venusian channels and valleys distribution and volcanological implications abstract nearly complete image coverage of venus by magellan enabled us to map various venusian channel and valley landforms and to examine their associations with other geological units global mapping reveals a nonrandom distribution the highest total concentration is in the equatorial regions characterized by highlands rift and fracture zones and associated volcanic features many channels associated with flow deposits are similar to typical terrestrial lava drainage channels they are observed to be associated with a wide range of volcanic edifices such as coronae shield volcanoes and rift and fracture zones one type of channel similar morphologically to lunar sinuous rilles is classified as a venusian sinuous rille based on the close associations of many venusian sinuous rilles with coronae we hypothesize that mantle plume or blob volcanism has caused high effusion and sustained lava eruptions essential for sinuous rille formation mantle derived high temperature low viscosity lava eruptions are responsible for the efficient erosional processes particularly for thermal erosion that seems to be at least partially required for some sinuous rille formation many valley networks are observed in highlands and in association with coronae fracture systems and source zones for low viscosity lavas both key to network formation were probably concentrated at highlands and coronae canali type channels which are morphologically unlike other known volcanic channels are limited to certain plains regions a wide range of low viscosity lava types is possible for the channel forming lava depending on the formation mechanism their lengths exceed the lengths of other common volcanic channel types on venus implying a large volume of lava and long duration of the eruption the close association of canali with plains regions implies that canali formation is probably related to the emplacement of plains a hypothesized global resurfacing event late in venusian history may be responsible for canali formation

venusian channel formation as a subsurface process 1 we constructed detailed geologic maps of channels within three widely separated regions using magellan synthetic aperture radar sar imagery and altimetry radar data in order to understand the operative processes involved in venusian channel formation channels in each region cut topographic highs including ridges and shield edifices with no deflection of the channel course by surface topography we argue that these relationships are difficult to reconcile with the widely held view that venusian channels represent surface processes whether constructional or erosional we conclude that the channels evolved through subsurface fluid flow which involved local stoping and transport of overlying material that is these channels were carved from below rather than from above we postulate that at least some venusian channels form because of subsurface fluid flow along a shallow crustal interface that forms the boundary between overlying low backscatter surface materials and underlying basal materials fluid movement along the interface may facilitate piecemeal stoping and erosion of the local surface materials from below eventually resulting in the formation of channel traces exposed at the surface the low backscatter material which hosts the channels in this study also hosts abundant coalescing shields and associated deposits which may represent shield terrain

geomagnetic paleointensity derived from single plagioclase crystals we suggest a new approach for the determination of absolute geomagnetic paleointensity that uses single plagioclase crystals to avoid problems caused by alteration during thellier thellier analyses transmission electron microscope imaging and rock magnetic data indicate that plagioclase crystals from a recent basalt flow in hawaii 1955 kilauea eruption contain pseudo single to single domain titanomagnetite inclusions these feldspars yield paleointensity values that agree within error with magnetic observatory data and paleointensity values reported from whole rock samples these results suggest that single plagioclase crystals from older igneous rocks may provide a viable source of paleointensity data

modeling dynamic recrystallization of olivine aggregates deformed in simple shear experiments by zhang and karato 1995 have shown that in simple shear dislocation creep of olivine at low strains an asymmetric texture develops with a 100 maximum rotated away from the shear direction against the sense of shear at large strain where recrystallization is pervasive the texture pattern is symmetrical and 100 is parallel to the shear direction the deformation texture can be adequately modeled with a viscoplastic self consistent polycrystal plasticity theory this model can be expanded to include recrystallization treating the process as a balance of boundary migration growth of relatively underformed grains at the expense of highly deformed grains and nucleation strain free nuclei replacing highly deformed grains if nucleation dominates over growth the model predicts a change from the asymmetric to the symmetric texture as recrystallization proceeds and stabilization in the easy slip orientation for the dominant 010 100 slip system this result is in accordance with the experiments and suggests that the most highly deformed orientation components dominate the recrystallization texture the empirical model will be useful to simulate more adequately the development of anisotropy in the mantle where olivine is largely recrystallized

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

the low temperature magnetic signature of fe rich serpentine in cm2 chondrites comparison with terrestrial cronstedtite and evolution with the degree of alteration 1 we have studied the low temperature magnetic properties of one ci and five cm2 chondrites well characterized in the literature and of cronstedtite the fe end member of the fe mg serpentine solid solution cronstedtite is highly anisotropic and its relaxation properties below 10 k suggest a glassy magnetic behavior frustration of the magnetic interactions in octahedral layers would account for the observed magnetic freezing with a possible role of the random distribution of ferric iron in tetrahedral layers based on the comparison between powders and crystals characterized by scxrd and empa we show that grain size and substitution of octahedral fe by mg induce changes in the position of the low tmagnetic susceptibility peak a low tpeak is also found in each of the cm2 chondrites but not in the ci chondrite it is interpreted as the signature of fe serpentines which are considered by most authors to have mostly formed during asteroidal aqueous alteration events the low tmagnetic properties of the less altered meteorites compare well with those of cronstedtite and the position of the susceptibility peak varies with the published degree of alteration by comparing magnetic data on reference cronstedtite magnetic and published mineralogical data on cm2 chondrites we suggest that the evolution of the low tmagnetic signature of the latter reflects bulk changes in fe serpentines chemical composition rather than only mixing between fe rich and mg rich serpentines this is analogous to previous observations on the changes in fe incorporation to mg serpentines during progressive serpentinization of abyssal peridotites

fayalite dissolution and siderite formation in water saturated supercritical co2 abstract olivines significant constituents of basaltic rocks have the potential to immobilize permanently co2 after it is injected in the deep subsurface due to carbonation reactions occurring between co2 and the host rock to investigate the reactions of fayalitic olivine with supercritical co2 scco2 and formation of mineral carbonates experiments were conducted at temperatures of 35 c to 80 c 90 atm pressure and anoxic conditions for every temperature the dissolution of fayalite was examined both in the presence of liquid water and h2o saturated scco2 the experiments were conducted in a high pressure batch reactor at reaction time extending up to 85 days the newly formed products were characterized using a comprehensive suite of bulk and surface characterization techniques x ray diffraction transmission emission mossbauer spectroscopy scanning electron microscopy coupled with focused ion beam and high resolution transmission electron microscopy siderite with rhombohedral morphology was formed at 35 c 50 c and 80 c in the presence of liquid water and scco2 in h2o saturated scco2 the formation of siderite was confirmed only at high temperature 80 c characterization of reacted samples in h2o saturated scco2 with high resolution tem indicated that siderite formation initiated inside voids created during the initial steps of fayalite dissolution later stages of fayalite dissolution result in formation of siderite in layered vertical structures columns or pyramids with a rhombus base morphology

