on the reliability of mesozoic dipole low new absolute paleointensity results from parana flood basalts brazil 1 thellier paleointensity experiments were carried out on early cretaceous parana flood basalts forty two samples from 11 lava flows yielded apparently reliable absolute intensity determinations the mean paleointensity values per flow obtained in this study are ranging from 19 4 4 0 to 46 7 7 0 t and the corresponding virtual dipole moments are ranging from 4 0 0 6 to 10 5 1 1 10 22 am 2 these yield a mean value of 7 2 2 3 x 10 22 am 2 which is about 92 of the present geomagnetic axial dipole our results in conjunction with tarduno et al s 2001 data suggest that the paleostrength during early cretaceous period might be comparable or even much higher than recent field intensity and not anomalousely low as previously suggested thus the mesozoic dipole low mdl may probably be considered as unreliable

directions and intensities of the earth s magnetic field during a reversal results from the permo triassic siberian trap basalts russia abstract an extensive palaeomagnetic study was carried out on an approximately 250 ma old reversed to normal transition of the earth s magnetic field emf recorded in 86 volcanic lava flows of the siberian trap basalts north siberia russia in addition to the investigation of the directional behaviour of the field 700 specimens a total of 298 specimens was subjected to thellier type palaeointensity determinations adding several modifications to the original thellier experiment such as tests for md tails and the additivity of partial thermoremanent magnetisation yields highly reliable palaeointensity estimates of the late permian early triassic emf transitional directions of the emf were obtained from 20 flows during the reversal a clustering of the virtual geomagnetic poles vgps is observed 15 flows palaeointensity estimates suggest that this feature is not an artifact due to rapid flow emplacement since the directional cluster is associated with a well defined increase in palaeointensity from 6 to 13 t subsequently the next vgps move towards the pole position of normal polarity departing in a sudden movement from normal polarity the vgps form a second directional cluster comprising the results of 14 flows this feature is interpreted as a post transitional excursion but lacks the characteristic intensity variation recorded during the first transitional cluster the rest of the section 41 flows is of normal polarity the characteristic features of this reversal low intensities and directional clustering during the reversal and an excursion shortly after the reversal were also observed in records of polarity transitions of younger age this suggests that the underlying reversal processes were similar the mean virtual dipole moment calculated for the stable normal part of the studied section yields a rather low value of 2 2 0 9 1022 am2 these findings confirm that the mesozoic dipole low extends at least down to the late permian early triassic the geomagnetic vgp dispersion was calculated for the stable normal part of the section and yields values similar to those for the last 5 ma considering that the intensity of the emf was considerably higher in this time interval than in the early triassic a direct relationship between intensity and secular variation seems unlikely

landsliding and the evolution of normal fault bounded mountains much of the tectonic and climatic history in high relief regions such as the mountains of the western u s basin and range province is contained in the morphology of hillslopes drainage networks and other landforms that range in scale from 10 1 to 101km to understand how these landforms evolve we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules bedrock landsliding long recognized as a significant geomorphic process in mountainous topography is for the first time explicitly included in the rule set in a series of numerical experiments we generate synthetic landscapes that closely resemble mountainous topography observed in the basin and range the production of realistic landscapes depends critically on the presence of bedrock landslides and landsliding yields rates of long term erosion that are comparable in magnitude to those of fluvial erosion the erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate limiting process in steady state experimental landscapes our experiments generate power law distributions of landslide sizes somewhat similar to both field and laboratory observations thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal fault bounded ranges

does the permo triassic geomagnetic dipole low exist studies of absolute paleointensity have been conducted on specimens from permo triassic sills from the eastern areas of the kimberlite pipes sytikanskaya yubileinaya and aikhal and northwestern extrusions near noril sk city parts of the siberian platform a total of 341 specimens have been subjected to a modified thellier thellier technique which included partial thermoremanent magnetization checks and multidomain tail checks the paleomagnetic directions derived from the present set of experiments d 107 5 i 75 5 k 82 8 a95 5 3 n 10 sills for the eastern localities and d 95 1 i 71 8 k 35 2 a95 15 7 n 4 outcrops for the northwestern locality agree with the directions from previous studies the specimens from the northwestern sites noril sk did not provide any suitable result of absolute paleointensity the mean virtual dipole moment obtained from only the eastern localities has a value close to 6 01 1 45 10 22 am 2 which is over 50 higher than the results obtained by heunemann et al 2004 and shcherbakova et al 2005 on northwestern specimens the discrepancy could possibly be explained by the presence of multidomain grains in northwestern specimens where previous studies were undertaken which would reduce the paleointensity estimates at high temperatures however the present results lead us to conclude that 1 the geomagnetic field intensity at the permo triassic boundary was very close to the observed present day values and as a consequence a dipole low is not characteristic during this period of time 2 the mesozoic dipole low did not extend to the permo triassic boundary contrary to previous studies

