calcified microbial reefs in cambrian series 2 north china platform implications for the evolution of cambrian calcified microbes abstract this study focuses on the microbial reefs of the zhushadong formation cambrian series 2 in shandong province china in order to understand the evolution of calcified microbes in the north china platform during the cambrian series 2 and 3 the microbial reefs occur in a thin unit ca 3 m thick over an area of 1 km 2 they consist of three types of thrombolite based on their mesostructures rimmed grainstone patch and dendritic the thrombolites mainly occur in various coarse grained carbonate facies including crudely stratified oolitic grainstone stromatolitic grainstone and disorganized limestone conglomerate calcified microbes in the thrombolites include epiphyton kordephyton a tubiform microbe bija tarthinia renalcis amgaina and razumovskia the zhushadong thrombolites were formed within a grainstone shoal and experienced repeated burial and exposure the rimmed thrombolite and grainstone patch thrombolite experienced abundant input of carbonate grains forming grainstone patches in contrast the dendritic thrombolite formed solely by calcification of microbes that mainly include epiphyton tarthinia and the tubiform microbe the outer crusts of the rimmed thrombolite were formed by amgaina under high energy conditions the diverse calcified microbes of the zhushadong formation form the earliest assemblage of their type in the north china platform their descendants mostly epiphyton subsequently thrived forming a ca 180 m thick microbialite oolite dominated succession during the cambrian series 3 zhangxia formation although the reefs in the zhushadong formation are much smaller than those of the overlying zhangxia formation their calcified microbes are more diverse this most likely reflects changes in depositional environments e g abundant siliciclastic input and tidal effects vs those of a stable carbonate platform and or global changes within reef environments e g end cambrian series 2 extinction of archaeocyaths and calcified microbes a decrease in diversity of calcified microbes in the north china platform where archaeocyaths were absent may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence

long term behaviour of australian stable continental region scr faults abstract australia boasts arguably the richest late neogene to quaternary faulting record in stable continental region scr crust anywhere in the world variation in fault scarp length vertical displacement proximity to other faults and relationship to topography permits division of the continent according to fault character six onshore neotectonic domains are recognised with an additional offshore domain proposed by analogy with the eastern united states each domain relates to a distinct underlying crustal type and architecture broadly considered to represent cratonic non cratonic and extended environments in general greater topographic expression associated with faults occurring in extended crust relative to non extended crust suggests a higher rate of seismic activity in the former setting consistent with observations worldwide using the same reasoning non cratonic crust might be expected to have a higher rate of seismic activity than cratonic crust this distinction together with the variation in fault character between domains should be recognised in attempts to identify analogous systems worldwide a common characteristic of morphogenic earthquake occurrence in australia appears to be temporal clustering periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence at the scale of a single fault and of proximal faults in several instances there is evidence for deformation at scales of several hundred kilometres switching on and off over the last several million years what is not clear from the limited palaeoseismological data available is whether successive active periods are comparable in terms of slip number of events magnitude of events etc regardless this apparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment psha these rely on the simplifying assumption that large earthquake recurrence for long return periods is not random i e poissonian presently our ability to incorporate such time dependent models is limited by available data

signature of enso signals in the coral growth rate record of arabian sea and indian monsoons we examine here three sets of recently published data 1 updated indian rainfall irf time series of the entire country covering the time span of 1826 1994 2 coral growth rate time series for a period of 42 years spanning 1948 1990 from the arabian sea and iii nino3 temperature records to investigate the signature of enso response of the indian monsoon multiple spectral techniques e g multi taper method mtm maximum entropy method mem wavelet and cross spectra are used to identify the coherent cyclic and nonstationary modes in these records mtm analysis of irf time series resolves statistically significant variability 90 c i i at multi decadal 66 70 year s scales related to the well known global temperature variability of internal atmospheric ocean origin ii relatively weak signals at 13 and 22 years solar cycles and iii the 2 5 to 7 5 year cycles associated with the enso frequency band the mtm spectra of the coral growth rate record also reveal statistically significant periodicities 90 c i within 1 8 4 2 enso frequency band and a relatively weak signal at 12 8 years mem analysis confirms the stability of above spectral peaks wavelet spectral analyses of the above time series reveal nonstationary localized modes of enso evolution corresponding to 2 7 years and higher order terms although matching periodicities are present in these records cross spectral analysis of irf and nino3 temperature records exhibits significant coherency 80 ci only at periods 5 4 years and 2 7 years suggesting the significant role of enso dynamics in organizing the subtle indian monsoon at these frequencies these results may provide significant implication for the modeling of indian monsoon