the magnetic stratification of layered mafic intrusions natural examples and numerical models abstract mafic ultramafic intrusions commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber this layering occurs at scales from several hundreds of meters in some large intrusions down to scales of a few millimeters layering forming processes are numerous and include both internal processes such as crystal settling double diffusive convection flow segregation and compaction and external processes such as magma recharge extraction or mixing mafic magma chambers may operate as a closed system or as an open system several recent investigations have been conducted on borehole cores through the insizwa sill south africa the great dyke zimbabwe and the bushveld complex south africa we present an overview of magnetic results on these intrusions the magnetic properties of mafic rocks are dominated by the presence of magnetite in most layers pyrrhotite the only ferrimagnetic sulfide is generally restricted to sulfide rich reefs merensky j m bohmke or to massive sulfide deposits at the base of the intrusion all intrusions display distinct variations of their magnetic properties anisotropy of magnetic susceptibility magnetic remanence magnetic saturation high field magnetic susceptibility across the layering the significance of these magnetic zones varies depending on the nature of variations from zone to zone for example a drop in magnetic susceptibility does not necessarily correspond to a new magma batch unless it is also associated with a change in magnetic anisotropy parameters in the studied cases all open systems the magnetic susceptibility and the total iron content estimated from high field measurements increase towards the top of these intrusions suggesting a general pattern of iron enrichment a numerical petrologic model for a tholeiitic magma predicts magnetic susceptibility variations across a statically differentiated closed system intrusion no convection the effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view conversely these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

the anisotropy of magnetic susceptibility in biotite muscovite and chlorite single crystals the anisotropy of magnetic susceptibility ams of single crystals of biotite muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock forming minerals the low field ams and the high field paramagnetic susceptibility are defined for the high field values it is necessary to combine the paramagnetic deviatoric tensor obtained from the high field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals this leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice the ellipsoid of paramagnetic susceptibility which was obtained for the three phyllosilicates is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c axes the anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements the degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite is smaller than previously published values the shape of the chlorite ams ellipsoid for all the samples is near perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks d 2003 elsevier science b v all rights reserved

determining the origin of ultrahigh pressure lherzolites the occurrence of coesite and diamond in regional ultrahigh pressure metamorphic rocks requires that rocks formed at pressures of 3 to 4 gpa 100 to 120 km below the earth s surface have been transported back to the surface 1 1 l dobrzhinetskaya et al suggest that garnet lherzolite from

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

paleomagnetism carried by crystal inclusions the effect of preferred crystallographic orientation abstract randomly oriented phlogopite crystals from a pegmatite contain submicroscopic inclusions of magnetite the inclusions have oblate magnetic fabrics subparallel to the ab crystal plane alternating field demagnetization of individual crystals shows that the remanence is influenced by the crystal symmetry of the phlogopite host the magnetizations isolated by alternating field demagnetization either cluster close to the a axis or scatter in the ab plane coarse grained igneous and metamorphic rocks can only record a paleofield direction accurately if there is a suitable orientation distribution of the rock forming minerals that host the ferromagnets an isotropic distribution of rock forming minerals is preferable but it is not obligatory rock forming minerals that have l fabrics or s fabrics with a strong l component are also suitable if they contain oblate inclusion fabrics of ferromagnets

late pleistocene glacial events in the central apennines italy the study of glacial evidence in the gran sasso massif of the central apennines italy has allowed the last maximum advance and the subsequent stadial phases to be dated and the mean annual temperature and quantity of precipitation in the form of snow to be assessed for a number of periods the glaciers probably reached their maximum extension campo imperatore stade ca 22 60014c yr b p and started to retreat ca 21 000 yr b p leaving behind three recessional moraines after a first interstade fornaca interstade the fontari stade appears to have taken place shortly after 16 000 yr ago ca 15 000 yr ago the glacier started retreating leaving behind four more recessional moraines an interstade venacquaro interstade preceded the mount aquila stade datable at ca 11 000 yr b p a strong correlation is evident between the glacial phases on land and the isotopic variations in cores from the tyrrhenian sea

possible pingo fields in the utopia basin mars geological and climatical implications abstract the presence of pingos on mars has been hypothesized since the period of the viking mission in fact a diverse range of pingo like features has been found at various martian sites including elysium chryse and utopia planitiae in the northern lowlands due to the morphology and the geological setting some of those features were interpreted in different ways creating some controversies as happened in athabasca valles this reflects the complexity of interpreting these features by remote sensing and multiple plausible interpretations of the same feature with the objective of identifying new possible pingos or rootless cones on mars we selected a study area in utopia planitia 10 55 n 210 260 w where the presence of both features is possible due to its geological history volcanic and hydrological we analyzed more than 2100 mars orbiter camera moc narrow angle images in addition to viking thermal emission imaging system themis and high resolution stereo camera hrsc images together with mars orbiter laser altimeter mola derived digital elevation models dems with a geographic information system gis we found in 94 moc narrow angle images dome cone and ring shaped features we analyzed them from morphological and morphometrical points of view in order to compare them with relevant features on mars and earth we tested different possible origins for those features following the approach of multiple working hypotheses we conclude that the dome cone and ring shaped features could be pingos which is in agreement with their geological settings regarding the driving heat source for the formation of the purported pingos we propose the existence of a heat source possibly a magma chamber underneath the surface of the utopia basin together with possible climatic shifts the past activities of the heat source may have caused melting of ground ice the pingo growth due to freezing of the water would have occurred during the following cold climatic conditions

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

paleointensity of the early geodynamo 2 45 ga as recorded in karelia a single crystal approach we report thellier paleointensity results from single plagioclase crystals derived from ca 2 45 ga border dikes of the burakovka layered intrusion karelia russia rock magnetic and transmission electron microscope analyses indicate that these plagioclase crystals contain pseudosingle to single domain magnetite inclusions fifteen determinations from four dikes meet reliability criteria and yield an average paleointensity of 43 2 10 8 t which corresponds to a virtual dipole moment of 8 43 2 11 x 10 2 2 am 2 although our result is unlikely to adequately represent the time averaged field the mean and range of values are similar to those of the present day field these values suggest that the inner core which may stabilize the geodynamo had started to grow by early proterozoic time

multi seismic cycle velocity and strain fields for an active normal fault system central italy abstract vertical offsets of late pleistocene holocene deposits and landforms are combined with slip direction indicators on dipping active normal faults in central italy to produce velocity and strain rate fields averaged since 12 18 ka for a 5 by 5 km grid maximum strain rates of 0 078 0 0156 ppm yr or 0 052 0 01 ppm yr across a distance of 63 639 m imply maximum ne sw extension rates of 5 0 1 0 mm yr or 3 3 0 66 mm yr depending on whether rates are averaged over 12 ka or 18 ka the high spatial resolution reveals that strain rates vary by up to 0 04 0 008 or 0 06 0 012 ppm yr over a distance of only c 7 km along the strike of the fault system again depending on the timescale chosen for the first time these data allow a comparison of rates derived over time periods that would contain numerous seismic cycles with those measured with gps that represent either interseismic strain or displacements due to a single large earthquake with some interseismic strain overall the rates compare reasonably well with those measured geodetically or derived from seismic moment summations

