local deformation in compacting flows development of pressure shadows abstract small scale structures produced by deformation and compaction are common in igneous metamorphic and sedimentary rocks and are often used to define foliation and lineation directions analytic solutions for models of flow round a rigid stationary sphere are obtained using the equations that describe viscous compaction the finite deformation associated with these flows shows many of the features observed in deformed rocks some of which resemble those generated when the ratio of the bulk viscosity to the shear viscosity is small under these conditions the particle paths of the matrix are almost unaffected by the presence of the sphere it is encouraging that the continuum compaction equations can describe the observed behaviour even though the length scale on which the deformation occurs is only mm to cm it is unlikely that the observations can be used to distinguish between d arcy flow of the low viscosity phase and mass transport by diffusion

evaluation of paleomagnetism as a correlation criterion for mogollon datil ignimbrites southwestern new mexico paleomagnetic and rock magnetic data show that 36 24 ma ignimbrite outflow sheets in the mogollon datil volcanic field carry thermoremanent magnetizations trms that provide useful reliable correlation criteria over 90 of 404 ignimbrite sites exhibit well grouped 95 500 m intracaldera ignimbrite sequences agree in polarity with associated outflow sheets but show less consistent trm directions probably reflecting syncooling secular variation and discrepancies between eutaxitic foliations and the syncooling paleohorizontal mean virtual geomagnetic pole positions vgps of normal and reverse polarity ignimbrites are nearly antipodal 175 6 apart showing no evidence of long term differences in the zonal harmonics of the normal and reverse polarity states less antipodal mean vgps 161 2 apart from normal and reverse polarity lavas are interpreted as defects in the data set probably including incompletely removed normal polarity viscous overprints the mean paleomagnetic pole position for mogollon datil ignimbrites n 68 independent unit mean vgps is located at 78 8 n 166 6 e k 19 8 a95 4 0 18 2

magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

cooling rate correction of paleointensity determination for volcanic glasses by relaxation geospeedometry the influence of the cooling rate on paleointensity estimates was investigated for samples from a vertical profile across a 600 a d obsidian lava flow ramp from lipari italy the natural cooling rates at the glass transition which were previously determined for the seven investigated samples by relaxation geospeedometry vary by a factor of more than 4 rock magnetic investigations indicate a magnetic microlite fraction in the single domain grain size range and strong magnetic anisotropy the thermoremanence anisotropy tensor was determined for each specimen to correct the paleointensity results for this anisotropy the cooling rate dependency of the thermoremanence was determined experimentally extrapolation to natural cooling rates indicate an overestimate of the paleointensity by 13 to 20 during experiments with typical laboratory cooling rates correcting for the different cooling rates and the cooling rate dependencies within the vertical profile significantly reduces the standard deviation of the average flow paleointensity the average paleointensity for the 543 19 a d flow ramp results in 52 4 1 1 t corresponding to a virtual axial dipole moment of 9 2 0 2 am2 uncertainties introduced by anisotropy correction and cooling rate extrapolation are considered by error propagation

the use of palaeomagnetism and rock magnetism to understand volcanic processes introduction this special publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques here we provide a context for the book placing individual chapters within the milieu of previous work including some magnetic techniques that were not used in the particular studies described herein thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks including estimating the temperature of emplacement of pyroclastic deposits which may allow us to better understand the rates of cooling during eruption and transport anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics and allow the interpretation of flow dynamics in dykes lava flows and pyroclastic deposits as well as the location of the eruptive vents rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits as the magnetic minerals can change with temperature in addition volcanic rocks may be key recorders of past magnetic fields allowing a better understanding of changes in field intensity and perhaps providing clues of how the magnetic field is formed over the past several decades palaeomagnetic and rock magnetic techniques have been applied to numerous problems in volcanology since aramaki akimoto s pioneering study in 1957 which used natural remanent magnetization nrm data to qualitatively estimate the temperature of emplacement of selected pyroclastic deposits in japan aramaki akimoto 1957 many studies have utilized and refined the approach of using progressive thermal demagnetization data to estimate emplacement temperatures of pyroclastic deposits and have demonstrated the utility of this approach in many settings another extensively utilized technique is that of anisotropy of magnetic susceptibility ams which was first applied to pyroclastic volcanic rocks by ellwood 1982 on ignimbrites in the san juan mountains sw colorado usa to estimate flow fabrics and in that study to infer source locations through triangulation it is staggering to think of the number and diversity of ams studies on volcanic rocks that have been undertaken since that time in a bit more than three decades remanent magnetization characteristics have long been used to correlate volcanic rocks and such data together with rock magnetic properties have been employed to study crystallization and cooling processes e g schlinger et al 1988 wells hillhouse 1989 as techniques have been refined and the sensitivity precision reliability and or speed of measurements for an array of modern instrumentation all have improved workers have been able to study volcanic problems in more detail pulling out details of the processes that have not previously been considered in addition the types of problems addressed have diversified considerably we intend this special publication to provide a representative suite of contributions that help to summarize the current array of studies that apply palaeomagnetism and rock magnetism to volcanic processes and to stimulate further research in this exciting science this book is a result of two sessions on volcanic processes and palaeomagnetism held at the international union of geodesy and geophysics congress in melbourne australia in july 2011 and at the american geophysical union annual meeting in san francisco usa in december 2011 from ort m h porreca m geissman j w eds 2015 the use of palaeomagnetism and rock magnetism to understand volcanic processes geological society london special publications 396 1 11 first published online january 20 2015 http dx doi org 10 1144 sp396 17 2015 the geological society of london for permissions http www geolsoc org uk permissions publishing disclaimer www geolsoc org uk pub ethics angharad hills at the geological society of london graciously and tactfully guided this book and the editors through the entire process from chapter solicitation to publication thermoremanent magnetization in volcanic rocks and emplacement temperature

analysis of foreshock sequences in california and implications for earthquake triggering we analyze foreshock activity in california and compare observations with simulated catalogs based on a branching aftershock triggering model we first examine foreshock occurrence patterns for isolated m ge 5 earthquakes in southern california from 1981 to 2011 and in northern california from 1984 to 2009 among the 64 m ge 5 mainshocks excluding 3 swarms and 3 doubles 53 of the rest are preceded by at least one foreshock within 30 days and 5 km foreshock occurrence appears correlated with mainshock faulting type and depth foreshock area is correlated with the magnitude of the largest foreshock and the number of foreshocks however it is not correlated with mainshock magnitude we then examine the occurrence pattern of all seismicity clusters without a minimum magnitude requirement and the possibility that they are foreshocks of larger mainshocks only about 30 of the small clusters lead to a larger cluster about 66 of the larger clusters have foreshock activities and the spatial distribution pattern is similar to m ge 5 mainshocks with lower occurrence rates in the transverse range and central california and higher occurrence rates in the eastern california shear zone and the bay area these results suggest that foreshock occurrence is largely controlled by the regional tectonic stress field and fault zone properties in special cases foreshock occurrence may be useful for short term forecasting however foreshock properties are not reliably predictive of the magnitude of the eventual mainshock comparison with simulated catalogs suggest that the swarmy features and foreshock occurrence rate in the observed catalogs are not well reproduced from common statistical models of earthquake triggering