geometry and rate of faulting in the north baikal rift siberia we present a detailed morphotectonic analysis of late quaternary faulting in the north baikal rift nbr a region characterized by ranges and basins distributed over more than 800 km along strike in eastern siberia remote sensing techniques spot meteor scenes and aerial photographs are used to map the active fault network which displays a general en echelon distribution from the northern lake baikal to the easternmost basin with 30 km spaced overstepping segments of 10 80 km in length most faults have a dominant dip slip component over their cenozoic history the inherited crustal fabric strongly influences the overall geometry of the rifted basins we use 54 14c ages of postglacial terraces near the foot scarps of the muya basin to date offsets measured inside alluvial fans the last main postglacial event in this area appears to be the early holocene optimum dated at 10 2 ka following the onset of deglaciation at 13 ka using these time constraints a detailed leveling across two terraces offset by the taksimo fault west muya basin shows consistent minimum vertical slip rates of 1 6 0 6 mm yr 1 using 30 other active scarps analyzed in the field we find a lower bound for horizontal velocity of 3 2 0 5 mm yr 1 across the nbr a rate close to the one found in the southern rift from global positioning system measurements we then compare directions of slip vectors from holocene field data and slip directions from earthquake fault plane solutions although local discrepancies appear the mean directions of lesser horizontal stress 3 inverted from theses values are n130 e and n155 e respectively which are comparable within uncertainties and favor a rifting obliquity of 30 40 extrapolating our holocene rates we estimate basin ages younger than those generally believed less than 7 ma and propose a spatial and temporal evolution of rifted basins consistent with experimental models of oblique rifting total amounts of extension and vertical throw 7 and 12 km respectively across major faults appear rather constant from the central to the northern rift these results favor a progressive development of asymmetric grabens in a rift zone that widens with times and they indicate a strong rheological control on deformation which seems enhanced by other contributions than the far field effects of the indo eurasian collision

mineralogical controls on mine drainage of the abandoned ervedosa tin mine in north eastern portugal the ervedosa mine in north eastern portugal has sn bearing quartz veins containing cassiterite and sulphides that cut silurian schists and a sn bearing muscovite granite these veins were mined for sn and as2o3 until 1969 cassiterite the main sn ore has alternate lighter and darker growth zones the darker zones are richer in fe nb ta and ti but poorer in sn than the adjoining lighter zones exsolution blebs of ferrocolumbite manganocolumbite ti ixiolite rutile ilmenite and rare wolframite were found in the darker zones arsenopyrite is the most abundant sulphide and contains inclusions of pyrrhotite bismuth bismuthinite and matildite other sulphides are pyrite sphalerite chalcopyrite and stannite secondary solid phases consisting mainly of hydrate sulphate complexes of al fe ca and mg aluminocopiapite copiapite halotrichite pickeringite gypsum and alunogen meta alunogen occur at the surface of the sn bearing quartz veins and their wall rocks granite and schist while oxides hydroxides arsenates and residual mineral phases albite muscovite and quartz occur in mining tailings toxic acid mine waters acid mine drainage amd which have high conductivity and significant concentrations of as so4 and metal cu zn pb fe mn cd ni and co occur in an area directly affected by the mine surface stream waters outside this area have low conductivity and a ph that is almost neutral metal and as concentrations are also lower stream waters within the impact area have an intermediate composition falling between that of the amd and the natural stream waters outside impact area waters associated directly with mineralised veins must not be used for human consumption or agriculture

drainage basin evolution in the rainfall erosion facility dependence on initial conditions four experiments in alluvial drainage basin evolution were carried out in the rainfall erosion facility ref at colorado state university to investigate the dependence of basin evolution on initial topography basins were initially undissected each experiment began with a unique initial condition representing various end members of relief and hypsometry drainage network development hillslope processes basin denudation and basin response to base level lowering all depended strongly on the initial topography no classic model of drainage network evolution was found to be generally applicable initially planar slopes first developed subparallel channels that extended headward dendritically during an early phase of extension channel incision occurred first in the interior of the basin where saturation overland flow was greatest not at the basin outlet as assumed in most classic models of network development channels widened over time initiating lateral migration and drainage capture in the downslope portion of the watershed before transferring lateral migration upslope planar basins of larger initial gradient grew headward more quickly and become more deeply entrenched inhibiting late stage lateral migration an experiment with initial relief concentrated at a plateau edge evolved in several unique ways a high ratio of subsurface to surface flow gave rise to mass movements at the plateau edge and outlet channels deep channels were quickly cut initially but did not extend far upslope because slope instability undermined channel head migration leaving the plateau undissected and hence very slow to erode these results suggest that the distribution of relief within a basin exerts an important control on drainage network pattern and basin denudation in addition erosional basins may evolve in several distinct modes characterized by particular combinations of hypsometry hillslope processes and mean denudation rate

further evidence for low intensity of the geomagnetic field during the early cretaceous time using the modified shaw method and microwave technique summary we report new absolute palaeointensity estimates using basalts from northeastern china k ar age 125 120 ma using the modified shaw method in conjunction with the microwave technique samples for the palaeointensity experiments were selected mainly based on their good reversibility of thermomagnetic curves and single primary magnetization characteristics using the modified shaw method 28 out of 45 measured samples from 10 cooling units give a virtual dipole moment of 3 1 1 0 10 22 am 2 and the microwave technique using 14 acceptable determinations out of 20 measured give an average value of 2 9 0 9 10 22 am 2 results using both the modified shaw method and the microwave technique demonstrate that the geomagnetic field strength recorded by these lavas was low this is in agreement with previous results of the same time interval obtained by the thellier method with partial thermal remanence p trm checks the fact that different techniques give qualitatively compatible low palaeointensity results provides greater confidence that the weak field features seen just prior to the cretaceous normal superchron cns are the result of the actual field recorded by the basalts as opposed to artefacts of the method analysis this study also demonstrates that the microwave technique can be used for very old basalts