late jurassic epiphyton like cyanobacteria indicators of long term episodic variation in marine bioinduced microbial calcification abstract epiphytaceans occur in late jurassic shallow marine reef limestones at several localities in the carpathian mountains of romania these calcified dendritic microfossils are well preserved in sparry calcite as sub millimetric radial clusters of narrow well defined filaments 10 30 m in diameter that show dichotomous branching and consist of dense dark micrite locally with tubiform structure epiphytaceans are widely interpreted as photosynthetic algae or bacteria but their precise affinities remain elusive and the group may be heterogeneous these late jurassic examples most closely resemble cambrian tubomorphophyton and late devonian paraepiphyton we interpret them to be calcified cyanobacterial sheaths post devonian records of epiphytaceans are extremely scarce the kimmeridgian tithonian specimens reported here represent one of the youngest known occurrences of epiphytaceans their highly sporadic geological distribution resembles that of marine calcified cyanobacteria which show phanerozoic abundance peaks in the early paleozoic late devonian mississippian and late jurassic early cretaceous we propose that kimmeridgian tithonian epiphytacean cyanobacteria reflect environmental conditions that favored bioinduced calcification in particular elevated seawater carbonate saturation state

holocene turbidites record earthquake supercycles at a slow rate plate boundary ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment especially where recurrence times overstep historical records we show that most turbidites emplaced at the africa eurasia plate boundary off west algeria over the past 8 k y correlate across sites fed by independent sedimentary sources requiring a regional trigger correlation with paleoseismic data inland and ground motion predictions support that m 7 earthquakes have triggered the turbidites the bimodal distribution of paleo events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults 13 paleo earthquakes underpin clusters of 3 6 events with recurrence intervals of 300 600 yr separated by periods of quiescence of 1 6 k y without major events on other faults over the study area this implies broad phases of strain loading alternating with phases of strain release our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models

temperature dependence of the oxygen isotopic fractionation between diatom silica and water abstract we present a new paleotemperature scale based on the oxygen isotopic ratio of the non exchangeable fraction of the oxygen from diatom silica the equation t 17 2 2 4 18osilica 18owater 40 0 2 18osilica 18owater 40 2 was derived using recent sediment samples from different oceanic areas the temperature and isotopic composition of the local surface water comparison of our results with other relationships established for quartz water or amorphous silica water at higher temperature suggests no difference in isotopic fractionation between quartz water and biogenic silica water couples

a diverse calcified cyanobacteria assemblage in the latest ediacaran abstract a three dimensionally preserved phosphatized cyanobacterial fossil assemblage tentatively identified as a distinct calcified cyanobacterial combination of oscillatorialeans and rivulariacean is recognized in the latest ediacaran in the southern shaanxi province china based on the morphological and preservation modes oscillatorialean cyanobacteria herein contain 3 genera which are girvanella obruchevella and subtifloria with thin walled tubes tangled coiled or aligned a kind of fan shaped fossils composed of filaments with tapered trichomes and pseudo branches cambricodium is clarified as calcified rivulariacean cyanobacteria prosperous calcified cyanobacteria co occurring with the earliest mineralized skeletal tubular fossils cloudina and sinotubulites in addition to contributing new morphological and preservation information the discovery of these cyanobacteria perhaps strengthens the case for prosperity in cyanobacterial calcification in the terminal neoproterozoic

