamic stress changes have similar although smaller effects on the local focal mechanisms and the spatial seismicity distribution dynamic stress effects are best observed at long periods 30 60 s and for metrics based on repeated stress cycling in the same direction this implies that dynamic triggering operates at least in part through cyclic shear stress loading in the direction of fault slip the background stress also strongly controls both the preshock and aftershock mechanisms while most aftershock mechanisms are well oriented in the background stress field 10 of aftershocks are identified as poorly oriented outliers which may indicate limited heterogeneity in the postmain shock stress field the fault plane orientations of the outliers are well oriented in the background stress while their slip directions are not implying that the background stress restricts the distribution of available fault planes

earliest triassic microbialites in curuk dag southern turkey composition sequences and controls on formation the permian triassic boundary sequence at curuk dag near antalya turkey begins with a major erosion surface interpreted as being the late permian lowstand on which lies ca 0 4 m of grainstone packstone composed of ooids peloids and bioclasts most ooids are superficial coats on fragments of calcite crystals presumed to be eroded from crystal fans which are no longer present the erosion surface is smooth and shows no evidence of dissolution the grainstone packstone contains intraclasts of the underlying wackestone proving erosion next are 15 m of microbialite comprised of interbedded stromatolites thrombolites plus beds of planar limestones with small scale erosion the latter comprise a complex interlayering of stromatolitic thrombolitic and peloidal fabrics and precipitated crystal fans which form a hybrid of microbialite and inorganic carbonate together with bioclastic debris and micrite the curuk dag microbialite sequence is repetitious the lower part is more complex with abundant stromatolites and hybrid microbialites some of the stromatolites are themselves hybrids composed of peloids and crystal fans in the upper part of the sequence stromatolites are missing and the rock is composed mostly of recrystallized thrombolites that develop upwards from tabular to domal form the domes form directly below small breaks in microbialite growth where very thin shelly micrites and grainstones packstones are deposited repetition of facies may be controlled by sea level change a deepening up model is consistent with the evidence stromatolites with abundant crystal fans dominate in shallower water deepening through hybrid microbialite and interlayered sediments to thrombolite probably no more than a few tens of metres deep followed by breaks and renewal of microbialite growth an interpretation of open marine fully oxygenated waters for microbialite growth is consistent with ongoing parallel work that has identified bairdioid ostracods in the microbialite a group known to be open marine however other researchers have proposed low oxygen conditions for permian triassic boundary facies globally so work continues to confirm whether the curuk dag microbialite grew in dysoxic or normally oxygenated conditions the principal stimulus for post extinction microbialites is likely to be carbonate supersaturation of the oceans the microbialite sequence is overlain by a further 25 m of grainstone packstone without microbialite followed by early triassic shales overall microbialites form a thin aggradational sequence during an overall relative sea level rise consistent with global eustatic rise following the late permian lowstand

seven years of postseismic deformation following the 2003 mw 6 8 zemmouri earthquake algeria from insar time series 1 we study the postseismic surface deformation of the mw 6 8 2003 zemmouri earthquake northern algeria using the multi temporal small baseline insar technique insar time series obtained from 31 envisat asar images from 2003 to 2010 reveal sub cm coastline ground movements between cap matifou and dellys two regions display subsidence at a maximum rate of 2 mm yr in cap djenet and 3 5 mm yr in boumerdes these regions correlate well with areas of maximum coseismic uplifts and their association with two rupture segments inverse modeling suggest that subsidence in the areas of high coseismic uplift can be explained by afterslip on shallow sections 5 km of the fault above the areas of coseismic slip in agreement with previous gps observations the earthquake impact on soft sediments and the ground water table southwest of the earthquake area characterizes ground deformation of non tectonic origin the cumulative postseismic moment due to 7 years afterslip is equivalent to an mw 6 3 earthquake therefore the postseismic deformation and stress buildup has significant implications on the earthquake cycle models and recurrence intervals of large earthquakes in the algiers area

stress transfer among en echelon and opposing thrusts and tear faults triggering caused by the 2003 mw 6 9 zemmouri algeria earthquake stress transfer among en echelon and opposing thrusts and tear faults triggering caused by the 2003 m w 6 9 zemmouri algeria earthquake 1 the essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated first through idealized examples and then by study of thrust faulting in algeria we calculated coseismic stress changes caused by the 2003 m w 6 9 zemmouri earthquake finding that a large majority of the zemmouri afterslip sites were brought several bars closer to coulomb failure by the coseismic stresses while the majority of aftershock nodal planes were brought closer to failure by an average of 2 bars further we calculated that the shallow portions of the adjacent thenia tear fault which sustained 0 25 m slip were brought 2 bars closer to failure we calculated that the coulomb stress increased by 1 5 bars on the deeper portions of the adjacent boumerdes thrust which lies just 10 20 km from the city of algiers both the boumerdes and thenia faults were illuminated by aftershocks over the next 6 years the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity the stress also increased by 0 4 bar on the east sahel thrust fault west of the zemmouri rupture algiers suffered large damaging earthquakes in a d 1365 and 1716 and is today home to 3 million people if these shocks occurred on the east sahel fault and if it has a 2 mm yr tectonic loading rate then enough loading has accumulated to produce a m w 6 6 6 9 shock today thus these potentially lethal faults need better understanding of their slip rate and earthquake history among en echelon and opposing thrusts and tear faults triggering caused by the 2003 m w 6 9 zemmouri algeria earthquake

