modeling mountain building and the seismic cycle in the himalaya of nepal a host of information is now available regarding the geological and thermal structure as well as deformation rate across the himalaya of central nepal these data are reconciled in a two dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes over geological timescale 5 ma the 20 mm yr estimated shortening rate across the range is accommodated by localized thrust faulting along the main himalayan thrust fault mht the mht reaches the surface along the foothills where it is called the main frontal thrust fault mft the mht flattens beneath the lesser himalaya and forms a midcrustal ramp at the front of the higher himalaya consistent with the river incision and the anticlinal structure of the lesser himalaya farther northward the mht roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust increasing the coulomb stress beneath the front of the higher himalaya where most of the microseismic activity clusters negligible deformation of the hanging wall requires a low apparent friction coefficient less than 0 3 on the flat portion of the mht on the ramp might be as high as 0 6 sensitivity tests show that a rather compliant quartz rich rheology and a high radioactive heat production in the upper crust of 2 5 w m 3 is required erosion affects the thermal structure and interplays with crustal deformation a dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure topography and deformation field using a linear diffusion model of erosion we constrain the value of the mass diffusivity coefficient to 0 5 1 6 l0 4 m 2 yr this study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period

shear and normal load perturbations on a two dimensional continuous fault 2 dynamic triggering 1 the influence of normal and shear stress static perturbations on a strike slip fault is addressed on the basis of a two dimensional continuous and quasi dynamic model friction along the fault plane is described using a rate and state friction law with depth variable properties normal and shear stress perturbations result in similar effects in terms of earthquake triggering iftm s is constant t ands being the amplitude of the shear and normal stress fluctuations respectively and m being a constant which can be interpreted as the static friction coefficient on the fault in a coulomb failure model therefore the coulomb stress changecff tm s is a useful tool to account simultaneously for normal and shear stress variations in our model we also show that when estimating the clock advance or clock delay of an earthquake the simple coulomb failure model is at first order in good agreement with our results during the first 90 of the earthquake cycle however it differs significantly during the last 10 due to the sharp velocity increase predicted by the rate and state friction law before rupture this suggests that as long as static variations of stress are concerned realistic fault models using rich laboratory based friction laws like rate and state friction laws may lead topredictions fairly close to the ones made using one of the simplest failure model i e the coulomb failure model this may explain why coulomb stress change computations although often based on drastic approximations have been able in many occasions to explain earthquake triggering sequences index terms 7209 seismology earthquake dynamics and mechanics 7215 seismology earthquake parameters 7260 seismology theory and modeling keywords earthquake triggering static triggering coulomb stress change rate and state friction laws clock advance delay

carbonate sedimentology of the early precambrian hamersley group of western australia abstract the early precambrian hamersley group of western australia contains both major and minor occurrences of well preserved carbonate sedimentary rock the carawine dolomite and the middle or paraburdoo member of the wittenoom formation are the two most extensive occurrences the carawine dolomite is restricted to the eastern part of the hamersley basin and contains abundant stromatolites oncolites and wave ripples as well as local occurrences of evaporite crystal pseudomorphs and oolitic to pisolitic textures these carbonate strata were deposited in a shallow water paleoenvironment herein referred to as the carawine platform in contrast the wittenoom formation is restricted to the centraland western parts of the hamersley basin and such shallow water features are entirely absent from its carbonate strata these strata consist largely of thinly laminated lutite but also include some thin carbonate turbidites the carbonate sedimentary rocks of the wittenoom formation as well as some in the carawine dolomite which display similar characteristics are therefore interpreted as sediments that were deposited off platform in deeper water paleoenvironments paleocurrent thickness and grain size trends of the carbonate turbidites indicate they were deposited by paleoflows moving south and west even though the carawine dolomite lies to the northeast of the wittenoom formation stratigraphic correlations using a newly recognized marker bed of probable impact origin suggest that the carbonate strata of the carawine dolomite are younger than the paraburdoo member of the wittenoom formation if true the carawine platform could not have been a source of sediment for most of the carbonate sedimentary rocks in the wittenoom formation but it may havebeen the sediment source for similar carbonate turbidites in younger formations such as the mt mcrae shale and the dales gorge member of the brockman iron formation this in turn suggests that carbonate sediments were accumulating in the shallower parts of the hamersley basin while some of the bifs were being deposited simultaneously in the deeper parts

the burgess shale and associated cambrian formations west of the fossil gully fault zone on mount stephen british columbia west of the fossil gully fault zone on mount stephen the three lowest members only of the burgess shale formation are preserved the kicking horse shale the yoho river limestone and the campsite cliff shale the formation rests unconformably upon the takakkaw tongue formation whose dark basinal limestones conformably overlie paler shelf like limestones of the mount whyte formation mapping has resolved a long standing problem and shown that the stratigraphical position of the famous mount stephen trilobite beds lies within the campsite cliff shale it has also revealed some of the complexities of the fossil gully fault zone among which different periods and directions of component fault movements are indicated faunal evidence shows that the plagiura kochaspis to albertella and albertella to glossopleura zonal boundaries lie within the takakkaw tongue sequence within the burgess shale three distinct soft bodied communities occur at different stratigraphical levels on this mountain slope the oldest

a viscoelastic coupling model for the cyclic deformation due to periodically repeated earthquakes at subduction zones vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic maxwell half space are obtained and compared with the observed deformation cycle coseismic strain release postseismic transients interseismic strain accumulation from japan the viscoelastic effects including the influence of buoyant restoring forces are obtained using the method developed by rundle and plate convergence and strain accumulation are incorporated following the procedure suggested by savage the resulting deformation cycle is compared with that of an analogous elastic half space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault cyclic deformation is similar but not identical for the two models and observations from southwest japan suggest the superiority of the viscoelastic coupling model in particular inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half space model and results in agreement with the observed interseismic movement pattern several aspects of the postseismic deformation its landward migration and its transition to the interseismic phase of the cycle are explained as well but the short duration of near trench transients relative to those observed farther inland is not matched the success of a buried slip model in explaining early postseismic near trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle elastic in the short term and ductile for longer term deformation and such a modification may reconcile remaining discordant observations however reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of japan and these regional differences remain unexplained