contemporary crustal deformation around the southeast borderland of the tibetan plateau 1 we derive a detailed horizontal velocity field for the southeast borderland of the tibetan plateau using gps data collected from the crustal motion observation network of china between 1998 and 2004 our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes separated by strike slip and transtensional faults most notably the regional deformation includes 10 11 mm yr left slip across the xianshuihe fault 7 mm yr left slip across the anninghe zemuhe xiaojiang fault zone 2 mm yr right slip across a shear zone trending northwest near the southern segment of the lancang river fault and 3 mm yr left slip across the lijiang fault deformation along the southern segment of the red river fault appears not significant at present time the region south and west of the xianshuihexiaojiang fault system whose eastward motion is resisted by the stable south china block to the east turns from eastward to southward motion with respect to south china resulting in clockwise rotation of its internal subblocks active deformation is detected across two previously unknown deformation zones one is located 150 km northwest of and in parallel with the longmenshan fault with 4 6 mm yr right slip and another is continued south southwestward from the xiaojiang fault abutting the red river fault with 7 mm yr left slip while both of these zones are seismically active the exact locations of faults responsible for such deformation are yet to be mapped by field geology comparing our gps results with predictions of various models proposed for tibetan plateau deformation we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger highly fragmented upper crust

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

phosphatized epiphyton from the terminal neoproterozoic and its significance abstract the fossil record of calcified cyanobacteria in the precambrian is scarce especially for fossils with dichotomous filaments epiphyton which have complex branching structures like cyanobacteria and are usually regarded as calcimicrobe with uncertain affinity and are widely considered to appear at the beginning of the cambrian exceptional phosphatized three dimensional epiphyton are reported from the gaojiashan member dengying formation of the terminal neoproterozoic lijiagou section southern shaanxi province of china based on data from scanning electron microscopy and x ray tomographic microscopy two preservational modes of epiphyton namely tubular and solid branches and their corresponding phosphatic processes are proposed branching forms of the thallus sizes and outer surface ornamentations of the filaments are characteristic of phanerozoic epiphyton and has allowed us to give a possible interpretation of this genus and to recognize its similarity to cyanobacteria the finding enhances our understanding of the diversity of cyanobacteria in the terminal neoproterozoic give us an opportunity to reassess the theories involving the coevolution of calcification of microbes and metazoans

earthquake synchrony and clustering on fucino faults central italy as revealed from in situ cl 36 exposure dating we recover the holocene earthquake history of seven seismogenic normal faults in the fucino system central italy we collected 800 samples from the well preserved limestone scarps of the faults and modeled their 36cl concentrations to derive their seismic exhumation history we found that 30 large earthquakes broke the faults in synchrony over the last 12 ka the seven faults released strain at the same periods of time 12 9 ka 5 3 ka and 1 5 1 ka on all faults the strain accumulation and release occurred in 3 6 ka supercycles each included a 3 5 ka phase of slow 0 5 2 mm yr strain accumulation in relative quiescence followed by a cluster of three to four large earthquakes or earthquake sequences that released most of the strain in 1 2 ka the large earthquakes repeated every 0 5 0 3 ka during the paroxysmal phases and every 4 3 0 9 ka between those phases earthquakes on the northern faults produced twice larger surface slips 2 m and had larger magnitudes mw 6 2 6 7 than those on the southern faults on most faults the relative strain level was found to control the amount of slip and the time of occurrence of the next large earthquake faults entered a phase of clustered activity once they had reached a specific relative strain threshold the tre monti fault is identified as the most prone to break over the next century our data document earthquake synchrony and clustering at a broader space and time scale than has been reported to date

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

temperature variability in the gulf of california during the last century a record of the recent strong el nino the oxygen isotopic ratio of diatom silica from a box core collected on the eastern slope of the guaymas basin gulf of california has been analyzed this core covers the last 90 years the isotopic record indicates a progressive and continuous temperature increase of about 8 c from the end of the last century until 1980 with superimposed rapid temperature fluctuations of 2 to 4 c although the gulf of california is not directly under the influence of el nino short temperature increases may be associated with the el nino events described by quinn et al 1987 the longer term changes are attributed to a decrease of upwelling intensity linked to weakening of the trade winds in the northern hemisphere