seafloor precipitated carbonate fans in the neoproterozoic rainstorm member johnnie formation death valley region usa cm sized carbonate seafloor fans occur in the neoproterozoic rainstorm member of the johnnie formation death valley usa the fans formed in a mixed carbonate clastic succession near storm wave base at the base of parasequences on a storm dominated ramp petrographic observations indicate that the fans were originally precipitated as aragonite and later inverted to calcite during diagenesis although not directly dated the rainstorm member preserves a large magnitude negative carbon isotopic anomaly down to 11 pdb tentatively correlated to the largest known carbon isotope excursion found in many stratigraphic successions around the world between 585 ma and 550 ma thus the age distribution of seafloor aragonite fans in neoproterozoic strata appears more widespread than previously thought occurring in strata significantly younger than the last widespread neoproterozoic glaciation rainstorm member carbonate fans and oolitic units representing time correlative shallower environments record similar carbon isotope ratios during the negative carbon isotopic anomaly the carbon isotopic homogeneity displayed between fans and other carbonate sediments implies that waters across the shelf were well mixed rather than stratified during the late neoproterozoic isotopic anomaly in addition the similarity of carbon isotope ratios shared among fans along a stratigraphic horizon on a m to cm scales suggests that the local source of alkalinity required for fan growth was derived from a well mixed reservoir likely seawater rather than local diagenetic fluids increased alkalinity and the presence of inhibitors to carbonate nucleation perhaps fe2 under anoxic conditions likely fostered precipitation of aragonite crystal fans on the seafloor

characteristics distribution and morphogenesis of subtidal microbial systems in shark bay australia abstract the distribution nature and extent of microbial deposits in hamelin pool shark bay have been investigated and mapped with emphasis on the occurrence external morphologies internal fabrics constructional mechanisms microbial communities growth rates and sediment associations in the intertidal and previously little researched subtidal zone detailed georeferenced substrate mapping revealed extensive subtidal microbial deposits occupying approximately 300 km 2 of the total holocene 1400 km 2 area of hamelin pool the microbial pavement covers 227 km 2 of the subtidal substrate that together with columnar structures reveals a subtidal microbial habitat which occupies an area 10 times larger than the area of the intertidal deposits microbial carbonate is composed of aragonite 80 98 that reveals high positive values of 13 c 4 46 to 5 88 and 18 o 3 06 to 3 88 as a characteristic of the highly evaporative environment with extensive microbial activity oldest dated heads are 1915 and 1680 14 c years bp and the overall system was deposited in two stages the first between 2000 and 1200 and the last from 900 years bp to the present slow growth rates vary from less than 0 1 mm year to 0 5 mm year different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities and lateral fabric relations have been established evidence of shallowing upward fabric sequences of microbial origin reflects relative falling sea levels during the late holocene and is likely useful in ancient environmental interpretation a sequence of events and mechanisms are described emphasizing differences between the stromatolitic thrombolitic and cryptomicrobial deposits in shark bay the new substrate map and depositional history for this distinctive and peculiar microbial habitat establish the significance of subtidal structures and emphasize the geoscientific importance of hamelin pool especially with respect to early life studies and ancient analogues for understanding microbial activity deposit characteristics fenestral fabrics and distribution

flexural behaviour of the north algerian margin and tectonic implications the algerian margin formed through back arc opening of the algerian basin mediterranean sea resulting from the roll back of the tethyan slab recent geophysical data acquired along the algerian margin showed evidence of active or recent compressive deformation in the basin due to the ongoing africa eurasia convergence published data from four wide angle seismic profiles have allowed imaging the deep structure of the algerian margin and its adjacent basins in this study we converted these velocity models into density models then into isostatic anomalies this allowed us to image an isostatic disequilibrium relative to a local isostasy model reaching a maximum amplitude at the margin toe converting isostatic anomalies into moho depth variations shows that the moho extracted from wide angle seismic data is deeper than the one predicted by a local isostasy model in the oceanic domain and shallower than it in the continental domain these anomalies can be interpreted by opposite flexures of two plates separated by a plate boundary located close to the margin toe we use a finite element model to simulate the lithospheric flexure the amplitude of the equivalent vertical moho deflection is larger in the central part of the study area 6 7 km than on the easternmost and westernmost profiles 3 km the effective elastic thickness used to best match the computed deflection is always extremely low always less than 10 km and probably reflects the relatively low strength of the lithosphere close to the plate boundary comparison with other wide angle seismic profiles across an active and a passive margin show that the north algerian margin displays isostatic anomalies close to that of an active margin finally plate flexure is highest at the southern tip of the ocean continent transition possibly indicating that a former passive margin detachment is reactivated as a crustal scale reverse fault pre dating a future subduction

peritidal carbonate platform growth and cyclicity in an early proterozoic foreland basin upper pethei group northwest canada abstract the upper pethei group is a 170 235 m thick largely limestone succession located in the east arm of great slave lake northwest canada strata accumulated in a paleoproterozoic foreland basin during the late stages of marine deposition rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages 1 marginal peritidal complex 2 back barrier lagoon 3 shelf margin 4 subtidal shelf and 5 slope and basin carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae water column precipitation of carbonate mud and generation and transport of nonskeletal grains stromaclasts ooids oncoids peloids and intraclasts the upper pethei platform is interpreted to have evolved through three basic platform geometries peritidal rimmed shelf subtidal rimmed shelf and subtidal open shelf in response to changes in the rate of creation of accommodation space the peritidal complex and rim behaved in a manner consistent with models of phanerozoic reefal rims and alternatively kept up gave up or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation three scales of cycle are recognized 1 overall formation scale upward deepening of the platform 2 laterally traceable decameter scale shallowing upward cycles separated by marine flooding surfaces and 3 laterally discontinuous meter scale shal owing upward peritidal cycles the first two cycle types are interpreted to have been allogenic controlled by changes in eustasy and subsidence rates meter scale cycles are interpreted as mainly autogenic having formed in an aggradational tidal island model the upper petrel platform is similar to other paleoproterozoic foreland basin carbonate platforms important differences however include 1 laterally discontinuous peritidal cycles 2 lack of widespread sand shoals inboard of the shelf rim 3 a thick predominantly carbonate slope and basin succession 4 moderate to low platformal micrite content 5 extensive lagoonal ooid sands and 6 low contents of terrigenous clastics