a geochemical characterization of cold water natural acid rock drainage at the zn pb xy deposit yukon canada abstract acid rock drainage ard is considered to be temperature limited due to the diminished activity of fe ii oxidizing microbes at low temperatures nonetheless ard streams are present in cold climates this study presents a geochemical characterization of a cold climate ard creek at the zn pb xy deposit in yukon canada which showed highly elevated concentrations of dissolved zinc up to 475 mg l acid rock drainage at the xy deposit is likely generated via subsurface abiotic and biotic oxidation of sulfide minerals and then exits as seeps at the headwaters of the creek the uppermost reaches of the creek have the lowest ph levels ph 3 3 and highest metal concentrations with prolific precipitation of iron hydroxysulfate and oxyhydroxide mineral precipitates schwertmannite jarosite and goethite present as terraced iron formations tifs at one sampling location the lower reaches of the creek show a progressive ph increase up to ph level 4 9 which occurs due to fe iii and al hydrolysis the neutralizing influence of carbonate rich strata and or ground waters and dilution by surface waters entering the creek progressive ph neutralization causes a change in precipitate mineralogy to x ray amorphous al hydroxysulfates with a composition similar to aluminite and hydrobasaluminite and amorphous al oh 3 natural attenuation of cd zn and pb occurred downstream from the headwater seeps which was likely influenced by adsorption reactions involving both metal sulfate anions and metal sulfate ternary complexes generally the concentrations of cd zn and pb in the mineral precipitates followed trends in ph aqueous concentration predicted speciation and mineralogy of the precipitates however anomalous trace metal concentrations were observed in the precipitates at 120 m downstream and potentially reflect a local ground water input overall the geochemistry of this cold climate ard creek was similar to ard in more temperate climates although low temperatures appear to retard the kinetics of ferrous iron oxidation and seasonal hydrodynamics may influence the formation of tifs in these highly saturated waters

permo carboniferous kiaman palaeointensity results from the western bohemian massif germany summary thellier palaeointensity analyses were carried out on samples from three quartz porphyry intrusions located within the western part of the bohemian massif close to the german czech republic border these intrusives have been dated at 28 275 ma which places them in the main part of the permo carboniferous kiaman reversed superchron p crs this age is consistent with the mean characteristic remanence direction calculated at d z 204 1 giving a vgp at 36 n 341 e palaeointensity values range from 6 6 pt to 22 1 pt with a mean value of 12 0 1 3 pt which has a corresponding vdm value of 3 0 i 1 2 x loz2 a m2 this suggests that the geomagnetic field strength recorded by the quartz porphyries is only 37 per cent of the current value supporting recent studies that report a field strength of between 25 and 40 per cent of the present day value during the p crs

paleointensity of the earth s magnetic field in early cretaceous time the parana basalt brazil paleointensity experiments were carried out with the coe and thellier methods on early cretaceous basalt 133 my old from the parana basin in southern brazil paleointensity estimates could be obtained from only six lava flows three of normal and three of reversed polarity of the 71 sampled moreover the quality of the determinations is fairly low with q ratio usually less than 10 mainly as a result of chemical and or physical changes of the magnetic oxides during heating this study strongly underscores the difficulty of finding natural rocks suitable for paleointensity experiments even so the directional results previously obtained were good the most reliable result give an intensity of 28 1 t while the others range from 20 8 t to 37 7 t the corresponding virtual dipole moments vdms range from 4 7 x 10 22 to 7 9 x 10 22 am 2 this dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation although the number of cooling units for which paleointensity estimates could be obtained is limited the results nevertheless indicate that the early cretaceous vdm was significantly lower than that for the recent field but greater than has been reported for this portion of the mesozoic dipole low period

the variation of intensity of earth s magnetic field with time abstract a new and larger database is established to assess the variation of paleointensity with time it is shown that submarine basaltic glass probably contains a grain growth chemical remanent magnetization in magnetite intensity estimates from these samples are used to establish a lower bound estimate on intensities statistical tests on various subsets of the database are used to establish a more reliable database for finding long term trends of the paleointensity we can obtain a reasonable estimate for intensity versus time for the cenozoic but not for the mesozoic or late paleozoic the distributions of data as a function of time also provide valuable information in particular the two well documented superchrons exhibit different intensity properties which suggests that there is not a simple correlation between reversal rate and intensity

the mineralized veins and the impact of old mine workings on the environment at segura central portugal abstract at segura granitic pegmatite veins with cassiterite and lepidolite hydrothermal sn w quartz veins and ba pb zn quartz veins intruded the cambrian schist metagraywacke complex and hercynian granites cassiterite from sn w quartz veins is richer in ti and poorer in nb and nb ta than cassiterite from granitic pegmatite wolframite from sn w quartz veins is enriched in ferberite component the sn w quartz veins contain pyrrhotite arsenopyrite sphalerite chalcopyrite stannite matildite and schapbachite and the ba pb zn quartz veins have cobaltite pyrite sphalerite chalcopyrite galena and barite which were analyzed by electron microprobe the presently abandoned mining area was exploited for sn w ba and pb until 1953 stream sediments and soils have higher concentrations of metals than parent granites and schists sn w b as and cu anomalies found in stream sediments and soils are associated with sn w quartz veins while ba pb and zn anomalies in stream sediments and soils are related to ba pb zn quartz veins sn w b as cu ba pb and zn anomalies in stream sediments and soils are also related to the respective old mining activities which increased the mobility of trace metals from mineralized veins to soils stream sediments and waters stream sediments and soils are sinks of trace elements which depend on their contents in mineralized veins and weathering processes but sn w and b depend mainly on a mechanic process soils must not be used for agriculture and human residence due to their sn b as and ba contents waters associated with mineralized veins were analyzed by flame atomic absorption spectroscopy faas and icp aes have high as fe and mn and should not be used for human consumption and agriculture activities the highest as values in waters were all related to sn w quartz veins and the highest fe and mn values were associated with the ba pb zn quartz veins no significant acid drainage was found associated with the old mine workings