depth profiles of venusian sinuous rilles and valley networks abstract more than 200 venusian channels and valleys have been mapped based on analyses of magellan sar images sinuous rilles are the most abundant channels among six types of venusian channels and they are widely distributed on venus morphological characteristics of venusian sinuous rilles include sinuous narrowing reaches source depressions and length of several 10s to a few 100s of km this type of channels is known to exist on the moon and possibly on mars valley networks on venus often occur in the vicinity of or in connection to sinuous rilles cross sectional morphologies of sinuous rilles and valley networks are of special importance in discussing their formation processes both qualitatively and quantitatively we reconstructed cross sectional profiles of 6 sinuous rilles and 2 valley networks using a new radar clinometric method reconstructed cross sections revealed that floors of the channels and valleys are clearly lower than the surrounding plains this finding implies that the sinuous rilles and the valley networks have erosional origins longitudinal depth profiles of the sinuous rilles show distinct decreasing trends toward the termini such decreasing trends of depths are qualitatively in agreement with theoretical models and laboratory experiments of thermal erosion in order to verify this assertion quantitatively we conduct simple 1 dimensional model calculations under the assumption that both channel forming lavas and ground substrate are tholeiitic basalt for initial lava thicknesses in the range 2 6 m the model calculations yield good matches to the depth profiles estimated duration of lava effusion ranges from several months to a few years these numerical results support thermal erosion of the sinuous rilles but do not necessarily exclude contributions from mechanical erosion processes valley networks seem to have formed under a strong structural control in comparison to sinuous rilles the valleys vary widely in characteristics of the depth profile and flow directions relative to surface slopes therefore valley networks appear to have originated from diverse formation mechanisms

magnetism of biotite crystals abstract we report mossbauer spectra magnetic hysteresis acquisition and demagnetization of isothermal remanent magnetization irm and low temperature irm warming curves of biotite crystals as well as a summary of domain observations on magnetic extracts the biotites are from 1 0 to 1 2 ga gneisses and 1 85 ga plutons of the grenville churchill and bear provinces canada most have paramagnetic susceptibilities in the range 0 45 1 2 10 3 si similar to reported values for iron rich biotites magnetite identified by its 120 k verwey transition is an inclusion in all the biotites sizes and domain states of inclusions correlate with magnetite content as measured by saturation magnetization ms the higher ms is the lower are the domain state indicators mrs ms mrs is saturation irm and coercive force hc added to a base population 0 003 0 1 of pseudo single domain psd grains with mrs ms from 0 1 to 0 3 and hc from 7 to 20 mt is 1 in two exceptional cases 7 and 14 of coarse grained multidomain md magnetite with much softer hysteresis properties the underlying psd magnetite fraction in biotites potentially retains useful paleomagnetic remanence in the more magnetic micas this stable fraction may be swamped by md magnetite with less stable behavior

silicate and oxide exsolution in pseudo spinifex olivine from metaultramafic rocks of the betic ophiolitic association a tem study transmission electron microscopy tem has been used to study submicroscopic particles in spinifex like textured olivine from secondary harzburgites collected from the cerro del almirez locality in the mulhacen complex betic cordillera se spain three main types of submicroscopic oxides have been identified 1 equidimensional fe rich spinel magnetite with average grain size in the order of 1 2 m 2 elongated cr bearing spinels fe chromite to cr magnetite with sizes ranging from 0 2 to 1 m long and 0 01 to 0 1 m thick and 3 equidimensional ti rich particles from 0 01 to 0 1 m and compositions ranging from ilmenite to fe cr ti oxide chromite and ilmenite particles form parallel chromite rich and ilmenite rich bands extending along the a axis of the host olivine both phases show a fixed orientation relationship with olivine with the approximately hexagonal close packed oxygen planes being parallel in both structures these textural relationships indicate that both chromite and ilmenite were formed during a common exsolution process magnetite particles also are orientated preferentially relative to the olivine but these particles are homogeneously distributed within the olivine suggesting either a primary origin or an exsolution process that was not contemporaneous with formation of chromite and ilmenite chromite particles commonly are accompanied by lamellae of talc and or enstatite both showing a consistent orientation relationship with olivine talc lamellae are twice as thick as the associated chromite crystals whereas enstatite lamellae show a greater thickness and moreover form single enstatite particles which consist of clino and orthoenstatite intergrowths talc formation may be explained by exsolution together with spinel from olivine containing oh groups probably related to incomplete dehydration of serpentine during olivine formation on the basis of these results and previously reported petrological data we have concluded that exsolution of chromite silicate and ilmenite occurred during the retrograde stage that followed the climax of the eo alpine metamorphic event

construction of a seismotectonic model the case of italy procedures for constructing a seismotectonic model of italy designed to be used as a basis for hazard assessment are described the seismotectonic analysis has essentially been based on a gis aided cross correlation of three data sets concerning the 3 d structural model of italy and surrounding areas the space distribution of historical and present seismicity the kinematic model of the central mediterranean region referred to the last 6 ma and including the available information on the present day plate motion and stress field the seismicity pattern in the study area is controlled by a quite complex geodynamic framework which includes continent continent convergence alps and dinarides with development of a neutral arc bordering the plate margins plate divergence across margins characterized by passive slab sinking northern apennines and calabrian arc with development of backarc basins northern tyrrhenian sea and southern tyrrhenian sea flanked by forelandward migrating thrust belt foredeep systems plate divergence across a margin previously characterized by lithosphere sinking and afterwards discharged from the subducted slab southern apennines with development of quite peculiar rift processes within the inactive thrust belt transpression northern sicily due to the combined effect of plate convergence africa europe and high rate flexure hinge retreat of an intervening plate adria microplate with high angles between the respective slip vectors intraplate strain partition and fault activity mainly combined strike slip and thrust motions possibly in correspondence of inverted structures the results of the seismotectonic analysis are synthesized in a zonation of italy in which every delimited zone corresponds to the surface projection of a kinematically homogeneous segment of a seismogenic fault system in cornell type hazard evaluations every polygon should be considered as a homogeneous source zone seat of randomly distributed earthquakes a homogeneous mechanical behaviour of an entire zone and a random earthquake distribution within a single source zone obviously represent oversimplified assumptions since every zone includes one or more master fault segments responsible for the greatest events in the area and several second order associated faults responsible for the background minor seismicity therefore major faults and background seismicity should be treated separately nevertheless the oversimplified assumption of homogeneous seismic zones was the price the authors consciously paid to produce in a reasonably short time a homogeneous product relative to the entire national territory suitable for earthquake hazard evaluation and for decisions regarding risk mitigatiton