feeding volcanoes of the kluchevskoy group from the results of local earthquake tomography 1 we present a seismic model of the area beneath the kluchevskoy volcano group kamchatka russia based on the tomographic inversion of more than 66000 p and s arrival times from more than 5000 local earthquakes that occurred in 2004 and that were recorded by 17 permanent stations below a depth of 25 km beneath the kluchevskoy volcano we observed a very strong anomaly in the vp vs ratio that reached as high as 2 2 this is a probable indicator of the presence of partially molten material with a composition corresponding to deeper mantle layers the upper part of this anomaly at a depth of 25 30 km coincides with a cluster of strong seismicity that can be explained by strong mechanical stresses in the lowermost crust due to magma ascension water release and or phase transitions in the crust we observed regular seismicity clusters that link the mantle anomaly with the kluchevskoy volcano and most likely indicate the paths of magma migration between depths of 8 and 13 km we see several patterns of high vp vs ratios interpreted as intermediate depth magma storages directly below the kluchevskoy volcano we observed a shallow body of high vp vs which probably represents the activated magma chamber just beneath the volcano cone which erupted in the beginning of 2005 the existence of three levels of magma storage based on results of local earthquake tomography may explain the variety of the lava composition and eruption regimes in different volcanoes of the kluchevskoy group

microbial biomineralization processes forming modern ca mg carbonate stromatolites abstract modern ca mg carbonate stromatolites form in association with the microbialmat in the hypersaline coastal lagoon lagoa vermelha brazil thestromatolites although showing diversi ed fabrics characterized by thin orcrude lamination and or thrombolitic clotting exhibit a pervasive peloidalmicrofabric the peloidal texture consists of dark micritic aggregates of veryhigh mg calcite and or ca dolomite formed by an iso oriented assemblage ofsub micron trigonal polyhedrons and organic matter limpid acicular crystalsof aragonite arranged in spherulites surround these aggregates unlike thearagonite crystals organic matter is present consistently in the dark micriticcarbonate comprising the peloids this organic matter is observed as sub micron at and lamentous mucus like structures inside the interspaces of thehigh mg calcite and ca dolomite crystals and is interpreted as the remains ofdegraded extracellular polymeric substances moreover many fossilizedbacterial cells are associated strictly with both carbonate phases these cellsconsist mainly of 0ae2to4lm in diameter sub spherical rod like and lamentous forms isolated or in colony like clusters the co existence offossil extracellular polymeric substances and bacterial bodies associated withthe polyhedrons of ca mg carbonate implies that the organic matter andmicrobial metabolism played a fundamental role in the precipitation of theminerals that form the peloids by contrast the lack of extracellular polymericsubstances in the aragonitic phase indicates an additional precipitationmechanism the complex processes that induce mineral precipitation in themodern lagoa vermelha microbial mat appear to be recorded in the studiedlithi ed stromatolites sub micron polyhedral crystal formation of high mgcalcite and or ca dolomite results from the coalescence of carbonatenanoglobules around degraded organic matter nuclei sub micron polyhedralcrystals aggregate to form larger ovoidal crystals that constitute peloids subsequent precipitation of aragonitic spherulites around peloids occurs asmicro environmental water conditions around the peloids change keywords bacterial fossils biomineralization microbialite nanoglobules peloids stromatolites

late proterozoic aragonitic cement crusts bambui group minas gerais brazil the late proterozoic pedro leopoldo facies bambui group in the vicinity of belo horizonte brazil comprises alternating laminated microsparitic limestones 10 35 mm thick beds and fibrous limestones 10 55 mm thick the latter are composed of a mosaic of sparry calcite crystals these irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared off growth zones ghosts of an original fibrous mineral hexagonal in cross section are visible the petrographic characteristics very high strontium content and low magnesium content of the fibrous beds as well as microspar beds strongly argue for an original aragonitic mineralogy the rays are interpreted as having formed by precipitation at the sediment water interface whereas the micrite was precipitated from the water column prior to deposition on the sea floor the lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions

fault location and source process of the boumerdes algeria earthquake inferred from geodetic and strong motion data the boumerdes earthquake occurred on a fault whose precise location offshore the algerian coast was unknown geodetic data are used to determine the absolute position of the fault the fault might emerge at about 15 km offshore accelerograms are used to infer the space time history of the rupture using a two step inversion in the spectral domain the observed strong motion records agree with the synthetics for the fault location inferred from geodetic data the fault plane ruptured for about 18 seconds the slip distribution on the fault indicates one asperity northwest of the hypocenter with maximum slip amplitude about 3 m this asperity is probably responsible for most of the damage another asperity with slightly smaller slip amplitude is located southeast of the hypocenter the rupture stops its westward propagation close to the thenia fault a structure almost perpendicular to the main fault

