the characteristics and formation mechanism of a regional fault in shale strata insights from the middle upper yangtze china abstract shale is a type of fracture prone rock but few regional faults are observed in shale strata here we investigate a regional detachment fault in the middle upper yangtze muy to understand the formation mechanism of a special regional fault in shale strata based on a lithological mineralogical and mechanical study as well as semi quantitative model of displacement we found that the formation of this detachment fault is closely related to the lithological association and flexural slip processes the lithological association of this detachment fault is characterized by two thick competent layers separated by a thin weak interlayer at both the macroscopic and microscopic scales when these layers are folded the detachment fault zone is commonly present in a thin weak interlayer namely the wufeng formation at the macroscopic scale and primarily along interfaces in laminar siliceous shales lss and secondarily in k bentonite interlayers within the lss at the microscopic scale the regional fault zone is characterized by parallel to strata surfaces and abrupt contacts with the protolith the fabric of the fault core is obviously controlled by the flexural slip intensity and the number of slip events as well as their direction and structural position the natural shale gas obtained from well hd 1 has proven that this regional detachment fault in shale strata is not only a migration pathway for hydrocarbons but also acts as an adsorption state reservoir for free gas from other areas this regional detachment fault in shale strata is likely to advance the shale gas industry and to expand the exploration of conventional gas

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

a geomagnetic record over the last 3 5 million years from deep tow magnetic anomaly profiles across the central indian ridge high resolution records of the geomagnetic field intensity over the last 4 myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short lived low field intensity features associated with excursions or short polarity intervals in order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events we have collected six deep tow 500 m above the seafloor and several sea surface magnetic anomaly profiles from the central indian ridge across the brunhes matuyama and gauss chrons i e from the ridge axis to anomaly 2a after removal of topography latitude and azimuth effects we converted distances into time sequences using well dated polarity reversal anomalies as tie points we calculated the average signal to test the robustness of the short wavelength anomalies the resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the central indian ridge the east pacific rise and the pacific antarctic ridge our results suggest that in addition to polarity reversals to previously suggested geomagnetic events subchrons or excursions within the brunhes and matuyama chrons a new small scale magnetic anomaly likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas we observe a dense succession of microanomalies that are correlated excursions ontong java 1 and 2 and gilsa is found after the olduvai chron the near bottom results support the existence of three geo magnetic features between the gauss matuyama boundary and olduvai they also suggest three geomagnetic events during the c2a i n subchron within the gauss chron this study emphasizes the potential of deep tow magnetic surveys in detecting fluctuations in geo magnetic field intensity and in particular short lived excursions a poorly constrained part of the geomagnetic field temporal variation spectrum

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

normal polarity magnetosubchrons in 24r and the age of the paleocene eocene boundary a late paleocene to early eocene sequence of flat lying continental strata occurs in an area known as obed mountain west central alberta the upper 110 m consist of interbedded fluvial channel sandstones and overbank mudrocks containing five back swamp coal seams two coreholes 3 5 km apart that extend through the entire coal zone were sampled for magnetostratigraphy and 13c isotope analysis bentonites in the no 1 lowest and no 5 coal seams and a tuff in the no 3 coal seam were sampled for u pb and or rb sr dating of zircon and biotite respectively magnetostratigraphic analysis of 520 samples identified the younger part of chron 25r the whole of chron 25n and the older half of chron 24r we find six normal polarity subzones in this part of chron 24r which we correlate to tiny wiggles 6 to11 in marine magnetic profiles carbon isotope analysis of 14 samples from two cores revealed a negative shift of about 2 peaking near the base of 24r 8r we interpret this as the carbon isotope excursion

geomagnetic dipole strength and reversal rate over the past two million years independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1 2 and thus provide information regarding the processes governing the geodynamo so far this procedure has been limited to the past 800 000 years 800 kyr ref 3 which does not include any geomagnetic reversals here we present a composite curve that shows the evolution of the dipole moment during the past two million years this reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas which were used for calibration we show that at least during this period the time averaged field was higher during periods without reversals but the amplitude of the short term oscillations remained the same as a consequence few intervals of very low intensity and thus fewer instabilities are expected during periods with a strong average dipole moment whereas more excursions and reversals are expected during periods of weak field intensity we also observe that the axial dipole begins to decay 60 80 kyr before reversals but rebuilds itself in the opposite direction in only a few thousand years

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

cyclostratigraphy and astronomical tuning of the late maastrichtian at zumaia basque country northern spain the standard geological time scale for the cretaceous is still largely based on seafloor anomaly profiles combined with radio isotopic tie points the astronomical tuned time scale with its much higher resolution and accuracy has recently been extended to the k pg boundary and is being extended into the cretaceous to construct such a time scale for the cretaceous we selected the upper maastrichtian of the zumaia section in the basque country northern spain which contains a cyclic alternation of limestones and marls deposited in a hemipelagic setting the paleogene portion of the zumaia section has previously been studied for a joint cyclostratigraphic radioisotopic intercalibration of the age of the k pg boundary here we present a high resolution cyclostratigraphic framework for the upper maastrichtian latest cretaceous of the zumaia section in the basque country northern spain with new biostratigraphic and magnetostratigraphic data bed to bed correlation with the nearby sopelana section provides additional bio and magnetostratigraphic constraints the stacking pattern of the lithologies shows a hierarchy that reflects the combined influence of the orbital parameters of precession and eccentricity this is confirmed by time series analyses of lithological and geochemical data indicating a strong influence of eccentricity modulated precession on latest cretaceous climate the expression of the 405 kyr eccentricity cycle serves as primary signal for astronomical tuning we provide two tuning options depending on absolute k pg boundary ages of 65 56 and 65 97 ma the logged part of the section encompasses nine and a half 405 kyr cycles in total and spans 3 9 myr the acquired cyclostratigraphic framework provides ages for characteristic planktonic foraminiferal events magnetic reversals and carbon isotope excursions and resolves the late maastrichtian time scale in unprecedented detail with relative age uncertainties o 100 kyr the high resolution and large amplitude of shifts in d 13 c on the 405 kyr and 1 2 myr scales allow for global