outflow channels with deltaic deposits in ismenius lacus mars abstract a connected series of outflow channels in the ismenius lacus quadrangle are identified for the first time and characterized using high resolution stereo camera images of mars express the context camera images of mars reconnaissance orbiter and the topography of the mars observer laser altimeter these channels named okavango valles which stretch over 400 km south to north and join the northern plains were identified from braided channels scour groove marks poorly sinuous valleys and depositional landforms discharge rates were estimated to 0 1 5 10 6 m 3 s 1 from analysis of the topography of scour marks pathways of channels segments were extracted from topography showing a unique source at a breached crater rim suggesting overflow from ponded depressions a series of delta fans are observed inside depressions along the channel pathways the presence of these deltas formed in former bodies of water is a compelling argument for formation of this outflow channel system by fluvial flows the similarity of these flows with other outflow channels on mars proves that volcanically related outflows cannot explain all such features in addition this study also shows that catastrophic floods are able to create fan deltas in transient lakes this example has to be taken into account in the interpretation of late stage fans associated with poorly branched valleys and single channels

toward a robust normalized magnetic paleointensity method applied to meteorites abstract we propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs alternating field rem method it provides an estimate of the absolute paleointensity with an uncertainty of about a factor two contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization it is applicable to multicomponent magnetizations artificial or natural isothermal remanent magnetizations can be recognized and relaxation of natural remanent magnetization can be taken into account it is applicable for magnetizations carried by magnetite feni alloys and pyrrhotite the rem method is of particular interest to estimate paleointensities in meteorites as these materials are often characterized by complex multicomponent magnetization of various origins impact magnetization thermoremanence isothermal remanent magnetization and metastable magnetic minerals not suitable for thellier experiments however like other paleointensity methods the rem method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale for l ordinary chondrites an upper limit of 1 t is proposed for the paleofield tentative paleofield estimates in the 0 05 0 5 t range are proposed for ll ordinary chondrites which is much lower than previous results the first paleofield estimates for rumuruti chondrites indicate the presence of magnetic fields around 6 t during the last major impact on their parent body aubrites and hed achondrites paleofields of at least 10 t may suggest the existence of a dynamo field during the cooling of their parent body results from martian meteorites are scattered between 1 and 24 t and may represent the crustal magnetic field of the planet after dynamo shutdown carbonaceous chondrites provide contrasting results with possible evidence of strong field mt processes that clearly require additional studies

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

geomagnetic field strength 3 2 billion years ago recorded by single silicate crystals analysis of silicate rocks from the archaean kaapvaal craton is used to obtain palaeodirections and intensities of the earth s magnetic field some 3 2 billion years ago and finds that the field strength was within 50 per cent of the present day value indicating that a viable magnetosphere sheltered the early earth s atmosphere from solar wind erosion

magnetic properties of olivine basalt application to mars abstract the natural remanent magnetisation nrm of basalt lava containing oxidised magnetic phases is usually assumed to be proportional to the weight percent of magnetic phases it is shown here that olivine basalt has a different behaviour the nrm intensity increases at an increasing rate with the amount of magnetic phases this is attributed to the oxidation of olivine during cooling of the basalt that leads to the exsolution of magnetite in a single domain state in this way olivine basalt is found to become an order of magnitude more magnetic than basalt that does not contain olivine a simple explanation for the magnetic anomalies on mars is offered based upon these findings and mossbauer spectroscopy data from gusev crater on mars

the holocene paleoearthquakes on the 1915 avezzano earthquake faults central italy implications for active tectonics in the central apennines abstract extensive paleoseismological research was conducted in the fucino plain central italy in order to better understand the seismogenic characteristics of the fault responsible for the 1915 avezzano earthquake ms 7 0 and improve the general knowledge regarding active tectonics in the central apennines evidence for late pleistocene holocene surface faulting events was obtained through the study of thirteen sites across four different fault branches the paleoseismological analysis outlined the occurrence of ten surface faulting events in the past 33 000 years seven of which occurred during the holocene radiocarbon thermoluminescence and archaeological dating permitted the definition of an event chronology and the estimation of a recurrence interval for surface faulting events ranging between 1400 and 2600 years on the basis of the observed offsets it was possible to calculate vertical slip rates for the individual fault branches ranging between 0 24 mm yr 1 and 0 5 mm yr 1 and an extension rate across the fucino plain ranging between 0 6 and 1 mm yr 1 the chronology of surface faulting events appears linearly distributed in time and the observed recurrence times are similar to those inferred by other paleoseismological studies along different active apennine faults the general consequence of the large time span between strong earthquakes is that even a 2000 year long historical record such as the italian earthquake catalogue does not cover the entire seismic cycle of all the active apennine faults a comparison of the extension rate across the fucino plain with that across the entire central apennines as inferred from the sum of seismic moments of earthquakes which occurred in the interval 1000 1992 shows a seismicity deficit in the period indicated this confirms that a number of seismogenic faults were not active during the past 1000 years

structure and petrology of peridotites clues to their geodynamic environment the first part of this paper is a review on the main mineralogical petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth s surface these criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history the unifying concept is that of adiabatic ascent rate related to rifting and spreading rates rates lower than 0 5 cm yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km this inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue higher rates probably not exceeding 1 cm yr correspond to oceanic rifts with the main melt extraction completed around 15 km generating a thin oceanic crust and plagioclase lherzolites as residue finally rates greater than 1 2 cm yr correspond to oceanic ridges with melt being extracted at moho depth thus generating a 6 km thick crust and leaving depleted harzburgites as residue thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin this conclusion must however be tempered by the fact that the spreading rate envisioned here as the principal controlling parameter can change with space and time in a given environment mantle diapirism rifting seafloor spreading crust generation

the magnetic properties of natural and synthetic fex mg1 x 2 sio4 olivines article i nfo olivine is an abundant orthosilicate in the solar system in the upper mantle of rocky planets in meteorites and interstellar dust the magnetic properties of fex m g1 x 2 sio4 olivines result from the silicate matrix and its iron rich inclusions and have not always been separated in previous studies the properties of the matrix are important to understand mantle rocks anisotropy and their deformation both in xenoliths and peridotite massifs while the inclusions are potential paleomagnetic and paleointensity recorders in this study we performed new measurements on 7 natural and 23 synthetic ferromagnesian olivines covering the whole range from forsterite mg2sio4 fo100 to fayalite fe2sio4 fo0 and from 4 to 310 k many of our specimens contain ferromagnetic inclusions magnetite or maghemite with magnetic sizes ranging from super paramagnetic tomultidomain the respectivecontributions of the matrix and the inclusions are systematically isolated using magnetic fields large enough to saturate the ferromagnetic component due to inclusions at room temperature as predicted by molecular field theory while the forsterite end member is diamagnetic xhf 6 8 10 10 m 3 kg and the fayalite end member is paramagnetic xhf 1 10 10 6 m 3 kg their magnetic properties do not vary linearly with iron content these olivines also exhibit one or two magnetic transitions at composition dependent low temperatures at the neel temperature tn olivines exhibit a first magnetic transition from paramagnetic to antiferromagnetic behavior indicated by negative paramagnetic curie temperatures a second transition tt occurs at lower temperatures for specimens with x 0 1 and could be attributed to a change from collinear to canted antiferromagnetic state in the range fo83 fo93 corresponding to the earth s upper mantle the anisotropy of magnetic susceptibility ams is directionally consistent with the ams principal axes k1 k2 and k3 respectively corresponding to the a c and b crystallographic axes at room temperature the degree of anisotropy increases from 1 028 to 1 302 with decreasing iron content the presence of small and submicroscopic iron rich inclusions significantly complicates the investigation of olivine physical properties but also constitutes opportunities to record paleomagnetic field intensities