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

paleomagnetism of the siberian traps new data and a new overall 250 ma pole for siberia the flood basalt province in siberia is one of the largest in the world but the number of reliable paleomagnetic data on these volcanics is still limited we studied lava flows and trap related intrusions from two areas in the north and west of the siberian platform a dual polarity characteristic component was isolated from most samples with the aid of stepwise thermal and alternating field demagnetization we then compiled all published paleomagnetic data on the siberian traps that have been obtained according to modern standards also included are presumably trap related overprint directions in paleozoic rocks although these overprints and trap results may locally differ the corresponding mean poles based on remagnetized sediments and volcanics show excellent overall agreement and justify pooling of both data types several ways of data grouping were attempted the trap mean pole proved to be rather insensitive to statistical treatment irrespective of the averaging procedure used the overall mean poles for the siberian traps nsp2 55 1 n 147 0 e n 8 k 123 a95 5 0 or nsp4 57 2 n 151 1 e n 8 k 192 a95 4 0 differ slightly but significantly from the coeval mean poles of baltica torsvik 2001 van der voo r and torsvik t h the quality of the european permo triassic paleopoles and its impact on pangea reconstructions in timescales of the paleomagnetic field j e t channell d v kent w lowrie and j g meert eds agu geophys monogr 2004 135 29 42 we consider possible causes for this difference and conclude that it could be explained either by persistent non dipole terms in the permo triassic geomagnetic field or widespread inclination shallowing in the european data

slope distributions threshold hillslopes and steady state topography digital elevation models of two steady state mountain ranges the olympic mountains om and oregon coast range ocr are used to examine relationships between slope distributions the development of threshold hillslopes and steady state topography plots of drainage area versus slope for these mountain ranges exhibit substantial scatter that complicates comparison of range form to analytical theories and landscape evolution models contour plots of the density of such data reveal an attractor at the scale of the transition from hillslope processes to channel processes and log bin averaging reveals trends that parallel predictions of steady state erosion laws but with different rate laws for five distinct process domains hillslopes valley heads and colluvial bedrock and alluvial valley segments slope histograms computed for 100 km2 areas defined by a 10 10 km grid throughout the om exhibit approximately normal or exponential distributions in areas of active rock uplift and depositional topography respectively local slope distributions in the ocr also tend to be normally distributed but some are left skewed in areas with gentler slopes mean slopes determined both over the above referenced grid and a 10 km diam moving window are relatively invariant in the core of the om in spite of strong contrasts in bedrock erodibility and gradients in long term rock uplift rates in contrast the mean slopes in the ocr parallel latitudinal gradients in rock uplift rates and bedrock erodibility hence the slope distributions in the om and ocr reflect distinct relationships between development of threshold bedrock and soil mantled hillslopes and steady state topography

hydrochemistry mineralogy and chemical fractionation of mine and processing wastes associated with porphyry copper mines a case study from the sarcheshmeh mine se iran abstract the sarcheshmeh is one of the largest oligo miocene porphyry cu deposits in the world comparative hydrochemical mineralogical and chemical fractionation associated with mining efflorescence salts and processing wastes of this mine are discussed hydrochemical results showed that rock waste dumps reject wastes and old impoundments of tailings are the main sources of acid mine drainage waters amd that contain potentially toxic metals such as cd co cu mn ni and zn as well as al episodic fluxes of highly contaminated acidic waters were produced in a tailings dam over a short period of time secondary soluble minerals provide important controls on the quality of amd produced especially in old dry tailings impoundments secondary sulfate minerals such as gypsum magnesiocopiapite hydronium jarosite kornelite and coquimbite were found in rock waste drainages and in old weathered reject wastes highly soluble secondary minerals such as gypsum eriochalcite and bonattite are also observed in an evaporative layer on old tailings impoundments chemical fractionation patterns of potentially toxic elements showed that the geochemical behavior of metals is primarily controlled by the mineralogical composition of waste samples elements such as co cr cu mn ni and zn are readily released into the water soluble fraction from efflorescence salts associated with rock waste drainages as well as from the evaporative layer of old tailings potentially toxic elements such as as mo and pb are principally adsorbed or co precipitated with amorphous and crystalline fe oxides but they may also be associated with oxidizing primary sulfides and residual fractions following the development of the dammed tailings pond the secondary minerals were dissolved producing acidic waters contaminated by al 154 mg l 1 cu 150 mg l 1 cd 0 31 m gl 1 co 2 13 mg l 1 mn 73 7 mg l 1 ni 1 74 mg l 1 zn 20 3 mg l 1 and cl 1690 mg l 1 therefore the potential use of recycled water from the sarcheshmenh dammed tailings pond is diminished by the presence of corrosive ions like cl in highly acidic fluids that promote corrosion of pipes and pumps in the water recycling system

