superplastic nanofibrous slip zones control seismogenic fault friction understanding the internal mechanisms controlling fault friction is crucial for understanding seismogenic slip on active faults displacement in such fault zones is frequently localized on highly reflective mirrorlike slip surfaces coated with thin films of nanogranular fault rock we show that mirror slip surfaces developed in experimentally simulated calcite faults consist of aligned nanogranular chains or fibers that are ductile at room conditions these microstructures and associated frictional data suggest a fault slip mechanism resembling classical ashby verrall superplasticity capable of producing unstable fault slip diffusive mass transfer in nanocrystalline calcite gouge is shown to be fast enough for this mechanism to control seismogenesis in limestone terrains with nanogranular fault surfaces becoming increasingly recognized in crustal faults the proposed mechanism may be generally relevant to crustal seismogenesis

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

periodicity of magnetic intensities in magnetic anomaly profiles the cretaceous of the central pacific summary spectral analyses of long magnetic anomaly profiles of cretaceous age in the phoenix and hawaiian lineations within the pacific basin show two to five non harmonic wavelength peaks with probabilities 90 per cent within the range of several tens to hundreds of kilometres for each profile for each profile with four peak wavelengths their ratios correlate strongly r 0 98 with the ratios for the earth s orbital eccentricity periodicities for this time and hence the profile peaks necessarily correlate with each other these observations suggest that the magnetic anomaly signal is influenced by the earth s changing orbital parameters probably through their influence on the internal geomagnetic field this implies that assumptions usually made to convert the magnetic signal to the magnetization of the oceanic crust may be suspect the observations if confirmed indicate that orbital periodicities can assist in improving the age calibration of the global polarity timescale and that the predicted orbital behaviour of the earth is confirmed for the last 150 ma

a microphysical model for fault gouge friction applied to subduction megathrusts a microphysical model is developed for the steady state frictional behavior of illite quartz fault gouge and applied to subduction megathrust p t conditions the model assumes a foliated phyllosilicate supported microstructure which shears by rate independent frictional slip on the aligned phyllosilicates plus thermally activated deformation of the intervening quartz clasts at low slip rates or high temperatures the deformation of the clasts is easy accommodating slip on the foliation without dilatation with increasing velocity or decreasing temperature the shear of the clasts becomes more difficult increasing bulk shear strength until slip is activated on inclined portions of the phyllosilicate foliation where it anastomoses around the clasts slip at these sites leads to dilation involving clast matrix debonding balanced at steady state by compaction through thermally activated clast deformation model predictions taking pressure solution as the thermally activated mechanism show three regimes of velocity dependent frictional behavior at temperatures in the range of 200 500 c with velocity weakening occurring at 300 400 c in broad agreement with previous experiments on illite quartz gouge effects of slip rate normal stress and quartz fraction predicted by the model also resemble those seen experimentally extrapolation of the model to earthquake nucleation slip rates successfully predicts the onset of velocity weakening behavior at the updip seismogenic limit on subduction megathrusts the model further implies that the onset of seismogenesis is controlled by the thermally activated initiation of fault rock compaction through pressure solution of quartz which counteracts dilatation due to slip on the fault rock foliation

cyclic geomagnetic changes in mid cretaceous rhythmites italy magnetic secular variation is recorded as a quasi rhythmical process of uncertain origin rhythmic changes recorded in the scisti a fucoidi in the central apennines of italy pelagic sediments of mid cretaceous late albian age show strong correspondence to patterns of orbital variation in the same strata cyclic magnetic variations are found with peaks in the same range 105 40 26 19 kyr these results appear to support earlier suggestions that secular variations may be related to these astronomical patterns

orbital forcing and sedimentary sequences abstract the variation of eccentricity is connected with the most celebrated attempts to determine a limited portion of geologic time in the elaboration of the theory of the ice age that bears his name croll correlated two important epochs of glaciation with epochs of eccentricity computed to have occurred 100 000 and 210 000 years ago as the analysis of the glacial history progresses these correlations will eventually be established or disproved and should they be established it is possible that similar connections may be made between events far more remote gilbert 1900 p 15 16 but the precessional motion pulses steadily on through the ages like the swing of a frictionless pendulum its throb may or may not be caught by the geological process which obtains in a particular province and in a particular era but whenever the conditions are favorable and the connection is made the record should reflect the persistence and the regularity of the inciting rhythm gilbert 1900 p 18

