drained shear strength parameters for analysis of landslides this paper presents recommendations for selecting the type and magnitude of drained shear strength parameters for analysis of landslides in particular the importance existence and use of the cohesion shear strength parameter is reviewed for slope stability analyses it is recommended that the shear strength be modeled using a stress dependent failure envelope or a friction angle that corresponds to the average effective normal stress acting on the slip surface passing through that particular material instead of using a combination of cohesion and friction angle to represent soil shear strength other recommendations for stability analyses include using an effective stress cohesion of zero for residual and fully softened strength situations to facilitate selection of shear strength parameters for landslide analyses empirical relationships for the drained residual and fully softened strengths are updated from the previous empirical relationships presented by stark and eid finally the paper presents torsional ring shear test results that indicate that pre existing shear surfaces exhibit self healing that results in increased shear resistance the magnitude of healing appears to increase with increasing soil plasticity and this increase could have implications for the size timing and cost of landslide remediation

cyclic behavior of fine grained soils at different ph values the effects of ph on the liquefaction susceptibility of fine grained soils were examined by performing undrained cyclic ring shear tests on artificial mixtures and a natural soil under different ph conditions solutions of diluted sulphuric acid h2 s o4 and dissolved sodium hydroxide naoh were used to create acidic and alkaline environments respectively while distilled water was used as a reference liquid low plasticity kaolin and illite sand mixtures and a medium plasticity bentonite sand mixture were selected to investigate the influence of plasticity and clay mineralogy on the ph dependent response of soil to cyclic loading the results showed that the effects of ph were more pronounced for the medium plasticity mixture and depended greatly on the mineralogy of clay fraction for example in an acidic medium the kaolin sand mixture became slightly more resistant to liquefaction while the illite sand mixture became more susceptible to liquefaction the bentonite sand mixture was observed to b

evolution of pangea paleomagnetic constraints from the southern alps italy abstract a new early late triassic paleopole for adria has been obtained from the val sabbia sandstone in the southern alps as early permian and jurassic cretaceous paleomagnetic data from para autochthonous regions of adria such as the southern alps are consistent with african apwps 1 2 paleomagnetic data from this region can be used to bolster the west gondwana apwp in the poorly known late permian triassic time interval the southern alpine paleopoles are integrated with the west gondwana and laurussia apwps of van der voo 1 and used to generate a tectonic model for the evolution of pangea the early permian overall mean paleopole for west gondwana and adria in conjunction with the coeval laurussia paleopole support pangea b of morel and irving 3 the late permian early triassic and the middle late triassic paleopoles from adria and laurussia support pangea a 2 of van der voo and french 4 the phase of transcurrent motion between laurasia and gondwana 5 that caused the pangea b to a 2 transition occurred essentially in the permian at the end of variscan orogeny with an average relative velocity of approximately 10 cm yr finally the late triassic early jurassic paleopoles from west gondwana and laurussia agree with pangea a 1 of bullard et al 6 the widely accepted pangea configuration at the time of the jurassic breakup

impact crater formation in icy layered terrains on mars we present numerical simulations of crater formation under martian conditions with a single near surface icy layer to investigate changes in crater morphology between glacial and interglacial periods the ice fraction thickness and depth to the icy layer are varied to understand the systematic effects on observable crater features to accurately model impact cratering into ice a new equation of state table and strength model parameters for h2o are fitted to laboratory data the presence of an icy layer significantly modifies the cratering mechanics observable features demonstrated by the modeling include variations in crater morphometry depth and rim height and icy infill of the crater floor during the late stages of crater formation in addition an icy layer modifies the velocities angles and volumes of ejecta leading to deviations of ejecta blanket thickness from the predicted power law the dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials our simulations suggest that many of the unusual features of martian craters may be explained by the presence of icy layers including shallow craters with well preserved ejecta blankets icy flow related features some layered ejecta structures and crater lakes therefore the cratering record implies that near surface icy layers are widespread on mars

martian impact crater ejecta morphologies as indicators of the distribution of subsurface volatiles 1 fresh martian impact craters display a variety of ejecta blanket morphologies the fluidized appearance of most fresh ejecta types is commonly ascribed to heating and vaporization of subsurface volatiles during crater formation we have conducted a study of the distribution of the three dominant fluidized ejecta morphologies single layer ejecta sle double layer ejecta dle and multiple layer ejecta mle within the 60 latitude zone on mars we have subdivided this region into 5 5 latitude longitude boxes and have computed the following for each box 1 percentage of craters showing any ejecta morphology as a function of total number of craters 2 percentage of sle craters as a function of craters with an ejecta morphology 3 percentage of dle craters as a function of craters with an ejecta morphology and 4 percentage of mle craters as a function of craters with an ejecta morphology we confirm previous reports that the sle morphology is the most common ejecta type within the study area constituting 70 of all ejecta morphologies over most of the study area the dle and mle morphologies are much less common but these morphologies are concentrated in localized regions of the planet using these results we discuss how subsurface volatile reservoirs may be distributed across the planet the regional variations found in this study generally correlate with the proposed locations of near surface h2o reservoirs detected by mars odyssey

ages of large lunar impact craters and implications for bombardment during the moon s middle age abstract standard lunar chronologies based on combining lunar sample radiometric ages with impact crater densities of inferred associated units have lately been questioned about the robustness of their interpretations of the temporal dependance of the lunar impact flux in particular there has been increasing focus on the middle age of lunar bombardment from the end of the late heavy bombardment 3 8 ga until comparatively recent times 1 ga to gain a better understanding of impact flux in this time period we determined and analyzed the cratering ages of selected terrains on the moon we required distinct terrains with random locations and areas large enough to achieve good statistics for the small superposed crater size frequency distributions to be compiled therefore we selected 40 lunar craters with diameter 90 km and determined the model ages of their floors by measuring the density of superposed craters using the lunar reconnaissance orbiter wide angle camera mosaic absolute model ages were computed using the model production function of marchi et al marchi s mottola s cremonese g massironi m martellato e 2009 astron j 137 4936 4948 we find that a majority 36 of 40 of our superposed crater size frequency distributions are consistent with the model production function a histogram of the original crater floor model ages indicates the bombardment rate decreased gradually from 3 8 ga until 3 0 ga implying an extended tail to the late heavy bombardment for large craters it also preliminarily suggests that between 3 0 and 1 0 ga bombardment may be characterized by long periods 600 myr of relatively few impacts lulls broken by a short duration 200 myr of relatively more impacts spike while measuring superposed craters we also noted if they were part of a cluster or chain named obvious secondary and analyzed these craters separately interestingly we observe a wide variety of slopes to the differential size frequency power law which demonstrates that there can be considerable variation in individual secondary crater field size frequency distributions finally four of the small superposed crater size frequency distributions are found to be inconsistent with the model production function possible reasons are resurfacing has modified these distributions unrecognized secondary craters and or the model production function has incorrect inputs such as the scaling law for the target terrain the degraded appearance of the superposed craters and indications of resurfacing suggest that the first cause is the most likely

determining rheological parameters of debris flow material abstract a 2 0 m diameter steel 30 inverted cone and plate viscometer rheometer was designed constructed and used to test the behaviour of coarse grained debris flow materials a 1 5 scale model machine was also constructed and used to test the internal flow dynamics of the viscometer rheometer and to obtain results for fluids grain fluid mixtures and debris flow fines for fluids and grain fluid mixes our results were similar to those obtained earlier using standard viscometric systems derived rheological parameters for debris flow materials and clay slurries agreed well with those determined from calculation direct measurement and field observation apparent viscosities were shear rate dependent extremely sensitive to water content and as high as 6000 pa s instantaneous stresses within shearing debris flow material varied over a wide range debris flow materials with bimodal grain size distributions had a dilatant plastic rheology those with a low content of coarse material and unimodal grain size distribution or exceptionally high fines content had a plastic or viscoplastic rheology