constitutive relationships and physical basis of fault strength due to flash heating 1 we develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement laboratory faulting experiments conducted at high sliding velocities which show dramatic strength reduction below the threshold for bulk melting are well fit by the model the predicted slip speed for the onset of weakening is in the range of 0 05 to 2 m s qualitatively consistent with the limited published observations for this model earthquake stress drops and effective shear fracture energy should be linearly pressure dependent whereas the onset speed may be pressure independent or weakly pressure dependent on the basis of the theory flash weakening is expected to produce large dynamic stress drops small effective shear fracture energy and undershoot estimates of the threshold slip speed stress drop and fracture energy are uncertain due to poor knowledge of the average contact dimension shear zone thickness and gouge particle size at seismogenic depths

detailed observations of california foreshock sequences implications for the earthquake initiation process we find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes in this study we examine in detail the evolution of six california foreshock sequences the 1986 mount lewis ml 5 5 the 1986 chalfant ml 6 4 the 1986 stone canyon ml 4 7 the 1990 upland ml 5 2 the 1992 joshua tree mw 6 1 and the 1992 landers mw 7 3 sequence typically uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution however the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations we use these results to identify small scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock in general these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer instead the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process fault zone heterogeneity may also be important in controlling the number of foreshocks i e the stronger the heterogeneity the greater the number of foreshocks the size of the nucleation region as measured by the extent of the foreshock sequence appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase we also find evidence for slip localization as predicted by some models of earthquake nucleation

occurrence patterns of foreshocks to large earthquakes in the western united states observations of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake unfortunately the apparent randomness of foreshock occurrence they precede some events and not others has severely hampered their use in reliable earthquake prediction understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short term prediction will ever be possible1 here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western united states the incidence of foreshocks decreases with increasing depth of the mainshock and also depends on the mainshock slip orientation this pattern of occurrence may be explained by a decrease in small scale crustal heterogeneity with increasing depth and suggests that increasing normal stress both regional tectonic stress and lithostatic load inhibits the occurrence of foreshocks no relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process or that foreshocks are not part of this process

an experimental study of pressure shadows in partially molten rocks abstract as a two phase solid melt material flows around rigid particles melt depleted and melt enriched regions i e pressure shadows develop due to the coupled fluxes of melt and solid driven by pressure gradients around the particles to study this compaction decompaction process samples composed of fine grained san carlos olivine plus mid ocean ridge basalt containing dispersed sub millimeter sized single crystal beads of olivine were deformed in torsion at a temperature of 1473 k and a confining pressure of 300 mpa indicated by melt distribution maps obtained from reflected light optical and backscattered electron microscopy melt enriched and melt depleted regions around the beads became observable at a local shear strain of 1 in samples with an initially homogeneously distributed melt fraction of 0 05 the melt enriched regions high 0 06 to 0 10 and the melt depleted regions low 0 02 to 0 04 extending as far as one radius of the bead were symmetrically distributed around the bead the flow field of the olivine matrix determined from crystallographic preferred orientations agrees with theoretical predictions based on two phase flow analysis these experiments are the first to produce pressure shadows in partially molten rocks one implication of this study is that it will be possible to constrain the ratio of bulk to shear viscosity which is inferred from the distribution of melt using a combination of experimental observations and numerical simulations

origin of stable remanent magnetization of siliceous sediments in the central equatorial pacific abstract siliceous sediments distributed in the central equatorial pacific have a stable remanent magnetization regardless of age or depth this is in contrast with the unstable remanence of unfossiliferous red clay defined as pelagic clay here which accumulates in the middle latitudes of the pacific we have conducted a rock magnetic study of the siliceous sediments to clarify the magnetic carriers of the stable remanence for this purpose we combined magnetic granulometry by the suspension method and observation of magnetic grains with a transmission electron microprobe tem the advantage of the former is that the magnetic extraction procedure is not required for obtaining size distribution of the magnetic grains in sediments the tem observations revealed that most of the magnetic extracts are identical in size and shape to bacterial magnetosomes the suspension method proved that the magnetic assemblages of the siliceous sediments of both quaternary and early miocene age have mean diameters of about 0 05 m which is within the single domain range of magnetite and have narrow size distributions compared with pelagic clay these characteristics can be explained if the magnetofossils observed by tem are the major constituent of the magnetic grains in the sediments and have been preserved for a long period of time the difference in the stability of the remanent magnetization between the siliceous sediments and the pelagic clay can be explained by the difference in the size distribution of magnetic grains which would reflect differences in their sources i e biogenic vs detrital eolian

energetics of a two phase model of lithospheric damage shear localization and plate boundary formation summary the two phase theory for compaction and damage proposed by bercovici et al 2001a j geophys res 106 8887 8906 employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a model for damage void generation and microcracking and a continuum description of weakening failure and shear localization here we examine further variations of the model which consider 1 how interfacial surface energy when averaged over the mixture appears to be partitioned between phases 2 how variability in deformational work partitioning greatly facilitates localization and 3 how damage and localization are manifested in heat output and bulk energy exchange microphysical considerations of molecular bonding and activation energy suggest that the apparent partitioning of surface energy between phases goes as the viscosity of the phases when such partitioning is used in the two phase theory it captures the melt compaction theory of mckenzie 1984 j petrol 25 713 765 exactly as well as the void damage theory proposed in a companion paper ricard bercovici submitted calculations of 1 d shear localization with this variation of the theory still show at least three possible regimes of damage and localization at low stress is weak localization with diffuse slowly evolving shear bands at higher stress strong localization with narrow rapidly growing bands exists and at yet higher shear stress it is possible for the system to undergo broadly distributed damage and no localization however the intensity of localization is strongly controlled by the variability of the deformational work partitioning with dilation rate represented by the parameter f or 1 extreme localization is allowed with sharp profiles in porosity weak zones nearly discontinuous separation velocities and effectively singular dilation rates finally the bulk heat output is examined for the 1 d system to discern how much deformational work is effectively stored as surface energy in the high stress distributed damage cases heat output is reduced as more interfacial surface energy is created yet in either the weak or strong localizing cases the system always releases surface energy regardless of the presence of damage or not and thus slightly more heat is in fact released than energy is input through external work moreover increased levels of damage represented by the maximum work partitioning f make the localizing system release surface energy faster as damage enhances phase separation and focusing of the porosity field thus yielding more rapid loss of net interfacial surface area however when cases with different levels of damage are compared at similar stages of development say the peak porosity of the localization it is apparent that increased damage causes smaller relative heat release and retards loss of net interfacial surface energy the energetics and energy partitioning of this damage and shear localization model are applied to estimating the energy costs of forming plate boundaries and generating plates from mantle convection