geomagnetic field variations between chrons 33r and 19r 83 41 ma from sea surface magnetic anomaly profiles abstract sea surface magnetic profiles exhibit coherent short wavelength micro anomalies or tiny wiggles superimposed to the main anomalies due to reversals in this study we investigate the nature and distribution of these tiny wiggles on oceanic crust formed during the 42 myr long period following the cretaceous normal superchron to this end we compute stacks of anomaly profiles from different areas in the indian and the pacific oceans using a simple method based on upward continuation we demonstrate that the tiny wiggles are consistent worldwide although their patterns exhibit different resolutions at different spreading rates they are therefore confidently ascribed to past fluctuations of the geomagnetic dipole moment a high resolution record of these fluctuations is obtained by selecting and stacking profiles from areas with the highest spreading rates modeling the micro anomalies as short magnetic polarity intervals yields durations for these intervals generally shorter than 10 kyr likely too short to be indeed true subchrons moreover the number of detected tiny wiggles clearly depends on the spreading rate these results support geomagnetic intensity fluctuations as being the cause of most tiny wiggles as also suggested by recent magnetostratigraphic data the tiny wiggles are uniformly distributed within chrons indicating that paleointensity fluctuations are neither inhibited after nor enhanced before a reversal beyond a blind zone of about 10 km corresponding to 80 to 250 kyr depending on the spreading rate for which the anomalies due to reversals prevent the detection of tiny wiggles most tiny wiggles probably represent a filtered record of a uniform secular variation regime as suggested by their uniform spatial distribution over the whole investigated period

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

central asian moisture modulated by proto paratethys sea incursions since the early eocene abstract the establishment and evolution of the asian monsoons and arid interior have been linked to uplift of the tibetan plateau retreat of the inland proto paratethys sea and global cooling during the cenozoic however the respective role of these driving mechanisms remains poorly constrained this is partly due to a lack of continental records covering the key eocene epoch marked by the onset of tibetan plateau uplift proto paratethys sea incursions and long term global cooling in this study we reconstruct paleoenvironments in the xining basin ne tibet to show a long term drying of the asian continental interior from the early eocene to the oligocene superimposed on this trend are three alternations between arid mudflat and wetter saline lake intervals which are interpreted to reflect atmospheric moisture fluctuations in the basin we date these fluctuations using magnetostratigraphy and the radiometric age of an intercalated tuff layer the first saline lake interval is tentatively constrained to the late paleocene early eocene the other two are firmly dated between 46 ma top magnetochron c21n and 41 ma base c18r and between 40 ma base c18n and 37 ma top c17n remarkably these phases correlate in time with highstands of the proto paratethys sea this strongly suggests that these sea incursions enhanced westerly moisture supply as far inland as the xining basin we conclude that the proto paratethys sea constituted a key driver of asian climate and should be considered in model and proxy interpretations

low temperature frictional characteristics of chlorite epidote amphibole assemblages implications for strength and seismic style of retrograde fault zones in retrograde faults exhuming mafic rocks shearing occurs in metamorphic and or hydrothermally altered mineral assemblages whose frictional properties are not well known here we present the results of laboratory shearing experiments on chlorite schist epidotite and hornblende dominated amphibolite and mixtures of these rocks and evaluate their frictional properties and microstructures the experiments were conducted on powdered rock samples with starting grain size of 125 m at room temperature under fluid saturated conditions and applied normal stress of 10 mpa the results show that chlorite schist is relatively weak friction coefficient of 0 36 whereas epidotite and amphibolite are strong friction coefficients of 0 63 and 0 67 respectively the friction of chlorite schist epidotite and chlorite schist amphibolite mixtures decreases nearly linearly with increasing chlorite content chlorite schist exhibits velocity strengthening behavior epidotite is velocity weakening and the amphibolite shows mostly velocity weakening friction mixtures show intermediate strength and velocity dependence of friction well developed striations formed on slip surfaces in samples with 50 chlorite schist the epidotite slip surface exhibits a mixture of very fine particles and coarser crystals amphibolite slip surfaces have less very fine grains and are composed of subhedral to euheral needles few intragranular fractures are preserved and we infer wear at contact asperities to be the likely cause of velocity weakening in our epidote gouges addition of chlorite to epidotite and amphibolite produces a striated slip surface and disrupts contacts between harder grains therefore retrograde chlorite growth is expected to facilitate frictional weakening and stable slip in higher grade mineral assemblages exhumed to low temperature conditions

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

ultrahigh resolution marine magnetic anomaly profiles a record of continuous paleointensity variations a distinctive pattern of small scale marine magnetic anomalies 25 100 nt amplitude 8 25 km wavelength tiny wiggles is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the indian ocean by normalizing and stacking multiple profiles it is demonstrated that this pattern of tiny wiggles is a high resolution recording of paleodipole field behavior between chrons c24 and c27 the pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading 82 mm yr half rate profile from the southeast pacific where a similar signal is observed confirming the paleodipole field origin of the anomalies two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations we conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles the implications of this study are that 1 tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo 2 the small scale anomalies observed in the jurassic quiet zones may be due to paleointensity fluctuations 3 tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the cretaceous quiet zones and 4 much of the variance in anomaly profiles normally attributed to crustal eraplacement processes particularly at fast and ultrafast spreading rates is actually due to intensity variations in the paleomagnetic field

magnetostratigraphic evidence that tiny wiggles in the oceanic magnetic anomaly record represent geomagnetic paleointensity variations the oceanic magnetic anomaly record contains low amplitude short wavelength anomalies tiny wiggles or cryptochrons of uncertain source a magnetostratigraphic investigation of their origin has been carried out in a 40 m long core the massicore drilled through the early oligocene grey greenscaglia cinerea marlstone and multi huedscaglia variegata limestone in the marches region of northern italy the massicore contains a complete magnetostratigraphy that matches the geomagnetic polarity sequence from chron c16n to chron c12r it contains no additional short polarity chrons that might account for tiny wiggles the marine magnetic record has several of these events within the chron sequence c13r c13n c12r these chrons fall in the 22 m thickscaglia cinerea part of the massicore this formation has a simple magnetic mineralogy dominated by magnetite and is suitable for paleointensity investigations magnetic susceptibility is rather uniform within the formation and is determined by paramagnetic minerals the intensities of natural nrm anhysteretic arm and isothermal irm remanent magnetizations vary coherently with depth power spectra of the arm irm and nrm variations show significant activity close to the 413 ka period of the earth s orbital elements arm and irm intensities are proportional except in two stratigraphic intervals the ratio of nrm intensity to arm intensity used as a proxy for relative variation of paleomagnetic field intensity fluctuates systematically its minimum values lie close to the reported positions of cryptochrons in the marine record the massicore results favour the interpretation that tiny wiggles represent paleointensity variations of the geomagnetic field rather than unresolved short polarity chrons