thrombolitic clots dominated by filamentous cyanobacteria and crusts of radio fibrous calcite in the furongian changshan formation north china abstract as a variety of non laminated microbial carbonate or calcified microbialite thrombolites commonly comprise diffusely clotted micrites as their basic structural unit the current study is aimed at discussing the composition and forming mechanism of thrombolitic clots which most likely represent the product formed by calcification of extracellular polymeric substances it provides instances of sophisticated calcification of thrombolite forming microbial mat in the calcite sea that is expressed by the distinctive composition of thrombolites thrombolites in the top part of the furongian changshan formation are chiefly composed of chaotic large and small micritic clots as observed under low microscopic resolution however high density preservation of fossils of calcified sheaths of cyanobacteria is observed within most of the clots under higher resolution their distribution as groups or colonies suggests that they can be interpreted as subtifloria of girvanella group these microbial components together with the enigmatic epiphyton reflect fundamental features of microbial growth and suggest a microbial origin of these clots the general abundance of these calcified fossils within individual clots offers an instance for the formation of thrombolitic clots from sophisticated calcification of photosynthetic biofilms it refutes the traditional view that both the thrombolites and the oncoids develop by accretion of microbial mats dominated by coccoid cyanobacteria aggregation of several individual clots to form millimetric agglutinated clots shows that their formation is genetically related to sophisticated calcification of extracellular polymeric substances making up multiple biofilms within relatively thick microbial mats this inference is strengthened by their encrustation by radio fibrous calcite which leads to enlargement of the clots besides the clots benthic ooids pyrite and dolomite also developed in the current thrombolites which are largely attributed to microbially mediated organomineralization

bacterially induced dolomite precipitation in anoxic culture experiments to study the process of microbial mediated dolomite formation growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in lagoa vermelha a hypersaline lagoon in brazil where dolomite precipitation occurs specifically we report the isolation of a particular strain of sulfate reducing bacteria lvform6 from lagoa vermelha sediment which apparently promotes the formation of nonstoichiometric dolomite sulfate reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 c the precipitates have morphologies similar to those observed in lagoa vermelha sediment our results demonstrate that sulfate reducing bacteria can influence dolomite precipitation under controlled lowtemperature anoxic conditions and imply that anaerobic microorganisms can play an important role in carbonate sedimentation they may have been particularly significant in earth s earliest history when a more reducing atmosphere existed

microbes produce nanobacteria like structures avoiding cell entombment microsedimentary structures referred to as nanobacteria like particles were described from modern carbonate environments where they form in close spatial association with sulfate reducing bacteria srb however the exact mechanism of their formation as well as their paleontological significance remains controversial here we report on an investigation of microbe mineral interactions in experimentally produced carbonate globules the experiments were carried out under anoxic conditions at 30 c with desulfovibrio brasiliensis a srb known to mediate dolomite formation we observed that extracellular polymeric substances eps secreted by the microbial community play a key role in the mineralization process nanobacteria like particles represent the early stage of carbonate nucleation within the eps which progressively evolve to larger globules displaying a grainy texture we excluded the possibilities that these structures are fossils of nanobacteria dissolution surfaces or artifacts created during sample preparation d brasiliensis cells are predominantly located outside of the eps aggregates where mineral growth takes place as a result they remain mobile and are rarely entombed within the mineral this self preservation behavior may not be limited to d brasiliensis other microbes may produce or may have produced during the geological past biogenic minerals through a similar process mineralization within eps explains why microbial relics are not necessarily present in biogenic carbonates

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

extremely low oxygen concentration in mid proterozoic shallow seawaters abstract the mid proterozoic 1 8 0 8 ga witnessed the first appearance but unusually low diversification of eukaryotes the stagnant biotic evolution during this billion year long period commonly referred to as the boring billion was arguably ascribed to low oxygen levels in atmosphere and ocean however evidence supporting low oxygen in shallow marine environments where early eukaryotes first evolved is generally lacking or insufficient here we report rare earth element and yttrium ree y data particularly cerium ce anomalies from a suite of mid proterozoic sedimentary rocks of the north china platform the new data from north china in combination with available ce anomaly data from other proterozoic successions demonstrate that during mid proterozoic negative ce anomalies did not occur until 1 54 ga and after 1 54 ga only episodic negative ce anomalies were present in shallow water carbonates trace element enrichments u ef v ef and mo ef remained at the average continental crust level before 1 54 ga but showed a sudden increase at 1 54 ga the data suggest that oxygen concentration in shallow marine environments of the mid proterozoic ocean was extremely low probably

microbialites of the neoproterozoic beck spring dolomite southern california ancient microbialites reflect interactions between microbial communities and environmental conditions however evaluating the relative roles of microbial community processes and environmental influences on microbialite morphology and internal fabric in the rock record can be challenging the neoproterozoic beck spring dolomite preserves diverse microbialites and thus provides an opportunity to explore the factors that influenced microbialite development locally stromatolitic thrombolitic and composite microbialites are abundant in subtidal to upper intertidal carbonates in the beck spring dolomite thrombolitic and composite microbialites have not been recognized previously in this unit but compose much of the newly defined thrombolitic member stratigraphic relationships demonstrate that these three types of microbialites formed in close spatial and temporal association in subtidal to intertidal environments the relative proportions and distributions of stromatolitic and thrombolitic microbialites vary with depositional environment stromatolitic microbialites dominate in deeper intertidal to subtidal facies whereas thrombolitic textures are more abundant in upper intertidal facies composite microbialites composed of intermingled clotted and laminated textures formed in all environments but are most abundant in intertidal facies the broad environmental distribution of stromatolitic thrombolitic and composite microbialites and the intermingling of textures suggest that laminated and clotted textures reflect diverse microbial community morphologies rather than environmental variations furthermore the ca 750 ma age of thrombolitic microbialites in the beck spring dolomite requires that they formed without the influence of calcimicrobes or metazoans colonizing and grazing the microbial mat surface thus these thrombolites provide further evidence that the biostratigraphic distribution of thrombolites cannot be uniquely attributed to evolution of calcifying and grazing organisms in the earliest cambrian and that older microbial communities were capable of producing clotted textures