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

quantitative extraction and analysis of carriers of magnetization in sediments we have applied an integrated procedure for quantitative magnetic mineral extraction based on the separation method of petersen von dobeneck vali 1986 to a range of sediment types to examine the efficiency and representative nature of the extraction process carriers of magnetization have been identified by rock magnetic measurements microscopy and x ray diffraction quantification of the extraction efficiencies is achieved by before and after extraction rock magnetic measurements susceptibility anhysteretic and isothermal remanences these magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types e g over 75 per cent for magnetite dominated sediments the extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin the amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction in terms of the mineralogies extracted we identify a diverse and complex range of mineral assemblages all sizes of discrete grains of magnetite are extracted including single domain and superparamagnetic grains and chains of bacterial magnetite other commonly extracted iron and titanium oxides are haematite and ilmenite ferrimagnetic chromites and sulphides were also obtained from some samples considerable amounts of quartz and feldspar are extracted due to the presence of magnetic inclusions within these diamagnetic host grains in the deep sea sediments we examined feldspars constitute a large proportion of the extracts but are significantly less abundant in other sediments where quartz is dominant a wide variety of paramagnetic minerals was identified in the extracts including pyroxenes amphiboles chlorites micas mg cr spinels garnets ti oxides apatites tourmaline and zircon many of which contain ferrimagnetic inclusions possibly less than 0 1 m in grain size dissolution of ultrafine grains of magnetite during pre extraction carbonate dissolution as suggested by sun jackson 1994 does not occur in our samples

repeated subaerial exposure of subtidal carbonate platforms triassic northern italy evidence for high frequency sea level oscillations on a 104 year scale triassic carbonate platforms in northern italy ladinian latemar carnian durrenstein norian dolomia principale consist of hundreds of stacked meter scale cycles each made of a subtidal unit with a cap rock that carries vadose cements pisoids and tepee structures these diagenetic caps are interpreted to be due to subaerial exposure of the subtidal sediments the absence of peritidal deposits rules out progradation as the cause of the exposure i e these are not ginsburg autocycles thus the repeated exposure must be the result of relative sea level oscillations which based on the number of cycles and time intervals involved occurred with an average frequency in the 104 year range we postulate a combination of small scale high frequency eustasy and local tectonic movements e g differential subsidence in the norian and infrequent pulses of uplift in the ladinian to explain the cyclicity and its internal variations

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

role of material properties in the cratering record of young platy ridged lava on mars 1 platy ridged surfaces in the elysium planitia region of mars exhibit different crater densities on rafted plates and polygonally patterned areas between them rather than being indicative of different ages these differences provide insight into the variable strength of different types of lava surface the sizes of small craters and the resulting size frequency distribution sfd depend on the material strength of target surfaces brecciated lava surfaces are likely to have higher crater densities than coherent lava

shallowing upward sequences in purbeckian peritidal carbonates lowermost cretaceous swiss and french jura mountains purbeckian carbonates in the swiss and french jura goldberg formation lower berriasian comprise shallow subtidal intertidal supratidal low energy high energy marine brackish freshwater and hypersaline facies these facies are arranged in small 0 2 1 5 m thick sequences which display a dominant shallowing upward component and which form the fundamental units of the highly structured purbeckian sedimentary record six types of small scale sequences can be recognized a intertidal to supratidal overprinting of shallow lagoonal facies b algal marsh sequence with frequent dolomitization c sabkha sequence often associated with collapse breccia d tidal flat sequence with desiccation features e lacustrine sequence f terrestrial overprinting of subtidal or intertidal facies episodic event deposits such as tempestites are superimposed thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences green marls and black pebbles are common at the top and indicate long subaerial exposure the sequences are often incomplete as subtidal facies may be absent or their upper part can be eroded lateral facies changes are common which is due to the very shallow and partly emergent purbeckian platform where various depositional environments were juxtaposed however many sequence boundaries are well developed and can be correlated over large parts of the study area the purbeckian shallowing upward sequences were generated by climatically controlled sea level changes autocyclic processes occurred locally but were overprinted by drops of sea level affecting the entire platform the small scale sequences are most probably related to the 20 000 year cycle of the precession of the equinoxes larger sequences with usually well developed emersion surfaces are attributed to the 100 000 and 400 000 year eccentricity cycles of the earth s orbit identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces which often cut across facies boundaries and thus to interpret in detail the palaeogeographic sedimentological and diagenetic evolution of the purbeckian peritidal carbonate environments