discrete single domain and pseudo single domain titanomagnetite particles in silicic glass of an ash flow tuff single domain and pseudo single domain titanium poor magnetite grains are present in silicic vitrophyre glass of an ash flow tuff from western nevada conventional transmission electron microscopy and analytical electron microscopy corroborate inferences made solely from rock magnetic and remanence data that the thermoremanent magnetization in the vitrophyre is carried by such grains the magnetite grains vary in size shape and distribution throughout the glass they probably crystal lized during violent eruption and welding of the ash flow at temperatures well in excess of 600 c

the holocene paleosecular variation record from elk lake minnesota the paleosecular variation record of the geomagnetic field from elk lake minnesota is presented and its implications are discussed the elk lake record is compiled from measurements of four parallel cores with a 2 cm sampling interval 15 years time control is provided by radiocarbon dates and by annual laminations varves in the sediment stratigraphic control is provided by the annual laminations and by fine scale 2 5 cm variations in magnetic susceptibility the varve chronology provides improved time control for the central north american paleosecular variation record back to about 4000 years the results of this study largely confirm the results of lund and banerjee from lake st croix and kylen lake and demonstrate the general utility of holocene sediment paleomagnetic records differences between the records appear to be due to experimental procedure so minimal procedures are recommended for future work comparison of north american and european records suggests that two drifting non dipole sources one having its advent during the last 5000 years can account for much of the secular variation

from sub rayleigh to supershear ruptures during stick slip experiments on crustal rocks supershear earthquake ruptures propagate faster than the shear wave velocity although there is evidence that this occurs in nature it has not been experimentally demonstrated with the use of crustal rocks we performed stick slip experiments with westerly granite under controlled upper crustal stress conditions supershear ruptures systematically occur when the normal stress exceeds 43 megapascals mpa with resulting stress drops on the order of 3 to 25 mpa comparable to the stress drops inferred by seismology for crustal earthquakes in our experiments the sub rayleigh to supershear transition length is a few centimeters at most suggesting that the rupture of asperities along a fault may propagate locally at supershear velocities in turn these sudden accelerations and decelerations could play an important role in the generation of high frequency radiation and the overall rupture energy budget

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

controlled biomineralization of magnetic minerals by magnetotactic bacteria magnetotactic bacteria produce two general types of minerals as the mineral phases of their magnetosomes iron oxides and iron sulfides the iron oxides include only ferrimagnetic magnetite fe3o4 and the iron sulfides ferrimagnetic greigite fe3s4 and non magnetic pyrite fes2 regardless of composition the magnetosome particles have a very narrow size range 35 120 nm and are single magnetic domains these intracellular particles confer a permanent magnetic dipole moment to the cell resulting in the cell s magnetotactic response i e a motile biomagnetic compass the morphology of the magnetosome particles is species and or strain specific thus the organism exerts significant control over the biomineralization process es involved in magnetosome formation however there is some accumulating evidence that suggests that in certain organisms under specific situations local environmental conditions appear to affect the magnetosome mineral phase in at least three different ways these include 1 variations in the stoichiometry of the metal and non metal components of the magnetosome mineral phase 2 the replacement of iron with other transition metal ions in the metal component of the magnetosome mineral phase and 3 the replacement of oxygen with sulfur or vice versa in the non metal component of the magnetosome mineral phase examples of each type of mineral change or variation are described and discussed

paleomagnetic evidence for high temperature emplacement of the 1883 subaqueous pyroclastic flows from krakatau volcano indonesia the paroxysmal 1883 eruption of krakatau volcano in indonesia discharged at least 6 5 km3 dense rock equivalent of pyroclastic material into the shallow waters of the sunda straits within a 15 km radius of the volcano progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single component remanence with mean inclination of 2 4 the partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation indicating that substantial cooling of clasts must have occurred following deposition estimated subaqueous emplacement temperature for such clasts is greater than 500 c rare two component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation these inflection points range from 350 to 550 c indicating a minimum subaqueous emplacement temperature of 350 c paleomagnetic evidence for high emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot subaerially generated pyroclastic flows into the sea similar deposits have been interpreted from the geologic record but this is the first documented example of submarine pyroclastic flows from a historic eruption the krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow seawater interactions

nature and origin of magnetic minerals within the middle jurassic shallow water carbonate rocks of the paris basin france implications for magnetostratigraphic dating summary the middle and upper jurassic bathonian oxfordian shallow water carbonate rocks from the paris basin france consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface despite a preliminary positive study these deposits have been considered to be largely remagnetized rochette private communication and hence not amenable to palaeomagnetic dating to establish their magnetic mineralogy and test this remagnetization hypothesis we have used an integrated investigation combining petrographic geochemical rock magnetic and palaeomagnetic measurements on samples extracted from five cores from the paris basin and from outcrops in burgundy magnetic minerals in the bathonian oxfordian carbonates include 1 primary biogenic single domain magnetite and detrital multi domain ti magnetite and their oxidized form maghemite 2 authigenic spheres of magnetite probably related to hydrocarbons and 3 goethite either restricted to ferruginous ooid layers or resulting from surficial alteration notably replacement of pyrite framboids rock magnetic experiments carried out on 68 samples reveal hcr hc and mrs ms ratios ranging from 1 88 to 5 58 and 0 017 to 0 314 respectively these values are clearly distinct from diagnostic values for a chemical remagnetization pyrrhotite was not identified within these sediments moreover the average hcr hc ratio of 3 14 is significantly different from the value of 1 333 for natural pyrrhotite dekkers 1988 these results have a direct implication for the preservation of the primary magnetization consequently these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations

premonitory acoustic emissions and stick slip in natural and smooth faulted westerly granite 1 a stick slip event was induced in a cylindrical sample of westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 mpa continuously recorded acoustic emission ae data and computer tomography ct generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault the dynamic stick slip event considered to be a laboratory analog of an earthquake generated an ultrasonic signal that was recorded as a large amplitude ae event first arrivals of this event were inverted to determine the nucleation site of slip which is associated with a geometric asperity on the fault surface ct images and ae locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault this experiment is compared with a stick slip experiment on a sample prepared with a smooth artificial saw cut fault surface nearly a thousand times more ae were observed for the natural fault which has a higher friction coefficient 0 78 compared to 0 53 and larger shear stress drop 140 compared to 68 mpa however at the measured resolution the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments suggesting a similar dynamic rupture process we propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field scale fault system in terms of the presence of asperities fault surface heterogeneity and interaction of branching faults