morphogenesis and facies relationships of thrombolites and siliciclastic stromatolites in a cambrian tidal sequence elat area southern israel abstract thrombolites and siliciclastic stromatolites occur within the lower cambrian marine timna formation in the elat area southern israel this rock succession consists of four consecutive stratal suites which together represent a transgressive regressive cycle within a peritidal depositional system the thrombolites are associated with the sea ingression stage and occur as a series of massive bedded strata with laminoid fenestral layers and with a microclotted peloidal fabric of the micritic material sem eds study of the clotted and peloidal micrite reveals a calcitic framework of pitted polygonally shaped units 2 m wide together with a few groups of compressed large hollow structures these forms are interpreted as the possible remnants of bacterial decomposers and of coccoid dominated mat forming unicells respectively the intimate structural connection between the carbonate fraction and the microbe structures suggest in situ carbonate production through bacterial decay of the cyanobacterial material during its self burial with carbonate precipitating as solid phases on vacant extracellular envelopes the siliciclastic stromatolites developed during seashore progradation and now occur as rare laterally linked columnar forms within channelled cross stratified sandstones at the top of the timna succession they are finely laminated with a low to moderate degree of inheritance of the laminae and are partly burrowed by skolithos textures and the geometry of the siliciclastic stromatolites suggest that they were formed through binding of transported quartz sand sized particles by bottom dwelling filamentous microorganisms sediment production and the growth pattern of the two structures within the timna sequence seem to have been regulated by different microbial assemblages which shifted in response to changes in environmental factors through time and space the differential distribution of the two buildups in the sedimentary sequence reflects the difference in balance between the biological attributes of the two biotas and the physical environment pitted architectures such as those displayed by the timna thrombolites could be an ontogenetic fingerprint of microbially constructed rocks which might help to recognize them in the fossil record when other evidence is lacking

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

spatial correlation of aftershock locations and on fault main shock properties 1 we quantify the correlation between spatial patterns of aftershock hypocenter locations and the distribution of coseismic slip and stress drop on a main shock fault plane using two nonstandard statistical tests test t1 evaluates if aftershock hypocenters are located in low slip regions hypothesis h1 test t2 evaluates if aftershock hypocenters occur in regions of increased shear stress hypothesis h2 in the tests we seek to reject the null hypotheses h0 aftershock hypocenters are not correlated with 1 low slip regions or 2 regions of increased shear stress respectively we tested the hypotheses on four strike slip events for which multiple earthquake catalogs and multiple finite fault source models of varying accuracy exist because we want to retain earthquake clustering as the fundamental feature of aftershock seismicity we generate slip distributions using a random spatial field model and derive the stress drop distributions instead of generating seismicity catalogs we account for uncertainties in the aftershock locations by simulating them within their location error bounds our findings imply that aftershocks are preferentially located in regions of low slip u umax and of increased shear stress 0 in particular the correlation is more significant for relocated than for general network aftershock catalogs however the results show that stress drop patterns provide less information content on aftershock locations this implies that static shear stress change of the main shock may not be the governing process for aftershock genesis

microbialites in a modern lagoonal environment nature and distribution tikehau atoll french polynesia abstract microbialites including stromatolitic structures gelatinous masses and mats develop in tikehau lagoon tuamotu french polynesia on the flanks and slopes of numerous pinnacles and motu islets between 0 and 25 m deep on coral colonies algal tufts or on lagoonal sediments individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive phormidium species symploca and possibly to one new species of schizothrix gelatinous masses hanging from the ceiling of caverns are produced by phormidium laysanense microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations the study of micro and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation in addition to phototrophic biomineralization pervasive carbonate precipitation occurred in association with decaying organic matter the basic constructional elements of the precipitated carbonate consist of 0 1 0 2 m rounded bodies that form very fine anhedral to subhedral micrite their surfaces comprise extremely small carbonate granules 20 50 nm gradual changes ageing in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities possibly induced by temporary and local nutrient pulses nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients

nucleation of calcium carbonate on bacterial nanoglobules nucleation of calcium carbonate on microbial cell material may have been the dominant mode of microbial carbonate formation during most of earth s history current knowledge predicts that nucleation takes place on the cell surface or on extracellular polymeric substances however the initial nucleation steps have not been described in detail and the process remains elusive here we describe the bacterial nucleation of calcium carbonate at the nanometer scale in our precipitation experiment with sulfate reducing bacteria srb the bulk of calcium carbonate precipitates on hundreds of individual globules 60 200 nm in diameter globules originate from the srb cell surface but calcify significantly only when released to the culture medium similar globules have been observed albeit at a much larger scale in other bacterial precipitation experiments and in many natural microbial carbonates suggesting that the process we describe could be an important step in microbial calcification