supposed permo triassic megashear between laurasia and gondwana a consensus has emerged that during the palaeozoic several landmasses progressively collided to form the supercontinent pangaea which subsequently split up in the mesozoic and tertiary to produce the present array of continents but whereas there is a high measure of agreement about the general shape of the early jurassic pangaea shortly before the initial atlantic opening three very different reconstructions of the end palaeozoic supercontinent have recently been proposed these differences which involve the relative positions of the northern and southern sectors of pangaea known respectively as laurasia and gondwana relate to the interpretation of palaeomagnetic data the three constructions are here tested with reference to geological and palaeobiogeographic data and arguments

atmospheric effects on ejecta emplacement laboratory experiments allow the investigation of complex interactions between impacts and an atmosphere although small in scale they can provide essential first order constraints on the processes affecting late stage ballistic ejecta and styles of ejecta emplacement around much larger craters on planetary surfaces the laboratory experiments involved impacting different fine grained particulate targets under varying atmospheric pressure and density different gas compositions during crater formation ballistic ejecta form the classic cone shaped profile observed under vacuum conditions as atmospheric density increases for a given pressure however the ejecta curtain bulges at the base and pinches above this systematic change in the ejecta curtain reflects the combined effects of deceleration of ejecta smaller than a critical size and entrainment of these ejecta within atmospheric vortices created as the outward moving wall of ejecta displaces the atmosphere additionally a systematic change in emplacement style occurs as a function of atmospheric pressure largely independent of density contiguous ejecta rampart superposing ballistically emplaced deposits 0 06 to 0 3 bar ejecta flow lobes 0 3 to 0 7 bar and radial patterns 0 8 bar underlying processes controlling such systematic changes in emplacement style were revealed by observing the evolution of the ejecta curtain by changing target materials including layered targets and low density particulates by varying atmospheric density by changing impact angle and by comparing the ejecta run out distances with first order models of turbidity flows three distinct ejecta emplacement processes can be characterized ejecta ramparts result from coarser clasts sorted and driven outward by vortical winds behind the outward moving ejecta curtain this style of wind modified emplacement represents minimal ejecta entrainment and is enhanced by a bimodal size distribution in the ejecta such eddy supported flows are observed to increase in run out distance scaled to crater size with increasing atmospheric pressure by analogy with turbidity flows this scaled distance should increase as r1 2 for a given atmospheric pressure and degree of entrainment ejecta flows with much greater run out distances develop as the turbulent power in atmospheric response winds increase such flows overrun and scour the inner ejecta facies thereby producing distinct inner and outer facies the degree of ejecta entrainment depends on the dimensionless ratio of drag to gravity forces acting on individual ejecta and the intensity of the winds created by the outward moving curtain entrainment increases with increasing atmospheric density and ejection velocity crater size but decreases with ejecta density and size the intensity of curtain generated winds increases with ejection velocity crater size the dimensionless drag ratio characterizing the laboratory experiments can be applied to mars since the reduced atmospheric density is offset by the increased ejection velocities for kilometer scale events for a given crater size ejection velocity and atmospheric conditions a wide range of nonballistic ejecta emplacement styles could occur simply by varying ejecta sizes even without the presence of water alternatively the onset crater diameter for nonballistic emplacement styles can reflect the range of ejecta sizes possible from the diverse martian geologic history massive basalts to fine grained aeolian deposits scaling considerations further predict that ejecta run out distances scaled to crater size on mars should increase as r1 2 hence long run out flows dependent on crater diameter need not reflect depth to a buried reservoir of water on venus however the dense atmosphere maximizes entrainment and results in ejecta flow densities approaching a constant fraction of the atmospheric density under such conditions ejecta run out distances should decrease as r 1 2

shear resistance variations in experimentally sheared mudstone granules a possible shear thinning and thixotropic mechanism we present results of ring shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone little rate dependency of shear resistance was observed in sand sized granules in any wet or dry test while saturated gravel sized granules exhibited significant and abrupt reversible rate weakening from 0 6 to 0 05 at about 2 mm s repeating resistance variations occurred also under constant shear displacement rate mudstone granules generate mud as they are crushed and softened shear thinning and thixotropic behavior of the mud can explain the observed behavior with the viscosity decreasing the mud can flow through the coarser soil pores and migrate out from the shear zone this brings new granules into contact which produces new mud thus the process can start over similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behaviour may play a role in some landslide kinematics

shear resistance of remoulded soils by viscometric and fall cone methods a comparison for the canadian sensitive marine clays the coaxial viscometer can be used to extend the measurement of the shear resistance of a soil below the range of applicability of the fall cone and other standard geotechnical methods the factors affecting the relationship between yield stress determined by the viscometer and remoulded shear strength determined by the fall cone are such that the correlation must be separately determined for each soil material approaches to determining this correlation for quick clays and other soils are proposed key words sensitive clays yield stress shear strength rheology

martian rampart crater ejecta experiments and analysis of melt water interaction abstract viking images of martian craters with rampart bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar sized craters on the moon and mercury topographic control of distribution lobate and terraced margins cross cutting relationships and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage it is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement the dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation ejecta particle size and the energy and mode of melt ejecta dispersal study of terrestrial lobate volcanic ejecta produced by steam blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse this deposition results from flow yield strength increasing above shear stress due to interparticle friction

global warming and the end permian extinction event proxy and modeling perspectives abstract the mass extinction event that occurred at the close of the permian period 252 million years ago represents the most severe biodiversity loss in the ocean of the phanerozoic the links between the global carbon cycle climate change and mass extinction are complex and involve a whole range of often inter related geochemical biological ecologic and climatic factors it has become widely accepted that the end permian mass extinction was associated with a global warming event because the age of the siberian trap eruption a potentially massive source of carbon dioxide coincides within error with the extinction event however geologic data that are in support of this global warming event are relatively sparse the chain of events and the causal relationship between the eruption of siberian traps and mass extinction is not well established global warming in particular has only been reported from limited proxy data and climate models for which the pco2 in the atmosphere just before and during the end permian extinction event is poorly known this study critically assesses both the proxy climate data and the existing paleoclimate simulations with the goals of assessing our current understanding of the link between mass extinction and climate change and providing some guidance for future studies proxies indicate that prior to the end permian extinction event tropical sea surface temperatures ranged from 22 to 25 c and possible pco2 values ranged from 500 to 4000 ppm before the extinction event during the peak extinction tropical temperatures rose up to 30 c while pco2 perhaps increased up to 8000 ppm climate models that use different pre event pco2 values show similar amount of co2 doubling to replicate the observed carbon isotope excursions we find that the temperature anomaly during the end permian extinction is consistent with 1 5 doublings of atmospheric pco2 and that the implied climate sensitivity is 5 6 c within the upper range of values produced by climate models

the stability of ground ice in the equatorial region of mars consideration of the partial pressure of h2o in the martian atmosphere and the range of mean annual temperatures at the martian surface suggests that the occurrence of ground ice in equilibrium with the atmosphere is restricted to latitudes poleward of 40 however there is a growing body of morphologic evidence that indicates that substantial quantities of ground ice may have been present in the equatorial regolith throughout martian geologic time the accepted explanation for this apparent contradiction has been that the h2o found near the equator is a relic emplaced very early in martian geologic history 3 5 b y ago and under substantially different climatic conditions it is generally believed that this fossil ground ice layer has been preserved to the present day by the diffusion limiting properties of a relatively shallow layer 10 m of fine grained regolith to evaluate this hypothesis the lifetime of a 200 m layer of ground ice buried below 100 m of ice free regolith has been examined for the latitudes between 30 on mars twelve model pore size distributions representative of silt and clay type soils found on earth were selected to simulate the pore structure of the martian regolith the parallel pore model of gaseous diffusion was then used to calculate the flux of escaping h2o molecules from the buried ground ice layer the potential effects of such factors as depth of burial the martian geothermal gradient regolith porosity adsorption surface diffusion and climatic change were also considered based on our analysis the most important factors affecting the stability of equatorial ground ice appear to be 1 soil structure 2 the magnitude of the geothermal gradient and 3 the climatic desorption of co2 from the regolith we conclude that while the conditions necessary for the long term survival of ground ice may exist in isolated regions within the equatorial regolith on the global scale it appears unlikely that a fossil ground ice layer has been preserved throughout martian geologic history if massive ground ice does presently exist in the equatorial regolith then its presence may imply the existence of some continuous or periodic process of replenishment