organic facies variations in the triassic shallow marine and deep marine shales of central spitsbergen svalbard abstract shallow marine shales of the lower triassic sticky keep member and deep marine shales of the middle triassic botneheia member of the barentsoya formation central spitsbergen have been subjected to detailed organic petrological and organic geochemical study an assessment based on organic facies characteristics has been carried out on these sediments in order to distinguish between those deposited in shallow vs deep marine depositional settings the methods employed include evaluation of organic carbon content rock eval pyrolysis extractable organic matter biomarker distributions and petrographic data the maturity assessment of the samples analysed is mainly based on vitrinite reflectance and tmax data organic facies criteria such as toc hi pp petroleum potential hydrocarbon yield kerogen assemblage and ratios of pristane n c17 phytane n c18 tm ts c24 tetracyclic c26 tricyclic c34 c29 hopane c35 c34 hopane hopanes steranes diasteranes steranes c21 c22 sterane and c21 c21 sterane were found to be useful in differentiating between the two depositional environments the biomarker ratios in particular are thought to reflect more of the depositional condition such as the extent of anoxicity oxicity and less on source input probably as a result of enhanced microbial degradation on the organic matter deposited

integration of macrofossil biostratigraphy and magnetostratigraphy for the pacific coast upper cretaceous campanian maastrichtian of north america and implications for correlation with the western interior and tethys new biostratigraphic data obtained from measured stratigraphic sections of santonian through maastrichtian age located along the west coast of north america necessitate changes to the currently accepted chronostratigraphic framework for this region of the north pacific biotic province we recognize and or define 12 molluscan zones over this interval of the upper cretaceous and propose revisions to the currently accepted integration of ammonite zones with global upper cretaceous magnetochrons our findings demonstrate that there was significantly more faunal interchange between the north american pacific coast and both the western interior and gulf coast regions of north america during the late cretaceous than has previously been recognized and because of this novel and direct biostratigraphic correlations can be made these new faunal correlations are augmented with the magnetostratigraphic record from pacific coast localities to arrive at better interregional correlation for the upper cretaceous globally the new integration of the global polarity time scale with the local west coast ammonite zonation now allows better correlation between sections both within the north pacific province but geographically far from our study areas as well as to sections outside of the province itself however we note here that previous correlations between biostratigraphy and the top and bottom of magnetochron 33r in west coast north american sections appear to have been in error due to unrecognized modern day normal field overprint of originally reversed polarity in upper cretaceous sections we reinterpret the position of this chron based on this new information

fracture and weakening of jammed subduction shear zones leading to the generation of slow slip events geodetic data have revealed that parts of subduction interfaces creep steadily or transiently transient slow slip events sses are typically interpreted as aseismic frictional sliding however sses may also occur via mixed visco brittle deformation as observed in shear zones containing mixtures melange of strong fractured clasts embedded in a weak visco brittle matrix we test the hypothesis that creep in a subduction melange occurs through distributed matrix deformation where flow is intermittently impeded by load bearing clast networks jamming our numerical models demonstrate that bulk melange rheology can be dominated by the strong clasts in the absence of fracturing while at high driving stresses or low frictional strength clast fracturing redistributes deformation into the matrix leading to high bulk strain rates because melange stress is heterogeneous fracturing of clasts occurs throughout jammed melange when the driving stresses are only 20 of the clast yield strength the effective rheology of jammed melange can be characterised by a logarithmic dependence of stress on strain rate which is used to explore strain rate transients caused by temporal variation in clast friction clasts must weaken significantly 75 in order to increase strain rate by 8 spring block slider models with a more idealized visco brittle rheology also demonstrate that strain rate transients can be generated when rate and state friction is incorporated we outline a model where high bulk strain rates are generated when pervasive fracturing occurs but further slip is limited by viscous processes incorporating such viscous damping into models may widen the conditions under which sses can occur while preventing development of seismic slip

can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation recent friction experiments carried out under upper crustal p t conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults these mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust upper mantle in this study displacement controlled experiments have been performed on carbonate gouges at seismic slip rates 1 m s 1 to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust at relatively low displacements 900 c grain boundary sliding gbs deformation mechanisms the observed micro textures are strikingly similar to those predicted by theoretical studies and those observed during experiments on metals and fine grained carbonates where superplastic behaviour has been inferred to a first approximation the measured drop in strength is in agreement with our flow stress calculations suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone rather than by frictional sliding along the main slip surfaces in the slip zone frictionally induced grainsize sensitive gbs deformation mechanisms can thus account for the self lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature

fluctuations of the paleomagnetic field during chron c5 as recorded in near bottom marine magnetic anomaly data near bottom magnetic data contain information on paleomagnetic field fluctuations during chron c5 as observed in both the north and south pacific the north pacific data include 12 survey lines collected with a spatial separation of up to 120 km and the south pacific data consist of a single long line collected on the west flank of the east pacific rise epr at 19 s the north pacific magnetic profiles reveal a pattern of linear short wavelength 2 to 5 km anomalies tiny wiggles that are highly correlated over the shortest 3 8 km to longest 120 km separations in the survey magnetic inversions incorporating basement topography show that these anomalies are not caused by the small topographic relief the character of the near bottom magnetic profile from anomaly 5 on the west flank of the epr formed at a spreading rate more than twice that of the north pacific displays a remarkable similarity to the individual and stacked lines from the north pacific survey area over distances corresponding to 1 m y 19 lows in the magnetic anomaly profile can be correlated between the north and south pacific lines modeling the lows as due to short polarity events suggests that they may be caused by rapid swings of the magnetic field between normal and reversed polarities with little or no time in the reversed state owing to the implausibly high number of reversals required to account for these anomalies and the lack of any time in the reversed state we conclude that the near bottom signal is primarily a record of paleointensity fluctuations during chron c5 spectral analysis of the north pacific near bottom lines shows that the signal is equivalent to a paleointensity curve with a temporal resolution of 40 to 60 kyr while measurements of the smallest separations of correctable dips in the field suggest a temporal resolution of 36 kyr

geomagnetic field variability during the cretaceous normal superchron during the cretaceous normal superchron 121 83 million years ago the polarity of the earth s geomagnetic field remained stable for an unusually long time deep tow magnetic data suggest that despite the stability of the polarity the field varied greatly throughout the interval

a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama we present a stack of geomagnetic paleointensity estimates for the matuyama chron 0 778 2 582 ma sedimentary magnetizations from 4 ocean drilling program odp cores from the ontong java plateau ojp are analyzed with thermal techniques thellier thellier experiments are used to determine magnetic properties and indicate that a thermal treatment to 250 c can be used for paleointensity estimates the correlated records form a coherent ojp stack which is dated at reversal boundaries a coherent feature with two other data sets spanning the matuyama chron is the occurrence of a 150 kyr cyclicity however the time control of the ojp stack is not accurate except for the 10 tie points at the reversals this could at least in part explain why the three matuyama paleointensity records show no other coherent frequencies