influence of static alternating field demagnetization on anisotropy of magnetic susceptibility experiments and implications anisotropy of magnetic susceptibility ams indicates the preferred orientation of a rock s constituent minerals however other factors can influence the ams e g domain wall pinning or domain alignment in ferromagnetic minerals therefore it is controversial whether samples should be alternating field af demagnetized prior to ams characterization this may remove the influence of natural remanent magnetization nrm or domain wall pinning on ams however it may also result in field induced anisotropy this study investigates the influence of stepwise af and low temperature demagnetization on mean susceptibility principal susceptibility directions ams degree and shape for sedimentary metamorphic and igneous rocks alternating fields up to 200 mt were applied along the sample x y and z axes rotating the order for each step to characterize the relationship between ams principal directions and the last af orientation the changes in anisotropy defined by the mean deviatoric susceptibility of the difference tensors are between 2 and 270 of the ams in nrm state variations in ams parameters range from small changes in shape to complete reorientation of principal susceptibility axes with the maximum susceptibility becoming parallel to the last af direction this is most prevalent in samples with low degrees of anisotropy in the nrm state no clear correlations were found between field induced anisotropy and hysteresis properties therefore we propose that future studies check any samples whose ams is carried by ferromagnetic minerals and low anisotropy degrees for af induced artifacts these results highlight the need for understanding the ams sources and carriers prior to any structural interpretation

active crustal extension in the central apennines italy inferred from gps measurements in the interval 1994 1999 we present the first gps estimate of crustal extension in the central apennines italy through the analysis of the deformation of a sub network of the national gps geodetic network igm95 in the interval 1994 1999 the selected sub network spans the entire active deformation belt perpendicularly to its axis and allows the evaluation of 1 the total extension rate absorbed in this sector of the apennines and 2 the seismogenic potential of the normal faults active in the late pleistocene holocene interval within the network results of this reoccupation are consistent with an extensional strain rate of 0 18 10 6 yr 1 concentrated in an area of about 35 km width giving an average extension rate of 6 2 mm yr across the central apennines the pattern of active deformation suggests active elastic strain accumulation on the westernmost of the two fault systems active in the late pleistocene holocene interval and may also suggest the presence of another active fault system not recognized so far

the magnetic anisotropy of mantle peridotites example from the twin sisters dunite washington abstract the twin sisters dunite massif washington state provides unweathered peridotites for magnetic analysis these rocks have been chosen to test a new magnetic fabric based on measuring the anisotropy of magnetic susceptibility ams in high field with a vibrating sample magnetometer although the dunites are fresh they host small quantities of secondary magnetite formed along cracks under low field conditions magnetite masks the magnetic fabric of olivine and thus the magnetic fabric does not reflect mantle flow deformation above the magnetic saturation at a field of 1 t the ferrimagnetic component is removed and the high field slope represents the paramagnetic ams only this high field ams originates from the magnetocrystalline anisotropy of olivine the high field ams are compared with lattice preferred orientation lpo fabric data obtained by electron backscatter diffraction lpo point distributions are formed by high temperature plastic flow and match the high field measurements in contrast girdle lpos present notable departures from the high field ams axes

valley networks on venus abstract valley networks on venus are classified as rectangular labyrinthic and pitted or irregular the venusian valley networks are structurally controlled as indicated by the morphological patterns of valley branches consistency between valley and fracture orientations and associations with the deformed terrains the morphologies resemble those of terrestrial and martian sapping valleys valley networks on venus probably formed initially from fracture systems and became enlarged by low viscosity lava sapping processes subsurface flow of lava may locally have been assisted by surface flows the lavas probably moved through permeable media and fractures venusian valley networks have a higher degree of network integration than do lunar sinuous rilles but they are less integrated than martian and terrestrial sapping valleys the viscosity of valley forming lavas must have been very low but was not low enough to exploit the permeable media so extensively as to attain a high degree of network integration the compositions of these lavas may have been mafic to ultramafic or mafic alkaline alternatively the lavas could have had more exotic compositions such as carbonatite and sulfur valley networks are often associated with corona and corona like features which are hypothesized to be the surface expressions of mantle plumes a plume association could mean that the lavas came from the mantle

fluvial geomorphology on earth like planetary surfaces a review morphological evidence for ancient channelized flows fluvial and fluvial like landforms exists on the surfaces of all of the inner planets and on some of the satellites of the solar system in some cases the relevant fluid flows are related to a planetary evolution that involves the global cycling of a volatile component water for earth and mars methane for saturn s moon titan in other cases as on mercury venus earth s moon and jupiter s moon io the flows were of highly fluid lava the discovery in 1972 of what are now known to be fluvial channels and valleys on mars sparked a major controversy over the role of water in shaping the surface of that planet the recognition of the fluvial character of these features has opened unresolved fundamental questions about the geological history of water on mars including the presence of an ancient ocean and the operation of a hydrological cycle during the earliest phases of planetary history other fundamental questions posed by fluvial and fluvial like features on planetary bodies include the possible erosive action of large scale outpourings of very fluid lavas such as those that may have produced the remarkable canali forms on venus the ability of exotic fluids such as methane to create fluvial like landforms as observed on saturn s moon titan and the nature of sedimentation and erosion under different conditions of planetary surface gravity planetary fluvial geomorphology also illustrates fundamental epistemological and methodological issues including the role of analogy in geomorphological geological inquiry

anisotropy of magnetic susceptibility in natural olivine single crystals mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties to interpret anisotropy in mantle rocks it is necessary to understand the anisotropy of olivine single crystals we determined anisotropy of magnetic susceptibility ams for natural olivine crystals high field ams allows for the isolation of the anisotropy due to olivine alone the orientations of the principal susceptibility axes are related to the olivine s crystallographic structure as soon as it contains 3 wt feo the maximum susceptibility is parallel to the c axis both at room temperature rt and at 77 k the orientation of the minimum axis at rt depends on iron content it is generally parallel to the a axis in crystals with 3 5 wt feo and along b in samples with 6 10 wt feo the ams ellipsoid is prolate and the standard deviatoric susceptibility k0 is on the order of 8 10210 m3 kg for the samples with 1wt feo and ranges from 3 1 1029 m3 kg to 5 7 1029 m3 kg for samples with 3 10 wt feo at 77 k the minimum susceptibility is along b independent of iron content the shape of the ams ellipsoid is prolate for samples with 5 wt feo but can be prolate or oblate for higher iron content the degree of anisotropy increases at 77 k with p0 7757 160 5 the results from this study will allow ams fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content