oceanic anoxia at the precambrian cambrian boundary the precambrian cambrian pc c boundary separates fossils representing two discrete evolutionary phases the neoproterozoic soft bodied ediacarian biotas and cambrian small shelly faunas the biological discontinuity is suspected to have been a result of mass extinction however recent discoveries of the ediacarian biotas in cambrian sediments have led to an understanding that the faunal change was gradual through the pc c transition th u ratios which are high in oxidizing conditions and low in reducing conditions show a considerable positive correlation with 13c values at all studied sites of the pc c boundary this correlation indicates that reported 13c variation across the pc c boundary from numerous localities corresponds to redox variation in the depositional environment the negative 13c anomaly that occurs worldwide at the pc c boundary therefore corresponds to the widespread development of an oxygen deficient shallow marine environment this finding suggests that widespread oceanic oxygen deficiency which has been interpreted to reflect phanerozoic mass extinction events also occurred immediately before the cambrian explosion

seismicity and deformation at convergent margins due to heterogeneous coupling studies of the mechanics of subduction as inferred from earthquake cycle observations suggest that the distribution and style of seismicity in the seafloor between the trench and the outer rise and in the slab at intermediate depth can in some cases serve to identify asperity locations along the thrust interface dmowska and lovison 1992 such asperities identified from seismic wavefield modeling are the zones of highest seismic moment release in large underthrusting events to the extent that asperity locations are relatively stationary from one event to the next their locations provide the zones of highest expected moment release in future large earthquakes and rupture often nucleates at the border of an asperity the region of the thrust interface outside such asperities is apparently less well coupled and releases moment throughout the great earthquake cycle in some combination of aseismic creep and moderate seismicity thus it is reasonable that stress and deformation rates associated with the earthquake cycle should be most pronounced near asperities and that this should have seismic and geodetic consequences three dimensional finite element modeling is used here to understand such stress and deformation patterns and their variation in time in relation to heterogeneity of coupling along thrust interfaces the stress field helps to explain the observed clustering of seafloor seismicity along the strike of the convergent margin in cases of convergence at approximately normal incidence like for the region of the valparaiso chile 1985 thrust event the modeling is consistent with the observation that areas of large earthquakes in the seafloor toward the outer rise and in the slab tend to lie within corridors through thrust zone asperities running perpendicular to the line of the trench we seek to learn if such model stress fields are consistent with observations for the strongly oblique subduction margin of the rat islands western aleutians 1965 event that active areas of the outer rise and slab at intermediate depth are offset along strike from asperity locations modeling results here for the stress in the seafloor raise the possibility that to explain this offset the asperity zones along the thrust interface may have to be strung out along the direction of oblique slip perhaps reflecting the contact path of subducting seamounts or geometric irregularities along the interface shear stress patterns created in the upper plate when there is oblique subduction suggest that favorable areas for back arc strike slip activity following underthrusting as in the adak island central aleutians region of the 1986 andreanof island earthquake ekstrom and engdahl 1989 will also be shifted along strike from asperity locations our analyses show how deformation patterns on the earth s surface above asperities differ from patterns above nonasperities and hence provide tools to identify inhomogeneous coupling from geodetic observations we discuss possible bathymetric topographic and structural signals of strength of coupling and of asperities particularly noting that the density and extent from the trench of seafloor normal faults correlates with seismically inferred zones of strongest coupling in the central aleutians

exceptional fossil record distribution of soft tissue preservation through the phanerozoic preservation of soft bodied fossil biotas konservat lagerstaten that preserve traces of volatile nonmineralized tissues readily degraded by bacteria are not evenly spaced through geologic time when compared to outcrop area exceptional faunas appear to be over represented in the cambrian and jurassic these concentrations in time correspond to particular environments indicating that controls on the distribution of exceptional faunas may have operated on a global scale the reduction in the number of exceptional faunas after the cambrian may reflect the evolution and diversification of deep bio turbators specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks

post cambrian closure of the deep water slope basin taphonomic window exceptional faunas konservat lagerstatten that preserve traces of volatile nonmineralized tissues are statistically overabundant in the cambrian period almost all examples preserved in continental slope and shelf basin environments are of this age the hypothesis that an increase in the amount and complexity of bioturbation was an important agent in the elimination of this deep water slope basin taphonomic window is supported post cambrian ichnofaunal assemblages contain a higher proportion of pascichnia and agrichnia ethologies produced by a mobile infauna they also illustrate the lateral partitioning of organisms into different environmental niches both opportunistic and equilibrium infaunas occur in low oxygen environments in which the preservation of nonbiomineralized tissues was favored direct consumption of carcasses by bioturbating organisms was less important than changes to sediment properties as a result of bioturbation notably enhanced microbial degradation of reactive organic matter increased permeability and the disruption of geochemical gradients necessary for mineral authigenesis

the middle cambrian burgess shale and its relationship to the stephen formation in the southern canadian rocky mountains the burgess shale has been an anomalous geologic unit ever since walcott named it in 1911 as the geographic equivalent of the ogygopsis shale in the middle cambrian stephen formation of southeastern british columbia but it has never been recognized outside of its type locality so its status relative to the stephen formation remained uncertain the geologic setting of the burgess shale was determined by aitken and fritz in 1968 when they recognized the cathedral escarpment and divided the stephen formation into a thin platformal succession on top of the escarpment and a thick basinal succession which included walcott s burgess shale in front fieldwork by royal ontario museum parties between 1982 and 1997 has now demonstrated that the thin and thick stephen successions lie within different facies belts and should be regarded as separate formations the stephen shale formation is part of the middle carbonate belt succession whereas the name burgess shale formation is applied to the thick basinal

postseismic relaxation driven by brittle creep a possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks application to the chi chi earthquake taiwan we evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth assuming a velocity strengthening brittle creep rheology we show that this model can help reconcile the time evolution of afterslip as measured from geodesy with aftershocks decay we propose an analytical expression for slip of the brittle creeping fault zone bcfz that applies to any dynamic or static stress perturbation including shear stress and normal stress changes the model predicts an initial logarithmic increase of slip with time postseismic slip rate decays over a characteristic time t r a that does not depend on the amplitude of the stress perturbation and it asymptotically joins the long term creep imposed by interseismic stress buildup given that the seismicity rate might be considered proportional to the sliding velocity of the bcfz the model predicts a decay rate of aftershocks that follows omori s law with a mathematical formalism identical to that of dieterich 1994 although based on a different mechanical rationale our model also differs from dieterich s model in that it requires that aftershock sequences and deep afterslip as constrained from geodetic measurements should follow the same temporal evolution we test this for the 1999 chi chi earthquake m w 7 6 and find that both sets of data are consistent with a model of afterslip due to the response of the bcfz the inferred relaxation time t r 8 5 years requires a value for a log v being the coefficient of friction in the range between 1 3 10 3 and 10 2