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

magnetite as a prokaryotic biomarker a review 1 over the years nanometer sized magnetite fe3o4 crystals have been recovered from many modern and ancient environments including sediments and soils and even meteorites in some cases these crystals have been used as magnetofossils for evidence of the past presence of specific microbes magnetite nanocrystals can be formed by a number of different biological and inorganic mechanisms resulting in crystals with different physical and magnetic characteristics prokaryotes bacteria biomineralize magnetite through two methods that differ mechanistically including biologically induced mineralization bim and biologically controlled mineralization bcm magnetite nanocrystals produced by bim are known to be synthesized by the dissimilatory iron reducing bacteria are deposited external to the cell and generally are physically indistinguishable from magnetite particles formed inorganically bcm magnetites in contrast are synthesized by the magnetotactic bacteria and some higher organisms and are precipitated intracellularly as membrane bounded structures called magnetosomes these magnetites appear to have unique crystal morphologies and a narrow size range leading to their original use as magnetofossils because of the discovery of nanometer sized crystals of magnetite in the martian meteorite alh84001 the use of these criteria for the determination of whether magnetite crystals could constitute a prokaryotic biomarker was questioned thus there is currently great debate over what criteria to use in the determination of whether specific magnetite crystals are biogenic or not in the last decade additional criteria have been established e g the magnetite assay for biogenicity and new tools and technologies have been developed to determine the origin of specific types of magnetite crystals

experimental investigation of flash weakening in limestone abstract flash heating and weakening could operate during earthquake nucleation and propagation we performed 27 friction experiments in a compression torsion apparatus on ring shaped limestone samples at sub seismic to seismic slip rates 340 mm s centimetric displacements and normal stresses of 8 mpa friction decreases dramatically at slip rates of 50 150 mm s flash weakening was contemporaneous with a peripheral temperature rise of 90 c measured with an infrared camera the peripheral temperature yields a lower limit to the slipping zone temperature the decrease in friction may result from weakening of the asperity contacts due to decarbonation of calcite induced by 1 flash heating or 2 mechanically activated reactions however x ray powder diffraction and raman spectroscopy analyses do not reveal the presence of decarbonation products in the slipping zone instead white light interferometry and field emission scanning electron microscope observations reveal the presence of a smooth sliding surface made of nanometric particles the mechanical data can be fit by the rate and state dependent friction model or by a quadratic model the latter proposed for powder lubrication we conclude that flash heating and weakening and powder lubrication may operate together to decrease dynamic friction in limestone in experiments and for the conditions investigated here in nature

a generalized formulation of interfacial tension driven fluid migration with dissolution precipitation abstract we present an extended formulation for the interfacial tension driven melt migration by taking into account dissolution precipitation and diffusive matter transport through the liquid phase our results indicate that the melt migration is caused by two mechanisms in the first mechanism a change in melt fraction is accommodated by compaction decompaction of solid matrix and in the second mechanism a change in melt fraction is accommodated by dissolution precipitation the latter mechanism is newly introduced in this study as spatial scale decreases the dominant mechanism changes from compaction decompaction to dissolution precipitation and when the second mechanism is dominant evolution of melt fraction is governed by a nonlinear diffusion equation therefore critical scale of this transition is called diffusion length d below which melt fraction evolves primarily by diffusion diffusion length d is usually smaller than the compaction length c important roles of the new mechanism are discussed on the basis of existing experimental data on melt infiltration shear induced melt segregation into melt rich bands and rehomogenization of melt in the bands due to static annealing melt distribution in the mantle is briefly discussed on the basis of the new model

elastohydrodynamic lubrication of faults the heat flow paradox provides evidence that a dynamic weakening mechanism may be important in understanding fault friction and rupture we present here a specific model for dynamic velocity weakening that uses the mechanics of well studied industrial bearings to explain fault zone processes an elevated fluid pressure is generated in a thin film of viscous fluid that is sheared between nearly parallel surface this lubrication pressure supports part of the load therefore reducing the normal stress and associated friction across the gap the pressure also elastically deforms the wall rock the model is parameterized using the sommerfeld number which is a measure of the lubrication pressure normalized by the lithostatic load for typical values of the material properties slip distance and velocity the sommerfeld number suggests that lubrication is an important process if the lubrication length scales as the slip distance in an earthquake the frictional stress during dynamically lubricated large earthquakes is 30 less than the friction with only hydrostatic pore pressure elastohydrodynamic lubrication also predicts a decrease in high frequency 1 hz radiation above a critical slip distance of a few meters this prediction is well matched by the strong motion data from the 1999 taiwan earthquake the observed 2 orders of magnitude variation in scaled radiated energy between small m w 6 is also predicted by the lubrication model

magnetic properties of marine magnetotactic bacteria in a seasonally stratified coastal pond salt pond ma usa summary magnetic properties of suspended material in the water columns of freshwater and marine environments provide snapshots of magnetic biomineralization that have yet to be affected by the eventual time integration and early diagenetic effects that occur after sediment deposition here we report on the magnetism geochemistry and geobiology of uncultured magnetite and greigite producing magnetotactic bacteria mb and magnetically responsive protists mrp in salt pond falmouth ma usa a small coastal marine basin 5 m deep that becomes chemically stratified during the summer months at this time strong inverse o2 and h2s concentration gradients form in the water column and a well defined oxic anoxic interface oai is established at a water depth of about 3 5 m at least four morphological types of mb both magnetite and greigite producers and several species of magnetically responsive protists are found associated with the oai and the lower sulphidic hypolimnion magnetic properties of filtered water were determined through the water column across the oai and were consistent with the occurrence of magnetite and greigite producing mb at different depths sharp peaks in anhysteretic remanent magnetization arm and saturation isothermal remanent magnetization sirm and single domain sd values of arm sirm occur within the oai corresponding to high concentrations of mb and mrp with magnetically derived cell densities of 10 4 10 6 ml 1 low temperature 1 per cent is present within magnetite magnetosomes produced either