petrology of the yamato nakhlites the yamato nakhlites y 000593 y 000749 and y 000802 were recovered in 2000 from the bare icefield around the yamato mountains in antarctica consisting of three independent specimens with black fusion crusts they are paired cumulate clinopyroxenites we obtained the intercumulus melt composition of the yamato nakhlites and here call it the yamato intercumulus melt yim the yim crystallized to form the augite rims the olivine rims and the mesostasis phases in the cumulates the augite rims consist of two layers inner and outer the crystallization of the inner rim drove the interstitial melt into the plagioclase liquidus field subsequently the residual melt crystallized pigeonites and plagioclase to form the outer rims and the mesostasis three types of inclusions were identified in olivine phenocrysts rounded vitrophyric angular vitrophyric and monomineralic augite inclusions the monomineralic augite inclusions are common and may have been captured by growing olivine phenocrysts the rounded vitrophyric inclusions are rare and may represent the composition of middle stage melts whereas the angular vitrophyric inclusions seem to have been derived from fractionated late stage melts glass inclusions occur in close association with titanomagnetite and ferroan augite halo in phenocryst core augites and the assemblages may be magmatic inclusions in augites we compared the yim with compositions of magmatic inclusions in olivine and augite the composition of magmatic inclusions in augite is similar to the yim phenocrystic olivines contain exsolution lamellae augite magnetite aggregates and symplectites in the cores the symplectites often occur at the boundaries between olivine and augite grains the aggregates symplectite and lamellae formed by exsolution from the host olivine at magmatic temperatures we present a formational scenario for nakhlites as follows 1 accumulation of augite olivine and titanomagnetite phenocrysts took place on the floor of a magma chamber 2 olivine exsolved augite and magnetite as augite magnetite aggregates symplectites and lamellae 3 the overgrowth on olivine phenocrysts formed their rims and the inner rims crystallized on augite phenocryst cores and finally 4 the outer rim formed surrounding the inner rims of augite phenocrysts and plagioclase and minor minerals crystallized to form mesostasis under a rapid cooling condition probably in a lava flow or a sill

a pyroclastic flow deposit on venus explosive volcanism on venus is severely inhibited by its high atmospheric pressure and lack of water this paper shows that a deposit located near 16 s 144 e here referred to as scathach fluctus displays a number of morphological characteristics consistent with a pyroclastic flow deposit these characteristics particularly its lack of channelisation and evidence for momentum rather than cooling limited flow length contrast with fissure fed lava flow deposits the total erupted volume is estimated to have been between 225 km3 and 875 km3 but this may have been emplaced in more than one event interaction between scathach fluctus and a small volcanic cone constrain the flow velocity to 48 m s 1 and plausible volatile concentrations to at least 1 8 wt h2o 4 3 wt co2 or 6 1 wt so2 the latter two implying magma sourced directly from the mantle the deposit has radar characteristics particularly an exponential backscatter function that are similar to those of nearly half the planetary surface implying that pyroclastic deposits may be much more common on venus than has been recognised to date and suggesting both a relatively volatile rich mantle and a volcanic source for atmospheric so2

palaeoseismicity in the gran sasso massif abruzzo central italy abstract the upper maone and venacquaro valleys on the northern side of the gran sasso massif apennines central italy have been studied in detail from the geological and geomorphological standpoints to date the activity of the faults that produced spectacular fault scarps in the late glacial and holocene sediments and morphological features the faults that produced the studied scarps have been active at least four times during the last 18 000 years and each time on the basis of the indications emerging from the study of the scarp heights displacements of similar size were probably produced the first faulting seems datable to a period slightly more recent than 18 000 years ago the second faulting occurred after the moraines of the second apennine stage these preceding a lacustrine deposit dated 11 760 160 bp could be put in the older portion of the period between 16 000 and 13 000 years ago the third faulting must be more recent than 6000 7000 years ago and slightly older than 5450 170 and 6110 180 bp a fourth phase is appreciably later than 3490 160 bp and older than the year 1000 a d for the venacquaro valley the size of the displacements more recent than 18 000 years ago indicated by the height of the fault scarps and by geophysical data is about 16 18 m for the val maone the size of the displacements more recent than the moraines of the second apennine stage is 10 11 m the respective slip rates are 0 88 1 mm year for the venacquaro valley and approximately 0 67 0 78 mm year for the maone valley it is very probable that the fault activity occurred during a seismic event or events that took place during a short time the intervals between the seismic events or seismic periods although these cannot be defined with precision appear longer than 2500 3000 years and shorter than 6000 7000 years it is possible that also in the gran sasso as in other parts of the abruzzo apennines seismic activity is characterized by events of great magnitude with long return periods

magnetic properties of fault pseudotachylytes in granites 1 we investigate the petrographic geochemical and magnetic properties of fault pseudotachylytes formed by frictional melting in granitic rocks from southern california the italian alps and kyushyu japan the main magnetic remanence carriers are mixtures of grain sizes of fine grained magnetite these ferrimagnetic grains record a stable multicomponent magnetization that consists of one or more of the following coseismic thermal remanent magnetization coseismic lightning isothermal remanent magnetization and post seismic chemical remanent magnetization fault pseudotachylytes from the three localities display contrasting magnetic properties which suggests that oxygen fugacity and host rock composition ultimately control the magnetic assemblage

new paleoseismological data from the gran sasso d italia area central apennines 1 paleoseismological analyses along the campo imperatore normal fault cif in the gran sasso massif which is an area characterized by the absence of significant historical earthquakes highlight the occurrence of surface faulting after the 5th 3rd cent bc and around the 6th 5th millenium bc these ages agree with those reported by giraudi and frezzotti 1995 on the western tip of the cif thus suggesting the possible entire rupture of the 30 km long cif during m7 events these data contribute to the re evaluation of the seismic hazard of a large surrounding area including the city of l aquila 100 000 inhabitants which is located 20 km from the cif in the hangingwall side index terms 7221 seismology paleoseismology 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 8107 tectonophysics continental neotectonics

the influence of t asio2 and fo2 on exsolution textures in fe mg olivine an example from augite syenites of the ilimaussaq intrusion south greenland two samples from the augite syenite unit of the 1 13 ga alkaline to peralkaline ilimaussaq intrusion in south greenland contain olivine fa 75 85 with micrometer sized exsolution lamellae of intergrown augite and magnetite the exsolved olivine contains only about 0 6 wt cao whereas the reintegrated magmatic composition contained about 1 4 cao corresponding to about 3 mol of a larnite la component the exsolved olivine bears no measurable na al or ti but the reintegrated magmatic olivine contained up to 0 4 wt na 2 o 0 2 wt al 2 o 3 and 0 3 wt tio 2 magmatic baddeleyite in the same samples indicates an extremely low silica activity of below 0 4 during the early stages of magmatic crystallization at 900 c and 1 kbar according to the equilibrium ca 2 sio 4 la in ol sio 2 2 casio 3 wo in cpx the low silica activity is thought to be responsible for the unusually ca rich compositions of the primary olivines which in these samples formed at 875 15 c and at oxygen fugacities between 1 2 and 2 3 log units below the qfm buffer the composite exsolution lamellae formed during cooling due to an overstep of the schematic reaction fa kst ca rich component in olivine 1 3 o 2 hd 2 3 mt phase equilibria indicate temperatures between 300 and 750 c and oxygen fugacities close to qfm for this exsolution process the lamellae only developed in samples where the olivines were both especially high in ca and presumably rapidly cooled although not chilled as the lamella bearing samples were collected at the outermost rim of the augite syenite unit and close to the top of the ilimaussaq intrusion schreinemakers analysis shows that ca rich olivine of the kirschsteinite monticellite solid solution series ca fe mg sio 4 would exsolve from ca bearing olivine of the fayalite forsterite series only at unusually low silica activities whereas at higher silica activities the observed exsolutions of clinopyroxene with magnetite are more stable hence both cooling history and evolution of intensive variables during cooling are important for the formation of olivine exsolution textures