interseismic re strengthening and stabilization of carbonate faults by non dieterich healing under hydrothermal conditions abstract friction experiments consisting of a velocity stepping stage a slide hold slide shs stage and a second velocity stepping stage have been performed to study the frictional healing behavior of carbonate fault gouge and the effects of healing on the velocity dependence of friction at 20 140 c and at 50 mpa effective normal stress dry experiments show classical dieterich healing characterized by a transient peak in friction after each hold period with no effects of shs testing on steady state friction or velocity dependence by contrast the wet tests show 1 an increase in apparent steady state friction upon re sliding after a hold period and 2 a pronounced increase in the velocity dependence parameter a b after the shs stage while the first of these non dieterich type healing effects has been observed in previous hydrothermal experiments on simulated quartz gouges it has not been reported previously for carbonate gouges the observed effect of shs testing on a b has never been reported our findings suggest that under in situ hydrothermal conditions interseismic fluid assisted deformation processes such as pressure solution can significantly promote fault re strengthening in carbonates and can cause slip stabilization if the results are applicable to active faults in carbonate terrains they have important implications for understanding how the extent of the seismogenic zone and how earthquake magnitude and aftershock distributions may evolve with repeated cycles of natural seismicity and interseismic healing

analysis of the early jurassic geomagnetic data recorded at the breggia gorge ticino switzerland abstract geomagnetic directions recorded in a lower jurassic stratigraphic sequence that outcrops at the breggia gorge are analyzed a new method to identify directions suitable for calculating a time averaged dipolar geomagnetic direction has been applied to calculate a hypothetical early jurassic geocentric axial dipole the data were separated to identify normal reverse directions corresponding to periods of stable field and intermediate directions from an analysis of the data in the palaeogeographic coordinates of the breggia gorge at the time of the acquisition of the remanence it is suggested that polarity transitions in the early jurassic behaved in a similar manner to that described for late cenozoic reversals with the vgps preferentially following longitudinal paths the preferred paths coincide with a time averaged location of the jurassic subduction area of pangea this agreement could indicate a connection between the geometry of the reversals and the subduction zones an analysis to test whether both n r and intermediate directions have similar preferred distributions suggests that the underlying structure of the early jurassic geomagnetic field was similar to that observed for the late cenozoic the dispersion of directions with inclinations around 0 is higher than those belonging to n r polarity directions suggesting that there was an enhancement of fluctuations of the geomagnetic field during intermediate states in the early jurassic

laboratory study of fault healing and lithification in simulated fault gouge under hydrothermal conditions abstract seismic data geologic observations and laboratory friction studies suggest that faults lithify and strengthen heal during the interseismic period of the earthquake cycle we report on experiments to investigate the influence of healing duration and temperature on the strength and healing rate of simulated faults layers of m sized quartz powder were used to simulate granular fault gouge gouge layers were sheared within sioux quartzite in a triaxial pressure vessel at elevated pressures p c 250 mpa elevated temperatures 230 636 c and in the presence of water p h 2 o 75 mpa we performed hold slide experiments in which samples were subjected to a period of healing under hydrostatic load followed by shear deformation healing times ranged from a few hundred s to 10 5 s to isolate the effects of lithification and temperature on friction we ran two types of hold slide experiments 1 samples were subjected to healing and shear deformation at elevated temperature and 2 following healing at elevated temperature temperature was reduced prior to shear we also ran slide hold slide experiments in which samples were healed under shear load at elevated temperature 636 c experiments in which deformation was carried out at a lower temperature than healing show that both the static and sliding coefficient of friction increase with heal time samples healed and deformed at higher temperature showed lower peak strengths and within the scatter of our data did not exhibit time dependent strengthening these data are interpreted to result from enhanced lithification rates at elevated temperature samples healed under shear load exhibited a log linear decrease in friction with hold time modeling using rate and state dependent friction laws indicates velocity strengthening behavior with friction a b values of 0 05 to 0 08 our data indicate that at high temperature the effective state evolution term b is negative moreover the data indicate the rate of frictional strengthening varies significantly for hydrostatic and non hydrostatic load conditions our measurements of healing are consistent with seismic estimates of fault healing rates if stress drop is a fraction of the total shear stress and effective fault normal stress is 60 to 100 mpa