laboratory generated m 6 earthquakes we consider whether mm scale earthquake like seismic events generated in laboratory experiments are consistent with our understanding of the physics of larger earthquakes this work focuses on a population of 48 very small shocks that are foreshocks and aftershocks of stick slip events occurring on a 2 0 m by 0 4 m simulated strike slip fault cut through a large granite sample unlike the larger stick slip events that rupture the entirety of the simulated fault the small foreshocks and aftershocks are contained events whose properties are controlled by the rigidity of the surrounding granite blocks rather than characteristics of the experimental apparatus the large size of the experimental apparatus high fidelity sensors rigorous treatment of wave propagation effects and in situ system calibration separates this study from traditional acoustic emission analyses and allows these sources to be studied with as much rigor as larger natural earthquakes the tiny events have short 3 6 s rise times and are well modeled by simple double couple focal mechanisms that are consistent with left lateral slip occurring on a mm scale patch of the precut fault surface the repeatability of the experiments indicates that they are the result of frictional processes on the simulated fault surface rather than grain crushing or fracture of fresh rock our waveform analysis shows no significant differences other than size between the m 7 to m 5 5 earthquakes reported here and larger natural earthquakes their source characteristics such as stress drop 1 10 mpa appear to be entirely consistent with earthquake scaling laws derived for larger earthquakes

paleomagnetic evidence for fossil biogenic magnetite in western crete abstract previous paleomagnetic research has shown that the concentration of fine grained magnetite preserved in miocene marine clays of western crete drops during two successive geomagnetic transitions although the mechanism responsible for this effect was not known magnetite precipitation by magnetotactic bacteria offers a straightforward explanation for this decrease reduced field strengths during a reversal will strain the competitive advantage of having intracellular magnetite and will force bacteria to precipitate correspondingly more magnetite if they are to maintain the same alignment in the geomagnetic field their ability to do this is limited by several magnetophysical constraints which should act to reduce the numbers of magnetite precipitating cells particularly in fields of less than about 12 of the present level this would lead to a decrease in the supply of bacterial magnitude to the sediments recovery of intact magnetotactic bacterial fossils may lead to a new method for estimating paleointensities from sedimentary rocks

influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions the influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions has been investigated by performing a series of high temperature triaxial compression experiments samples with melt fractions of 0 02 0 12 were deformed under water saturated conditions at temperatures between 1373 and 1473 k and a confining pressure of 300 mpa in a gas medium apparatus at constant differential stress and temperature the rate of deformation increased rapidly but systematically with increasing melt fraction in the diffusion creep regime at a given differential stress samples with melt fractions of 0 02 and 0 12 deformed a factor of 2 and 20 respectively faster than a melt free sample in the dislocation creep regime a sample with a melt fraction of 0 12 deformed a factor of 40 faster than a melt free sample for partially molten olivine basalt aggregates deformed under hydrous conditions the dependence of creep rate on melt fraction can be expressed in the form 0 exp where 26 for diffusion creep and 31 for dislocation creep the results of this study combined with reasonable estimates for the spatial variation in the concentrations of water and melt as well as for the geotherm and the activation volume for creep provide constraints on the viscosity structure of earth s upper mantle as an example we present a viscosity profile for the mantle wedge above a subducting plate demonstrating that the viscosity in that region can vary by 3 orders of magnitude over a depth of 60 km due to the combined effects of water and melt weakening

a reinterpretation of the precursory seismic b value anomaly from fracture mechanics summary a new model is presented which can explain the major temporal fluctuations in seismic b value in terms of the underlying physical processes of time varying applied stress and crack growth under conditions of constant strain rate the model predicts two minima in the b value separated by a temporary maximum of inflexion point a corollary being that a single broad maximum would be expected in scattering attenuation these fluctuations in b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence we present preliminary results from controlled laboratory experiments and report recent field observations both of which are consistent with the predicted form and in particular exhibit the distinctive second short term anomaly which reaches a value bc 0 5 during the critical phase

role of pore fluids in the generation of seismic precursors to shear fracture a systematic study of temporal changes in seismic b values defined as the log linear slope of the earthquake frequency magnitude distribution has shown that large earthquakes are often preceded by an intermediate term increase in b followed by a decrease in the months to weeks before the earthquake1 the onset of the b value increase can precede earthquake occurrence by as much as 7 years a recently proposed fracture mechanics model of the earthquake source2 explains these temporal fluctuations in b in terms of the underlying physical processes of time varying applied stress and crack growth the model predicts two minima in b separated by a short lived maximum here we report the results of controlled laboratory deformation experiments done in simulated upper crustal conditions on both air dried and water saturated rock specimens as found in previous experiments3 5 shear fracture in dry specimens is characterized by a decline in b during anelastic deformation to a single minimum reached just before failure but in water saturated specimens when pore fluid volume is kept constant by servo control we also observe a second intermediate term b value minimum so reproducing the double b value anomaly predicted by the model2

electron optical characterization of bacterial magnetite abstract magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy electron diffraction and electron microprobe analysis observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although chains are also seen electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure electron microprobe analysis shows the magnetite to be slightly titaniferous electron microscopy indicates that the grains rather than being flake shaped are parallelepiped crystals with a mean length of 99 3 8 7nm a mean width of 62 3 6 1nm yielding a width to length ratio of 0 63 these data support the contention that the magnetic bacterial grains are single domain crystals capable of producing a natural remanent magnetization in sediments

compaction around a rigid circular inclusion in partially molten rock conservation laws that describe the behavior of partially molten mantle rock have been established for several decades but the associated rheology remains poorly understood constraints on the rheology may be obtained from recently published experiments involving deformation of partially molten rock around a rigid spherical inclusion these experiments give rise to patterns of melt segregation that exhibit the competing effects of pressure shadows and melt rich bands such patterns provide an opportunity to infer rheological parameters through comparison with models based on the conservation laws and constitutive relations that hypothetically govern the system to this end we have developed software tools to simulate finite strain two phase flow around a circular inclusion in a configuration that mirrors the experiments simulations indicate that the evolution of porosity is predominantly controlled by the porosity weakening exponent of the shear viscosity and the poorly known bulk viscosity in two dimensional simulations presented here we find that the balance of pressure shadows and melt rich bands observed in experiments only occurs for bulk to shear viscosity ratio of less than about five however the evolution of porosity in simulations with such low bulk viscosity exceeds physical bounds at unrealistically small strain due to the unchecked exponential growth of the porosity variations processes that limit or balance porosity localization should be incorporated in the formulation of the model to produce results that are consistent with the porosity evolution in experiments