magnetic petrofabric of igneous rocks lessons from pyroclastic density current deposits and obsidians abstract measurement of the anisotropy of magnetic susceptibility ams of igneous rocks can provide clues concerning their mechanism of formation and in particular are very helpful as flow direction indicators unlike other igneous rocks however pyroclastic density current deposits pdcds present a challenge in the interpretation of ams measurements due to the complexity of their mechanism of emplacement in this paper we review the most common assumptions made in the interpretation of the ams of pdcd taking advantage of key lessons obtained from obsidians despite the complexities on the mechanism of formation of pdcds it is shown that a key element for the fruitful interpretation of ams is to give proper attention to the various components likely to be involved in controlling their general petrofabric the anisotropies of ferromagnetic crystals whether as free phases or embedded within clasts or shards and those of paramagnetic minerals mainly ferrosilicates need to be taken into consideration when interpreting the ams measurements of pdcds variations of the deposition regime both as a function of position and of time also need to be considered on the interpretations nevertheless if a suitable sampling strategy is adopted the potential of the ams method as a petrofabric indicator is maximized

coseismic magnetization of fault pseudotachylytes 1 thermal demagnetization experiments fault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine grained magnetite this confirms previous studies on friction melt experiments stoichiometric magnetite produced during earthquakes by the breakdown of ferromagnesian silicates records the ambient magnetic field during seismic slip we find that most fault pseudotachylytes exposed in the santa rosa mountains southern california a classic pseudotachylyte locality acquired their natural remanent magnetization nrm upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization nrm intensities typical of most pseudotachylyte veins range from 1 to 60 10 4 am2 kg a few specimens however contain magnetizations significantly higher than that caused by the earth s field as well as magnetization directions that are highly variable over short distances other magnetization processes possibly related to coseismic electric currents may be involved during the seismogenic process to control nrm acquisition

evidence for extensive olivine rich bedrock on mars color infrared observations of an area around the nili fossae acquired by the mars odyssey thermal emission imaging system have been used to map the regional distribution and geologic context of olivine rich materials that first were identified from mars global surveyor thermal emission spectrometer data spectral thermal geomorphic and topographic data demonstrate that the majority of olivine rich material is in the form of inplace layered rock covering an area roughly four times larger than previously recognized some high olivine concentrations are associated with spatially small areas dominated by sediments we favor olivine rich basalts as the origin of the observed olivine enrichment although several hypotheses are viable

quantitative 3d measurement of ilmenite abundance in alpe arami olivine by confocal microscopy confirmation of high pressure origin a critical aspect of the debate about the origin and conditions of metamorphism of the alpe arami aa peridotite is the disagreement over how much ilmenite is contained in the older generation of olivine and therefore how much tio 2 might have been dissolved at high pressure and temperature we have now determined quantitatively the 3 dimensional distribution of ilmenite in aa olivine by confocal laser scanning microscopy clsm the clsm measurements show an average concentration of 0 31 vol of ilmenite in olivine with individual grains containing up to 1 2 vol this translates into average and maximum concentrations of 0 23 and 0 9 wt tio 2 in olivine respectively and confirms the original estimation of maximum concentration of 1 vol tio 2 the vast majority of ilmenite in aa olivine is distributed randomly although topotactically oriented and in all cases is accompanied by chromite in a ratio of 4 1 these observations are consistent with an origin of the ilmenite and chromite by exsolution from an olivine solid solution at p 9 12 gpa and temperatures above the stability field of titanian clinohumite but are not consistent with suggested breakdown of titanian clinohumite combining these results with other recent findings suggests that exsolution followed deformation under relatively high fugacity of h 2 o and that the high solubility of tio 2 is probably explained by pressure induced accommodation of ti in the tetrahedral site of silicates

deformation and magnetic fabrics in ductile shear zones a review abstract the anisotropy of magnetic susceptibility ams is a well established petrofabric tool for indicating relative strain and microstructural character and has been validated on various rock types and different structural settings the magnetic susceptibility of a rock k depends primarily on the nature and abundance of magnetic minerals the physical arrangement and lattice preferred orientation of these magnetic minerals give rise to magnetic anisotropy the ams scalar parameters most commonly used to constrain strain include the corrected degree of anisotropy p 1 a proxy for fabric intensity and the shape factor 1 t 1 an indicator of the magnetic fabric symmetry prolate vs oblate a number of studies have shown that a positive correlation generally exists between p and strain thus the ams shows a great potential as a tool for examining deformation in geologic structures characterized by large strain gradients such as shear zones however a number of caveats exist i the increase of p with strain cannot be solely attributed to deformation because p also increases with k regardless of deformation ii strain across shear zones is typically heterogeneous and is often localized in units of different lithology thus making the separation of the lithological and strain controls on ams difficult also deformation is commonly accompanied by mineral segregation or fluid rock interaction that induces changes in magnetic mineralogy iii even if the undeformed lithology was uniform across a shear zone variations in strain rate or temperature may result in different deformation mechanisms hence the relationship between p and strain depends strongly on both the mineral carriers of ams and on deformation mechanisms and iv the ams is unable to resolve composite fabrics such as those resulting from s c structures where minerals on the c and s planes respectively contribute to ams

tracing deep fluid source contribution to groundwater in an active seismic area central italy a combined geothermometric and isotopic δ13c perspective abstract an understanding of the origin and migration pathways of geofluids in an active seismic area is of paramount importance in terms of societal challenges such as mitigation of seismic hazards this study investigates the relationship between the stable isotope ratio 13c 12c of deep co2 and geothermometry in selected groundwater samples located close to extensional faults in central italy the temperature range is inferred from an equation based on the na li ratio and chemical geothermal modelling 113 130 c globally the calculated temperature range agrees with that resulting from deep boreholes in the northern apennines an alternative method is also included to better evaluate the difference in isotopic composition between the aqueous and gaseous 13c co2 at deep condition a review of previously published data shows that monitoring of the isotopic composition of the dissolved inorganic carbon dic in springs fed by meteoric origin located in seismically active areas must take into account mass and isotopic balance to correctly evaluate the component and the role of deep fluid during seismic events in particular the coupled and corrected isotope and geothermometrical monitoring of the springs could help to distinguish between deep gas or deep fluid contributions to shallow aquifers the results of this study indicate that faults play a crucial role in controlling the migration of crustal fluids in addition they reveal that possible evaluation of potential seismic precursors mandatorily requires a long period of monthly monitoring