shear strength in preexisting landslides drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kpa or less the test results also show that the recovered strength even at effective normal stresses of 100 kpa or less is lost after a small shear displacement i e slope movement an effective normal stress of 100 kpa corresponds to a shallow depth so the observed strength gain has little if any impact on the analysis of deep landslides this paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses

collision and rifting in the tethys ocean geodynamic implication abstract the tethys ocean was formed 250 m y ago when gondwana collided with laurasia since then and up to its closure about 40 50 m y ago the development of the ocean was dominated by repeated jumps of the spreading axes to the passive margin of gondwana splitting of microcontinents their transportation across the ocean and accretion to the active margin of eurasia it appears that jumps of the spreading centre and rifting of the passive margin coincided with periods of major collisions on the active margin several epochs of collision and rifting are described the late permian mid late triassic late jurassic early cretaceous end of the early cretaceous late eocene the simultaneous occurrence of rifting and collision has important geodynamic implications and it cannot be explained if only the forces applied to plate margins i e slab pull and ridge push are considered as the plate drying forces slab pull did not act in times of collision when the subduction zone was blocked by continental fragments and the ridge push creates compression but not extension on the passive margin it is suggested instead that the conveyor belt process of transportation of continental masses from the passive to the active margin of tethys was governed by north directed convective flow in the asthenosphere in times of collision when lithospheric plates were unable to move with the flow the latter created great tensile stresses in the overlying lithosphere that led to formation of rifts and then to new spreading centres

the late permian climate what can be inferred from climate modelling concerning pangea scenarios and hercynian range altitude abstract a major unsolved geodynamic problem is the permian pangea plate configuration at the end of paleozoic era around 255 ma while consensual geology indicates a plate arrangement close to that of the jurassic prior to the atlantic opening pangea a paleomagnetic data indicates a nearly 3000 km more eastward position of gondwana with respect to laurussia leading to major plate and relieves reorganisation during permo triassic times pangea b using an atmospheric general circulation model agcm we simulate the climatic response to both configurations we also test the fundamental role of paleo elevations through sensitivity experiments each simulated climate is then compared with the aim of constraining the best fit to a particular paleogeographic scenario main trends of simulated late permian climate in agreement with paleoclimatic indicators are 1 a warm temperate climate accompanied by a monsoon circulation over the eastern side of gondwana 2 a cold temperate climate marked by strong annual thermal amplitude at high latitudes in gondwana and in siberia 3 an arid belt in the subtropics over the western side of gondwana and laurussia and 4 strong climatic differences over both hemispheres respectively main variance is found over laurussia with a simulated tropical climate while an arid climate is suggested by paleodata simulating different paleo elevations of the appalachian and the variscan fold belts can solve this discrepancy the best model data fit is reached for a mean altitude of 4500 m in the appalachians and appears to be dependent of pangea configuration with a southern europe variscan range of some 3000 m in a pangea a configuration or only 2000 m in for pangea b taking into account the geodynamic context we argue that pangea b appears to be the more probable configuration

standardizing the nomenclature of martian impact crater ejecta morphologies the mars crater morphology consortium recommends the use of a standardized nomenclature system when discussing martian impact crater ejecta morphologies the system utilizes nongenetic descriptors to identify the various ejecta morphologies seen on mars this system is designed to facilitate communication and collaboration between researchers crater morphology databases will be archived through the u s geological survey in flagstaff where a comprehensive catalog of martian crater morphologic information will be maintained

paleomagnetism and the evolution of pangea we present new apparent polar wandering apw paths for the latest carboniferous to middle jurassic of europe northern asia north america and gondwana the paths have been constructed by ordering the paleopoles into a time sequence and calculating running averages globally this statistical analysis cannot be carried back before the latest carboniferous because the paleomagnetic record becomes inadequate the polar paths have all been rotated along with the land masses from which they have been observed into the classical configuration of wegener pangea a as quantified by later workers the mean paleopoles agree in the early jurassic but disagree in the latest carboniferous permian and early triassic this result is insensitive to reasonable choices made regarding the arrangement of the continents within gondwana it is true either for results from gondwana as a whole or from africa and south america alone this disagreement in apw paths suggests but does not prove that pangea a lasted for only a short interval of time in the early to middle jurassic 190 170 m y and perhaps the latest triassic 200 m y alternatively we may say either that the paleomagnetic record is biassed by sampling inadequacies or by systematically incorrect age assignments or that the earth s field has been non dipolar we regard these explanations as less likely although they cannot be discounted the polar paths can be reconciled by rotating gondwana anticlockwise by about 35 relative to laurasia placing africa beneath europe and south america beneath eastern north america this configuration is called pangea b pangea b could be a valid reconstruction for the latest carboniferous and early permian approximately 290 260 m y pangea b provides a statistically satisfactory resolution of the paleomagnetic problem it also provides a symmetrical arrangement of the continents bordering the appalachian hercynian fold belt for the late paleozoic which pangea a does not it provides a satisfactory framework for the early evolution of the gulf of mexico and it provides an explanation of the absence of geological evidence for a tethys ocean in the paleozoic the transition between b and a cannot be accurately reconstructed from the paleomagnetic data because the resolution of the record is as yet inadequate but it presumably occurred by a 3500 km minimum dextral megashear between laurasia and gondwana during the late permian and triassic approximately 250 200 m y without the formation of ocean between them and was accompanied by lesser motions between north america and europe the sense of megashear is the same as that invoked by arthaud and matte 1977 on geological grounds but the magnitude and timing are different our interpretation favors the idea that continental drift is a continuous process not confined to jurassic and later times but extending backwards into the triassic and permian we suggest that pangea was not an immobile configuration as envisaged by wegener and most subsequent workers but that it evolved more or less continuously

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

on the secular retention of ground water and ice on mars tropical ground ice on mars undergoes long term sublimation and likely exospheric escape without restriction of sublimation the cryosphere would eventually breach leading to massive loss of any underlying groundwater we seek to understand the conditions under which the ground ice seal groundwater and subsurface habitability are preserved using multi reservoir models for the evolution of deuterium to hydrogen ratios d h we derive a median estimate of the hesperian amazonian h2o loss of 60 m interquartile range 30 120 m global equivalent layer gel neglecting magmatic degassing most of the loss may have been early with consequent low loss in the modern cold and dry climate we modeled global h2o transport within mars following an assumed abrupt transition to modern conditions at 3 ga sublimation is retarded in order of decreasing priority by higher obliquity smaller porosity higher tortuosity lower heat flow and smaller pore radius higher obliquity reduces h2o loss by decreasing tropical surface temperatures and raising global atmospheric water vapor time variable obliquities do not overly influence outcomes as long as the total duration 30 obliquity is 1 gyr with no interval longer than a few hundred myr decreased solar luminosity or a thicker co2 atmosphere can also retard loss of subsurface h2o whereas lower atmospheric water vapor accelerates sublimation if mars post noachian crustal h2o inventory was a few hundred meters gel or more then the modest loss since then implies that the cryospheric seal has been maintained following top down freezing and that groundwater likely exists globally on mars today