herringbone calcite petrography and environmental significance abstract herringbone calcite is a previously undescribed carbonate cement and sea floor precipitate that is common in archean carbonates but rare in proterozoic and phanerozoic rocks it is abundant in the 2520 ma campbellrand malmani platform south africa where field relationships such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones demonstrate that it precipitated from ambient marine water this interpretation is supported by depositional relationships in the 2 6 ga huntsman limestone of the bulawayo greenstone belt zimbabwe the 2 6 ga carawine dolomite australia the 1 90 ga rocknest formation and the 1 8 1 2 ga dismal lakes group canada the ordovician porterfi ld carbonate buildup virginia and various silurian carbonate buildups in the midcontinent united states each of these occurrences is associated with anaerobic depositional environments or organic rich sediments herringbone calcite consists of alternating light and dark crenulated bands each light dark pair is 0 5 1 0 mm thick microscopically each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement the bases of the crystals are optically unoriented but upwards in each crystal the optical c axis rotates until it is perpendicular to crystal elongation the tops of the elongate crystal are thus optically aligned and length slow the light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals whereas the dark bands correspond to the optically unoriented lower parts of the elongate crystals microspar crystals are also present in some dark ba ds a mg calcite precursor for herringbone calcite now preserved as low mg calcite or dolomite is supported by the presence of micro dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low mg calcite precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface since herringbone calcite is associated with anaerobic depositional environments the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water thus the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time

tepees in peritidal carbonates origin via earthquake induced deformation with example from the middle cambrian of western canada abstract tepee structures are common in many ancient carbonate tidal flat successions and because analogues are known from tropical coastal areas they are well established as evidence for synsedimentary lithification of subaerially exposed carbonate sediment existing explanations involving desiccation thermal expansion and contraction fluctuating groundwater pore pressure or diagenetic force of crystallization do not adequately account for the style and degree of deformation seen in some examples such as those in the middle cambrian cathedral formation of the southern rocky mountains western canada these tepees provide evidence for essentially instantaneous events causing brittle failure of decimetre thick crusts of ooidal and pisoidal grainstone followed by their dilation rupturing separation tilting and shingling they can be explained as breccias generated by sporadic episodes of strong ground motion from earthquakes that emanated from a syndepositionally active fault system nearby variably directed tensile and compressive stresses broke crusts shifted them upwards and shuffled them laterally subsequent microbial encrustation and vigorous flushing by seawater in the resulting shelter cavities led to rapid stabilization by cementation recognition of the role of seismically induced deformation in explaining some tepee structures furnishes a proxy for synsedimentary tectonism and adds another member to the family of seismites which reflect a range of rheological properties of unlithified and lithifying sediments

a new hypothesis for organic preservation of burgess shale taxa in the middle cambrian wheeler formation house range utah cambrian konservat lagerstatten are the most significant fossil deposits for our understanding of the initiation of phanerozoic life although many modes of preservation may occur these deposits most frequently contain nonmineralized fossils preserved in the form of kerogenized carbon films a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model the middle cambrian wheeler formation of utah one of these lagerstatten contains abundant kerogenized preservation of nonmineralized tissues which occurs within a distinctive taphofacies that accumulated under the following conditions 1 domination of the siliciclastic fraction by clay sized particles 2 close proximity to a carbonate platform which resulted in mixed carbonate clay sediments 3 a well developed oxygen minimum precluding benthic colonization and burrowing and 4 relative proximity to oxic bottom waters facilitating transport of organisms from a habitable environment to one that favored their preservation we propose that preservation of nonmineralized tissues in the wheeler formation may have resulted from a combination of influences that reduced permeability and thus lowered oxidant flux which in turn may have restricted microbial decomposition of some nonmineralized tissues those influences include near bottom anoxia preventing sediment irrigation by restriction of bioturbation reducing conditions near the sediment water interface that may have acted to deflocculate aggregations of clay minerals resulting in low permeability face to face contacts early diagenetic pore occluding carbonate cements and an absence of coarse grains such as silt skeletonized microfossils fecal pellets or bioclasts this model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstatten

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

stress transfer and strain rate variations during the seismic cycle 1 the balance of forces implies stress transfers during the seismic cycle between the elastobrittle upper crust and the viscoelastic lower crust this could induce observable time variations of crustal straining in the interseismic period we simulate these variations using a one dimensional system of springs sliders and dashpot loaded by a constant force the seismogenic zone and the zone of afterslip below are modeled from rate and state friction the ductile deeper fault zone is modeled from a viscous slider with newtonian viscosity n the force per unit length f must exceed a critical value fc to overcome friction resistance of the fault system this simple system produces periodic earthquakes the recurrence period tcycle and the duration of the postseismic relaxation phase which is driven dominantly by afterslip then both scale linearly with n between two earthquakes interseismic strain buildup across the whole system is nonstationary with the convergence rates vi just after each earthquake being systematically higher than the value vf at the end of the interseismic period we show

comparison of geochemical and distinctive mineralogical features associated with the kinzers and burgess shale formations and their associated units article i nfo article history accepted 12 august 2007 this paper compares two north american occurrences of fossils that exhibit burgess shale type preservation bst the middle cambrian burgess shale formation of southeastern british columbia and the lower to middle cambrian kinzers fm of southeastern pennsylvania redox sensitive trace elements mo ni co v sc indicate that palaeo oxygenation conditions varied with respect to faunal assemblage and location ogygopsis bearing fauna in both the burgess shale and kinzers formations are associated with a dysoxic metal signature burgess shale type preservation in the kinzers fm was deposited in a fluctuating oxycline whereas the bst fossil beds of fossil ridge and mt stephen burgess shale fm yield a geochemical signature that consistently indicates oxic conditions thus sustained low oxygen conditions do not correlate with higher quality of fossil preservation at these sites ogygopsis is associated with dysoxic conditions in both localities and may therefore be a faunal indicator of such environments in middle cambrian strata carbonate units associated with both the burgess shale and kinzers formations contain large generally concordant near surface cavities that contain herringbone calcite cements that show no significant enrichment in the potential calcite inhibitor ions such as reduced fe sr s pb and zn the abundance of micro inclusions of dolomite suggests that mg may have been the calcite inhibitor ion responsible for herringbone cement in these cambrian occurrences another distinctive mineralogical feature is the presence of doubly terminated euhedral zoned authigenic quartz as a conspicuous part of a carbonate host rock replacement assemblage the association of both herringbone calcite and zoned quartz crystals in units associated with the burgess shale and kinzers formations suggest a common feature in the depositional and diagenetic environment of these two regions in which burgess shale type fossil preservation occur syn depositional brine flow is the likely cause of these distinct mineralogical features brine seeps in the depositional environment may have provided nutrients for stable high abundance faunal communities in a slope setting thereby accounting for the abundance of fossil material furthermore high salinity brine in the pore waters of carcass bearing cambrian muds may have further limited shallow burrowing scavenging and decomposition rates accounting for the exceptional preservation in these two localities