a new experimental material for modeling relief dynamics and interactions between tectonics and surface processes abstract we developed a new granular material mativ to study experimentally landscape evolution in active mountain belt piedmonts its composition and related physical properties have been determined using empirical criteria derived from the scaling of deformation erosion transport and sedimentation natural processes mativ is a water saturated composite material made up with 4 granular components silica powder glass microbeads plastic powder and graphite whose physical mechanical and erosion related properties were measured with different laboratory tests mechanical measurements were made on a modified hubbert type direct shear apparatus erosion related properties were determined using an experimental set up that allows quantifying the erosion sedimentation budget from tilted relaxation topographies for mativ we also investigated the evolution of mean erosion rates and stream power erosion law exponents in 1d as a function of slope our results indicate that mativ satisfies most of the defined criteria it deforms brittlely according to the linear mohr coulomb failure criterion and localizes deformation along discrete faults its erosion pattern is characterized by realistic hillslope and channelized processes slope diffusion mass wasting channel incision during transport eroded particles are sorted depending on their density and shape which results in stratified alluvial deposits displaying lateral facies variations to evaluate the degree of similitude between model and nature we used a new experimental device that combines accretionary wedge deformation mechanisms and surface runoff erosion processes results indicate that mativ succeeded in producing detailed morphological and sedimentological features drainage basin channel network terrace syntectonic alluvial fan geometric kinematic and dynamic similarity criteria have been investigated to compare precisely model to nature although scaling is incomplete it yields particularly informative orders of magnitude with all these characteristics mativ appears as a very promising material to investigate experimentally a wide range of scientific questions dealing with relief dynamics and interactions between tectonics erosion and sedimentation processes

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

testing the accuracy of absolute intensity estimates of the ancient geomagnetic field using copper slag material abstract the middle eastern copper slag is a promising new material for studying intensity variations in the geomagnetic field with high resolution and precision the purpose of this study is to test the accuracy of archaeointensity estimates determined using copper slag by addressing two questions 1 does slag material display the magnetic properties required for valid thellier experiments and 2 what is the accuracy of the archaeointensity estimates derived from thellier style experiments on optimal samples we address the first question through a comprehensive microscopic and magnetic study of representative archaeological slag samples in order to identify the properties responsible for optimal behavior in thellier experiments to address the second question we reproduced slag samples in the laboratory under controlled magnetic fields and analyzed them using the same izzi paleointensity technique used for the ancient slag microscopic analyses of the archaeological slag show that ferromagnetic phases occur as three dimensional dendritic structures whose branches consist of submicron elongated particles magnetic analyses show that these dendrites behave as an assemblage of shape controlled single domain like particles and that their magnetization is thermoremanent we conclude that slag material can be magnetically suitable for valid thellier experiments the laboratory produced slag material demonstrated similar magnetic and mineralogical properties as the archaeological slag izzi experiments showed that non linear trm acquisition even at field strengths similar to earth s and trm anisotropy are important factors to monitor during paleointensity studies of slag material anisotropy and non linearity are probably related to the dendritic shape of the oxide grains intensity estimates derived from three laboratory produced slag samples demonstrated accuracy to within 5 after applying the required corrections

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

long term trends in paleointensity the contribution of dsdp odp submarine basaltic glass collections abstract the deep sea drilling project and the ocean drilling program have been collecting fresh appearing submarine basaltic glass from the world s oceans for over three decades this glass has proved nearly ideal for estimating paleointensity variations of the earth s magnetic field we compile here data for 726 paleointensity experiments from six publications on paleointensity using dsdp odp glass we also include new data for an additional 225 specimens these were obtained through the so called izzi paleointensity experiment of tauxe l staudigel h 2004 strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite geochem geophys geosyst 5 2 q02h06 doi 10 1029 2003gc000635 whereby infield zerofield steps are alternated with the zerofield infield steps to enhance quality assessment of the resulting data the entire collection of data from 951 experiments was prepared for uploading to the magic data base http earthref org including original measurements interpretations and useful metadata excellent results were obtained throughout the depth 1400 mbsf and age 0 160 ma range sampled dsdp odp glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1 160 ma paleolatitudes were estimated for all cooling units in a self consistent manner for use in calculating virtual axial dipole moments we conclude 1 there is about a 20 difference in mean values between the sbg and the lava flow data 48 36 and 57 29 zam2 respectively the difference is caused by the fact that there are more higher values in the lava flow data than in the sbg data set rather than a difference in the minimum values lava flows cooling over a periods of days to months can account for the discrepancy 2 the positive relationship between polarity interval length and average paleofield intensity first hypothesized by cox a v 1968 lengths of geomagnetic polarity intervals j geophys res 73 3247 3260 is supported by data compiled here the brunhes data for which we have only a minimum estimate for polarity interval length are consistent with a long polarity interval suggesting that instead of racing toward reversal hulot g eymin c langlais b mandea m olsen n 2002 small scale structure of the geodynamo inferred from oersted and magsat satellite data nature 416 620 623 we could instead be in the midst of a long stable polarity interval 3 because the average value appears to be a function of polarity interval length it is probably not useful to calculate a mean value nonetheless it appears that most of the time apart from the brunhes and the longest polarity intervals the average dipole moment is substantially less than the present day value as suggested by juarez t tauxe l gee j s pick t 1998 the intensity of the earth s magnetic field over the past 160 million years nature 394 878 881