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

evolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode laboratory and theoretical studies provide insight into the mechanisms that control earthquake nucleation when fault slip velocity is slow 0 001 cm s and dynamic rupture when fault slip rates exceed centimeters per second the application of these results to tectonic faults requires information about fabric evolution with shear and its impact on the mode of faulting here we report on laboratory experiments that illuminate the evolution of shear fabric and its role in controlling the transition from stable sliding v 0 001 cm s to dynamic stick slip v 1 cm s the full range of fault slip modes was achieved by controlling the ratio k k k c where k is the elastic loading stiffness and k c is the fault zone critical rheologic stiffness we show that k controls the transition from slow and silent slip k 0 9 to fast and audible k v 3 cm s slip duration 0 003 s slip events microstructural observations show that with accumulated strain deformation concentrates in shear zones containing sharp shear planes made of nanoscale grains which favor the development of frictional instabilities once this fabric is established fault fabric does not change significantly with slip velocity and fault slip behavior is mainly controlled by the interplay between the rheological properties of the slipping planes and fault zone stiffness

slip rate dependent friction as a universal mechanism for slow slip events a growing body of observations worldwide has documented fault slip transients that radiate little or no seismic energy the mechanisms that govern these slow slip events sses and their wide range of depths slip rates durations stress drops and recurrence intervals remain poorly known here we show that slow slip can be explained by a transition from rate weakening frictional sliding at low slip rates towards rate neutral or rate strengthening behaviour at higher slip rates as has been observed experimentally we use numerical simulations to illustrate that this rate dependent transition quantitatively explains the experimental data for natural fault rocks representative of materials in the source regions of sses with a standard constant parameter rate and state friction law sses arise only near the threshold for slip instability the inclusion of velocity dependent friction parameters substantially broadens the range of conditions for slow slip occurrence and produces a wide range of event characteristics which include stress drop duration and recurrence as observed in nature upscaled numerical simulations that incorporate parameters consistent with laboratory measurements can reproduce geodetic observations of repeating sses on tectonic faults we conclude that slip rate dependent friction explains the ubiquitous occurrence of sses in a broad range of geological environments a transition from rate weakening to rate strengthening frictional behaviour with increasing slip rate could explain the observed diversity of slow slip events on faults according to numerical simulations

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

the nature of a cryptochron from a paleomagnetic study of chron c4r 2r recorded in sediments off the antarctic peninsula abstract the magnetostratigraphy from ocean drilling program odp site 1095 off the pacific margin of the antarctic peninsula contains an extra normal polarity event that occurs near the base of chron 4r 2r 8 072 8 699 ma which we interpret to be cryptochron c4r 2r 1 owing to the relatively high sedimentation rates about 90 m m y this event is particularly well recorded at the site spanning 4 99 m of the stratigraphic section this allows the characteristics of the cryptochron to be investigated in greater detail than possible from marine magnetic anomalies where it was originally identified or from other sedimentary sections in which it has been recorded at much lower resolution our observations suggest that the cryptochron is a full geomagnetic reversal in which both the direction and paleointensity attain levels similar to that of other normal polarity chrons at the site based on its position within chron 4r 2r the cryptochron started at 8 622 ma and terminated 56 k y later at 8 566 ma at the transition zones bounding the cryptochron the paleointensity collapses to near zero but recovers within a few thousand years our results as well as paleomagnetic observations from other thick sedimentary units indicate that cryptochrons are not always purely paleointensity variations instead they are a record of short term geomagnetic variability that includes short geomagnetic reversals excursions intervals of high paleosecular variation and paleointensity lows all of which are part of a vector field that varies in both strength and direction over time

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

magnetic structure of a slow spreading ridge segment insights from near bottom magnetic measurements on board a submersible 1 near bottom magnetic measurements on board submersible nautile were carried out on the mid atlantic ridge 21 40 n segment and deep sea geomagnetic vector anomalies along 19 dive tracks were obtained by applying the processing method for shipboard three component magnetometer data a forward modeling technique using short wavelength components of the anomalies arising from local topography and vertical motion of the submersible was designed to estimate the absolute magnetization intensity of the seafloor in the vicinity of the spreading axis a considerable number of magnetization estimations are reliably confirmed by the high correlation between observed and modeled anomalies whereas less reliable estimations are obtained off axis probably because the sediment buries the basement topography the natural remanent magnetization nrm measured on basalt samples collected during these dives is compared with the magnetization from anomalies though both results give a similar range of magnetization intensity no correlation is confirmed between them possibly because the magnetization from anomalies represents laterally averaged seafloor magnetization whereas the nrm has variations at the scale of individual pillow or lava pile equivalent magnetization inverted from the sea surface magnetic anomalies shows axial magnetization increases significantly from the segment center to the segment ends however the results of eight dives conducted near the spreading axis at different locations along the segment show much less variation in magnetization intensity along the axis we ascribe the high equivalent magnetization at segment ends to preferential serpentinization of peridotite near the segment ends and the associated formation of magnetite the results of three across axis transects composed of 15 dives running in the spreading direction can be consistently interpreted as recording geomagnetic paleointensity variations during the brunhes epoch although magnetization lows are generally correspondent to periods of low paleointensity they show deeper drop than predicted from the paleointensity variation

antarctic tectonics constraints from an ers 1 satellite marine gravity field a high resolution gravity field of poorly charted and ice covered ocean near west antarctica from the ross sea east to the weddell sea has been derived with the use of satellite altimetry including ers 1 geodetic phase wave form data this gravity field reveals regional tectonic fabric such as gravity lineations which are the expression of fracture zones left by early 65 to 83 million years ago pacific antarctic sea floor spreading that separated the campbell plateau and new zealand continent from west antarctica these lineations constrain plate motion history and confirm the hypothesis that antarctica behaved as two distinct plates separated from each other by an extensional bellingshausen plate boundary active in the amundsen sea before about 61 million years ago