observations of martian layered ejecta craters and constraints on their formation mechanisms the formation mechanism of layered ejecta craters on mars has remained a topic of intense debate since their discovery in this study we perform a global morphological analysis of martian layered ejecta craters using thermal emission imaging system themis images and mars orbiter laser altimeter mola data the study focuses on the ejecta morphologies and well defined distal rampart characteristics associated with 9945 layered ejecta craters with a diameter greater than 1 5 km distributed across the entire martian surface data analysis based on the new database provides new information on the distribution and morphological details of the three major layered ejecta morphologies single layer ejecta sle double layer ejecta dle and multiple layer ejecta mle global analysis is applied to the latitudinal distribution of characteristic parameters including the ejecta mobility lobateness values and onset diameter our survey of the distribution and characteristics of layered ejecta craters reveals that strong correlations exist between ejecta mobility and latitude and there is a latitudinal dependence of onset diameter our study of martian layered ejecta craters provides more detailed information and insights of a connection between the layered ejecta morphologies and the subsurface volatiles

origin of discrepancies between crater size frequency distributions of coeval lunar geologic units via target property contrasts abstract recent work on dating copernican aged craters using lunar reconnaissance orbiter lro camera data re encountered a curious discrepancy in crater size frequency distribution csfd measurements that was observed but not understood during the apollo era for example at tycho copernicus and aristarchus craters csfds of impact melt deposits give significantly younger relative and absolute model ages amas than impact ejecta blankets although these two units formed during one impact event and would ideally yield coeval ages at the resolution of the csfd technique we investigated the effects of contrasting target properties on csfds and their resultant relative and absolute model ages for coeval lunar impact melt and ejecta units we counted craters with diameters through the transition from strength to gravity scaling on two large impact melt deposits at tycho and king craters and we used pi group scaling calculations to model the effects of differing target properties on final crater diameters for five different theoretical lunar targets the new csfd for the large king crater melt pond bridges the gap between the discrepant csfds within a single geologic unit thus the observed trends in the impact melt csfds support the occurrence of target property effects rather than self secondary and or field secondary contamination the csfds generated from the pi group scaling calculations show that targets with higher density and effective strength yield smaller crater diameters than weaker targets such that the relative ages of the former are lower relative to the latter consequently coeval impact melt and ejecta units will have discrepant apparent ages target property differences also affect the resulting slope of the csfd with stronger targets exhibiting shallower slopes so that the final crater diameters may differ more greatly at smaller diameters besides their application to age dating the csfds may provide additional information about the characteristics of the target for example the transition diameter from strength to gravity scaling could provide a tool for investigating the relative strengths of different geologic units the magnitude of the offset between the impact melt and ejecta isochrons may also provide information about the relative target properties and or exposure degradation ages of the two units robotic or human sampling of coeval units on the moon could provide a direct test of the importance and magnitude of target property effects on csfds

early history of the atlantic ocean and gas hydrates on the blake outer ridge results of the deep sea drilling project leg 76 leg 76 of the deep sea drilling project achieved two major scientific objectives the first objective was met at site 533 where on the blake outer ridge gas hydrates were identified by geophysical geochemical and geological studies gas hydrate decomposition produced a volumetric expansion of 20 1 of gas volume to pore fluid volume this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions the gas hydrate includes methane ethane propane and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography for the first time marine gas hydrates were tested with a pressure core barrel the second objective was achieved when coring at site 534 in the blake bahama basin sampled the oldest oceanic sediments yet recovered the sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the north atlantic ocean in detail the oldest oceanic sediments are red claystones and laminated green and brown claystones of middle callovian age this finding supports the interpretation that the beginning of the modern north atlantic occurred in the early callovian 155 m y b p as much as 20 m y later in time than often previously thought

triassic magmatism of the dinarides in yugoslavia abstract the triassic magmatism of the dinarides had a polyphase character it took place during a period of about 50 ma during the initial stages of the mesozoic wilson cycle their earliest phases might have been connected with rifting processes and the subsequent ones preceeded directly the opening of the dinaridic part of the tethys and the formation of a spreading ridge the triassic magmatism is represented by basic and more commonly by intermediate and acid plutonic and volcanic rocks with calc alkaline affinity the triassic magmatic rock association can be correlated on major element evidence with calc alkaline rocks of recent convergent plate margins however field relationships and the data on immobile trace elements indicate that lavas poured out over triassic sediments on a carbonate platform which was underlain by rocks of the continental crust the basic diversity of the triassic magmatism of the dinarides was influenced by partial melting other processes crystallization contamination and others must have played a secondary role

preflow stresses in martian rampart ejecta blankets a means of estimating the water content abstract measurements of extents of rampart ejecta deposits as a function of the size of the parent craters support models which for craters larger than about 6 km diameter constrain ejecta blankets to all have a similar maximum thickness regardless of the crater size these volatile rich ejecta blankets may have failed under their own weights then flowed radially outward assuming this to be so we can then determine some of the physicomechanical properties of the ejecta deposits at the time of their emplacement finite element studies of the stress magnitudes distributions and directions in hypothetical martian rampart ejecta blankets reveal that the material most likely failed when the shear stresses were less than 500 kpa and the angle of internal friction was between 26 and 36 these figures imply that the ejecta has a water content between 16 and 72 whether the upper limit or the lower limit is more appropriate depends on the mode of failure which one presumes namely viscous flow or plastic deformation

laboratory investigation on reactivated residual strength recent research has outlined that mobilized in situ strength at the reactivation of displacements along a preexisting sliding surface may be slightly greater than laboratory residual strength although geotechnical literature explains this difference in various ways for instance strength gain due to soil aging little work has been devoted to this topic bromhead ring shear tests were carried out on remolded samples of the cormons flysch a typical geological formation in northeastern friuli venezia giulia italy the specimens came from the shallow weathered crust of the cormons flysch near the areas of montona and rosazzo this experimental study aimed at highlighting the effect of a limited number of parameters on strength gain at the reactivation of displacements along aged sliding surfaces the results of this research confirm that aging may cause a rising of reactivated shear strength above the residual value depending on time of aging and vertical stress the longest aging period about a month showed a 20 strength increase for the rosazzo flysch and a 30 increase for the montona flysch reactivated strength increases according to a power law almost 50 develops over a short time i e after 24 h of aging in the long term strength growth is characterized by a deviatoric creep rate gradually approaching zero

variations in the onset diameter for martian layered ejecta morphologies and their implications for subsurface volatile reservoirs we investigated regional variations in the onset diameter of craters displaying a single layer ejecta morphology within 30 latitude using viking imagery our results generally agree with those of previous studies which show onset diameters of 5 to 6 km in the equatorial region but we have identified localized regions with unusually small onset diameters the largest region is located in solis and thaumasia planae the 3 5 km onset diameter range in this area indicates a near surface ice rich reservoir depth 110 m this unusual concentration of near surface ice may have resulted from magmatic driven uplifts associated with the tharsis rise which modified parts of a regional aquifer drainage basin system and resulted in the transfer and concentration of subsurface volatiles in this region

thixotropic behavior during shear of a fine grained mud from eastern canada the quantitative determination of the hysteresis loop area developed during viscometer tests on shear rate shear stress reverse paths has been used to describe the thixotropic behavior of a clayey mud under flow conditions tests were conducted for liquidity indexes between 1 8 and 4 0 and for six pore water salinities ranging from 0 1 to 30 g l admitting that a low hysteresis loop area denotes a well developed thixotropic behavior for a given liquidity index the thixotropy of the slurry is likely to be more strongly marked for salinities greater than the flocculation threshold at a constant salinity the results also indicate the existence of a simple linear relationship between the hysteresis loop area and the yield stress and the viscosity of the slurry the restrictions of this approach and the possible implications of a strengthening softening process on mudflow kinematics are also discussed

martian surface near surface water inventory sources sinks and changes with time today a 34 m global equivalent water layer gel lies in the martian polar layered deposits and shallow ground ice during the amazonian 3 m was outgassed and 31 m was lost to space and to the surface leaving 62 m at the end of hesperian during the hesperian volcanic outgassing added 5 m 7 m was lost and 40 m gel of groundwater was added to form outflow channels leaving 24 m carryover of surface water from the noachian into the hesperian the hesperian budget is incompatible with a northern ocean during this era these figures are for near surface water substantial amounts of water may have existed as deep ground ice and groundwater our estimate of approximately 24 m near surface water in the late noachian is insufficient to support an ocean at that time also and favors episodic melting of an icy highlands to produce the fluvial and lacustrine features