uniform california earthquake rupture forecast version 2 ucerf 2 the 2007 working group on california earthquake probabilities wgcep 2007 presents the uniform california earthquake rupture forecast version 2 ucerf 2 this model comprises a time independent poisson process earthquake rate model developed jointly with the national seismic hazard mapping program and a time dependent earthquake probability model based on recent earthquake rates and stress renewal statistics conditioned on the date of last event the models were developed from updated statewide earthquake catalogs and fault deformation databases using a uniform methodology across all regions and implemented in the modular extensible open seismic hazard analysis framework the rate model satisfies integrating measures of deformation across the plate boundary zone and is consistent with historical seismicity data an overprediction of earthquake rates found at intermediate magnitudes 6 5 m 7 0 in previous models has been reduced to within the 95 confidence bounds of the historical earthquake catalog a logic tree with 480 branches represents the epistemic uncertainties of the full time dependent model the mean ucerf 2 time dependent probability of one or more m 6 7 earthquakes in the california region during the next 30 yr is 99 7 this probability decreases to 46 for m 7 5 and to 4 5 for m 8 0 these probabilities do not include the cascadia subduction zone largely north of california for which the estimated 30 yr m 8 0 time dependent probability is 10 the m 6 7 probabilities on major strike slip faults are consistent with the wgcep 2003 study in the san francisco bay area and the wgcep 1995 study in southern california except for significantly lower estimates along the san jacinto and elsinore faults owing to provisions for larger multisegment ruptures important model limitations are discussed

a 20 year varve record of siliceous phytoplankton variability in the central gulf of california abstract year to year variability in the structure of the preserved siliceous phytoplankton assemblage is reconstructed from a box core retrieved from the varved sediments along the guaymas slope of the gulf of california a 20 year record from 1953 through 1972 is obtained from interpretation of the varve stratigraphy supported by 210 pb and 228 th dating ecological time series derived from microfossil analysis show a strong coherence with interannual sea level anomalies indicating a response to large scale climatic variability associated with the el nino southern oscillation phenomenon el nino periods are generally marked by increases in preserved abundances of the total siliceous assemblage and in particular by greater numbers of individuals within species whose distribution is limited to tropical and subtropical waters

major earthquakes occur regularly on an isolated plate boundary fault the scarcity of long geological records of major earthquakes on different types of faults makes testing hypotheses of regular versus random or clustered earthquake recurrence behavior difficult we provide a fault proximal major earthquake record spanning 8000 years on the strike slip alpine fault in new zealand cyclic stratigraphy at hokuri creek suggests that the fault ruptured to the surface 24 times and event ages yield a 0 33 coefficient of variation in recurrence interval we associate this near regular earthquake recurrence with a geometrically simple strike slip fault with high slip rate accommodating a high proportion of plate boundary motion that works in isolation from other faults we propose that it is valid to apply time dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide

present day crustal deformation around sagaing fault myanmar 1 global positioning system gps measurement campaigns in myanmar conducted in 1998 and 2000 allow quantifying the present day crustal deformation around the sagaing fault system in central myanmar both a regional network installed at four points within the country and a local 18 station network centered on the city of mandalay across the sagaing fault demonstrate that active deformation related to the northward motion of india is distributed across myanmar in a platelet that extends from the western edge of the shan plateau in the east to the andaman trench in the west in this platelet deformation is rather diffuse and distributed over distinct fault systems in the east the sagaing shan scarp fault system absorbs 10 mm yr this gps study combined with an on land geotectonic survey demonstrate that oblique slip of india along the rigid sundaland block is accommodated by a partitioned system characterized by distribution of deformation over a wide zone index terms 1206 geodesy and gravity crustal movements interplate 8155 1243 geodesy and gravity space geodetic surveys 8110 tectonophysics continental tectonics general 0905 8158 tectonophysics plate motions present and recent 3040 keywords tectonics gps fault southeast asia