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

a paleomagnetic power spectrum we construct a power spectrum of geomagnetic dipole moment variations or their proxies that spans the period range from some tens of million down to about 100 years empirical estimates of the spectrum are derived from the magnetostratigraphic time scale from marine sediment relative paleointensity records and from a time varying paleomagnetic field model for the past 7 kyr the spectrum has the most power at long periods reflecting the influence of geomagnetic reversals and in general decreases with increasing frequency decreasing period the empirical spectrum is compared with predictions from simple models discrepancies between the observed and predicted spectra are discussed in the context of i changes in reversal rate ii overall average reversal rate iii cryptochrons iv the time taken for a reversal to occur and v long term paleosecular variations and average estimates of the field strength and variance from other sources 2005 elsevier b v all rights reserved

strain localization and the onset of dynamic weakening in calcite fault gouge abstract to determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates single slide and slide hold slide experiments were conducted on 2 3 mm thick layers of calcite gouge at normal stresses up to 26 mpa and slip rates up to 1 m s 1 microstructures were analyzed from short displacement 35 cm experiments stopped prior to and during the transition to dynamic weakening in fresh calcite gouge layers dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness strain is initially distributed across the full thickness of the gouge layer but within a few millimeters displacement the strain becomes localized to a boundary parallel high strain shear band c 20 m wide during the strengthening phase which lasts between 3 and 30 cm under the investigated conditions the shear band broadens to become c 100 m wide at peak stress the transition to dynamic weakening in calcite gouges is associated with the nucleation of micro slip surfaces dispersed throughout the c 100 m wide shear band each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite interpreted to result from grain welding driven by local frictional heating in the shear band by the end of dynamic weakening strain is localized to a single 2 3 m wide principal slip surface flanked by layers of recrystallized gouge calcite gouge layers re sheared following a hold period weaken nearly instantaneously much like solid cylinders of calcite marble deformed under the same experimental conditions this is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide reducing the effective gouge layer thickness to a few microns our results suggest that formation of a high strain shear band is a critical precursor to dynamic weakening in calcite gouges microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate the delayed onset of dynamic weakening in fresh calcite gouge layers particularly at low normal stresses may inhibit large coseismic slip at shallow crustal levels in calcite bearing fault zones

slow slip slow earthquakes period two cycles full and partial ruptures and deterministic chaos in a single asperity fault abstract the assimilation of geological geophysical and laboratory data in physics based models of fault dynamics promises increasingly realistic simulations of the seismic cycle to assist this effort i explore the dynamics of a single velocity weakening asperity to delineate the relationship between a fault physical properties and the style of faulting during the seismic cycle based on a micro physical model of rate and state friction from a dimensional analysis four non dimensional parameters control the characteristics of slip events and the behavior during the interseismic period including the dieterich ruina rice number ru which is proportional to the ratio of asperity to critical nucleation size w h and rb b a b where a and b control the direct and local evolutionary effects respectively for sufficiently large rb the ru number controls the spectrum of fault slip from periodic slow slip events to aperiodic fast ruptures in finite faults but not in semi infinite faults the transition between these end members includes a bifurcation that involves period two period four period six cycles and deterministic chaos with increasing ru macroscopic fault slip is stabilized as rb vanishes even as ru 1 for finite faults but not for semi infinite faults the transition between slow and fast ruptures for increasing rb includes another type of bifurcation with period two and period four cycles of slow and fast ruptures for 0

evolution of the late cretaceous crust in the equatorial region of the northern indian ocean and its implication in understanding the plate kinematics summary analysis of 3100 km of newly acquired marine magnetic data constrained by satellite and shipborne free air gravity anomalies in the corridor between the 86 e fracture zone and ninetyeast ridge north of the equator reveals the evolutionary history of the late cretaceous crust characterised by anomaly 34 through 31 83 5 68 7 ma under complex tectonic settings seafloor spreading model studies suggest that the crust particularly between the chrons 33r and 33 79 0 73 6 ma was formed with variable and slightly higher half spreading rates 4 8 7 1 cm yr than the crust of similar age either in the regions west of 86 e fracture zone east of the ninetyeast ridge or the southern crozet basin further the interpretation of magnetic anomalies suggests the presence of fossil spreading ridge segments and extra oceanic crust on the indian plate that has been transferred from the antarctica plate by discrete southward ridge jumps these ridge jumps are caused by thermal instability of the spreading center as the indian plate moved northward over the kerguelen mantle plume the present study indicates that the spreading ridge plume interaction is the prime mechanism for these ridge jumps which have occurred since 75 8 ma the newly identified magnetic anomalies 34 through 31 and the inferred n3 e trending fracture zones refined the plate reconstruction models for that period

geomagnetic reversal behaviour since 100 ma abstract the new magnetic polarity time scale of cande and kent j geophys res 97 13917 13951 1992a provides an opportunity to analyse further the random character of the geomagnetic polarity sequence since 100 ma large differences in the peak amplitudes of changes in magnetic reversal frequency are observed when the previous harland et al a geological time scale cambridge university press 1990 and new cande and kent sequences are compared these differences illustrate remaining uncertainties in the significance of these changes when the successive lengths of polarity intervals are displayed as a function of their order of occurrence in the sequence a change in behaviour is observed midway through the series flat white noise like behaviour is observed for the last 100 intervals from about 25 ma to the present for older intervals a quasi periodic pattern in interval duration is observed this evolution is more apparent in the cande and kent time scale the quasi periodicity of interval duration as a function of occurrence does not translate into periodicity of reversal frequency as a function of time from 85 ma to about 25 ma our description of the sequence is reminiscent of the findings of dubois and pambrun c r acad sci 311 643 650 1990 who suggested that low dimensional deterministic chaos between 85 and 25 ma preceded a random process occurring since then the details of a long term control or memory of the geomagnetic field remain an open question addition of independent information such as palaeointensity data will probably be necessary to uncover any determinism in reversal chronology