natural nakmgcl solutions and solid derivatives trapped in euhedral quartz from danish zechstein salt abstract highly saline mgcl2 kcl metamorphic solutions trapped in euhedral quartz from the top of the zechstein 2 salt the deck halite in the mors dome denmark have been studied by means of microthermometry the inclusions are classified after their content of solution and daughter minerals or solid derivatives type i contains much solution and little carnallite kmgcl3 6h2o which dissolves under vapour pressure from 80 to 320 mm hg in the temperature range 75 140 c the salinity is between 35 and 50 eq wt nacl type ii contains more carnallite than solution and the carnallite dissolves in the temperature range 121 169 c under pressures up to 77 mpa the salinity ranges from 38 to 52 eq wt nacl based on the measurements of type ii inclusions a pressure dependent melting curve of carnallite melting incongruently is proposed the curve is expressed by the equation t m car c 167 5 c 0 1p m car where dt dp 0 1 c mpa 1 the types iiia b and c contain the parageneses carnallite sylvite halite carnallite mgcl2 tetrahydrate sylvite halite and tetrahydrate sylvite halite respectively the salinity ranges from fully 50 to 60 eq wt nacl in several of these inclusions grains of carnallite sylvite or halite were trapped in suspension in the solutions the incipient melting temperatures of tetrahydrate in negative crystals 175 8 180 6 c are minimum crystallization temperatures of the host quartz type iv inclusions consist of trapped dry grains of carnallite the melting temperatures of which 172 4 179 6 c are maximum crystallization temperatures of the host quartz these crystals probably crystallized during early cretaceous times at a depth of more than 5000 m four quartz crystals with inclusions containing concentrated seawater probably formed in the span of time from the compaction of the bedded rock salt in triassic times to the initial diapirism in late jurassic times

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

middle cambrian carbonate platform collapse southeastern canadian rocky mountains regional erosion features termed megatruncation surfaces at the seaward margin of a middle cambrian carbonate platform in southeastern british columbia are inferred to be the proximal parts of gravity slide scars comparable in scale and morphology to modern examples in the caribbean region the surfaces truncate up to 200 m of outer platform strata and have low angle to near vertical headwalls they can be traced a minimum of 56 km along depositional strike and appear to extend at least 8 km basinward of the platform edge they therefore profoundly affect regional stratigraphy

origin of meter scale submarine cavities and herringbone calcite cement in a cambrian microbial reef ledger formation u s a abstract meter scale submarine cavities in middle cambrian shelf margin microbial reef strata indicate large scale dewatering processes in conjunction with substrate instability related to interreef channeling and shelf edge downslope creep and slip syndepositional cement precipitation within the cavities preserved delicate microbial fabrics and stabilized the reef system radiaxial fibrous calcite and herringbone calcite cements line the cavity interiors isopachously the two phases cannot be discriminated on the basis of fe mn or sr contents but do have different isotopic signatures slightly more negative 13c values in herringbone calcite suggest that abrupt transitions between radiaxial fibrous and herringbone calcite cement are the result of rapid and repeated changes in pore fluid oxygen levels storm driven pore water circulation renewed oxygenated seawater flow into the cavities resulting in precipitation of radiaxial fibrous calcite a threshold level of oxygen reduction resulted in the change to herringbone calcite precipitation the pore fluids associated with herringbone calcite did not have elevated mn or fe concentrations as suggested in previous studies herringbone calcite appears to be more susceptible to diagenetic alteration than radiaxial fibrous cement however as indicated by greater resetting of oxygen isotope values

source of silica and silicification of the lowermost cambrian yanjiahe formation in the three gorges area south china abstract the rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the ediacaran cambrian e c interval even if the exact timing and biological triggers of this event are still debated although ediacaran cambrian chert depositions are widely distributed multiple evidences converge to support their hydrothermal origin in this study we provide petrographic study supplemented by elemental concentration sem and raman spectral data on the siliceous rocks from the yanjiahe formation the three gorges area south china in order to ascertain the origin of included silica and discuss the involved processes of silica deposition contrary to most previous studies our analyses suggest that studied cherts mainly resulted from biogenic silica and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input the deposition model of silica includes direct deposition of siliceous bioclasts secondary replacement of carbonate by dissolved biosilica and microbial induced silicification from seawater the sink of silica recorded in the yanjiahe formation was characterized of a transition from typical precambrian to phanerozoic process in addition micropalaeontological and taphonomical analyses of the yanjiahe formation revealed two pulses of microfossil abundance and diversity in the fortunian and stage 2 respectively they can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux

carbonate ramp to rimmed shelf evolution lower to middle cambrian continental margin virginia appalachians drill cores through the lower to middle cambrian shady dolomite carbonate platform 600 to 1 200 m thick in the austinville virginia region have provided critical platform margin facies relations not exposed in outcrop the control provided by the closely spaced drill cores has allowed detailed cross sections to be constructed through the margin which allow the evolution of the carbonate platform from a gently sloping carbonate ramp to a high relief reef rimmed shelf to be interpreted the shady dolomite records the inception of carbonate sedimentation on the lower cambrian passive margin and forms the initial carbonate foundation for the over lying cambrian ordovician carbonate shelf sequence carbonate sedimentation was initiated on a submerged lower cambrian clastic shelf resulting in deposition of a thick sequence 300 m of deep ramp nodular limestones this facies shallows upward into a carbonate ramp fringed by low relief stromatactoid mud mound buildups as much as 30 m thick and 3 5 km wide downslope these buildups pass into slope conglomerate and deep ramp carbonate facies landward of the buildups cyclic peritidal sequences developed by latest early cambrian time as the ramp increased in relief skeletal algal bioherms and lime sands were established on the high energy margin to form a high relief as much as 500 to 700 m rimmed shelf with thick 600 m off platform deposits immediately seaward of the shelf edge in the shelf interior a major regression during early to middle cambrian time resulted in development of a major unconformity and in deposition of red beds on the more rapidly subsiding outer platform subsidence rates exceeded sea level fall rates allowing continued upbuilding of the reefal rimmed shelf this rimmed shelf persisted for about 30 m y and influenced carbonate sedimentation throughout the cambrian until it was destroyed by incipient collision during the early ordovician period read in press