variations of upwelling intensity recorded in varved sediment from the gulf of california during the past 3 000 years circulation in the pacific ocean is periodically affected by el nino events which are a direct response to intensity changes in trade winds1 2 the appearance of anomalously warm surface water in the eastern equatorial pacific ocean is one of the most spectacular manifestations of this event warm surface water anomalies are also observed in the northern hemisphere along the californian coast and in the gulf of california in the gulf strengthened trade winds induce upwelling during the winter whereas in the summer prevailing southerly winds provoke an input of warm water from the south3 we have sampled laminated sediments from the slopes of guaymas basin to reconstruct with high resolution variations of the hydrology of surface waters in the central part of the gulf of california sedimentation rate is of the order of 4 mm yr 1 at the top and 1 2 mm yr 1 down core by analysing the oxygen isotope ratio of biogenic silica of diatoms4 we have recorded the surface water temperature variations in the guaymas basin the comparison of these fluctuations and variations of silicoflagellate abundance has allowed us to reconstruct changes in upwelling intensity during the past three thousand years the present conditions of low upwelling activity are seen to be abnormal in comparison with those prevailing in the past

irregular recurrence of large earthquakes an analysis of historic and paleoseismic catalogs assessment of a region s seismic hazard requires knowledge of the repeat times and the regularity of earthquake recurrence for most areas the earthquake catalog is too short to determine recurrence parameters reliably using a large set of events for many regions however it should be possible to constrain the average characteristics of earthquake recurrence behavior this study compiles a set of the most complete historic and paleoseismic series of m 7 earthquakes from the middle american trench alaska chile japan and the san andreas fault the 52 recurrence series show a large average aperiodicity that is variations in recurrence time are larger than 40 of the average recurrence interval aperiodicity may decrease with increasing magnitude neither the use of different magnitude thresholds the exclusion of foreshocks and aftershocks the type of statistics normal or lognormal used nor the choice of segmentation results in a significantly smaller aperiodicity the irregular recurrence behavior exhibited by the data implies that earthquakes are more unpredictable than often assumed in hazard analyses

assemblages of calcareous algae near the precambrian cambrian boundary in siberia and mongolia calcareous algae make their first appearance near the base of the tommotian the oldest confirmed occurrences are of gemma obruchevella and renalcis in the nemakit daldyn horizon and late yudomian of siberia a little higher in the sequence in the aldanocyathus sunnaginicus zone of the earliest tommotian angulocellularia bajanophyton batenevia botomaella girvanella hedstroemia korilophyton proaulopora subtifloria and tarthinia appear immediately above this in the dokidocyathus regularis zone of the middle tommotian chabakovia appears with the epiphytacean genera epiphyton gordonophyton and tubomorphophyton kordephyton appears in the dokidocyathus lenaicus zone of the late tommotian and razumovskia in the early atdabanian these fossils constitute three calcareous algae assemblages caa 1 2 3 characteristic of the nemakit daldyn horizon late yudomian early tommotian and middle tommotian atdabanian respectively most of the genera are long ranging but their rapid appearance and diversification near the base of the tommotian represents an important event in the history of calcareous algae and augments faunal data concerning evolution and biostratigraphy near the precambrian cambrian boundary modified journal abstract

mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria filamentous and shrub like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca 1200 ma society cliffs formation baffin and bylot islands arctic canada are 400 m y older than previously reported examples in vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms ccms and is a direct ecophysiological link to atmospheric co 2 concentration ccms are induced in present day cyanobacteria under experimental conditions when pco 2 is below 0 36 10 times present atmospheric level pal society cliffs calcified cyanobacteria consequently imply pco 2 levels of 2 of 7 10 pal in the late mesoproterozoic combined petro graphic experimental and modeling results therefore suggest that mesoproterozoic pco 2 concentrations were not substantially different from phanerozoic values and were significantly less than previous estimates of up to 200 pal assuming 10 lower solar luminosity in the late mesoproterozoic pco 2 levels of 10 pal or less require the presence of additional greenhouse gases for maintenance of an ice free earth at 10 pal pco 2 methane concentrations of 100 200 ppm would have been sufficient to sustain warm earth surface conditions the low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of earth9s proterozoic biosphere