deconvolution of long core paleomagnetic data of ocean drilling program by akaike s bayesian information criterion minimization deconvolution of long core paleomagnetic data of the ocean drilling program odp was developed based on bayesian statistics samples of deep sea sediments obtained by drilling with an advanced piston corer are subjected to pass through measurements by a cryogenic magnetometer aboard the odp s research vessel joides resolution which provide continuous paleomagnetic records deconvolution of the magnetometer output was developed in order to obtain more detailed variations of the magnetization than permitted by the spatial resolution of the pickup coil approximately 11 cm the magnetization vector was modeled as a smoothly changing sequence parametrized by l2 norm of second order difference and then deconvolved using a matrix calculation as a smoothness constrained least squares method the optimum smoothness was obtained by minimizing akaike s bayesian information criterion which is a measure of the logarithm of the likelihood this deconvolution scheme was applied to pass through data artificially produced by convolving synthetic reversal record and adding a gaussian noise the amplitude of the noise was estimated and the variations shorter than the spatial resolution of the sensor were obtained the deconvolution also was applied to the real pass through data from holes 769a and 769b measured at intervals of 5 mm the deconvolution revealed variations of the magnetizations with a maximum spatial resolution of about 2 cm the magnetizations after deconvolution were in good agreement with magnetization values measured separately on cube samples taken at intervals of 5 or 10 cm

geomagnetic long term secular variations in italian lower cretaceous shallow water carbonates summary the remanent magnetic properties of an 88 m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis individual readings represent time intervals of c 720 32 yr and after 40 mt partial demagnetization they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3 17 myr the magnitude of directional secular variation is twice that of the present day despite being smoothed and the secular variations appear to grade into polarity transitions suggesting no diverence in their mechanisms the rates of change in direction between subjacent levels in the core have a log normal distribution which extends smoothly beyond 90 and has a median value of 13 700 yr the same as for unsmoothed european secular variation during the last 2000 yr the intensity of remanence after 40 mt partial demagnetization appears to provide a reasonable approximation to geomagnetic field intensity this tends to be weaker when the direction is moving faster reflecting averaging but is unrelated to the distance of the vector from the mean direction that is it depends on the rate of change and not on the virtual pole latitude the virtual poles after correction for tectonic rotations about horizontal and vertical axes have latitudes that form a strongly platykurtic fisherian distribution while their longitudes have a circular distribution on which are superimposed two gaussian peaks 180 apart this bore core thus provides detailed information of smoothed geomagnetic secular variation in the lower cretaceous 127 3 ma which shows clear regularities in behaviour some related to changes in the earth s orbital parameters

influence of subduction zone conditions and gouge composition on frictional slip stability of megathrust faults to understand the temperature depth distribution of destructive earthquakes in subduction megathrusts and the mechanisms of nucleation of these events data on the frictional behaviour of phyllosilicate quartz rich megathrust fault gouges under in situ conditions are needed we performed rotary shear friction experiments at effective normal stresses of 25 200 mpa pore fluid pressures of 50 200 mpa at 140 600 c and sliding velocities of 1 100 m s using gouge mixtures with an illite quartz ratio between 65 35 and zero experiments on 65 35 mixtures deformed at an effective normal stress neff of 170 mpa a pore fluid pressure pf of 100 mpa and 150 500 c provided a reference dataset this showed three temperature dependent slip stability regimes with potentially unstable velocity weakening behaviour at 250 400 c and velocity strengthening at lower and higher temperatures the velocity weakening regime was found to shift towards higher temperatures with decreasing neff being located at 350 600 c at 50 mpa increasing quartz content and decreasing sliding velocity also displaced the velocity weakening regime towards lower temperatures increasing pf increased a b at all temperatures narrowing the temperature extent of the velocity weakening regime we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and a b with velocity and temperature are brought about by changes in the relative importance of deformation of the clast phase by thermally activated stress corrosion cracking and pressure solution versus athermal granular flow of the mixture accompanied by dilatation our results imply that the depth extent of the seismogenic zone on subduction megathrusts depends not only on temperature and that to predict its location it is essential to have well constrained depth profiles for pore fluid pressure and effective normal stress

nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation crystal plasticity at sub seismic slip rates at 18 150 c a central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic seismic transitions in fault gouges and to identify microstructural indicators for such transitions we present new data on the slip stability of calcite fault gouges and on microstructural development down to the nanometer scale our experiments consisted of direct shear tests performed dry at slip rates of 0 1 10 m s at a constant normal stress of 50 mpa at 18 150 c the results show a transition from stable to potentially unstable slip above 80 c all samples recovered showed an optical microstructure characterized by narrow 30 40 m wide riedel and boundary shear bands marked by extreme grain comminution and a crystallographic preferred orientation cpo boundary shear bands sectioned using fib sem focused ion beam scanning electron microscopy revealed angular grain fragments decreasing from 10 to 20 m at the outer margins to 0 3 m in the shear band core where dense aggregates of nanograins also occurred transmission electron microscopy applied to foils extracted from boundary shears using fib sem combined with the optical cpo showed that these aggregates consist of calcite nanocrystals 5 20 nm in size with the 104 201 dislocation glide system oriented parallel to the shear plane and direction our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity because crystal plasticity is strongly thermally activated we infer that the transition to velocity weakening slip is likely due to enhanced crystal plasticity at 80 c this implies that tectonically active limestone terrains will tend to be particularly prone to shallow focus seismicity

the magnetic polarity stratigraphy of a hauterivian barremian carbonate sequence from southern italy summary remanence directions measured at 2 cm intervals along a composite 88 m bore core enable mean palaeomagnetic poles to be defined at 13 6 s 25 2 w and 13 6 n 154 8 e the directions of remanence vary very smoothly away from each palaeomagnetic pole extending more than 90 from them this raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles for practical purposes intermediate polarity is defined as directions whose virtual poles lie more than 25 from the mean pole enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three quarters of the core the stratigraphic thickness between these subchrons shows a very high linear correlation r 0 99 with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age the analysed sequence contains wavelength spectra which when transformed to the temporal realm match periodicities determined for three marine magnetic anomaly profiles of similar age these also match planetary orbital periodicities for the cretaceous these observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically

carbon isotope stratigraphy a tool for basin to carbonate platform correlation the early cretaceous carbonate carbon isotope stratigraphy established in pelagic limestones is marked by several pronounced excursions towards positive 13c values we investigated a biostratigraphically and palaeomagnetically calibrated aptian section to see whether c isotope stratigraphy could be recognized in shallow water carbonates the chosen carbonate platform sections are located in southern italy and have been dated by biostratigraphy bulk samples chosen from the barremian albian part of the sequence were analysed for their o and c isotope compositions the c isotope curve established shows two major positive excursions which can be correlated with the synchronous and globally recognized aptian c isotope events the data provide evidence that c isotope stratigraphy can be used as a powerful correlation tool between pelagic and shallow water limestone sequences

frictional heterogeneities on carbonate bearing normal faults insights from the monte maggio fault italy observations of heterogeneous and complex fault slip are often attributed to the complexity of fault structure and or spatial heterogeneity of fault frictional behavior such complex slip patterns have been observed for earthquakes on normal faults throughout central italy where many of the mw 6 to 7 earthquakes in the apennines nucleate at depths where the lithology is dominated by carbonate rocks to explore the relationship between fault structure and heterogeneous frictional properties we studied the exhumed monte maggio fault located in the northern apennines we collected intact specimens of the fault zone including the principal slip surface and hanging wall cataclasite and performed experiments at a normal stress of 10 mpa under saturated conditions experiments designed to reactivate slip between the cemented principal slip surface and cataclasite show a 3 mpa stress drop as the fault surface fails then velocity neutral frictional behavior and significant frictional healing overall our results suggest that 1 earthquakes may readily nucleate in areas of the fault where the slip surface separates massive limestone and are likely to propagate in areas where fault gouge is in contact with the slip surface 2 postseismic slip is more likely to occur in areas of the fault where gouge is present and 3 high rates of frictional healing and low creep relaxation observed between solid fault surfaces could lead to significant aftershocks in areas of low stress drop