authigenic euhedral megaquartz crystals in a quaternary dolomite abstract autochthonous doubly terminated euhedral megaquartz crystals as well as overgrowths on detrital quartz grains are present throughout a gypsum and anhydrite bearing pleistocene sabkha dolomite accumulation from the arabian gulf the siliceous precipitates whose source was probably recycled biogenic silica formed within meters of the sediment surface these quaternary precipitates provide the first modern analog for euhedral megaquartz crystals a relatively common constituent in ancient evaporite bearing tidal flat carbonate strata in the quaternary dolomites studied megaquartz is present as individual crystals and clusters of crystals as well as overgrowths on detrital quartz grains individual crystals commonly attain lengths of 1 mm and many show shallow dolomite mold impressions on their crystal facies and or partly or completely engulf dolomite rhombohedra the common occurrence of dolomite molds within the quartz crystals indicates that precipitation of the quartz took place either concurrently with or more likely after dolomitization of the host carbonates additionally the oft cited association of authigenic megaquartz and evaporite minerals plus the existence of void spaces into which the quartz precipitated imply a causal relationship between dissolution of the evaporites and or dolomites and precipitation of the megaquartz thus these findings demonstrate that authigenic megaquartz crystals as well as overgrowths on detrital quartz can originate under near surface temperatures and pressures and very early in the diagenetic history of evaporite associated carbonate strata

strain budget of the ecuador colombia subduction zone a stochastic view the 2016 pedernales earthquake m w 7 8 ruptured a portion of the colombia ecuador subduction interface where several large historical earthquakes have been documented since the great 1906 earthquake m 8 6 considering all significant ruptures that occurred in the region it has been suggested that the cumulative moment generated co seismically along this part of the subduction over the last century exceeds the moment deficit accumulated inter seismically since 1906 such an excess challenges simple models with earthquakes resetting the elastic strain accumulated inter seismically in locked asperities these inferences are however associated with large uncertainties that are generally unknown the impact of spatial smoothing constraints on co seismic and inter seismic models also prevents any robust assessment of the strain budget we propose a bayesian kinematic slip model of the 2016 pedernales earthquake using the most comprehensive dataset to date including insar and gps offsets tsunami waveforms and kinematic records from high rate gps and strong motions in addition we use inter seismic geodetic velocities to produce a probabilistic inter seismic coupling model of the subduction interface our stochastic co seismic and inter seismic solutions include the ensemble of all plausible models consistent with our prior information and that fit the observations within uncertainties the analysis of these model ensembles indicates that an excess of co seismic moment during the 1906 2016 period is likely in central ecuador only if we assume that 1942 and 2016 earthquakes are colocated if this assumption is relaxed we show that this conclusion no longer holds given uncertainties in co and inter seismic processes the comparison of 1942 and 2016 teleseismic records reveals large uncertainties in the location of the 1942 event hampering our ability to draw strong conclusions on the unbalanced moment budget in the region our results also show a heterogeneous coupling of the subduction interface that coincides with two slip asperities in our co seismic model for the 2016 pedernales earthquake and with the location of historical ruptures in 1958 1979 and 1998 the spatial variability in coupling and complexity in earthquake history suggest strong heterogeneities in frictional properties of the subduction megathrust

comparison of postseismic afterslip models with aftershock seismicity for three subduction zone earthquakes nias 2005 maule 2010 and tohoku 2011 we focus on the relation between seismic and total postseismic afterslip following the maule mw 8 8 earthquake on 2010 february 27 in central chile first we calculate the cumulative slip released by aftershock seismicity we do this by summing up the aftershock regions and slip estimated from scaling relations comparing the cumulative seismic slip with afterslip models we show that seismic slip of individual aftershocks exceeds locally the inverted afterslip model from geodetic constraints as the afterslip model implicitly contains the displacements from the aftershocks this reflects the tendency of afterslip models to smear out the actual slip pattern however it also suggests that locally slip for a number of the larger aftershocks exceeds the aseismic slip in spite of the fact that the total equivalent moment of the afterslip exceeds the cumulative moment of aftershocks by a large factor this effect seen weakly for the maule 2010 and also for the tohoku 2011 earthquake can be explained by taking into account the uncertainties of the seismicity and afterslip models in spite of uncertainties the hypocentral region of the nias 2005 earthquake is suggested to release a large fraction of moment almost purely seismically therefore these aftershocks are not driven solely by the afterslip but instead their slip areas have probably been stressed by interseismic loading and the mainshock rupture in a second step we divide the megathrust of the maule 2010 rupture into discrete cells and count the number of aftershocks that occur within 50 km of the centre of each cell as a function of time we then compare this number to a time dependent afterslip model by defining the afterslip to aftershock ratio asar for each cell as the slope of the best fitting line when the afterslip at time t is plotted against aftershock count although we find a linear relation between afterslip and aftershocks for most cells there is significant variability in asar in both the downdip and along strike directions of the megathrust we compare the spatial distribution of asar with the spatial distribution of seismic coupling coseismic slip and bouguer gravity anomaly and in each case we find no significant correlation