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

growth patterns of the cambrian microbialite phototropism and speciation of epiphyton abstract microbes started constructing shallow marine stromatolitic bioherms in the archean but they transferred their role as a major buildup maker to metazoans in phanerozoic microbial buildups often recovered their predominance in the carbonate platform when reefal metazoan communities collapsed epiphyton an extinct taxon of calcified microbe that possessed branching filamentous trichomes was an important reef builder in the shallow marine carbonate platform during middle cambrian aftermath of an extinction of archaeocyath sponges which were major reef building sessile organisms in the early cambrian here we present direct evidence of phototropism of epiphyton found from fossilized behavior in micro and macro structures of meter scale microbial bioherms of the zhangxia formation middle cambrian north china platform shandong province china the bioherms consist of stacked growth layers with the inner and outer divisions divided by distinct boundary the inner division of growth layers of the epiphyton bioherm is dominated by dense uniform bush shaped epiphyton thalli whereas the curved outer division has layered texture normal to the surface comprised of elongated and chambered thalli it suggests that photosynthetic epiphyton reacted actively to the spatial changes in intensity of sunlight controlled by angle of illumination on the curved growth surface of the bioherm the inner and the outer divisions comprise different morpho types of epiphyton the spatial distributions of different morpho types in variously illuminated divisions of epiphyton might have caused further speciation of epiphyton

long aftershock sequences within continents and implications for earthquake hazard assessment one of the most powerful features of plate tectonics is that the known plate motions give insight into both the locations and average recurrence interval of future large earthquakes on plate boundaries plate tectonics gives no insight however into where and when earthquakes will occur within plates because the interiors of ideal plates should not deform as a result within plate interiors assessments of earthquake hazards rely heavily on the assumption that the locations of small earthquakes shown by the short historical record reflect continuing deformation that will cause future large earthquakes here however we show that many of these recent earthquakes are probably aftershocks of large earthquakes that occurred hundreds of years ago we present a simple model predicting that the length of aftershock sequences varies inversely with the rate at which faults are loaded aftershock sequences within the slowly deforming continents are predicted to be significantly longer than the decade typically observed at rapidly loaded plate boundaries these predictions are in accord with observations so the common practice of treating continental earthquakes as steady state seismicity overestimates the hazard in presently active areas and underestimates it elsewhere

varve formation in the gulf of california insights from time series sediment trap sampling and remote sensing the varved sediments that accumulate in the central gulf of california guaymas and carmen basins provide a record that allows for the resolution of annual to decadal scale climate variability in this region time series sediment trapping combined with remotely sensed observations of sea surface temperature avhrr and color czcs have been used to examine the mechanism of varve formation in the central gulf sst and surface pigment concentration records for both the mainland and baja sides of the gulf display similar seasonal trends high temperatures and low pigment concentrations occur synchronously on both sides of the central gulf from june through to october in association with the 1991 92 el nino warm temperatures persist in the central gulf until december low ssts typically occur from december through may with high pigment concentrations marking the period from november through april the summer early fall june october is a time of high terrigenous and biogenic sediment fluxes during this time of year the total flux is dominated by eolian transported lithogenic material and dark laminae are deposited during this period during el nino years high terrigenous fluxes may extend into winter light laminae represent deposition from november through may during which time biogenic material dominates the total sediment flux high opal fluxes beginning in november are attributed to seasonal cooling of surface water and upward mixing of nutrients true upwelling conditions do not develop in the gulf until late winter spring during this time the total flux is still dominated by opal but the magnitude of the flux is reduced this may be an artifact of phytoplankton grazing and not lower production rates

stromatolitic bioherms and microphytolites from the late proterozoic draken conglomerate formation spitsbergen abstract the late proterozoic draken conglomerate formation akademikerbreen group in eastern spitsbergen displays several prominent stromatolitic bioherms in its lowermost beds the dolomitic bioherms are comprised principally of columnar stromatolites cf gymnosolenoides but include cross bedded microphytolitic grainstones the excellent exposures of the bioherms and microphytolitic grainstones along the margin of petrovbreen glacier and the exquisite preservation of structural and microscopic details permit the documentation and interpretation of these sedimentary and biological associations locally favorable ecologic circumstance or chance colonization initiated stromatolitic mat development which then became self perpetuating to form bioherms until rapid siliciclastic influx overwhelmed the mat building populations intermediate energy conditions in concert with microbial activity and inorganic processes generated the microphytolitic grainstones the interpretation of the biological and sedimentological interactions within these biohermal carbonates microphytolitic grainstones and siliciclastic rocks affords new insights into proterozoic paleoenvironments