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

mesoproterozoic biogenic thrombolites from the north china platform thrombolites are abundant in the subtidal dolostones of the mesoproterozoic wumishan formation ca 1 50 1 45 ga in the north china platform three major components are identified within the thrombolites irregular mesoclots micritic matrix and spar filled voids the mesoclot generally comprises a relatively organic rich micritic core and a microsparitic outer layer that consists of fibrous aragonite pseudocrystals with less organic matter in the core of mesoclots abundant fossilized organic remnants such as putative coccoidal and filamentous bacteria and mucus to film like extracellular polymeric substance eps are closely associated with organomi nerals including nanoglobules and submicron scale poly hedrons in exceptionally well preserved mesoclots their outer layers commonly contain micropores displaying as bacterial molds and filamentous bacteria fossils the matrix of mesoclots consists mainly of micropeloids 20 30 l mi n diameter and minor terrigenous detritus some mesoclots have denticulate edges and their matrix shows growth laminations that envelope the outlines of mesoclots these features indicate that the mesoclots are primary and they were mineralized earlier than the surrounding matrix the mineralization of mesoclots may have proceeded in two stages 1 organomineralization of the cores through replacement of organic matter by minute organominerals resulting from anaerobic degradation of bacteria and eps and 2 inorganic precipitation of the outer layers fostered by an increase in carbonate alkalinity in micro environ ment due to organic matter decomposition the thrombo lites from the mesoproterozoic wumishan formation may have formed through complex interactions between microbes and environments and represent the earliest known precambrian biogenic thrombolites

bacterial fossils and microbial dolomite in triassic stromatolites triassic stromatolitic dolomite from italy preserves mineralized bacterial remains one of the first unequivocal identifications of such structures in the geological record they consist of empty spheroids 1 0 m diameter resembling coccoid bacteria and smaller 150 400 nm objects interpreted as dwarf bacterial forms occurring within and between syn sedimentary dolomite crystals moreover gently folded sheets 100 200 nm thick and several micrometers long form a sub polygonal network reminiscent of eps extracellular polymeric substance their granular textured surfaces suggest bacterial degradation of original organic matter these features confirm a biological origin for the stromatolites as in modern microbial mats and the preserved original geochemical signatures indicate early precipitation of mg carbonates induced through microbial sulfate reducing metabolic activities

full aftershock sequence of the m w 6 9 2003 boumerdes earthquake algeria space time distribution local tomography and seismotectonic implications we present a detailed analysis of the aftershocks of the may 21 2003 boumerdes earthquake m w 6 9 recorded by 35 seismological stations and 2 obs deployed in the epicentral area this network recorded the aftershock activity for about 1 year and resulted in locating about 2500 events the five main aftershocks 4 7 m 5 8 display thrust faulting consistent with the main shock except for the second event m5 8 29 05 2003 which depicts a strike slip focal solution at the western tip of the rupture zone most aftershocks are clustered near the main rupture plane in the footwall or at the westernmost tip of the 2003 boumerdes rupture area many aftershocks last over the whole seismic crisis ahead north of the main rupture zone forming a diffuse low angle surface within the footwall where the coseismic static stress change is predicted to increase at the sw tip of the rupture short lived clusters locate at intersections of faults near the contact between the inner kabylia and outer tell zones the tomographic inversion depicts high velocity p and s wave anomalies coinciding with miocene magmatic intrusive bodies in the upper crust partially hidden by surrounding basins the area of the main shock is associated with a large low velocity body subdivided into sub domains including neogene basins on land and offshore our results support a rupture model strongly controlled by geological inhomogeneities and extending as ramp flat ramp systems upward favoring heterogeneous slip and segmentation in the fault plane with strong afterslip toward the surface the diffuse aftershock activity in the footwall evidences an inherited discontinuity at mid crustal depth that we interpret as the contact of kabylian and african tethyan continental crusts that were stacked during the upper miocene collision

earthquake induced flooding and paleoseismicity of the el asnam algeria fault related fold late quaternary activity of the el asnam thrust fault responsible for the october 10 1980 ms 7 3 earthquake and 36 km coseismic faulting is explored through diverse paleoseismological field methods we trenched through the main and secondary 1980 coseismic scarps made an analysis of earthquake induced flood deposits conducted a geomorphic study of leveling profiles and offset geological units across thrust and normal scarps and considered the geomorphology of the active fold twenty five radiocarbon ages from colluvial and alluvial deposits along with conventional fossils and artefacts dating of geological units show the timing of successive net vertical displacements on the thrust related fold and bending moment faults previous work showed that eight pre 1980 earthquakes occurred in clusters during the late holocene cumulative net vertical slip that amounts to 3 times the 1980 displacement is observed on multiple thrust fault scarps and offsets of geological units on bending moment faults maximum cumulative vertical displacements which are visible in different zones along the fault segments can reach 10 to 25 times the 1980 coseismic displacements the late quaternary uplift rate on the el asnam fault is 0 6 mm yr and taking into account slip variation along the fault this rate may range between 0 25 1 6 mm yr from which we obtain a shortening rate of 0 17 1 2 mm yr because the tell atlas mountains exhibit comparable seismogenic fault related folds with parallel and sequential structures of active faults a total average shortening rate of 2 05 mm yr during the late quaternary represents part of the convergence accommodated along the africa eurasia plate boundary

coastal uplift and thrust faulting associated with the mw 6 8 zemmouri algeria earthquake of 21 may 2003 1 a shoreline uplift marked by a continuous white band visible at rocky headlands occurred during the 21 may 2003 earthquake mw 6 8 in northern algeria we measured the amount of coastal uplift on a white band emerged algae and harbors quays between boumerdes and dellys most of measured points were collected using tape and differential gps on rocky headlands with 0 15 m error bar tidal prism leveling lines running parallel and orthogonal to the coast also provide the precise amount of uplift in the epicentral area the uplift distribution shows an average 0 55 m along the shoreline with a maximum 0 75 m east of boumerdes and a minimum close to 0 near cap djinet the active deformation related to a thrust fault is modeled along the 55 km coastline the dislocation model predicts surface slip on a n 54 e trending reverse fault dipping 50 se in agreement with cmt solution and coastal uplift the faulting characteristics imply a fault geometry with possible sea bottom ruptures between 5 to 10 km offshore