an integrated model for transitional pressure solution in sandstones abstract a key aspect of the deformation of sedimentary rocks during diagenesis is pressure solution however it is often not clear why some rocks of a given mineralogy depth and pressure exhibit a great deal of pressure solution while very similar rocks under similar conditions do not to address such difficulties which we believe arise from the natural non linear physico chemical processes a quantitative model for pressure solution applied to sandstones has been developed this model subsumes various factors 1 pressure temperature and burial rate 2 grain size shape and packing of the mineral grains 3 the variability of water film thickness and coefficient of diffusion between any two grains in contact as a function of stress across the contact pore fluid pressure salinity ph and temperature in this study we distinguish several types of surface sites over quartz grains in a sandstone with corresponding differences in chemical potential thus allowing the diffusive transfer of matter from one site to another the contact between two grains is divided into two parts the actual true contact between two grains is a site of water film diffusion and the inclusions inside the contact the free face inclusion contacts are sites of free face pressure solution on the pore surface site precipitation occurs it is shown that a kinetics transition from a reaction limited deformation to a diffusion limited deformation occurs in the upper crust a geometric transition occurs too due to the textural evolution of the rock with deformation the main goal of this paper is to show the dynamics of the shifting importance of actual and free face inclusion contacts using a unified model including both features the result is a predictive model for porosity variations with depth due to pressure solution in sandstones

new paleomagnetic pole for the upper jurassic chon aike formation of southern argentina south america testing the tectonic stability of patagonia with respect to south america and implications to middle jurassic early cretaceous true polar wander abstract a paleomagnetic pole and a new ar ar date of 156 08 0 05 ma are reported for the chon aike formation an extensive plateau of ignimbrites outcropping in the deseado massif southern patagonia argentina the geographic coordinates and statistical parameters for the paleomagnetic pole validated by a reversal test are lat 84 3 s long 191 3 e a95 8 6 k 13 3 n 23 this pole shows a complete coherence with late jurassic and early to mid cretaceous poles of south america obtained exclusively from igneous rocks avoiding any potential inclination shallowing it is suggested that a southern patagonia was an integral part of south america in the late jurassic b any reported vertical axis tectonic rotations must pre date the late jurassic in this region c according to updated paleomagnetic data from igneous rocks south america had very limited latitudinal drift between about 160 and 140 ma and therefore d its apparent polar wander apw path when compared with global apparent polar wander paths does not support a large global shift or true polar wander tpw between 160 and 140 ma

the effect of loading rate on static friction and the rate of fault healing during the earthquake cycle the seismic cycle requires that faults strengthen heal between earthquakes and the rate of this healing process plays a key role in determining earthquake stress drop1 2 3 4 rupture characteristics5 6 and seismic scaling relations2 3 4 7 frictional healing as evidenced by increasing static friction during quasi stationary contact between two surfaces1 8 9 10 11 12 is considered the mechanism most likely to be responsible for fault strengthening2 3 13 14 previous studies however have shown a large discrepancy between laboratory and seismic field estimates of the healing rate2 3 4 14 15 in the laboratory rock friction changes by only a few per cent per order of magnitude change in slip rate whereas seismic stress drop increases by a factor of 2 to 5 per order of magnitude increase in earthquake recurrence interval but in such comparisons it is assumed that healing and static friction are independent of loading rate here i summarize laboratory measurements showing that static friction and healing vary with loading rate and time as expected from friction theory16 17 18 applying these results to seismic faulting and accounting for differences in laboratory seismic and tectonic slip rates i demonstrate that post seismic healing is expected to be retardedfor a period of several hundred days following an earthquake in agreement with recent findings from repeating earthquakes13 14 19 20

neural net aided detection of astronomical periodicities in geologic records abstract astronomically controlled variations in the earth s climate induce cyclic trends in the sedimentary process and record milankovitch periodicity one of the main difficulties to be solved in order to choose among the registered periodicities is the conversion from the spatial i e recurrent variations along the stratal sequences to the temporal domains of the astronomically induced frequencies present in the rock record we discuss here how this problem can be circumvented by teaching a neural net how to recognize periodicities in the signal the application to two sequences of shallow water carbonate deposits from the cretaceous of southern italy has shown this approach to be particularly effective confirming the existence of milankovitch type periodicities in the records examined where climate sediments and biota concomitantly react to the variation in the solar constant induced by secular perturbations of the earth s orbital elements