how does trench coupling lead to mountain building in the subandes a viscoelastoplastic finite element model 1 subduction induced crustal shortening is believed to be the primary cause of the andean mountain building the present day crustal shortening in the andes is clear from the gps measurements but the rate 30 40 mm a is higher than the geological shortening rate 15 mm a suggesting that much of the present day crustal shortening may recover during future earthquakes in the subduction zone using a two dimensional viscoelastoplastic finite element model we investigate here how the cyclic trench coupling leads to long term mountain building which has been concentrated in the subandes in the past few million years our results show that much of the gps measured crustal shortening in the andes can be explained by the interseismic and postseismic viscoelastic strain that does not contribute to mountain building the apparently discordant gps and geological data can be reconciled in the geodynamic model with a weakened crust in the subandes which allows accumulation of plastic strains that leads to mountain building the localized crustal weakening and mountain building in the subandes may have been a consequence of the uplifted high andes the accelerated westward drift of the south american plate the possible delamination of mantle lithosphere under the eastern cordillera and the increasing trench coupling

paleoseismic event dating and the conditional probability of large earthquakes on the southern san andreas fault california we introduce a quantitative approach to paleoearthquake dating and apply it to paleoseismic data from the wrightwood and pallett creek sites on the southern san andreas fault we illustrate how stratigraphic ordering sedimentolog ical and historical data can be used quantitatively in the process of estimating earth quake ages calibrated radiocarbon age distributions are used directly from layer dating through recurrence intervals and recurrence probability estimation the method does not eliminate subjective judgements in event dating but it does provide a means of systematically and objectively approaching the dating process date dis tributions for the most recent 14 events at wrightwood are based on sample and contextual evidence in fumal et al 2002 and site context and slip history in weldon et al 2002 pallett creek event and dating descriptions are from published sources for the five most recent events at wrightwood our results are consistent with pre viously published estimates with generally comparable or narrower uncertainties for pallett creek our earthquake date estimates generally overlap with previous results but typically have broader uncertainties some event date estimates are very sensitive to details of data interpretation the historical earthquake in 1857 ruptured the ground at both sites but is not constrained by radiocarbon data radiocarbon ages peat accumulation rates and historical constraints at pallett creek for event x yield a date estimate in the earliest 1800s and preclude a date in the late 1600s this event is almost certainly the historical 1812 earthquake as previously concluded by sieh et al 1989 this earthquake also produced ground deformation at wrightwood all events at pallett creek except for event t about a d 1360 and possibly event i about a d 960 have corresponding events at wrightwood with some overlap in age ranges event t falls during a period of low sedimentation at wrightwood when conditions were not favorable for recording earthquake evidence previously pro posed correlations of pallett creek x with wrightwood w3 in the 1690s and pallett creek event v with w5 around 1480 fumal et al 1993 appear unlikely after our dating reevaluation apparent internal inconsistencies among event layer and dating relationships around events r and v identify them as candidates for further inves tigation at the site conditional probabilities of earthquake recurrence were estimated using poisson lognormal and empirical models the presence of 12 or 13 events at wrightwood during the same interval that 10 events are reported at pallett creek is reflected in mean recurrence intervals of 105 and 135 years respectively average poisson model 30 year conditional probabilities are about 20 at pallett creek and 25 at wrightwood the lognormal model conditional probabilities are somewhat higher about 25 for pallett creek and 34 for wrightwood lognormal variance rln estimates of 0 76 and 0 70 respectively imply only weak time predictability conditional probabilities of 29 and 46 respectively were estimated for an em pirical distribution derived from the data alone conditional probability uncertainties are dominated by the brevity of the event series dating uncertainty contributes only secondarily wrightwood and pallett creek event chronologies both suggest varia tions in recurrence interval with time hinting that some form of recurrence rate modulation may be at work but formal testing shows that neither series is more ordered than might be produced by a poisson process