mass occurrence of oncoids at the cambrian series 2 series 3 transition implications for microbial resurgence following an early cambrian extinction abstract the traditional lower middle cambrian transition cambrian series 2 series 3 transition is marked by the first major biotic extinction of the phanerozoic eon this biotic crisis has been arguably linked to changes in ocean chemistry and or marine environments but their casual relationships remain controversial to better understand the microbial responses to paleoceanographic changes across this critical transition we have studied the environmental conditions for mass occurrence of oncoids in the western north china platform oncoids at the lower middle cambrian transition form 1 to 3 m thick massive beds show spherical subspherical morphology and contain 8 14 light dark cortical laminar couplets the light laminae are thicker and contain densely intertwined filamentous cyanobacteria that have calcified sheaths and a prostrate growth pattern the dark laminae are thinner and rich in organic matter relics pyrite framboids and heterotrophic bacteria in most oncoids cortical laminae show the same growth orientation for more than five light dark laminar couplets suggesting much less frequent grain overturning than generally thought both light and dark laminae contain well preserved organomineralization fabrics textures including extracellular polymeric substances epss nanoglobules polyhedrons and micropeloids suggesting oncoid formation in shallow marine environments with high alkalinity and active sulfate reduction the presence of pyrite framboids and heterotrophic bacterial relics implies anoxic dysoxic bottom water conditions stratigraphic correlation indicates that time equivalent oncolites and other microbialites are widespread not only in north china but also in other early cambrian successions globally the mass occurrence of oncoids coincides with the kalkarindji large igneous province of australia a prominent negative 13 c excursion roece or redlichiid olenellid extinction carbon isotope excursion a significant increase in 87 sr 86 sr and a large positive shift in 34 s the coincidence of these events suggests that the early cambrian biotic crisis may have been caused by an ocean anoxic event resulting from enhanced volcanic release of co 2 global warming and increased continental weathering preservation of massive oncolites is likely related to decreased metazoan grazing following an early cambrian biotic crisis during which archaeocyathids and many early cambrian trilobite taxa went to extinction the oncoid mass occurrence provides evidence for the resurgence of microbial life in anoxic dysoxic marine shelf environments concomitant with the early cambrian extinction event

biosedimentology of a silurian thrombolite reef with meter scale growth framework cavities a completely dolomitized silurian reef at maumee ohio u s a is uncommon among early paleozoic reefs in that its framework consists of about 95 thrombolite boundstone and 5 corals the reef lithified syndepositonally as evidenced by centimeter to meter scale growth framework cavities within it that are completely filled with internal sediment the largest cavities 2 3 5 m in diameter are prob ably the largest ever recorded in a thrombolite reef four main types of components are distinguished in the reef frame work 1 dense bushy mesoclots 2 dense rounded to subrounded mesoclots 3 saccate microstructures and 4 crusts mostly struc tureless but also containing dense bushy mesoclots the dense bushy components are abundant rounded to sub rounded mesoclots less so and the remaining components are rare the dense bushy and rounded to subrounded mesoclots are interpreted to represent the calcified re mains of what were initially either aggregates of the probable filamen tous cyanobacterum angulocellularia or aggregates of renalcid calci microbes or both and the saccate mesoclots as the calcimicrobe ren alcis sensu stricto it is speculated that syndepositonal lithification of the reef framework was accomplished by impregnation of unlithified protomesoclots by micrite to create mesoclots along with the formation of micritic cement around and between mesoclots that served to fuse together successive generations of mesoclots the direction of branching of bushy mesoclots in the reef indicates that unlithified protomesoclots were able to grow upward laterally and downward this ability along with syndepositional lithification to create mesoclots and to cement them together enabled the protome soclots and mesoclots to create reef knobs and eventually to roof over spaces that were centimeters to three or more meters in width and height to create growth framework cavities features indicative of rap id deposition including graded bedding poor sorting of bioclasts and textural inversion are common in internal sediment in meter size cav ities these features suggest that storms possibly hurricanes were at least as important and probably more so than everyday wave en ergy in transporting sediment into all cavities in the reef regardless of size

micromorphology and genesis of quaternary stromatolitic tufa crust within the exposed cretaceous rayda formation in al jabal al akhdar oman abstract this study aimed to improve the understanding of the causes of lamination in terrestrial tufa using samples of terrestrial quaternary stromatolitic tufa deposits occurring within narrow karst grooves carved along the bedding planes of the exposed cretaceous rayda formation in al jabal al akhdar oman these stromatolitic tufa deposits consist wholly of low mg calcite and are formed of vertically stacked laminated planar and digitate like columnar structures lamination is exhibited as vertically stacked couplets of dark micritic and light sparitic laminae significant variations in the abundance and thickness of the micritic laminae as well as progressive changes in the size and fabric of calcite in the sparitic laminae were recognised thorough microscopic and nanoscopic examinations revealed that these laminations initially developed as stacked indistinct micritic laminae formed of calcitic microbialite that had initially precipitated within a biologically active microbial biofilm which thrived on the wet bedrock surface this was followed by the growth of microglobules at certain horizons within the microbialitic laminae forming incipient discontinuous thin sparitic laminae consisting of laterally stacked biogenic skeletal disphenoidal calcite microcrystals the latter were further growing via abiogenic syntaxial precipitation of calcite forming coarsely crystalline dagger shape bladed calcite crystals these processes resulted in the formation of well developed lamination it is therefore suggested that the stromatolitic lamination has been generated through five consecutive stages of biogenic and abiogenic precipitation of calcite within initially developed microbial biofilms stable isotope values 13c and 18o suggest that these tufa deposits had precipitated from continuously renewed freshwater enriched with bicarbonate dissolved from the marine limestone bedrock during a cold pluvial period