evidence for self secondary cratering of copernican age continuous ejecta deposits on the moon crater size frequency distributions on the ejecta blankets of aristarchus and tycho craters are highly variable resulting in apparent absolute model age differences despite ejecta being emplaced in a geologic instant crater populations on impact melt ponds are a factor of 4 less than on the ejecta and crater density increases with distance from the parent crater rim although target material properties may affect crater diameters and in turn crater size frequency distribution csfd results they cannot completely reconcile crater density and population differences observed within the ejecta blanket we infer from the data that self secondary cratering the formation of impact craters immediately following the emplacement of the continuous ejecta blanket by ejecta from the parent crater contributed to the population of small craters 300 m diameter on ejecta blankets and must be taken into account if small craters and small count areas are to be used for relative and absolute model age determinations on the moon our results indicate that the cumulative number of craters larger than 1 km in diameter per unit area n 1 on the continuous ejecta blanket at tycho crater ranges between 2 17 10 5 and 1 0 10 4 with impact melt ponds most accurately reflecting the primary crater flux n 1 3 4 10 5 using the cratering flux recorded on tycho impact melt deposits calibrated to accepted exposure age 109 1 5 ma as ground truth and using similar crater distribution analyses on impact melt at aristarchus crater we infer the age of aristarchus crater to be 280 ma the broader implications of this work suggest that the measured cratering rate on ejecta blankets throughout the solar system may be overestimated and caution should be exercised when using small crater diameters i e 300 m on the moon for absolute model age determination

on the paucity of amorphous minerals in the sensitive postglacial marine clays the abundance and role of amorphous minerals in the sensitive marine clays of eastern canada has been a subject of investigation since 1974 unfortunately the segalen selective extraction procedure alternating hcl and naoh which was used in most of the experiments is very aggressive by attacking both oxide minerals which were presumed to be amorphous and crystalline phyllosilicates its use led to major overestimates of the possible amorphous mineral content less aggressive selective extraction procedures indicate that less than 1 of the sensitive champlain sea sediments is oxide minerals and mossbauer spectroscopy has detected only crystalline iron oxides which are probably of detrital origin the amount of amorphous oxide minerals is at most a small fraction of 1 crystalline iron oxides in the concentrations naturally present have been demonstrated to influence the rheology of the sensitive marine clays but it seems improbable that the very small amounts of amorphous minerals that m

possible precipitation of ice at low latitudes of mars during periods of high obliquity most of the old cratered highlands of mars are dissected by branching river valleys that appear to have been cut by running water1 2 yet liquid water is unstable everywhere on the martian surface in the equatorial region where most of the valleys are observed even ice is unstable3 4 it has been suggested therefore that mars had an early denser atmosphere with sufficient greenhouse warming to allow the existence of liquid water5 here we suggest instead that during periods of very high obliquities ice could accumulate at low latitudes as a result of sustained sublimation of ice from the poles and transport of the water vapour equatorwards at low latitudes the water vapour would saturate the atmosphere and condense onto the surface where it would accumulate until lower obliquities prevailed the mechanism is efficient only at the very high obliquities that occurred before formation of tharsis very early in the planet s history but limited equatorial ice accumulation could also have occurred at the highest obliquities during the rest of the planet s history partial melting of the ice could have provided runoff to form the channels or replenish the groundwater system

global distribution and migration of subsurface ice on mars a thermal diffusive model of h2o kinetics and equilibrium was developed to investigate the long term evolution and depth distribution of subsurface ice on mars the model quantitatively takes into account 1 obliquity variations 2 eccentricity variations 3 long term changes in the solar luminosity 4 variations in the argument of subsolar meridian in planetocentric equatorial coordinates 5 albedo changes at higher latitudes due to seasonal phase changes of co2 and the varying extent of co2 ice cover 6 planetary internal heat flow 7 temperature variations in the regolith as a function of depth time and latitude due to the above factors 8 atmospheric pressure variations over a 104 year time scale 9 the effects of factors 1 through 5 on seasonal polar cap temperatures and 10 knudsen and molecular diffusion of h2o through the regolith the migration of h2o into or out of the regolith is determined by two boundary conditions the h2o vapor pressure at the subsurface ice boundary and the annual average h2o concentration at the base of the atmosphere these are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap starting from an arbitrary initial uniform depth distribution of subsurface ice h2o fluxes into or out of the regolith are calculated for 100 selected obliquity cycles each representing a different epoch in mars history the h2o fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time the results show that obliquity variations influence annual average regolith temperatures in varying degrees depending on latitude with the greatest effect at the poles and almost no effect at 40 lat insolation changes at the pole due to obliquity argument of subsolar meridian and eccentricity variations can produce enormous atmospheric h2o concentration variations of 6 orders of magnitude over an obliquity cycle superimposed on these cyclic variations is a slow monotonic change due to the increasing solar luminosity albedo changes at the polar cap due to seasonal phase changes of co2 and the varying thickness of the co2 ice cover are critically important in determining annual average atmospheric h2o concentrations despite the strongly oscillating character of the boundary conditions only small amounts of h2o are exchanged between the regolith and the atmosphere per obliquity cycle 10 g cm2 the net result of h2o migration is that the regolith below 30 40 lat is depleted of subsurface ice while the regolith above 30 40 lat contains permanent ice due to the depth of penetration of the annual thermal wave this result is supported by recent morphological studies the rate of migration of h2o is strongly dependent on average pore capillary radius for which we have assumed values of 1 and 10 m we estimate that the h2o ice removed from the regolith would produce a permanent ice cap with a volume between 2 106 and 6 106 km3 this generally agrees with estimates deduced from deflationary features at lower latitudes depositional features at higher latitudes and the mass of the polar caps

paleomagnetic results from saudi arabia and the permo triassic pangea configuration the pangea configuration for the interval of late carboniferous to early jurassic is still debated was it similar to the well known wegenerian pangea or did gondwanaland move with respect to laurussia along a major right lateral lineament of several thousand kilometres during the triassic as suggested by irving we try here to answer this important question with the paleomagnetic study of well dated permo triassic sediments from a borehole in saudi arabia sediments of the same age have also been sampled at the surface but proved to be remagnetized probably by a strong desert alteration the paleomagnetic study of the bore hole core yielded a high temperature component magnetization thought to be of primary origin carried either by magnetite late permian limestones or hematite early triassic red shales with antipodal normal and reversed inclinations its co latitude and pole directions compare well with those from a selection of south american african madagascar and moroccan poles a new reliable pole west gondwana with a mean age of 244 11 m y is derived 53 4 n 259 4 e a95 3 6 in west african coordinates and its comparison with the laurussian poles of same age window strongly suggests a pangea in which the northern part of south america was facing the east of north america pangea b of irving at the permo triassic boundary a large right lateral strike slip movement of some 3500 km during the triassic is thus required to reconstruct pangea to its jurassic pre atlantic opening position

shear rate dependent behavior of clayey bimaterial interfaces at landslide stress levels the behavior of reactivated and first failure landslides after large displacements is controlled by the available shear resistance in a shear zone and or along slip surfaces such as a soil bedrock interface among the factors influencing the resistance parameter the dependence on the shear rate can trigger catastrophic evolution rate weakening or exert a slow down feedback rate strengthening upon stress perturbation we present ring shear test results performed under various normal stresses and shear rates on clayey soils from a landslide shear zone on its parent lithology and other lithologies and on clay rock interface samples we find that depending on the materials in contact the normal stress and the stress history the shear rate dependent behaviors differ we discuss possible models and underlying mechanisms for the time dependent behavior of landslides in clay soils