earthquake nucleation on model faults with rate and state dependent friction effects of inertia laboratory studies suggest that earthquake nucleation involves a transition from quasi static slip when inertial effects are negligible to inertia driven dynamic motion this transition occurs via quasi dynamic motion during which the effects of inertia become increasingly important the characteristics of this transition which depend on frictional properties of the fault determine the observability of earthquake nucleation by seismic geodetic or other means by investigating the role of inertia during nucleation we obtain a quantitative definition of the limiting velocity vin which marks the end of quasi dynamic motion and the onset of instability for reasonable friction parameters and fault widths we obtain estimates of vin for crustal faults to study the roles of inertia stiffness and friction parameters on preseismic motion we simulate triggered instabilities in a one dimensional model of a homogeneous fault with rate and state variable friction in most of our simulations triggering is achieved by applying a stress perturbation to an initially creeping fault under steady state friction we also investigate triggering on faults which are initially locked and overstressed compared to their nominal frictional strength due to time dependent healing we study the amount up and duration tp of preseismic slip as a function of system mass m and other model parameters for crustal faults we interpret the relevant mass as a product of density and fault width and find that wider fault zones result in smaller up and larger tp both up and tp are proportional to the system stiffness k the characteristic slip distance dc and the friction constitutive parameter a and inversely proportional to the size of the triggering event we find greater up and tp for constitutive laws which allow strengthening at zero velocity compared to laws that require slip or a combination of slip and aging for state evolution in contrast to quasi static modeling our simulations suggest a minimum stress perturbation criterion for instability which may be interpreted in terms of a strain threshold for triggered seismicity

fault stability inferred from granite sliding experiments at hydrothermal conditions seismicity on crustal faults is concentrated in the depth interval 1 3 to 12 15km tse and rice 1986 suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening destabilizing to velocity strengthening stabilizing friction they inferred this transition from friction data for dry granite however pore fluids exist at elevated temperatures throughout the crust and may strongly influence strength and sliding behavior we present new data from sliding experiments on granite at elevated t 23 to 600 plus elevated ph2o 100 mpa our results show velocity strengthening at room temperature but velocity weakening from 100 to 350 c except at 250 from 350 to 600 there are systematic trends from velocity weakening to strong velocity strengthening and from high to low friction neither trend was seen in tests on dry granite the velocity dependence data imply the potential for unstable slip in the interval 100 to 350 using a geotherm to map temperature to depth this interval closely matches the observed earthquake distribution

geochemical evidence for oxygenated bottom waters during deposition of fossiliferous strata of the burgess shale formation abstract deciphering the oxygenation potential of bottom waters during deposition of the fossiliferous strata of the burgess shale is key to understanding the paleoecology of the organisms preserved as fossils and the processes involved in their preservation methods of paleo redox determination that are based upon trace fossils organic carbon content or size analysis of pyrite framboids are problematic when applied to the burgess shale fortunately patterns in redox sensitive trace elements hold great potential for determining paleo redox conditions for these greenschist facies rocks because they are independent of both age and metamorphic grade four burgess shale type bst fossil bearing sections were studied three lie within the burgess shale formation whereas one lies within the slightly younger duchesnay unit of the middle chancellor formation the geochemical proxies of seawater redox conditions that were used are mo u th v cr v v ni ni co and v sc each of these redox sensitive indices suggests that oxygenated conditions prevailed in the overlying seawater during deposition of beds within which bst preservation developed non fossiliferous beds at the base of the mt stephen trilobite beds yield three geochemical indices v v ni v sc ni co that indicate deposition under dysoxic to anoxic conditions thus the only beds to have been deposited under anoxic conditions are barren of fossils of soft bodied taxa further contradicting the notion that bottom water anoxia is a factor in bst preservation the laminated nature of the burgess shale and evidence for subsurface anoxia coupled with evidence for oxygenated bottom waters presence of bacterial mats and surficial trace fossils suggest that the burgess shale was deposited in the exaerobic zone oxic anoxic boundary at the water sediment interface implications of this reinterpretation of the depositional environment of the burgess shale include 1 taphonomic models for bst preservation must account for the presence of oxygen in bottom waters 2 paleoecological interpretations of the burgess shale need to account for a probable autochthonous benthic component in what is a complex assemblage of fossils that includes elements derived from the platform and 3 the possibility of burgess shale communities based upon chemosynthesis and or bacteria grazing must be considered

a new type of shelf margin deposit rigid microbial sheets and unconsolidated grainstones riddled with meter scale cavities middle cambrian microbial limestone contains a network of unusual predominantly horizontal cavities up to 2 m in length and 0 5 m in height the microbialite experienced rapid syndepositional lithification but adjacent grainstone sediments remained unlithified during deposition this juxtaposition contributed to sediment instability resulting in fracturing and brecciation of the lithified microbialite while unconsolidated grainstones underwent slumping and injection into some cavities remaining space within the cavities was colonized by a series of encrustations thin crusts 2 8 mm of laminated algal mats followed by several generations of calcified renalcis like cyanobacteria up to 45 mm thick remaining void space was partially filled by internal sediment and then sequentially occluded by banded radiaxial fibrous calcite herringbone calcite and finally saddle dolomite cements the radiaxial and herringbone calcite cements precipitated from porewaters derived from seawater that became anoxic through the breakdown of organic matter in the microbialite noteworthy is the presence of herringbone calcite cement not as a seafloor precipitate but as an early cavity fill we propose that the unusual bedding parallel fractures were caused by gravity collapse along a shallow platform margin coeval foreslope sediments show syndepositional slumping faulting and mass flow deposits these redeposited sediments contain boulders of microbialite and grainstone of platform margin provenance 1999 elsevier science b v all rights reserved

seepage problems in the right abutment of the shahid abbaspour dam southern iran abstract in this study seepage phenomena through the right abutment of shahid abbaspour dam are investigated the shahid abbaspour dam is a 200 m high arch dam which regulates the waters of the karun river serves power generation and flood control and irrigation needs the dam site lies in the zagros mountains of southern iran this region presents continuous series of mainly of karstic limestone marl shale and gypsum ranging in age from jurassic to pliocene the region has subsequently been folded and faulted seepage from the shahid abbaspour reservoir occurs mainly through the karstic limestone the basic foundation treatment of the dam consisted of consolidation grouting a high pressure grout curtain and a drainage curtain moreover a 144 m high and 30 m wide concrete cutoff wall was built to prevent reservoir seepage through a clay filled fracture zone in the right abutment the grout curtain penetrates the principal vuggy zone only beneath the central portion of the dam and below the cutoff wall in the right abutment fan curtains were constructed to reduce drainage flows but the seepage problem could not be solved in order to determine the seepage direction and karstification pattern hydrogeological studies have been carried out additional investigation boreholes have been drilled to monitor fluctuations in groundwater level besides these water chemistry dye tracer pinhole and xrf tests have been carried out as a result of these studies seepage paths have been identified in the karstic limestone in the right abutment of the dam