modulation of fault strength during the seismic cycle by grain size evolution around contact junctions abstract earthquake prediction is fundamentally hampered by insufficient knowledge of the constitutive behavior of faults laboratory and theoretical work by dieterich 1978 and ruina 1983 formed the basis of rate and state friction a constitutive framework that captures the healing and weakening of frictional contacts the rate and state law successfully explains the frictional behavior of a wide range of rocks and many phenomena associated with fault slip the physics underlying the rate and state friction law is not fully understood but it is generally accepted that several micro mechanisms operate depending on the texture maturity fluid content and other physical and kinematic conditions of the fault here i formulate a constitutive framework that unifies a number of laboratory experiments on silicate rocks and the gouges derived from these rocks that describe the effect of grain size temperature real area of contact gouge thickness and fault roughness fundamental to the model is that fault strength is controlled by the area of the interfacial contact junctions that support the shear and normal loads the curvature of asperities weakly controls the area of their contact junction and the resulting fault strength as a result the dynamics of grain size evolution around micro asperities enables the seismic cycle the model quantitatively explains the correlation between the characteristic weakening distance and the gouge thickness marone and kilgore 1993 the correlation of the static friction coefficient with the rate dependence parameters ikari et al 2011 the dependence of frictional resistance to sub solidus temperatures chester 1994 and predicts a range of grain sizes that is compatible with observations at exposed fault zones however the dynamics of grain size evolution is still challenging to verify experimentally the model features a regularization at vanishing slip speeds that implies distinct dynamics at low strength for friction coefficients lower than 0 1 seismic cycle simulations predict the emergence of frictional instabilities with complex source time functions this is a significant departure from the predictions of rate and state friction that is most relevant to the dynamics of decollements serpentinized shear zones and other low strength faults

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

geomagnetic intensity variations over the past 780 kyr obtained from near seafloor magnetic anomalies knowledge of past variations in the intensity of the earth s magnetic field provides an important constraint on models of the geodynamo a record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences but a long term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust here we show that near seafloor magnetic anomalies recorded over the southern east pacific rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr moreover the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well documented dipole intensity pattern for the past 50 kyr a comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region the record of geomagnetic intensity preserved in the oceanic crust should provide a higher time resolution record of crustal accretion processes at mid ocean ridges than has previously been obtainable

the cretaceous opening of the south atlantic ocean abstract the separation of south america from africa during the cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field the cretaceous normal superchron cns lasted between 121 and 83 6 myr ago we present a new identification of magnetic anomalies located within the southern south atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field using these anomalies together with fracture zone locations we calculate the first set of magnetic anomalies based finite rotation parameters for south america and africa during that period the kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them the rotations indicate that seafloor spreading rates increased steadily throughout most of the cretaceous and decreased sharply at around 80 myr ago a change in plate motion took place in the middle of the superchron roughly 100 myr ago around the time of the final breakup i e separation of continental oceanic boundary in the equatorial atlantic prominent misfit between the calculated synthetic flowlines older than anomaly q1 and the fracture zones straddling the african plate in the central south atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion

evolution of the carlsberg ridge between 60 and 45 ma ridge propagation spreading asymmetry and the deccan reunion hotspot despite the availability of numerous geophysical data no convincing magnetic anomaly identification and structural map of a major part of the arabian and eastern somali basins have been published to date i propose a new interpretation of the available magnetic data based on the identification of tiny wiggles i e singular second order patterns of the magnetic signal only one fracture zone and a set of propagating rifts existed between the owen fracture zone and the chagos laccadive ridge between anomalies 26 and 20 at anomalies 26 25 at least two eastward propagating rifts are identified while at anomalies 24 20 seven westward propagating rifts are confidently recognized the major consequence of this systematic ridge propagation is a tremendous regional asymmetry between anomalies 26 and 25 about 65 of the crust formed at the carlsberg ridge was accreted to the african plate while at anomalies 24 20 more than 75 benefited to the indian plate i suggest that these asymmetries result from the relative position of the carlsberg ridge and the nearby deccan reunion hotspot the ridge tending to remain near the hotspot

a 9 million year long astrochronological record of the early middle eocene corroborated by seafloor spreading rates the early middle eocene ca 56 41 ma is recorded in the pelagic scaglia rossa and variegata formations of the umbria marche basin central italy geochemical and magnetostratigraphic alignment between the bottaccione section gubbio central italy and the smirra core cagli central italy allows us to generate a continuous and wellpreserved new record that combined with previously published data from the same area creates a continuous high resolution record from the paleocene eocene thermal maximum ca 56 ma to the lower part of chron c21n comparison with carbon isotope records from ocean drilling program sites 1258 and 1263 reveals a satisfactory match providing further evidence of the global significance of the long term trend and superposed perturbations captured by the 13c records the identification of astronomically forced geochemical cycles allows us to develop a 405 k y tuned age model thereby extending the astrochronology from ca 56 0 to ca 47 5 ma marine magnetic anomaly profiles from major oceanic basins characterized by high seafloor spreading rates were used to independently test the astronomical polarity time scale associated with our tuning as well as other polarity time scales our age model suggests the existence of periods of relatively constant seafloor spreading rates separated by rapid changes while the other time scales generate more gradual variations and also include large and short term deviations in spreading rates that occur simultaneously in different oceanic basins implying errors in polarity reversal ages the umbria marche age model further contributes to the closure of the middle eocene gap in the astronomical time scale

experimental and modeling study of the effect of fault roughness on dynamic frictional sliding abstract friction governs many processes in nature from the motion of glaciers and landslides to tremors and earthquakes the roughness of the sliding interfaces presumably affects their dynamics in particular the condition for stable or unstable sliding we investigate this issue by modeling numerically tri axial stick slip experiments on westerly granite samples with different levels of roughness the model accounts for surface roughness at the millimeter scale and assumes rate and state friction with enhanced dynamic weakening it reproduces key characteristics observed in the experiments such as sequences of fast stick slip events for smooth surface and sequence of slow slip events with small stress drop followed by a large stress drop unstable slip event for the rough interface the increase in loading stiffness due to the additional resistance from roughness can explain the initial slow slip the fast unstable event may be the result of the higher critical stiffness due to the increase of local normal tractions with slip as well as of the reduction of the loading stiffness due to contacts losses the study allows examining numerically the relationship between the frictional behavior on the interface and the apparent friction estimated from external loads we find that because of dynamic overshoot the reduction in apparent friction is larger than that of the actual friction coefficient on the interface