martian craters viewed by the thermal emission imaging system instrument double layered ejecta craters 1 the thermal emission imaging system themis visible vis images provide new insight into the nature and development of the unique ejecta deposits of martian craters this study focuses on double layered ejecta dle craters to date over 100 dle craters have been examined using mainly themis vis data our observations suggest that emplacement of dle crater ejecta occurred in two stages with the inner ejecta layer emplaced similar to single layered ejecta sle crater ejecta this may have involved both ballistic and flow processes in contrast the outer ejecta layer of dle craters appears to have been emplaced through the high velocity outflow of materials from tornadic winds generated by the advancing ejecta curtain or base surge remarkably dle craters lack secondary craters which suggests that the large ejecta blocks that normally produce such craters may have either been entrained and or crushed by these winds or fragmented as a result of the presence of water in the target materials these observations suggest that volatiles either trapped in the subsurface or in the atmosphere have played a key role in the emplacement of the ejecta of dle craters and leaves open the question as to what role volatiles play in the emplacement of ejecta of other types of fluidized ejecta craters i e sle and mle craters because dle craters are found in many different regions of mars often in close proximity to other types of craters conditions e g atmospheric density that produce dle craters must fluctuate or the martian crust must be unexpectedly heterogeneous laterally and vertically while the degree of heterogeneity has yet to be recognized recent suggestions about possible martian climate change raises the possibility of impact into target materials that are periodically wet or that a significantly higher atmospheric pressure may be periodically present

fast shearing of pre existing shear zones in soil the paper presents results from a laboratory investigation into the influence of fast rates of displacement on the residual strength of soil this influence was studied in the ring shear apparatus shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement results from a wide range of natural soils are presented all soils show initially an increase in strength when resheared at fast rates of displacement the strength then tends to decrease with fast displacement to a minimum value the fast residual strength three types of rate effects on the residual strength are identified a positive rate effect in soils showing a fast residual strength higher than the slow residual a neutral rate effect in soils showing a constant fast residual strength equal to the slow residual irrespective of rate of displacement and a negative rate effect in soils showing a significant drop in fast residual strength below the slow residual when sheared at rates higher

ages of rampart craters in equatorial regions on mars implications for the past and present distribution of ground ice we are testing the idea of squyres et al 1992 that rampart craters on mars may have formed over a significant time period and therefore the onset diameter minimum diameter of a rampart crater only reflects the ground ice depth at a given time we measured crater size frequencies on the layered ejecta of rampart craters in three equatorial regions to derive absolute model ages and to constrain the regional volatile history nearly all rampart craters in the xanthe terra region are 3 8 gyr old this corresponds to the noachian fluvial activity that region rampart crater formation declines in the hesperian whereas onset diameters minimum diameter increase no new rampart craters formed after the end of the hesperian 3 gyr this indicates a lowering of the ground ice table with time in the xanthe terra region most rampart craters in the valles marineris region are around 3 6 gyr old only one large probably amazonian aged 2 5 gyr rampart crater exists these ages indicate a volatile rich period in the early hesperian and a lowering of the ground ice table with time in the valles marineris study region rampart craters in southern chryse planitia which are partly eroded by fluvial activity show ages around 3 9 gyr rampart craters superposed on channels have ages between 1 5 and 0 6 gyr the onset diameter 3 km at 1 5 gyr in this region may indicate a relatively shallow ground ice table loss of volatiles due to diffusion and sublimation might have lowered the ground ice table even in the southern chryse planitia region afterwards in general our study implies a formation of the smallest rampart craters within and or shortly after periods of fluvial activity and a subsequent lowering of the ground ice table indicated by increasing onset diameter to the present these results question the method to derive present equatorial ground ice depths from the onset diameter of rampart craters without information about their formation time

impact craters in the northern hemisphere of mars layered ejecta and central pit characteristics mars global surveyor mgs and mars odyssey data are being used to revise the catalog of large martian impact craters analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of mars this report focuses on the ejecta morphologies and central pit characteristics of these craters the results indicate that single layer ejecta sle morphology is most consistent with impact into an ice rich target double layer ejecta dle and multiple layer ejecta mle craters also likely form in volatile rich materials but the interaction of the ejecta curtain and target produced vapor with the thin martian atmosphere may be responsible for the large runout distances of these ejecta pancake craters appear to be a modified form of double layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions pedestal craters are proposed to form in an ice rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated central pits likely form by the release of vapor produced by impact into ice soil mixed targets therefore results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the martian northern hemisphere

small scale lunar farside volcanism abstract we identified and mapped 28 mare basalt occurrences between the australe and south pole aitken basins on the southern lunar farside and determined their absolute model ages amas by performing crater size frequency distribution csfd measurements our study area can be subdivided into seven major mare basalt occurrences in and around bolyai roche v rosseland pauli and roche craters south of both rosseland and coblentz craters as well as mare patches between eotvos and roche craters the amas of the mare basalts in and around bolyai crater range from 2 1 ga to 3 5 ga two units varying drastically within short distances the mare patches south of coblentz crater contain nine units that have amas ranging from 2 1 ga to 3 8 ga the mare basalts in roche v crater show an ama of 2 2 ga we mapped seven volcanic units in rosseland crater and derived amas for five of these units the mare basalts in rosseland crater show the youngest amas found in our study area ranging from 1 5 ga to 2 9 ga the mare basalt occurrence south of rosseland crater shows significantly older ages with an ama of 3 3 ga the mare basalts in pauli and roche craters show amas from 1 7 ga to 3 1 ga the mare patches between eotvos and roche craters show a similar range of amas from 1 9 ga to 3 1 ga the amas of the mare basalts in our study show that the lunar farside was volcanically active for nearly as long as the lunar nearside 1 2 ga ago or at least longer than previously thought 2 5 ga ago and predicted by models of the ascent and eruption of lunar basalts in addition we calculated thicknesses and volumes of the investigated mare basalts with thicknesses between 21 m and 172 m and volumes from 0 1 km3 to 379 km3 the mare basalts in our study area show a wide range of dimensions similar to other mare basalts of the lunar near and farsides

late palaeozoic to early mesozoic evolution of pangaea several possible configurations of the pangaea supercontinent have been suggested for the interval from late carboniferous to early triassic here we re examine the palaeomagnetic basis for these models emphasizing the trends of the paths of apparent polar wander for the individual components of the supercontinent rather than simply averaging poles of presumed similar age two of the alternatives pangaea b and c may result from averaging poles of dissimilar age along common polar wander paths giving rise to spurious tectonic displacements the most likely model appears to be the formation in late devonian time of a modified pangaea a2 followed by evolution to the traditional configuration a during the triassic

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

layered ejecta craters and the early water ice aquifer on mars a model for emplacement of deposits of impact craters is presented that explains the size range of martian layered ejecta craters between 5 km and 60 km in diameter in the low and middle latitudes the impact model provides estimates of the water content of crater deposits relative to volatile content in the aquifer of mars these estimates together with the amount of water required to initiate fluid flow in terrestrial debris flows provide an estimate of 21 by volume 7 6 x 10 7 km 3 of water ice that was stored between 0 27 and 2 5 km depth in the crust of mars during hesperian and amazonian time this would have been sufficient to supply the water for an ocean in the northern lowlands of mars the existence of fluidized craters smaller than 5 km diameter in some places on mars suggests that volatiles were present locally at depths less than 0 27 km deposits of martian craters may be ideal sites for searches for fossils of early organisms that may have existed in the water table if life originated on mars

planetary surface dating from crater size frequency distribution measurements multiple resurfacing episodes and differential isochron fitting abstract the analysis of crater size frequency distributions and absolute densities forms the basis of current approaches for estimating the absolute and relative ages of planetary surfaces users of the neukum system of crater dating have conventionally used a cumulative presentation of the data but because of the recent proliferation of interest in identifying resurfacing ages it is worth emphasising the utility of the differential presentation of crater data in identifying resurfacing events and particularly in distinguishing the signature of short lived events such as volcanic flows from long acting processes such as aeolian erosion the work describes some additional considerations for making isochron fits to differentially plotted crater populations with respect to the removal of a binning bias for incrementally differentially plotted data the hartmann approach has not typically employed the fitting of isochrons but differential fitting would be a natural choice for this system and is implemented in the craterstats software a revised calculation of the martian epoch boundary times in both chronology systems is provided