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

silicon in magnetite high resolution microanalysis of magnetite ilmenite intergrowths magnetite ilmenite oxidation exsolution intergrowths from an original titanomagnetite microphenocryst from an ash flow tuff unit have been studied using conventional transmission electron microscopy and analytical electron microscopy silicon has been found to be in solid solution in all of the magnetite studied and in some of the coexisting ilmenite the average value in magnetite is 1 2 wt si equivalent to solid solution of 9 mole fe2sio4 silicon is also present in very small silicate inclusions and as unusual si rich domains of uncertain origin in magnetite the inclusions and domains may be irregularly distributed through the magnetite in sizes well below those resolvable with the electron microprobe microprobe analyses for si in magnetite generally reflect these heterogeneities in addition to a component presumably in solid solution the petrologic implications of the data can be assessed only when relevant thermochemical data become available and the distribution of si in magnetite is better understood

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

pervasive foreshock activity across southern california foreshocks have been documented as preceding less than half of all mainshock earthquakes these observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation which both suggest that foreshock activity should be nearly ubiquitous here we use a state of the art high resolution earthquake catalog to study foreshock sequences of magnitude m4 and greater mainshocks in southern california from 2008 2017 this highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity seventy two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate foreshock sequences vary in duration from several days to weeks with a median of 16 6 days the results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes

magnetism and magnetic mineralogy of ash flow tuffs from yucca mountain nevada the magnetic susceptibility and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement spatial variations in these properties therefore provide a potentially useful tool for geological studies maxima and minima in measured along profiles at yucca mountain nevada identify persistent magnetic marker horizons within vitric portions of the tiva canyon and underlying topopah spring members of the paintbrush tuff the correlatable variations of with vertical position in these units exist over the 8 km of strike examined the observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of fe ti oxide phenocrysts and the presence shape size and mineralogy of magnetic fe oxide microcrystals that precipitated at high temperature after emplacement of each sheet vitrophyre near the top of the topopah spring member has a distinctive maximum in due to titanomagnetite phenocrysts susceptibility in the lower portion of the overlying tiva canyon member reaches maximum values of about 1 3 10 2 si near the base of the member beneath the columnarly jointed vitrophyre due to the presence of abundant superparamagnetic cubic fe oxide these elongate microcrystals are several tens of nm long and are typically deformed and tapered up section in the tiva canyon member the susceptibility decreases going to a minimum within the columnar vitrophyre at this stratigraphic level the precipitated fe oxides are larger and carry intense nrm of up to 30 a m is only 3 10 3 the whisker like microcrystals of fe oxide which typically nucleated on what appears to be chain silicate grew as needles up to 25 m in length the size variations of the precipitated fe oxides which were established using transmission electron microscopy tem and petrographic observation are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization several interpretations of the shape anisotropy of the precipitated feoxide are possible including growth by a dislocation mechanism additionally the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field susceptibility variations as established at the outcrop as well as in the borehole offer a potentially useful tool for stratigraphic correlation of ash flow sheets

distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism the cougar point tuffs mid miocene southern snake river plain idaho usa in this paper we present paleomagnetic geochemical mineralogical and geochronologic evidence for correlation of the mid miocene cougar point tuff cpt in southwest snake river plain srp of idaho the new stratigraphy presented here significantly reduces the frequency and increases the scale of known srp ignimbrite eruptions the cpt section exposed at the black rock escarpment along the bruneau river has been correlated eastward to the brown s bench escarpment six common eruption units and cassia mountains three common eruption units regions of southern idaho the cpt records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events to obtain reliable paleomagnetic directions the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization

on the evolution of elastic properties during laboratory stick slip experiments spanning the transition from slow slip to dynamic rupture the physical mechanisms governing slow earthquakes remain unknown as does the relationship between slow and regular earthquakes to investigate the mechanism s of slow earthquakes and related quasi dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration we reproduced the full spectrum of slip behavior from slow to fast stick slip by altering the elastic stiffness of the loading apparatus k to match the critical rheologic stiffness of fault gouge kc our experiments show an evolution from stable sliding when k kc to quasi dynamic transients when k kc to dynamic instabilities when k kc to evaluate the microphysical processes of fault weakening we monitored variations of elastic properties we find systematic changes in p wave velocity vp for laboratory seismic cycles during the coseismic stress drop seismic velocity drops abruptly consistent with observations on natural faults in the preparatory phase preceding failure we find that accelerated fault creep causes a vp reduction for the complete spectrum of slip behaviors our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults depending on frictional properties in agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by vp changes during the seismic cycle the observed reduction in vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

a new formulation for coupled magma mantle dynamics many open problems in the earth sciences can only be understood by modelling the porous flow of melt through a viscously deforming solid rock matrix however the system of equations describing this process becomes mathematically degenerate in the limit of vanishing melt fraction numerical methods that do not consider this degeneracy or avoid it solely by regularising specific material properties generally become computationally expensive as soon as the melt fraction approaches zero in some part of the domain here we present a new formulation of the equations for coupled magma mantle dynamics that addresses this problem and allows it to accurately compute large scale 3 d magma mantle dynamics simulations with extensive regions of zero melt fraction we achieve this by rescaling one of the solution variables the compaction pressure which ensures that for vanishing melt fraction the equation causing the degeneracy becomes an identity and the other two equations revert to the stokes system this allows us to split the domain into two parts and to solve the coupled magma mantle dynamics system only in cells where melt is present we have implemented this formulation in the open source geodynamic modelling code aspect and illustrate the improved performance compared to the previous three field formulation showing numerically that the new formulation is optimal in terms of problem size and only minimally sensitive to model parameters beyond that we demonstrate the applicability to realistic problems by showing large scale 2 d and 3 d models of mid ocean ridges with complex rheology hence we believe that our new formulation and its implementation in aspect will prove a valuable tool for studying the interaction of melt segregating through and interacting with a solid host rock in the earth and other planetary bodies using high resolution three dimensional simulations

changes in lake stratification and oxygen distribution inferred from two contrasting records of magnetotactic bacteria and diatoms 1 magnetotactic bacteria mtb and diatoms produce hard parts on seasonal timescales that continuously becomes embedded in various soft sediment archives consequently they can and are frequently used to reconstruct past environmental conditions in oceans fjords and freshwater basins in this study we observe how these two cosmopolitan creatures can be used to track vertical shifts in the distribution of oxygen in a small mountain lake situated in western norway several short and long sediment cores have been retrieved from the lake allowing for a detailed reconstruction that spans the last 2500 years during periods of enhanced runoff from the catchment the oxic anoxic transition zone oatz is lowered down to the lake sediment interface due to increased convection we posit that this condition reduces the concentration of microaerophilic magnetite producing mtb an interpretation partly supported by rock magnetic analyzes preservation of diatom frustules is however improved whenever this oxygen rich scenario prevails similarly when runoff is low and the oatz is lifted above the lake sediment interface mtb concentration seems to increase whereas dissolution of diatoms is more common this consistent antiphase relationship between mtb concentration and diatom preservation occurring on a multidecadal timescale during lowest reconstructed lake water ph in the lake s history suggest a coupled response to subtle changes in external environmental variables that affect catchment and lake hydrology