source of tiny wiggles in chron c5 a comparison of sedimentary relative intensity and marine magnetic anomalies 1 in addition to the well established pattern of polarity reversals short wavelength fluctuations are often present in both sea surface data tiny wiggles and near bottom anomaly data while a high degree of correlation between different geographical regions suggests a geomagnetic origin for some of these wiggles anomaly data alone cannot uniquely determine whether they represent short reversals or paleointensity variations independent evidence from another geomagnetic recording medium such as deep sea sediments is required to determine the true nature of the tiny wiggles we present such independent evidence in the form of sedimentary relative paleointensity from chron c5 we make the first comparison between a sedimentary relative paleointensity record odp site 887 at 54 n 148 w and deep tow marine magnetic anomaly data 43 n 131 w bowers et al 2001 for chron c5 the sediment cores are densely sampled at 2 5 kyr resolution the inclination record shows no evidence for reverse intervals within the 1 myr long normal chron c5n 2n rock magnetic measurements suggest that the primary magnetic carrier is pseudo single domain magnetite we choose a partial anhysteretic magnetization parm as our preferred normalizer and the resulting relative paleointensity record is used as input to a forward model of crustal magnetization we then compare the results of this model with the stacked deep tow anomaly records the two records show a significant degree of correlation suggesting that the tiny wiggles in the marine magnetic anomalies are likely produced by paleointensity variations an analysis of our sampling density suggests that if any reverse intervals exist at this site they are likely to be 5 kyr in duration furthermore we suggest that reverse intervals during chron c5n 2n documented in other locations are unlikely to be global

mechanical amorphization flash heating and frictional melting dramatic changes to fault surfaces during the first millisecond of earthquake slip the evolution of fault strength and behavior during the initial stages of slip plays an important role in driving the onset of instability and fault weakening using small displacement triaxial experiments on quartz sandstone this study highlights the rapid onset of microstructural change on fault interfaces and identifies new evidence for an evolution in physical processes with increasing slip and velocity pre ground fault surfaces have been slipped over a range of velocities 0 36 m s 1 to 18 cm s 1 and at normal stresses comparable to upper to mid crustal conditions 92 287 mpa microstructural analysis of the fault interfaces reveals the formation of amorphous material at displacements 170 m and slip durations 1 ms mechanical and microstructural observations have been combined with numerical modeling to present the first documented examples of a transition from mechanical amorphization to flash heating then frictional melting with changes in slip conditions the sequence of processes activated during the initial stages of fault movement may provide new insights into factors that influence the onset of slip in the seismogenic crust

creep and slip seismic precursors to the nuugaatsiaq landslide greenland precursory signals to material s failure are predicted by numerical models and observed in laboratory experiments or using field data these precursory signals are a marker of slip acceleration on weak regions such as crustal faults observation of these precursory signals of catastrophic natural events such as earthquakes and landslides is necessary for improving our knowledge about the physics of the nucleation process furthermore observing such precursory signals may help to forecast these catastrophic events or reduce their hazard i report here the observation of seismic precursors to the nuugaatsiaq landslide in greenland time evolution of the detected precursors implies that an aseismic slip event is taking place for hours before the landslide with an exponential increase of slip velocity furthermore time evolution of the precursory signals amplitude sheds light on the evolution of the fault physics during the nucleation process

skewness of marine magnetic anomalies created between 85 and 40 ma in the indian ocean we have performed a detailed analysis of the skewness of marine magnetic anomalies in indian ocean basins created between 85 and 40 ma as a result of the northward motion of india visual and semiautomated methods of skewness determination were applied to the data both provide consistent results but the visual method is prefered for its ability to deal with noisy data plots of apparent effective remanent inclination or skewness corrected for present geomagnetic field inclination versus time for conjugate basins display the combination of three effects a gradual increase with time related to the northward motion of the ridges attached to india in the geomagnetic reference frame a gap between conjugate curves which represents anomalous skewness senso stricto and short period fluctuations which represent the sequence effect i e the effect of neighboring magnetic sources on the skewness of a given anomaly the anomalous skewness decreases with faster spreading rate and completely disappears above 50 km m y an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness

integrated stratigraphy of the smirra core umbria marche basin apennines italy a new early paleogene reference section and implications for the geologic time scale pelagic sections of the umbria marche basin in the northern apennines italy have provided key geological archives for studying critical intervals of early paleogene time in addition to classical sections the smirra coring project provides a new record of relatively undisturbed sediments 120 m from 4 overlapping holes of the upper scaglia formation paleocene middle eocene here we present a new high resolution integrated magneto bio stratigraphy of the 93 m composite section drilled in smirra holes 1 and 2 the correlation of the magnetostratigraphy to published geomagnetic polarity timescales gptss ck95 gpts2004 gpts2012 as constrained by calcareous nannofossil biostratigraphy reveals that the cores range from chron c26r to c21n thanetian lutetian 60 to 46 ma sedimentation rates range from 0 6 to 1 cm kyr and are comparable with those from coeval sections in the umbria marche basin they point to significant discrepancies between existing time scales which mainly involve the duration of chron c23 and subchrons within c23n and c24n application of the average spectral misfit method on the magnetic susceptibility record reveals a strong signature of orbital forcing and allows an independent estimate of the sedimentation rates this analysis offers additional insights into solving time scale issues which mainly concern the duration of chron c23n our results imply a significantly longer duration for c23n 2n which would accommodate the presence of an extra third 405 kyr eccentricity related cycle in c23n the smirra record thus provides new constraints on the early paleogene time scale a key record for further cyclostratigraphic and astrochronological research and a reference framework for paleo climatic and oceanographic studies

the galapagos axial magnetic anomaly evidence for the emperor event within the brunhes and for a two layer magnetic source the axial brunhes magnetic anomaly over the galapagos spreading center shows consistent short wavelength anomalies that are not consistent with a uniformly magnetized layer we interpret this anomaly to confirm the presence of the emperor reversed event at 0 49 0 05 mybp with a duration about 0 01 m y further this anomaly is consistent with a two layer magnetic source with the lower layer having very broad transition zones offset away from the axis from those of the upper layer by about 10 km

cryptochrons and tiny wiggles new magnetostratigraphic evidence from chrons 32 and 33 in western canada abstract magnetostratigraphic results are reported for a deep core that penetrates campanian maastrichtian strata in south central alberta canada chrons 32n 32r and 33n are represented fourteen short reversed polarity magnetozones are present 10 of which are interpreted as new polarity subchrons four in 32n 2n and six in 33n their durations are about an order of magnitude longer than that inferred for the corresponding tiny wiggles observed in marine magnetic anomaly patterns