post failure analysis of the maierato landslide using the material point method abstract the 15 february 2010 maierato landslide has become the symbol of the hydrogeological instability of the calabria region southern italy it occurred after a long period of rain in an area prone to landslides some videos filmed during the event documented the high velocity and the flow like character of this landslide after the slope failure several studies focused on the pre failure and failure stages of the maierato landslide in order to interpret its failure mechanism and to establish the main factors of triggering however none of these studies analysed the post failure stage of this landslide in the present study the material point method mpm is used to simulate the run out process of the maierato landslide and provide a possible explanation of this process different scenarios are considered and for each of them the simulated post failure configuration of the displaced material is compared to that detected after the event numerical results match fairly well the observed ones when a severe reduction in the shear strength of the involved soil is accounted for in the analysis this strength reduction is attributed herein to the high excess pore water pressures generated by the large distortions of the moving soil mass moreover the back calculated post failure strength is consistent with that found in several case histories of slope failures involving liquefied materials

martian impact craters correlations of ejecta and interior morphologies with diameter latitude and terrain abstract the premise of this study is that global analysis of crater morphologies can discriminate between the effects of impact energy atmosphere and subsurface volatiles based on the correlation between morphology and latitude diameter and terrain this study focuses on seven ejecta and nine interior morphologies associated with 3819 craters 8 km in diameter distributed across the entire martian surface this data set is sufficiently large to provide statistically viable results on a global scale the results of the analyses indicate that changes in ejecta and interior morphology correlate with increases in crater diameter excavation depths corresponding to these diameters are calculated from depth diameter relationships and are compared to the theoretical distribution of subsurface volatiles the different ejecta morphologies can be described by impact into material with varying proportions of volatiles and the latidudinal variations seen for rampart crater morphologies correlate well with the proposed latitude depth relationship for ice and brines across the planet many of the interior structres show distributions indicative of terrain dependent influences for example the concentrations of central pit craters around ancient impact basins and on alba patera suggest that volatiles preferentially collect in these areas however central peak and peak ring interior morphologies show little relationship to planetary properties and probably result from changes in impact energy thus many of the unique morphologies associated with martian impact craters result from the presence of subsurface volatiles

the distribution and behavior of martian ground ice during past and present epochs mars undergoes significant oscillations in its orbit which will have a pronounced effect on its climate and in particular on the behavior of subsurface water ice we explore and map the behavior of ice in the martian near surface regolith over the past 1 m y using a diffusion and condensation model presented in an earlier paper with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance we find that the past behavior of ground ice differs significantly from that at the present epoch primarily the result of high amplitude oscillations in obliquity presently 25 in midlatitude and equatorial regions ground ice will condense from atmospheric water during times of higher obliquity filling the top few meters of the regolith with significant amounts of ice at an obliquity of 32 ground ice becomes stable globally during times of lower obliquity ground ice will sublime and diffuse back into the atmosphere dessicating the regolith to a depth of about 1 to 2 m equatorward of 60 to 70 latitude in the high latitude regions these oscillations are considerably subdued below this depth of cyclic saturation and dessication a long term stability of ice exists in some geographic regions we present a map of the distribution of ice expected at the present epoch cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layered deposits

shear strength of clay at different shear rates abstracta test procedure that involved shearing soil specimens at different shear rates was proposed to study the effect of shear rate on the strength recovery of three natural clays the obtained results indicated that an increase in shear strength occurred when the shear rate decreased this strength regain was more pronounced for higher plasticity clays which were sheared at a relatively lower confining pressure of 100 kpa

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

carboniferous palaeomagnetic investigations in morocco permian remagnetizations and possible large carboniferous rotations occurring in mesetian sedimentary and jebilets intrusive rocks summary carboniferous limestone and sandstone formations from the moroccan meseta and visean gabbros from the jebilets massif have been investigated palaeomagnetic results reveal the occurrence of different magnetic components these are i a permian component dm 141 5 zm 1 7 n 4 virtual geomagnetic pole vgp 33 s 43 9 e similar to that found in the anti atlas and in other parts of the hercynian chain ii a carboniferous component dm 116 6 zm 25 lo n 9 vgp 7 4 s 54 5 e consistent with other carboniferous data from central sahara and morocco and iii a component not previously described from africa and still undated with a direction dm 77 4 im 1 5 this direction and it s opposite polarity counterpart were found only at one site 21 specimens in the jebilets and at two sites 20 specimens in the meseta opposite polarity it may represent an older magnetization possibly of lower carboniferous age which would imply important rotations of africa during this period

a palaeomagnetic test of the axial dipole assumption and implications for continental distribution through geological time the distribution of the magnetic inclination i in the global palaeomagnetic data is examined over broad windows of geological time extending from the cenozoic back to the archaean distributions are compared with the theoretical predictions from the uniform global sampling of dipolar and higher order geomagnetic field sources the distributions are found to accord more closely with a dipolar source than with a multipolar source over most of geological time reinforcing a conclusion by evans based on 10 of the present data base the exception is the interval 600 300 ma broadly coincident with palaeozoic times when the data critically fail to accord with any simple multipole assumption we interpret this to be a symptom of the remagnetisation effect the sedimentary derived and to a much lesser extent the igneous and metamorphic derived record from these times has been extensively remagnetised in late palaeozoic times as the result of a unique incidence of diagenetic tectonic and thermal processes otherwise we conclude that the palaeomagnetic data interpreted on the basis of the axial dipole assumption can be used reliably to evaluate tectonic models for older 600 ma and younger 250 ma parts of geological time models for the global distribution of the continental crust for those cases that have postulated episodic or long term accretion of the crust into quasi rigid supercontinents are also evaluated the proterozoic supercontinent yields the most acceptable solution to the i distribution observed over the long interval of precambrian times 3000 600 ma it also explains the variable ratios of high latitude palaeomagnetic poles from the ancient shield segments of continental crust owing to the failure of the palaeozoic data set to accord with the dipole assumption the i distributions cannot be used to test the relationship between the supercontinent of pangaea and the geoid symmetry during the earlier part of its history but after 250 ma it was most closely constrained to the form of the geoid positive over geological time as a whole the continental crust has shown a marked preference for the i distribution defined by the present geoid positive rather than the distribution defined by the geoid negative we therefore conclude that the deep mantle effects responsible for the geoid have on average had a surface response which has tended to move the continental crust towards this sector of the globe

investigating the interactions between an atmosphere and an ejecta curtain 1 wind tunnel tests when an asteroid impacts a planetary surface ejecta are excavated along ballistic trajectories whose loci define an inverted cone shape the outward advancing motion of this curtain displaces atmosphere to generate a ring vortex whose winds can entrain transport and deposit ejecta and fine grained surface materials curtain width and velocity particle concentration size distribution and motion parallel to the curtain and the density viscosity and compressibility of the surrounding atmosphere all influence the vortex circulation strength as analogs to an advancing ejecta curtain we tested the effect of inclined solid and porous plates on vortex formation in a low speed wind tunnel we found that hydraulic resistance a measure of energy losses for 1 d porous flow governs the position along a porous plate where it becomes effectively permeable and flow separation occurs the resulting flow field is similar to that over an inclined solid plate of the same effective length energy losses through the top permeable portions of the plate reduce circulation strength by only 7 relative to flow over a solid plate the two parameters needed to estimate circulation strength curtain velocity and impermeable height can thus be determined by coupling an impact model with published hydraulic resistance data these tests also served to calibrate a numerical model which we then applied to investigate the influence of atmospheric compressibility and particle motion parallel to the curtain see part 2 barnouin jha et al this issue these two studies provide a method to predict the curtain induced flow and consequent patterns of debris deposition associated with impacts on planets with atmospheres