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

mid carboniferous deglaciation of the protoprecordillera argentina recorded in the agua de jaguel palaeovalley abstract the agua de jaguel formation near uspallata mendoza province argentina was deposited within a palaeovalley along the southeastern margin of the calingasta uspallata basin adjacent to the protoprecordillera the basal glacigenic sequence of the formation was deposited in the serpukhovian early bashkirian near the beginning of the late palaeozoic ice age the sequence is composed of diamictite conglomerate sandstone and mudrock and records four depositional stages within a palaeofjord 1 morainal bank deposition by a wet based tidewater glacier 2 glacial retreat succession where ice retreated up the fjord out of the immediate area allowing iceberg deposition of dropstones and dump deposits 3 continued glacial retreat with ice receding onto land allowing a shoreface to develop within the palaeovalley and 4 transgression across the shoreface and resumption of iceberg deposition the thickness and facies of this succession are similar to deposits within modern alaskan fjords housing temperate tidewater glaciers comparison of the agua de jaguel formation with alaskan fjords indicates that sequence stratigraphy used for low latitude deposits must be applied with caution as glacimarine ice retreat within a fjord produces strata surfaces that are easily misidentified as flooding surfaces resulting from changes in water depth likewise the transgression during stage 4 is significant as it indicates a rise in sea level that occurred during glaciation of the protoprecordillera thus suggesting that the transgression between stages 3 and 4 was not driven by input of glacial meltwater additionally the sediment geochemistry suggests that the bottom waters of the palaeovalley were anoxic which may help explain the absence of bioturbation in the sequence as well as in other ancient glacimarine palaeovalleys the deglaciation succession and transgression are also recorded in the nearby hoyada verde and tramojo formations so it is proposed that all three formations record one glacial event in the protoprecordillera overlying fluvial and shallow marine strata in the agua de jaguel hoyada verde and tramojo formations show no indication of continued glaciation in the protoprecordillera following stage 4 and equivalent strata the deglaciation succession of the agua de jaguel formation affirms the emerging concept that the late palaeozoic ice age was characterized by alpine glaciers ice caps and small ice sheets that were not massive enough to have driven eustatic fluctuations of 100 m as previously understood and that ice never covered westernmost gondwana during later lpia events

ree and trace element patterns from organic rich rocks of the ediacaran cambrian transitional interval abstract it has been established that important changes in the marine environment and the biosphere occurred during the cambrian however the relationships between the so called cambrian explosion and the concomitant environmental changes are not yet fully understood this study presents new geochemical data from the black shale successions from different facies belts of the yangtze platform in south china variations in the concentrations of ree and trace elements varying ce ce th u v sc and v cr ratios in kerogen as well as in bulk rocks from different depositional environments along a transect from platform to basin indicate two oxidation events which led to the oxygenation of the water column in shallow marine environments and euxinic conditions weak correlation between toc v u and mo in the deeper sea during the first oxidation event in the late terreneuvian anoxic conditions in bottom waters rapidly changed to euxinic conditions subsequently the second oxidation event during the early epoch 2 of the cambrian led to oxic suboxic conditions in deeper seawater

nucleation and early seismic propagation of small and large events in a crustal earthquake model 1 earthquake nucleation and early seismic propagation are studied in a two dimensional strike slip fault model with depth variable properties the fault is governed by the dieterich ruina rate and state friction law we use an efficient and rigorous numerical procedure for elastodynamic analysis of earthquake sequences on slowly loaded faults developed by lapusta et al 2000 we find that for decreasing values of the characteristic slip distance of the friction law small events appear at the transition from the locked to creeping behavior toward the bottom of the seismogenic zone small and large events have very similar nucleation phases in our simulations here by nucleation phase we mean gradually accelerating aseismic slip in a small slowly expanding zone before the breakout of the dynamic seismically detectable event moment acceleration to which velocity seismograms are proportional in early stages of seismic propagation exhibits irregular fluctuations in the form of speedups and slowdowns in the moment release rate consistently with observations as reported by ellsworth and beroza 1995 our simulations show that such irregular moment acceleration can at least in part be due to the heterogeneous stress distribution imprinted on the fault by the arrest of previous small events and by stress concentrations at the borders of creeping regions and to partial arrest of the rupture in velocity strengthening fault regions which inhibit seismic slip index terms 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 3220 mathematical geophysics nonlinear dynamics 7230 seismology seismicity and seismotectonics 3230 mathematical geophysics numerical solutions 8164 tectonophysics stresses crust and lithosphere keywords earthquake nucleation phase event clustering irregular moment release stress concentrations rate and state friction earthquake sequences

taphonomy and depositional circumstances of exceptionally preserved fossils from the kinzers formation cambrian southeastern pennsylvania abstract the emigsville member of the kinzers formation cambrian southeastern pennsylvania is a deposit of exceptional preservation containing three main lithofacies that were part of a mixed siliciclastic carbonate debris fan developed seaward of a carbonate shelf along the appalachian margin of laurentia fossils were exceptionally preserved in a shelf environment subject to tempestite deposition most remains are fragmentary which emphasizes the importance of predation as a taphonomic process where emigsville sediments were deposited remains are preserved mostly as organic carbon films pyrite crusts and aluminosilicate films these preservation styles apparently depended upon the development of microbial consortia in isolated microenvironments and some of the microbes were autolithified sedimentary and biological evidence suggests that anoxia salinity fluctuations and sedimentary obrution played relatively minor roles in the emigsville member of the kinzers formation the abundance of exceptionally preserved remains suggests that exceptional sedimentary conditions were not necessary in the cambrian in order for exceptional preservation to occur the abundance of predation evidence in a deposit of exceptional preservation reinforces the concept that predation was the primary taphonomic filter during cambrian time

forecast model for great earthquakes at the nankai trough subduction zone an earthquake instability model is formulated for recurring great earthquakes at the nankai trough subduction zone in southwest japan the model is quasistatic two dimensional and has a displacement and velocity dependent constitutive law applied at the fault plane a constant rate of fault slip at depth represents forcing due to relative motion of the philippine sea and eurasian plates the model simulates fault slip and stress for all parts of repeated earthquake cycles including post inter pre and coseismic stages calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the m 8 1 1946 nankaido earthquake in model simulations accelerating fault slip has two time scales the first time scale is several years long and is interpreted as an intermediate term precursor the second time scale is a few days long and is interpreted as a short term precursor accelerating fault slip on both time scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace

palaeontology and depositional environments of ancient brine seeps in the middle cambrian burgess shale at the monarch british columbia canada abstract the monarch a mountain in southeastern british columbia exposes the cathedral escarpment which abuts the basinal burgess shale and an underlying limestone the upper takakkaw tongue at one section a dolomitized megabreccia representing an intraformational collapsed cavern forms the cathedral escarpment wall the megabreccia is cross cut by the subsequent megatruncation event marking the collapse of the cathedral margin which supports previous hypotheses that dolomitizing brines were active at the cathedral margin prior to and during deposition of the burgess shale epiphyton rich carbonate megablocks within the basinal strata previously interpreted as olistoliths spalled off the escarpment are unlike the peritidal strata comprising the cathedral margin and are more likely deep water carbonate mud mounds that grew in the basin near the escarpment face evidence for early brines in basinal environments at this locality includes lenses of dark magnesium rich clinochlore 30 wt mgo vs 2 in background shales within the burgess shale at the escarpment wall these clinochlores are interpreted as precipitates from seafloor brine seeps analogous to ones currently found at the foot of the florida escarpment and on the distal shelf and slope of the northern gulf of mexico like the gulf examples the brines formed standing pools on the cambrian seafloor and hosted abundant organisms in a narrow fringe around their peripheries but not within the pools sponges priapulid worms hyoliths linguliform brachiopods and trilobites predominate shales only a few metres basinward from the pools are nearly barren of animal fossils increased biologic productivity localized at the seeps would account for the patchy distribution of fossils in the burgess shale in a narrow zone close to the escarpment the discovery of brine seep deposits and associated fringing communities at the monarch shows that at least some assemblages in the burgess shale were preserved within their habitat and were supported by chemosynthesis

the initiation of the early paleozoic cordilleran miogeocline evidence from the uppermost proterozoic lower cambrian hamill group of southeastern british columbia the uppermost proterozoic lower cambrian hamill group of southeastern british columbia contains geologic evidence for a phase of extensional tectonism that led directly to the onset of thermally controlled subsidence in the cordilleran miogeocline moreover the hamill group contains the sedimentological record of the passage of the ancient passive margin from unstable tectonic conditions associated with rifting and or the earliest phases of thermal subsidence to post rift conditions characterized by stabilization of the margin and dissipation of the thermal anomaly generated during the rift phase the rift to post rift transition widespread uplift that occurred prior to and during the deposition of the lower hamill group is indicated by an unconformable relation with the underlying windermere supergroup and by stratigraphic relations between middle and upper proterozoic strata and unconformably overlying upper lower cambrian quartz arenites upper hamill group in the southern borderlands of the hami

an early cambrian rift to post rift transition in the cordillera of western north america the upper proterozoic and lower palaeozoic wedge of miogeoclinal strata in the north american cordillera is widely regarded as evidence for a proto pacific passive margin1 3 the rifting history of this margin appears to have been protracted possibly spanning 200 myr in a tectonic setting that is not well understood3 quantitative subsidence analyses of lower palaeozoic strata between british columbia and utah however provide indirect evidence that the transition from rifting to post rift cooling occurred within a relatively short interval of time although probably not synchronously between 600 and 555 myr refs 4 6 this age is significantly younger than that implied in previous studies1 2 we describe here new field data which together with published geological data provide direct evidence of a latest proterozoic or early cambrian age for the rift to post rift transition the data support recent arguments for widespread initiation of passive margins around the edge of the north american craton close to the cambrian precambrian boundary7

coupling of tectonic loading and earthquake fault slips at subduction zones because of the viscoelastic behaviour of the earth accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales if the lithospheric plate is acted upon by distant tectonic forces strain accumulation must occur in a broad region however an earthquake releases strain only in a region comparable to the size of the rupture area a two dimensional finite element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips a fault lock and unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed but determined by the accumulated stress the amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation as well as the rheology of the rock material the larger the region of strain accumulation is compared to the earthquake rupture the smaller is the earthquake fault slip the reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate since rapid permanent plate shortening is not observed at subduction zones there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles the former can be achieved only by significant aseismic fault slip between large subduction earthquakes the most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault which requires significant aseismic fault slip between asperities

viscoelastic simulation of earthquake cycle using a simple spring dashpot mass system with a friction law we examine the viscoelastic effect on earthquake cycle using a simple spring dashpot mass system with a laboratory derived friction law the temporal change of the frictional stress is large at the early interseismic stage and gradually decreases in contrast to the linear change in an elastic medium this temporal change depends on the maxwell time the smaller relaxation time leads to the larger change of the stress accumulation rate such a rate change in stress accumulation may affect the seismicity at subduction zones moreover we find that the smaller relaxation time produces the smaller period of earthquake cycle and that if the system relaxation time becomes smaller than the period of earthquake cycle the bifurcation phenomenon where the period splits into two occurs it is also found that the smaller relaxation time causes the earlier occurrence of the precursory stress decrease preceding the rapid stress drop

high resolution geochemical evidence for oxic bottom waters in three cambrian burgess shale type deposits abstract the cambrian radiation of complex animals is a fundamental event in the history of life on earth much of our understanding of this event is made possible through the study of exceptionally preserved fossils in burgess shale type bst deposits based on bioturbation levels in bst deposits they are often interpreted as representing restricted oxygen settings this study tests the low oxygen interpretation through analysis of geochemical paleoredox proxies and bioturbation levels in three bst deposits the early cambrian maotianshan shale china the middle cambrian wheeler shale usa and the middle cambrian spence shale usa results from 96 samples show fine scale geochemical evidence for oxic bottom water conditions during the deposition of these three bst deposits trace element paleoredox indices pi give consistent oxic signals in all three shales in addition total organic carbon toc and total sulfur ts levels in the spence shale are extremely low the average c s ratio is 3 1 comparable to values for modern oxic marine water bioturbation levels in all three shales are low with rare development of minimal to moderate horizontal bioturbation while short term oxygen fluctuations or poikiloaerobic conditions could still have sporadically existed in these settings this study shows that these bst deposits were deposited in a dominantly oxic paleoredox setting evidence for low bioturbation levels coupled with paleoredox geochemistry indicative of oxic bottom waters suggests that pervasive exaerobic conditions with an oxic water column positioned above oxygen depleted sediment pore waters may have existed in many cambrian bst deposit paleoenvironments