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

monitoring local changes in granite rock under biaxial test a spatiotemporal imaging application with diffuse waves diffuse acoustic or seismic waves are highly sensitive to detect changes of mechanical properties in heterogeneous geological materials in particular thanks to acousto elasticity we can quantify stress changes by tracking acoustic or seismic relative velocity changes in the material at test in this paper we report on a small scale laboratory application of an innovative time lapse tomography technique named locadiff to image spatio temporal mechanical changes on a granite sample under biaxial loading using diffuse waves at ultrasonic frequencies 300 khz to 900 khz we demonstrate the ability of the method to image reversible stress evolution and deformation process together with the development of reversible and irreversible localized micro damage in the specimen at an early stage using full field infrared thermography we visualize stress induced temperature changes and validate stress images obtained from diffuse ultrasound we demonstrate that the inversion with a good resolution can be achieved with only a limited number of receivers distributed around a single source all located at the free surface of the specimen this small scale experiment is a proof of concept for frictional earthquake like failure e g stick slip research at laboratory scale as well as large scale seismic applications potentially including active fault monitoring

precursory changes in seismic velocity for the spectrum of earthquake failure modes temporal changes in seismic velocity during the earthquake cycle have the potential to illuminate physical processes associated with fault weakening and connections between the range of fault slip behaviors including slow earthquakes tremor and low frequency earthquakes1 laboratory and theoretical studies predict changes in seismic velocity prior to earthquake failure2 however tectonic faults fail in a spectrum of modes and little is known about precursors for those modes3 here we show that precursory changes of wave speed occur in laboratory faults for the complete spectrum of failure modes observed for tectonic faults we systematically altered the stiffness of the loading system to reproduce the transition from slow to fast stick slip and monitored ultrasonic wave speed during frictional sliding we find systematic variations of elastic properties during the seismic cycle for both slow and fast earthquakes indicating similar physical mechanisms during rupture nucleation our data show that accelerated fault creep causes reduction of seismic velocity and elastic moduli during the preparatory phase preceding failure which suggests that real time monitoring of active faults may be a means to detect earthquake precursors

a comparison between rate and state friction and microphysical models based on numerical simulations of fault slip rate and state friction rsf is commonly used for the characterisation of laboratory friction experiments such as velocity step tests however the rsf framework provides little physical basis for the extrapolation of these results to the scales and conditions of natural fault systems and so open questions remain regarding the applicability of the experimentally obtained rsf parameters for predicting seismic cycle transients as an alternative to classical rsf microphysics based models offer means for interpreting laboratory and field observations but are generally over simplified with respect to heterogeneous natural systems in order to bridge the temporal and spatial gap between the laboratory and nature we have implemented existing microphysical model formulations into an earthquake cycle simulator through this numerical framework we make a direct comparison between simulations exhibiting rsf controlled fault rheology and simulations in which the fault rheology is dictated by the microphysical model even though the input parameters for the rsf simulation are directly derived from the microphysical model the microphysics based simulations produce significantly smaller seismic event sizes than the rsf based simulation and suggest a more stable fault slip behaviour our results reveal fundamental limitations in using classical rate and state friction for the extrapolation of laboratory results the microphysics based approach offers a more complete framework in this respect and may be used for a more detailed study of the seismic cycle in relation to material properties and fault zone pressure temperature conditions

a method for determining absolute ultrasonic velocities and elastic properties of experimental shear zones abstract laboratory experiments are a vital tool for assessing elastic properties of rock and determining the underlying geomechanical processes that inform large scale predictive models in some cases relative values of elastic properties are sufficient but absolute values are necessary when comparing between locations and to address upscaling from lab to field settings however determining absolute values of ultrasonic velocity and elastic parameters in laboratory experiments is often complex and hampered by apparatus design moreover measuring the evolution of elastic properties with shear deformation has proven especially difficult here we describe a method that allows measurements of p and s wave velocity as a function of shear deformation under stresses of 10 s of mpa the approach includes rigorous calibration experiments and accounts for the evolution of impedance contrasts at sample interfaces as a function of strain we describe our method by applying it to sheared layers that represent simulated fault zones composed of clay quartz mixtures and fault rocks recovered from drilling p wave arrival times range from 10 to 20 s and wave speeds are 2 4 km s during shear of layers a few mm in thickness subject to normal stress of 25 mpa and shear strains 30 travel time data for apparatus calibration are fit with rational functions and root mean square error is used to assess uncertainty wave speed varies systematically with shear stress and increases with shear strain due to comminution compaction internal strain localization and shear fabric development

mantle plumes link magnetic superchrons to phanerozoic mass depletion events abstract the four most recent large mass extinction events in the phanerozoic the cretaceous tertiary kt the triassic jurassic tj and the permo triassic pt and guadalupian tatarian gt doublet are associated with a major flood basalt eruption with the timing of peak volcanic activity corresponding within measurement uncertainties to the extinction event three magnetic superchrons precede the four largest phanerozoic extinctions the cretaceous long normal superchron duration 35 myr precedes the kt and the permian kiaman long reversed superchron 50 myr precedes the pt gt doublet in addition the newly recognized ordovician moyero long reversed superchron 30 myr precedes the end ordovician extinction event there is a 10 20 myr delay between the end of each superchron and the subsequent mass depletion event both of which represent distant outliers from their respective populations we propose that deep mantle plumes link these seemingly unrelated phenomena long term 200 myr variations in mantle convection possibly associated with the wilson cycle induce temporal and spatial variations in heat flow at the core mantle boundary polarity reversals are frequent when core heat flow is high and infrequent when it is low thermal instabilities in the d layer of the mantle increase core heat flow end the magnetic superchron and generate deep mantle plumes the plumes ascend through the mantle on a 20 myr time scale producing continental flood basalt trap eruptions rapid climatic change and massive faunal depletions

11 million years of oligocene geomagnetic field behaviour summary an 11 million year long record of the oligocene geomagnetic field has been obtained from pelagic sediments of dsdp hole 522 an average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr the rock magnetics are remarkably consistent across the entire interval previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite the natural remanent magnetism nrm carries an oligocene polarity timescale that is in excellent agreement with the oligocene reversal record as determined from marine magnetic anomalies mmas including many of the so called crypto chrons normalized nrm intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences these features include consistent major decreases in palaeointensity dips at reversal boundaries and occasional dips between reversal boundaries that could correspond to lineated tiny wiggles in the mma records the data set suggests that the overall field strength was 40 per cent higher in the first half of the oligocene when the average reversal frequency was 1 6 myr l than in the second half when the reversal frequency was 4myr there is also a weak dependence of average field strength on length of polarity interval finally in the three cores suited to spectral analysis of coherent polarity and relative intensity independent of lithological contamination there is a persistent ca 30 50 ka periodicity in the variations of the relative intensity suggesting that the geomagnetic field pulses at about this frequency not only during the brunhes as demonstrated by tauxe shackleton 1994 but in the oligocene as well