martian subsurface volatile concentrations as a function of time clues from layered ejecta craters 1 martian layered ejecta morphologies are characterized using a new preservation classification system and through measurement of ejecta mobility em ratios em the ratio of ejecta extent to crater radius is believed to provide information about ejecta material fluidity during emplacement this study compares em and preservation classification to determine if subsurface volatile concentrations have changed measurably over time results from both regional and local analyses suggest that concentrations of subsurface volatiles have remained approximately constant at the depths and over the time periods recorded by these craters

minimum effective area for high resolution crater counting of martian terrains the acquisition of high resolution imagery for the surface of mars has enabled mapping of spatially limited order of 200 m on four type terrains using mars reconnaissance orbiter mro context camera ctx imagery that span the noachian hesperian and amazonian epochs the counts from each location include a region covering 10 000 km2 ten 1000 km2 subsets of that larger area and approximately one hundred 100 km2 samples the data demonstrate significant variation in the crater size frequency and derived model ages across a single terrain type for the 100 km2 samples the crater size frequency at this area scale varies across a single uniform geologic unit by up to a factor of 2 3 on the four different terrains at 1000 km2 the local pattern variations that are relevant at the 100 km2 scale become less important and the age variations are tighter in all four terrain cases the 10 000 km2 and 1000 km2 samples capture distinct crater populations km sized craters that formed before and after resurfacing event s however due to the relatively high mean distance between km sized craters the 100 km2 size area samples more commonly than not exclude a statistically significant sample at the kilometer size range masking important information about the pre resurfacing history of the terrain we therefore suggest that due to the effect of pattern variability in cratering over 100 km2 and the susceptibility of smaller craters to resurfacing crater counts derived from small area samples are suspect to major uncertainties

small rampart craters in an equatorial region on mars implications for near surface water or ice small onset diameters 1 km of rampart craters were identified in high resolution stereo camera hrsc imagery in the equatorial region of the valles marineris plateaus this is in contrast to previous global studies based on viking imagery which showed onset diameters in the range of 4 km to 7 km in equatorial regions the observed small rampart craters show morphologically eroded ejecta blankets this indicates a formation in the early martian history when erosion rates were higher volatile rich target material might have been available shortly after the formation of the hesperian aged plateaus surrounding valles marineris and this is in agreement with the reported hesperian fluvial activity in this region the lack of fresh small rampart craters indicates a lowering of the ground ice table with time

paleomagnetism of permian sedimentary rocks from tanzania and the permian paleogeography of pangea abstract we report new paleomagnetic results from karoo permian rocks in tanzania k1 and k3 redbeds and k5 siltstones and limestones from the songwe kiwira basin ndeke redbeds from the mikumi basin and group viii redbeds from the nyakatitu basin were sampled and thermally demagnetized palynologic data and lithologic correlations suggest that the k1 redbeds are lower permian and that the k3 ndeke and group viii redbeds as well as the k5 siltstones and limestones are upper permian stable magnetic directions were obtained from the k3 ndeke and group viii redbeds yielding an upper permain pole at 26 n 267 e characteristic remanence directions were also obtained from the k5 siltstones and limestones however because these directions define a pole 66 n 269 e that is very close to the upper cretaceous pole for africa and significantly different from the upper permian redbed poles these rocks may have been remagnetized the k1 redbeds did not yield stable results using our new results together with published permian poles for africa and the other gondwanan continents we revise the mean lower and upper permian poles for africa west gondwana and gondwana and investigate whether or not these revised poles can discriminate between the various pangea reconstructions proposed for the permian with any greater certainty than previous unrevised mean poles when compared to the mean lower and upper permian poles for laurussia we find that our revised west gondwanan and gondwanan poles do not necessarily provide stronger support for any particular pangea reconstruction than the previous poles therefore even though this study improves the paleomagnetic data base for gondwana for the permian it is not clear that the combination of our results with other paleomagnetic data from the gondwanan continents yields significant new insights about the permian configuration of pangea as suggested previously strong non dipole fields during the permian may provide an explanation for why it is not easy to discriminate with certainty between the various permain pangea reconstructions even with new paleomagnetic results

mapping rays and secondary craters from the martian crater zunil 1 zunil a 10 1 km rayed crater in elysium planitia mars produced more than 7 107 secondary craters 15 m in diameter we mapped zunil s rays from thermal ir themis nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters candidate secondaries up to 3600 km range ray segments were mapped up to 450 km east of zunil and up to 1700 km to the west both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia which are abundant west of but not east of zunil nevertheless our interpretation is that zunil was created by a moderately oblique impact from the east zunil secondaries are abundant over all terrain types except the medusae fossae formation mff given the likely age of zunil 100 ma parts of the mff must be eroding at 0 08 m ma the size frequency distribution of the secondaries in zunil s rays and probable distant secondaries 750 1700 km to the west of zunil have a cumulative power law exponent near 5 whereas secondaries between the major rays have an exponent near 3 4 we modeled the size velocity relationship for zunil s ejected fragments it is consistent with the predictions of melosh 1984 for spallation of a strong surface layer and demonstrates that the inverse size velocity correlation continues up to at least 2 km s

pangea in permian to jurassic time the apparent polar wander paths apwps for the northern and southern continents have been redetermined using revised time scales and data when plotted on the conventional pangea pangea a the paths are distinct but essentially linear and parallel from late carboniferous to late permian time both paths then undergo a sharp change in direction and converge so as to overlap in early to middle jurassic time if the permo triassic magnetic field was an axial dipole then paleomagnetic data show that pangea was mobile during most and possibly all of the mid triassic to mid jurassic interval because of the uncertainties in pole ages and positions and errors in measurements the new apwps are consistent with a range of pangeas in particular the data are compatible with two paleomagnetic pangeas a2 and a new class of pangea here named the d pangeas these resemble morel and irving s 1981 pangea b and the c pangeas of smith et al 1981 unlike the a2 pangeas the b c and d pangeas all require large 2000 km relative motion between the northern and southern continents geological data require the transition from an earlier pangea to pangea a to take place along a transform zone here named the pangea transform zone or ptz the ptz is considered to be technically analogous to the great glen fault of scotland four types of possible ptz are examined passing through either the western or eastern end of the brunswick magnetic anomaly of eastern north america or north and south of iberia the geological data suggest that those pangeas that require large displacements on ptzs that pass east of the brunswick magnetic anomaly are improbable b and c pangeas are snapshots giving precise paleomagnetic reconstructions d pangeas fit long stretches of the apwps as well as possible a2 pangeas fit the apwps moderately well but also require the geological displacement to be smaller the differences in displacement between the d and a2 pangeas reflect geometrical relationships between euler poles ptz curvature and the apwps the general geological evidence suggests that d pangeas are less probable than a2 pangeas but that they cannot be rejected transition from a2 to a is most likely to have occurred via a sequence of rotations about different poles rather than a single rotation in detail the conventional pangea requires modification africa must be broken into at least two fragments along the benue trough and its northern continuation northwest africa then fits more tightly into northern south america which modifies the apwps and reduces the gap between them by including the effects of continental extension and using detailed geological data the gap can be reduced to one in which the a63 circles touch or overlap non dipole fields or an expanding earth need not be invoked to explain the mismatch of the apwps on the conventional pangea the finite rotation required for the older pangea to pangea a is very close to van der voo and french s 1974 single rotation for a2 to a but is actually the sum of three smaller rotations that reflect the modification to pangea a continental extension and the postulated ptz displacement of the possible ptzs a ptz parallel to the west african continental magnetic anomaly with a displacement of about 170 km gives a better fit of this anomaly to the east coast magnetic anomaly of north america further refinements to pangea a2 will depend on a better knowledge of stretching factors on the passive continental margins of the modified pangea a and incorporation of rotations due to major faults such as the south atlas fault in morocco