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

mineral water interactions in tailings from a tungsten mine mount pleasant new brunswick abstract the pore water geochemistry and mineralogy of tailings derived from a granitic tungsten deposit were characterized by collecting pore water samples at discrete depth intervals throughout the tailings for the analysis of major and minor element concentrations mineralogical samples from the oxidation zone were analyzed by x ray diffraction scanning electron microscopy combined with energy dispersive x ray spectroscopy sem eds electron microprobe emp combined with wavelength dispersive x ray spectroscopy wds and transmission electron microscopy tem the oxidation of sulfide minerals in the near surface tailings leads to a decrease in pore water ph and elevated so 4 as and metal concentrations the unusual mineralogy of this deposit compared with that of commonly studied base metal and gold deposits results in several unique geochemical characteristics the dissolution of fluorite releases f into the pore water the f forms strong complexes with al and enhances the dissolution of aluminosilicate minerals within the oxidation zone as a result high al concentrations up to 151 7 mg l are detected in the near neutral pore water in the oxidation zone the combined dissolution of aluminosilicates and carbonate minerals maintains the ph near 10 in the pore water at depth elevated concentrations of w up to 7 1 mg l are detected in the pore water throughout the tailings likely as a result of the dissolution of wolframite consistent with geochemical model calculations results from sem eds emp wds and tem eds analyses indicate that secondary minerals which occur as orange brown coatings on grains of primary minerals are fe oxyhydroxides examples of these secondary minerals display a fibrous habit at high resolution in the tem one of these minerals which contains substantial amounts of al as and si as impurities was identified by selected area electron diffraction saed analyses to be goethite another mineral contains relatively high amounts of si pb bi and as and saed analyses suggest that the mineral is two line ferrihydrite

the timing and extent of the eruption of the siberian traps large igneous province implications for the end permian environmental crisis we present new high precision 40ar 39ar ages on feldspar and biotite separates to establish the age duration and extent of the larger siberian traps volcanic province samples include basalts and gabbros from noril sk the lower tunguska area on the siberian craton the taimyr peninsula the kuznetsk basin vorkuta in the polar urals and from chelyabinsk in the southern urals most of the ages except for those from chelyabinsk are indistinguishable from those found at noril sk cessation of activity at noril sk is constrained by a 40ar 39ar age of 250 3 1 1 ma for the uppermost kumginsky suite the new 40ar 39ar data confirm that the bulk of siberian volcanism occurred at 250 ma during a period of less than 2 ma extending over an area of up to 5 million km2 the resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates or is synchronous with the end permian mass extinction further strengthening an association between volcanism and the end permian crisis a sanidine age of 249 25 0 14 ma from bed 28 tuff at the global section and stratotype at meishan china allows us to bracket the p tr boundary to 0 58 0 21 myr and enables a direct comparison between the 40ar 39ar age of the traps and the permo triassic boundary section younger ages 243 ma obtained for basalts from chelyabinsk indicate that volcanism in at least the southern part of the province continued into the triassic

dem based morphometry of range front escarpments in attica central greece and its relation to fault slip rates in this paper we apply current geological knowledge on faulting processes to digital processing of digital elevation models dem in order to pinpoint locations of active faults the analysis is based on semiautomatic interpretation of 20 and 60 m dem and their products slope shaded relief in northern eastern attica five normal fault segments were recognized on the 20 m dem all faults strike wnw ese the faults are from west to east thriassion thfs fili fifs afidnai affs avlon avfs and pendeli pefs and range in length from 10 to 20 km all of them show geomorphic evidence for recent activity such as prominent range front escarpments v shaped valleys triangular facets and tilted footwall areas however escarpment morphometry and footwall geometry reveal systematic differences between the bexternalq segments pefs thfs and avfs and the binternalq segments affs and fifs which may be due to mechanical interaction among segments and or preexisting topography in addition transects across all five escarpments show mean scarp slope angles of 22 18f0 78 for both carbonate and metamorphic bedrock the slope angle equation for the external segments shows asymptotic behaviour with increasing height we make an empirical suggestion that slope angle is a function of the long term fault slip rate which ranges between 0 13 and 0 3 mm yr the identified faults may rupture up to magnitude 6 4 6 6 earthquakes the analysis of the 60 m dem shows a difference in fault patterns between western and northern attica which is related to crustal rheology variations d 2004 elsevier b v all rights reserved

the stability of sulfate and hydrated sulfate minerals near ambient conditions and their significance in environmental and planetary sciences abstract sulfate and hydrated sulfate minerals are abundant and ubiquitous on the surface of the earth and also on other planets and their satellites the humidity buffer technique has been applied to study the stability of some of these minerals at 0 1 mpa in terms of temperature relative humidity space on the basis of hydration dehydration reversal experiments updated phase relations in the binary system mgso 4 h 2 o are presented as an example to show how reliable thermodynamic data for these minerals could be obtained based on these experimental results and thermodynamic principles this approach has been applied to sulfate and hydrated sulfate minerals of other metals including fe both ferrous and ferric zn ni co cd and cu metal sulfate salts play important roles in the cycling of metals and sulfate in terrestrial systems and the number of phases extends well beyond the simple sulfate salts that have thus far been investigated experimentally the oxidation of sulfide minerals particularly pyrite is a common process that initiates the formation of efflorescent metal sulfate minerals also the overall abundance of iron bearing sulfate salts in nature reflects the fact that the weathering of pyrite or pyrrhotite is the ultimate source for many of these phases many aspects of their environmental significance are reviewed particularly in acute effects to aquatic ecosystems related to the dissolution of sulfate salts during rain storms or snow melt events hydrous mg ca and fe sulfates were identified on mars with wide distribution and very large quantities at many locations on the basis of spectroscopic observations from orbital remote sensing and surface explorations by rovers however many of these findings do not reveal the detailed information on the degree of hydration that is essential for rigorous interpretation of the hydrologic history of mars laboratory experiments on stability fields reactions pathways and reaction rates of hydrous sulfates likely to be found on mars enhance our understanding of the degrees of hydration of various sulfates that should currently exist on mars at various seasons and locations and during various atmospheric pressure and obliquity periods two sets of systematic experiments were described one on hydrous mg sulfates and the other on hydrous fe 3 sulfates also their implications to mars sulfates mineralogy were discussed