rate and state frictional and healing behavior of carbonate fault gouge explained using microphysical model classical rate and state friction rsf laws are widely applied in modeling earthquake dynamics but generally using empirically determined parameters with little or no knowledge of or quantitative account for the controlling physical mechanisms here a mechanism based microphysical model is developed for describing the frictional behavior of carbonate fault gouge assuming that the frictional behavior seen in lab experiments is controlled by competing processes of rate strengthening intergranular sliding versus contact creep by pressure solution by solving the controlling equations derived from kinematic and energy entropy balance considerations and employing a microphysical model for rate strengthening grain boundary friction plus standard creep equations for pressure solution we simulate typical lab frictional tests namely velocity stepping and slide hold slide test sequences for velocity histories and environmental conditions employed in previous experiments the modeling results capture all of the main features and trends seen in the experimental results including both steady state and transient aspects of the observed behavior with reasonable quantitative agreement to our knowledge ours is the first mechanism based model that can reproduce rsf like behavior in terms of microstructurally verifiable processes and state variables since it is microphysically based we believe that our modeling approach can provide an improved framework for extrapolating friction data to natural conditions

hierarchy of high frequency orbital cycles in cretaceous carbonate platform strata abstract lower cretaceous valanginian hauterivian carbonate platform strata which are well exposed in the matese mountains of the southern apennines italy have been analyzed on a microstratigraphic cm scale to exemplify the high frequency cyclic nature of the mesozoic shallow water sedimentation in southern italy based on the vertical organization of lithofacies and their early diagenetic signature 46 metre scale cycles are recognized these elementary cycles are hierarchically organized into groups bundles and superbundles the elementary cycles are prevalently formed by subtidal deposits and show emersion related features at the top suggesting repeated sea level fluctuations the number of elementary cycles compared to time as constrained by biostratigraphy and their grouping into sets implying higher order complexity in the environmental oscillation suggest an orbital control on sedimentation spectral analysis of recurrent thickness in stratal features defines and quantifies the cyclicity which is characterized by lithofacies thickness periodicities of 1271 391 159 124 and 69 cm linear correlation of the relative ratio set rrs of these periodicities with the corresponding rrs of the orbital perturbation periodicities shows that 1271 and 391 cm fit the 404 220 and 94 890 year cycles of early cretaceous orbital eccentricity 159 and 124 cm fit the 49 010 and 38 030 year cycles of the earth s axial obliquity whereas 69 cm corresponds to the precession periodicity thus the san lorenzello section required not less than 1 6 ma to form at an average rate of 26 mm ka each 400 ka cycle can be viewed as a depositional sequence in which sequence boundaries maximum flooding surfaces and system tract equivalents have been recognized the microstratigraphic approach is very promising for understanding the fine structure of climate related environmental fluctuation recorded by the 3000 m thick carbonate platform successions of the southern apennines over a time span of about 150 ma upper triassic upper cretaceous and for deriving a more precise chronostratigraphy for global correlation

fluid assisted healing processes in gouge bearing faults insights from experiments on a rock analogue system on the basis of both experimental and field studies solution transfer processes are expected to be important in contributing to fault strength recovery healing under the hydrothermal conditions prevailing in large parts of the seismogenic zone however most experimental work on healing effects in faults has been done using quartzo feldspathic sliding surfaces or fault gouges under conditions in which solution transfer processes are very slow mechanisms of fluid assisted fault healing are accordingly rather poorly understood we have performed slide hold slide fault healing experiments on simulated faults containing brine saturated granular halite as a fault rock analogue halite was used because solution transfer processes are known to be rapid in this system under easily accessible conditions we studied the effects of hold time duration shear stress during hold pre hold sliding velocity and pore fluid composition on strength evolution the results show rapid fluid assisted compaction and stress relaxation during hold periods and a significant hold time dependent strengthening upon re shear the data reveal that healing resulted from both a packing density increase and a contact area strength increase during hold periods the general type of behaviour observed is similar to that observed in quartzo feldspathic gouges at room temperature although important differences were observed as well notably the time dependence of healing deviates from the log linear trend observed in gouges where solution transfer processes are absent this means that if pressure solution is an important healing mechanism in natural faults applying a log linear trend will underestimate natural fault healing rates