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

evidence for bacterial palaeoecological origin of mineral magnetic cycles in oxic and sub oxic tasman sea sediments abstract sediments from the tasman basin in the southwest pacific ocean show a cyclic pattern of variation in mineral magnetic properties including susceptibility and remanence over the brunhes chron the magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by tem investigation of the magnetic fraction the intervals of low bacterial magnetite concentration occurring in glacial periods are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage there is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped either as the result of variation in bottom water oxygen concentration or sedimentation rate the variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration the bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick

evolution of b value during the seismic cycle insights from laboratory experiments on simulated faults abstract we investigate the evolution of the frequency magnitude b value during stable and unstable frictional sliding experiments using a biaxial shear configuration we record broadband acoustic emissions ae while shearing layers of simulated granular fault gouge under normal stresses of 2 8 mpa and shearing velocity of 11 m s ae event amplitude ranges over 3 4 orders of magnitude and we find an inverse correlation between b and shear stress the reduction of b occurs systematically as shear stress rises prior to stick slip failure and indicates a greater proportion of large events when faults are more highly stressed for quasi periodic stick slip events the temporal evolution of b has a characteristic saw tooth pattern it slowly drops as shear stress increases and quickly jumps back up at the time of failure the rate of decrease during the inter seismic period is independent of normal stress but the average value of b decreases systematically with normal stress for stable sliding b is roughly constant during shear however it exhibits large variability during irregular stick slip we see a mix of both behaviors b decreases during the interseismic period between events and then remains constant when shear stress stabilizes until the next event where a co seismic increase is observed our results will help improve seismic hazard assessment and ultimately could aid earthquake prediction efforts by providing a process based understanding of temporal changes in b value during the seismic cycle

fabrics facies and flow through a large volume ignimbrite pampa de oxaya chile the miocene oxaya formation exposed along the western andean slope in northern chile represents one of the largest ignimbrite provinces on earth in this study magnetic fabric data were acquired from a 1 km long core drilled vertically through a single cooling unit of the oxaya formation the ca 22 ma cardones ignimbrite samples for magnetic analysis were obtained every 20 m from the fine grained matrix of the core detailed measurements of the variation in bulk magnetic properties including natural remanent magnetization nrm susceptibility and anisotropy of magnetic susceptibility ams were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow ams ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large volume ignimbrite after utilizing the magnetic remanence to correct for rotation about the core axis well grouped and imbricated petrofabric orientations reveal a well defined sw 247 transport direction down a proto western andean slope indicating syn or post welding flow and confirming the deformed lauca caldera as the likely source of the eruption systematic variations in fabric shape t and intensity p with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center these vertical changes in fabric reveal that this massive apparently homogeneous deposit has a systematic layering this layering reflects depth dependent temperature variations temporal changes in the flow boundary zone during deposition and post depositional processes

high velocity frictional properties of a clay bearing fault gouge and implications for earthquake mechanics 1 frictional properties of natural kaolinite bearing gouge samples from the median tectonic line sw japan have been studied using a high velocity rotary shear apparatus and deformed samples have been observed with optical and electron scanning and transmission microscopy for a slip velocity of 1 m s 1 and normal stresses from 0 3 to 1 3 mpa a dramatic slip weakening behavior was observed x ray diffraction analysis of deformed samples and additional high velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip the critical slip weakening distance dc is of the order of 1 to 10 m these values are extrapolated to higher normal stresses assuming that dc is rather a thermal parameter than a parameter related to a true characteristic length the calculation shows that dimensionally dc 1 n2 where n is the normal stress applied on the fault the inferred dc values range from a few centimeters at 10 mpa normal stress to a few hundreds of microns at 100 mpa normal stress microscopic observations show partial amorphization and dramatic grain size reduction down to the nanometer scale localized in a narrow zone of about 1 to 10 m thickness fracture energy gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction and energies of mineralogic transformations we show that most of the fracture energy is either converted into heat or radiated energy the geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust

nucleation of the 1999 izmit earthquake by a triggered cascade of foreshocks much of what we know about the nucleation of earthquakes comes from the temporal and spatial relationship of foreshocks to the initiation point hypocentre of the mainshock the 1999 mw 7 6 izmit turkey earthquake was preceded by a 44 minute long foreshock sequence which built in intensity as the time of the mainshock approached here we apply a series of high resolution methods to the sparse seismological observations to determine the spatial and temporal relation of the foreshocks to the mainshock hypocentre we find that the foreshocks form a contiguous series of ruptures that progressed systematically from west to east towards the mainshock hypocentre located at the extreme eastern edge of the foreshocks the izmit foreshock sequence occurred as a triggered cascade in which one foreshock loads the adjacent fault patch causing it to fail with the mainshock initiation no different than another foreshock we find no evidence to support a hypothesized precursory aseismic driving process if we are to resolve the role of aseismic deformation and possible role of fluid overpressure in the earthquake nucleation process it will likely require measurements made in the near field of the hypocentre the magnitude 7 6 izmit earthquake that struck turkey in 1999 was nucleated by an eastward migrating cascade of foreshocks according to high resolution analyses of seismic data

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

torsion of a cylinder of partially molten rock with a spherical inclusion theory and simulation the processes that are involved in migration and extraction of melt from the mantle are not yet fully understood gaining a better understanding of material properties of partially molten rock could help shed light on the behavior of melt on larger scales in the mantle in this study we simulate three dimensional torsional deformation of a partially molten rock that contains a rigid spherical inclusion we compare the computed porosity patterns to those found in recent laboratory experiments the laboratory experiments show emergence of melt rich bands throughout the rock sample and pressure shadows around the inclusion the numerical model displays similar melt rich bands only for a small bulk to shear viscosity ratio five or less the results are consistent with earlier two dimensional numerical simulations however we show that it is easier to form melt rich bands in three dimensions compared to two the addition of strain rate dependence of the viscosity causes a distinct change in the shape of pressure shadows around the inclusion this change in shape presents an opportunity for experimentalists to identify the strain rate dependence and therefore the dominant deformation mechanism in torsion experiments with inclusions