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

calcified algae as sediment contributors to early paleozoic limestones evidence from deep water sediments of the cow head group western newfoundland abstract calcified algae are important elements of deep water carbonate sediments in the cow head group large algal boundstone boulders in debris flow conglomerates consist of girvanella sheets and arborescent clusters of epiphyton silt and sand sized particles in fine grained thin bedded slope limestones include abundant girvanella as single tubules rafts of intertwined tubules oncolites and intraclasts integration of petrographic information from the boulders and fine grained limestones indicates that many micritic peloids of silt to very fine sand size the single most abundant particle type in the cow head group are a combination of comminuted girvanella tubules and broken epiphyton rods much of the structure grumeleuse in intraclasts is also diagenetically modified calcified algae these findings suggest that many peloids and intraclasts in early to middle paleozoic carbonates in both shallow and deep water may have been derived from calcified algae calcified algae may have been just as important in producing carbonate sediment during this time as benthic calcified algae are in the modern ocean the difficulty in recognizing the algal contribution in most ancient sediments is probably caused by the simple microfabric of the resultant grains filling and obliteration of tubules with micrite cement or micritization

quasi periodic recurrence of large earthquakes on the southern san andreas fault it has been 153 yr since the last large earthquake on the southern san andreas fault california united states but the average interseismic interval is only 100 yr if the recurrence of large earthquakes is periodic rather than random or clustered the length of this period is notable and would generally increase the risk estimated in probabilistic seismic hazard analyses unfortunately robust characterization of a distribution describing earthquake recurrence on a single fault is limited by the brevity of most earthquake records here we use statistical tests on a 3000 yr combined record of 29 ground rupturing earthquakes from wrightwood california we show that earthquake recurrence there is more regular than expected from a poisson distribution and is not clustered leading us to conclude that recurrence is quasi periodic the observation of unimodal time dependence is persistent across an observationally based sensitivity analysis that critically examines alternative interpretations of the geologic record the results support formal forecast efforts that use renewal models to estimate probabilities of future earthquakes on the southern san andreas fault only four intervals 15 from the record are longer than the present open interval highlighting the current hazard posed by this fault

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

gps and geologic estimates of the tectonic activity in the marmara sea region nw anatolia a dense network of 52 global positioning system gps sites is used to determine the velocity field and strain rate pattern at the western end of the north anatolian fault zone nafz in the marmara sea region nw anatolia based on four gps campaigns carried out biannually between 1990 and 1996 the detailed kinematic field of crustal motion is calculated the southernmost station shows an average rate of 22 3 mm yr oriented westward relative to istanbul eurasia this value is indicative of the dextral strike slip motion of nw anatolia relative to the black sea the gps velocity field reveals that the sites located on the thrace black sea block show negligible motion relative to istanbul the gps based deformation pattern of three sections crossing the westward prolongation of the nafz is compared with neotectonic data both the gps and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow e w oriented zone this belt extends from the single fault trace of the nafz through the gulf of izmit the marmara sea the arkoy region and the gulf of saros into the north aegean trough nat in addition the cross sections show the increasing influence of the n s oriented extension in the aegean sea region from the comparison of the directions and rates of shear and normal strain deduced from gps data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along mudurnu valley the northern marmara sea and the saros trough where it joins the nat

the radiocarbon record in varved sediments of carmen basin gulf of california a measure of upwelling intensity variation during the past several hundred years the decrease in 14c 12c with depth in a varved sediment core raised from the eastern flank of carmen basin gulf of california is greater than expected from varve and 210pb chronology the discrepancy is ascribed to a virtually monotonic decrease in upwelling intensity from several hundred years ago to the present this radiochemical study presents an approach independent of previously used biostratigraphic techniques for evaluating temporal variations in upwelling intensity in the gulf of california