geochemical and stable isotope signatures of proterozoic stromatolitic carbonates from the vempalle and tadpatri formations cuddapah supergroup india implications on paleoenvironment and depositional conditions abstract spectacular stromatolitic dolomites with varied morphological features are preserved in the vempalle and tadpatri formations of the proterozoic cuddapah basin eastern dharwar craton they exhibit stratifera columnar conical domal and spheroidal lobate morphologies in which columns are branched at some places in tadpatri formation based on their trace elements such as rey and stable isotope compositions carbon and oxygen the vempalle stromatolites have been classified as three types vempalle i stromatolites exhibit pronounced positive eu anomalies with small scale positive ce anomalies whereas vempalle ii counterparts have slight negative to positive eu feeble positive ce and positive gd anomalies vemaplle iii stromatolites display positive ce eu and gd anomalies the tadpatri formation stromatolites have pronounced gd anomalies with low order positive ce and eu anomalies significant depletion in 18o is observed in vempalle ranging from 10 70 to 7 07 and 20 73 to 11 87 in tadpatri formations the tadpatri stromatolites display negative 13c varying from 4 56 to 1 26 contrarily the vempalle stromatolites recorded both positive and negative 13c ranging from 0 24 to 2 00 thereby reflecting their deviation from the popular belief of the boring billion concept of displaying midproterozoic 13c stasis the flat ree patterns along with ce eu and gd anomalies chondritic to superchondritic y ho ratios collectively reflect on marine water conditions for their deposition with variable inputs of siliciclastic terrigenous materials and hydrothermal signatures these cherty dolomitic stromatolites were deposited in subtidal intertidal supratidal zones of a shallow shelf of proterozoic open ocean under warm alkaline and anoxic conditions

implications for neoarchaean ocean chemistry from primary carbonate mineralogy of the campbellrand malmani platform south africa the precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform building process during deposition of the 2560 2520 ma campbellrand malmani carbonate platform south africa aragonite fans and fibrous coatings are common in unrestricted shallow subtidal to intertidal facies they are also present in restricted facies but are absent from deep subtidal facies decimetre thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies the proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0 to 65 depending on the facies subtidal facies commonly contain 20 35 in situ precipitated carbonate demonstrating that neoarchaean sea water was supersaturated with respect to aragonite carbonate crystal growth rates were rapid compared with sediment influx rates and the dynamics of carbonate precipitation were different from those in younger carbonate platforms the abundance of aragonite pseudomorphs suggests that sea water ph was neutral to alkaline whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans

plio quaternary reactivation of the neogene margin off nw algiers algeria the khayr al din bank the algiers region northern algeria is known to be seismically active with recurrent large m 6 earthquakes because of the lack of high resolution bathymetry the offshore structures remained for a long time poorly known thanks to a new marine data base maradja 2003 cruise the offshore part of the margin is accurately mapped and new active and recent structures are described west of the bay of algiers the margin enlarges forming the khayr al din bank interpreted as a tilted block of the passive margin born during the opening of the algero provencal basin at the slope break a 80 km long fault tip quaternary fold namely the khayr al din fault extends at the foot of the margin off nw algiers and represents the largest active structure of the coastal area together with the sahel anticline we also map for the first time a set of overlapping en echelon active folds in the upper part of the khayr al din bank located off previously known active structures on land most of these faults represent actually a threat for the algiers region in terms of seismic hazard but also geological hazards such as tsunamis as most of them depicts significant dimensions and slip rates the highest long term horizontal shortening rate is found on the khayr al din fault and is estimated at 0 5 0 1 mm yr with a maximal magnitude of 7 3 which provides one of the highest seismogenic potential in the region a new tectonic framework for the algiers region is proposed in which the main south dipping offshore structure of opposite vergence relative to most thrusts on land appears to be nowadays the main driving fault system as also found further east in the boumerdes m 6 8 2003 rupture zone the overall apparent pop up structure of the recent and active faults may result from a progressive migration of the plate limit from the late miocene north dipping suture zone on land to the quaternary south dipping main khayr al din fault at sea suggesting a process of subduction inception

nanotextures of aragonite in stromatolites from the quasi marine satonda crater lake indonesia stromatolites have been extensively used as indicators of ancient life on earth although much work has been done on modern stromatolites the extent to which biological processes control their structure and the respective contributions of biological and abiotic processes in their formation are however still poorly constrained a better description of the mineralogical textures of these formations at the submicrometre scale may help improve our understanding of how carbonates nucleate and grow in stromatolites here we used a combination of microscopy and microspectroscopy techniques to study the chemical composition and the texture of aragonite in lacustrine stromatolites from the alkaline crator lake in satonda indonesia several textural features are described including morphological variations of aragonite from nanosized grains to micrometre sized fibres the presence of striations in the aragonite laminae showing a striking similarity with growth bands in corals and clusters of small aragonite crystals sharing a common crystallographic orientation these nanotextural features are compared with those observed in scleractinian corals and possible processes involved in their formation are discussed the geological society of london 2010