diagenetic and metamorphic history of the umm nar bif eastern desert egypt the umm nar bif was formed in a sedimentary environment it is confined to an upper stratigraphic zone of pre pan african metamorphosed shelf deposits during the pan african deformational history the bif and the host metasediments were tectonically overlain by ophiolitic melange succession the metasediments and the melange were subjected to a major folding phase and then thrust over the shaitian sheared granite prior to the intrusion of syn to late orogenic granitoids the bif is divisible into two main types oxide bands including magnetite and hematite and oxide silicate bands including magnetite hematite and stilpnomelane the associated gangues are quartz calcite epidote garnet plagioclase graphite and muscovite rhythmic banding and lamination cross lamination and flaser structure are the most prominent primary features in the if bands the iron minerals and the associated gangue show a variety of textural aspects and microscopic interrelationships which indicate successive episodes of mineral accumulation and formation involving deposition recrystallization blastic growths overgrowths replacement and deformations during continuous burial and subsequent tectonic deformations

paleomagnetic evidence for the timing of collapse and resurgence of the lake city caldera san juan mountains colorado rocks of the 23 1 m y old lake city caldera consist of the compositionally zoned sunshine peak tuff postcollapse intracaldera lava flows and resurgent quartz syenite intrusions declinations of reversely magnetized i 45 to 75 sunshine peak tuff change from easterly d 93 130 throughout most of the tuff to southerly d 195 207 within the late eruptive phases the postcollapse lava flows are also reversely magnetized and their emplacement appears to have spanned resurgence of the caldera in contrast the uppermost levels of the resurgent intrusion record two distinct components of magnetization a reversed component isolated by thermal demagnetization typically at temperatures greater than 400 c and a normal component defined by thermal demagnetization at lower temperatures a baked contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data the time for caldera development from eruption of the ash flow tuffs to emplacement of the resurgent intrusion was less than about 300 000 years on the basis of best estimates of durations of reversed polarities in the early miocene the estimated 300 000 year time span for the development of the lake city caldera is very close in duration to the radiometrically determined time spans of much younger pleistocene calderas having similar dimensions and similar volumes of erupted material

micromechanics of asperity rupture during laboratory stick slip experiments 1 acoustic emissions and tremor like signals are widely recorded in laboratory experiments we are able to isolate the physical origins of these signals using high resolution nanoseismic analysis the use of a picometer sensitive wide band sensor array permits us to determine force time functions and focal mechanisms for discrete events found amid the noise of friction similar to how low frequency earthquakes are found buried within tremor we interpret these localized events to be the rupture of m sized contacts known as asperities we performed stick slip experiments on plastic plastic and rock rock interfaces and found a systematic difference between the nano earthquakes the rock interface produces very rapid 1 s implosive forces indicative of brittle failure and fault gouge formation while rupture on the plastic interface releases only shear force and produces a nano quake more similar to earthquakes commonly recorded in the field the difference between the mechanisms is attributed to the vast differences in the hardness and melting temperatures of the two materials which affect the distribution of asperities as well as their failure behavior with proper scaling the strong link between material properties and laboratory earthquakes will aid in our understanding of fault mechanics and the generation of earthquakes and tectonic tremor

chemical signature of magnetotactic bacteria there are longstanding and ongoing controversies about the abiotic or biological origin of nanocrystals of magnetite on earth magne totactic bacteria perform biomineralization of intracellular magne tite nanoparticles under a controlled pathway these bacteria are ubiquitous in modern natural environments however their identification in ancient geological material remains challenging together with physical and mineralogical properties the chemical composition of magnetite was proposed as a promising tracer for bacterial magnetofossil identification but this had never been explored quantitatively and systematically for many trace elements here we determine the incorporation of 34 trace elements in magnetite in both cases of abiotic aqueous precipitation and of production by the magnetotactic bacterium magnetospirillum magneticum strain amb 1 we show that in biomagnetite most elements are at least 100 times less concentrated than in abiotic magnetite and we provide a quantitative pattern of this depletion furthermore we propose a previously unidentified method based on strontium and calcium incorporation to identify magnetite produced by magnetotactic bacteria in the geological record magnetotactic bacteria magnetite biomineralization trace element incorporation

ultrafine grained magnetite in deep sea sediments possible bacterial magnetofossils a new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep sea sediments morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types some of which closely resemble those formed by the magnetotactic bacteria the apparently biogenic magnetite particles are of single domain size and dominate the population in calcareous deep sea sediments bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments these objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

acoustic emissions document stress changes over many seismic cycles in stick slip experiments 1 the statistics of large earthquakes commonly involve large uncertainties due to the lack of long term robust earthquake recordings small scale seismic events are abundant and can be used to examine variations in fault structure and stress we report on the connection between stress and microseismic event statistics prior to the possibly smallest earthquakes those generated in the laboratory we investigate variations in seismic b value of acoustic emission events during the stress buildup and release on laboratory created fault zones we show that b values mirror periodic stress changes that occur during series of stick slip events and are correlated with stress over many seismic cycles moreover the amount of b value increase associated with slip events indicates the extent of the corresponding stress drop consequently b value variations can be used to approximate the stress state on a fault a possible tool for the advancement of time dependent seismic hazard assessment citation goebel t h w d schorlemmer t w becker g dresen and c g sammis 2013 acoustic emissions document stress changes over many seismic cycles in stick slip experiments geophys res lett 40 2049 2054 doi 10 1002 grl 50507

paleointensities of phonolitic obsidian influence of emplacement rotations and devitrification 1 a paleomagnetic study on phonolitic obsidian from six sites in tenerife spain is presented two sites are located at the 750 ka el pasajiron at the southern wall of las canadas caldera four sites correspond to the 115 17 bce montana blanca complex paleointensity determinations are performed with a modified thellier technique using checks for alteration and domain state additionally the anisotropy of the thermoremanence and the magnetic cooling rate dependency of each specimen are measured it was not possible to obtain good quality paleointensity data for el pasajiron thermal alteration was observed in most measurements systematic changes in rock magnetic properties and paleointensities within the profile of one site also suggest the presence of a chemical remanence crm which was probably acquired during devitrifiction of the upper part of the flow this crm cannot be seen in the arai plots themselves but only by comparison of data from different samples this underlines the importance of sampling only fresh glassy looking obsidians and not devitrified ones paleointensity determinations at montana blanca are of very good quality leading to an high success rate of 82 samples at one site show acquisition of trm during emplacement rotations however it could be shown that paleointensity data of rotated samples are reliable if the remanence carriers are in the single domain range an atrm anisotropy tensor of thermoremanent magnetization and cooling rate corrected paleointensity of 48 4 2 1 t and a vdm of 9 64 0 42 1022am2 were determined which are in very good agreement with other data from spain and morocco and to various geomagnetic field models