magnetic anisotropy of chloritoid 1 the magnetocrystalline anisotropy of monoclinic chloritoid a relatively common mineral in aluminum rich metapelitic rocks has been determined for the first time by measuring the high field anisotropy of magnetic susceptibility hf ams using two independent approaches i e a directional magnetic hysteresis measurements and b torque magnetometry on a collection of single crystals collected from different tectonometamorphic settings worldwide magnetic remanence experiments show that all specimens contain ferromagnetic s l impurities being mainly magnetite the determined hf ams ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c axis of chloritoid in the basal plane of chloritoid though the hf ams can be considered isotropic the degree of anisotropy is found to be 1 47 which is significantly higher than the anisotropy of most paramagnetic silicates and even well above the frequently used upper limit i e 1 35 for the paramagnetic contribution to the ams of siliciclastic rocks the obtained values for the paramagnetic curie temperature parallel and perpendicular to the basal plane indicate that this pronounced magnetocrystalline anisotropy is related to strong antiferromagnetic exchange interactions in the direction of the crystallographic c axis 0 as a consequence chloritoid bearing metapelites with a pronounced mineral alignment can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic s l minerals the newly discovered magnetocrystalline anisotropy of chloritoid thus calls for a revised approach of magnetic fabric interpretations in chloritoid bearing rocks

enhanced magnetization of the marlboro clay as a product of soil pyrogenesis at the paleocene eocene boundary abstract the kaolinite rich marlboro clay was deposited on the inner shelf in the salisbury embayment of the u s atlantic margin at the onset of the carbon isotope excursion marking the 56 ma paleocene eocene boundary and is characterized by an anomalously high concentration of magnetic nanoparticles of enigmatic origin that give rise to notably intense bulk magnetization recent studies point to a magnetic assemblage that is dominated by single domain magnetite particles that tend to be isolated rather than arranged in chains the most distinguishing feature of magnetotactic bacteria fossils on the other hand it is very unlikely that the nanoparticles can be condensates of an impact plume given the meter scale thickness of the marlboro clay we obtained new data from a landward proximal site at wilson lake on the new jersey coastal plain and find that the abrupt increase in magnetite nanoparticles is virtually coincident stratigraphically with the recently reported impact spherule layer at the base of the marlboro clay in the same core yet the high field magnetic susceptibility a measure of total iron concentration and strontium isotope values on bulk sediment an indicator of sediment weathering provenance are not different in the marlboro clay from the immediately underlying vincentown formation we suggest that the distinctive magnetic properties of the marlboro clay originated from pyromagnetic soil enhancement by widespread wildfires on the adjoining drainage area the pyrogenetic products were soon washed from the denuded landscape and rapidly deposited as mud waves across the shelf becoming the marlboro clay a few percent of incinerated biomass ends up as calcite known as wood ash stone and can inherit its light carbon isotope composition disseminated wood ash stone entrained in the marlboro clay could contribute to the landward increase in amplitude of the carbon isotope excursion in bulk carbonate data a plausible trigger for the initial conflagration is a fireball from the impact of a sizable extraterrestrial object at moderate range

a method for the separation of paramagnetic ferrimagnetic and haematite magnetic subfabrics using high field torque magnetometry summary in this study the contribution of the paramagnetic ferrimagnetic and haematite components to the magnetic anisotropy is separated by means of high field torque magnetometry torque measurements at different fields which are high enough to saturate the ferrimagnetic minerals however still low enough that the torque resulting from the haematite is linear with field allow for the separation of the three magnetic anisotropy components the method has been applied to haematite single crystals in which no paramagnetic or ferrimagnetic components have been found contribute to the torque signal the mean direction of the poles to the crystallographic basal plane in the haematite single crystals is subparallel to the minimum susceptibility direction measured in low field the separation analysis has also been applied to highly deformed red beds from the lower glarus nappe complex switzerland no ferrimagnetic phases are present in the rocks and therefore they cannot contribute to the anisotropy of magnetic susceptibility the magnetic fabric arises from a paramagnetic subfabric carried by the phyllosilicate minerals and haematite in which the basal planes of both phases are in the cleavage plane the measured magnetic lineation seen in low field anisotropy of magnetic susceptibility appears to be an apparent lineation that arises from a weak girdling of haematite and the paramagnetic minerals conforming the rock

magnetic fabric interpretation complicated by inclusions in mafic silicates abstract the expected relationships between anisotropy of low field magnetic susceptibility ams and crystal symmetry of mafic silicates are disturbed by the presence of magnetite inclusions measurements of ams anisotropy of anhysteretic remanence aarm and theoretically predicted bulk susceptibilities from chemical composition all favour the exercise of great caution in the interpretation of preferred orientation distributions of silicates from a rock s ams these results pertain mainly to the mafic silicates of lower crustal rocks pyroxene orthopyroxene amphibole and some of their low grade metamorphic alterations serpentine epidote

late glacial and holocene aeolian deposits and geomorphological features near roccaraso abruzzo central italy abstract aeolian deposits and geomorphological features due to wind erosion have been recorded in the aremogna plain near roccaraso central apennines at this site three different aeolian deposits can be distinguished 1 silty loess deposits composed mainly of quartz 2 aeolian sand deriving from the reworking of tephra which lies on a peat level dated to 12 850 200 bp and 3 aeolian silt coming from the deflation of the deposits 2 which is associated to deflation geomorphological features in part still active the onset of these processes is more recent than 4690 200 bp the first and the second aeolian deposits are related to cold and dry phases of the late glacial period the third set of aeolian deposits and deflation features corresponds to wind erosion on a denudated landscape as a consequence of a dry climatic phase the effect of which was amplified by winter freezing thawing activity

low field variation of magnetic susceptibility as a tool for magnetic mineralogy of rocks abstract low field variation of magnetic susceptibility was investigated on a collection of several hundreds specimens of various minerals and rocks using the kly 4s kappabridge the measurement is fully automated being executed in 21 distinct fields ranging from 2 to 450 a m in one frequency of 875 hz the measurement is rapid 7 min per specimen so that large collections of specimens can be investigated the results can be processed both graphically and mathematically for the latter processing parameters of two kinds were introduced one characterizes the susceptibility change with field the other one characterizes the field in which the susceptibility no longer obeys the rayleigh law and starts becoming more complex the results were evaluated statistically remarkable differences were revealed between individual minerals and between some rock types for example the field variation of susceptibility of pyrrhotite is in general an order of magnitude larger than that of titanomagnetite the susceptibility increase in pyrrhotite starts at the field an order of magnitude lower than that of titanomagnetite low field variation of susceptibility then appears as an interesting phenomeon that helps in the identification of magnetic minerals and in some cases also in assessing the compositional variation of them