acid mine drainage in the iberian pyrite belt odiel river watershed huelva sw spain geochemistry mineralogy and environmental implications abstract this work reports the physical properties and water chemistry of 64 amd discharges from 25 different mines in the ipb draining to the odiel river watershed which have been investigated during the hydrologic year 2003 2004 these amd solutions vary largely in flow rate and chemical composition both spatially between the different mine sites suggesting a strong geologic control on amd chemistry and seasonally due to marked hydrologic variations and include cases with very low ph mostly in the range 1 4 4 and extreme sulphate up to 44 g l so 4 2 and metal content e g up to 7 7 g l fe 2 6 g l al or 1 4 g l zn different hydrogeochemical facies of amd namely fe ii anoxic fe iii suboxic and aluminous oxic are recognized in the field as a response to the continuous oxidation and hydrolysis of dissolved fe relevant geochemical aspects of these amd environments are discussed including i the redox chemistry of the fe ii fe iii couple ii the reaction rates for bacterially catalyzed oxidation of fe ii and hydrolysis of fe iii iii the role played by dissolved fe and al in the acidity and chemical buffering of the amd systems and iv the solubility and trace metal retention capacity of the fe oxyhydroxysulphate and hydrated sulphate minerals commonly associated with amd in addition the mineralogy and chemistry of the fe precipitates schwertmannite jarosite goethite ferrihydrite al phases e g basaluminite and mg fe al efflorescent so4 salts e g epsomite hexahydrite copiapite halotrichite rozenite coquimbite present in the amd generating mine sites have also been studied the mineralogy of the fe precipitates is well correlated with the water ph with jarosite at ph 2 schwertmannite at ph 2 4 basaluminite at ph 4 5 and ferrihydrite at ph 6 schwertmannite appears to be the most important mineral phase both in controlling the fe solubility at ph 2 4 and as sorbent of trace elements as cu zn which favours natural attenuation finally a basin scale environmental perspective is given in order to evaluate the impact of amd on the water quality including calculation of metal loadings transported by amd from the most important mine districts in the province

the siberian lithosphere traverse mantle terranes and the assembly of the siberian craton abstract the kimberlite fields scattered across the ne part of the siberian craton have been used to map the subcontinental lithospheric mantle sclm as it existed during devonian to late jurassic time along a 1000 km traverse ne sw across the archean magan and anabar provinces and into the proterozoic olenek province 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe major elements and proton microprobe trace elements these data and radiometric ages on the kimberlites have been used to estimate the position of the local paleo geotherm and the thickness of the lithosphere and to map the detailed distribution of specific rock types and mantle processes in space and time a low geotherm corresponding approximately to the 35 mw m2 conductive model of pollack and chapman tectonophysics 38 279 296 1977 characterised the devonian lithosphere beneath the magan and anabar crustal provinces the devonian geotherm beneath the northern part of the area was higher rising to near a 40 mw m2 conductive model areas intruded by mesozoic kimberlites are generally characterised by this higher but still cratonic geotherm lithosphere thickness at the time of kimberlite intrusion varied from ca 190 to ca 240 km beneath the archean magan and anabar provinces but was less 150 180 km beneath the proterozoic olenek province already in devonian time thinner devonian lithosphere 140 km in parts of this area may be related to riphean rifting near the northern end of the traverse differences in geotherm lithosphere thickness and composition between the devonian toluopka area and the nearby mesozoic kimberlite fields suggest thinning of the lithosphere by ca 50 60 km related to devonian rifting and triassic magmatism a major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data extended from outcrops in the anabar shield represent major lithospheric sutures which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock type distribution between 100 and 250 km depth the presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the magan and anabar provinces is concordant with their archean surface geology the lack of harzburgitic garnets and the chemistry of the lherzolitic garnets beneath most of the other fields are consistent with the proterozoic surface rocks mantle sections for different terranes within the archean portion of the craton show pronounced differences in bulk composition rock type distribution metasomatic overprint and lithospheric thickness these observations suggest that individual crustal terranes of both archean and proterozoic age had developed their own lithospheric roots and that these differences were preserved during the proterozoic assembly of the craton data from kimberlite fields near the main archean proterozoic suture the billyakh shear zone suggest that reworking and mixing of archean and proterozoic mantle was limited to a zone less than 100 km wide

analysis of long term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics summary the 1167 published cooling unit cu palaeointensity estimates contained in the 400 10 ma portion of the pint global database were rigorously filtered according to accurate age determinations palaeodirectional reliability recognition of polarity and the method of palaeointensity acquisition the remaining 865 estimates group 1 were further filtered to ensure self consistency reducing the data set to 425 estimates group 2 group 1 and 2 data were clustered into temporally and or spatially distinct rock suites rs enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation psv the record was segmented according to the distribution of the data rather than using arbitrary time windows to ensure quasi consistent behaviour within each segment differences between these segments clearly indicate that a significant long timescale 10 7 and 10 8 yr variation of the mean geomagnetic poloidal field intensity gpfi occurred during the 400 10 ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field both the mean dipole moment and its standard deviation appear to be a function of the range of values each cu may adopt at one particular time this range is itself controlled by the variation of the maximum limit of dipole moment while the value of the minimum limit remains relatively constant tentative support is provided for the recent suggestion that psv may have been reduced during the cretaceous normal superchron cns though more data are needed in the range 120 60 ma to confirm this no conclusive evidence was found to support the suggestion that the gpfi record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt ptrm checks indeed offsets caused by unreliable data in well represented parts of the record are likely to be random and cancel one another out when gpfi variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency rf it is not possible to confirm whether the two parameters are anticorrelated decoupled or related in some more complex way however it is clear that gpfi and rf are definitely not positively correlated as has been previously suggested the present database documents sharp increases in gpfi around the onset times of the two recognized superchrons itself implying an anticorrelation the implications for geodynamo and mantle modelling of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly a generic geodynamic model is proposed to explain the relationship between observed longterm changes in gpfi and global geodynamic processes the model predicts that changes in gpfi result from a chain of geodynamic processes extending from crust to core beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model four time stages are proposed to describe the major long term changes in gpfi since the early devonian 400 350 350 250 250 175

application of multivariate statistical methods to indicate the origin and geochemical behavior of potentially hazardous elements in sediment around the sarcheshmeh copper mine se iran multivariate statistical techniques i e correlation coefficient analysis principal components analysis pca and hierarchical cluster analysis ca were applied to the total and water soluble concentrations of potentially hazardous metals in sediments associated with the sarcheshmeh mine one of the largest oligo miocene porphyry copper deposits in the world the samples were analyzed for hazardous metal concentration levels by inductively coupled plasma mass spectrometry method results indicate that the contaminant metals as cd cu mo s sb sn se pb and zn were positively correlated with the total concentrations these hazardous metals also have strong association in the pca and ca results different anthropic versus natural sources of contaminant metals were distinguished by using ca method water soluble fraction of hazardous metals showed that the hydro geochemical behavior of these metals in sediments is different considerably elements such as cd co cr cu fe mn ni s and zn are readily water soluble from contaminated samples especially from evaporative mineral phases while the release of as mo sb and pb into the water is limited by adsorption processes results obtained from the application of multivariate techniques on the water soluble fraction data set show that the hazardous metals are categorized into three groups including 1 ni s co cu cr and fe 2 se mn cd and zn and 3 sb as mo and sn this classification describes the hydro geochemical behavior of hazardous metals in water sediment environments of the sarcheshmeh porphyry copper mine and can be used as a basis in remedial and treatment strategies

evidence for weak geomagnetic field intensity prior to the cretaceous normal superchron abstract a palaeomagnetic and geochronologic investigation has been carried out on a sequence of 19 lava flows at sihetun in northeastern china age determinations by k ar dating indicate that the sequence spans 124 133 ma that is the time just prior to cretaceous normal superchron cns rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo single domain magnetite stepwise thermal demagnetization successfully isolated a well defined direction of characteristic magnetization chrm in all the lavas the mean palaeodirection determined is d i 5 4 58 7 95 2 4 and d i 175 9 60 0 95 6 4 for 15 normally magnetized flows and 3 reversely magnetized flows respectively one lava was found to possess an intermediate direction of magnetization the inferred palaeolatitude of the sampling site is very similar to the present day position supporting the contention that suturing between the mongolia and north china plates was probably completed by the late jurassic and that subsequently little rotation or translation occurred in the region west of the tanlu fault zone thellier thellier palaeointensity experiments with systematic partial thermoremanent magnetization ptrm checks were conducted on 66 samples from 13 lava flows results from the 29 samples 44 which met strict reliability criteria indicate a range of virtual dipole moments vdms from 2 83 to 4 17 1022 a m2 with an average of 3 53 0 02 1022 a m2 this finding helps confirm that a weak geomagnetic field probably occurred just prior to the cns

some global features of palaeointensity in geological time summary a global palaeointensity data base was constructed from all published data from volcanic rocks in geological time older than 0 03 ma the data base contains a total of 1123 flow mean data retrieved from 83 original papers various features of the earth s dipole moment were examined from the data which are based on thellier and shaw methods long term variation of the earth s dipole moment seems to have existed in the past 300 ma with a broad minimum at 120 180 ma as suggested by prevot et al 1990 however due to limited site distribution we cannot regard this mesozoic dipole low as being completely established precambrian palaeointensity data are still insufficient to conclude any long term variation in this time range although the geodynamo processes of moderate magnitude definitely existed in the early time of the earth s history the 2 test was applied to the distribution of the virtual dipole moment for the past 5 ma in which the transitional data were excluded the results indicate that the distribution of virtual dipole moment is better represented by a log normal distribution rather than a normal distribution and this tendency seems to be true for the past 20 ma the relation of mean palaeointensity versus palaeomagnetic colatitude was examined for the past 10 ma for the data excluding transitions the relation is concordant with a theoretical curve from a geocentric axial dipole this reconfirms that the dipole field was dominant in the past geomagnetic field which is the dipole hypothesis in palaeomagnetism on the other hand the virtual dipole moment is much smaller for transitional palaeomagnetic fields and a virtual geomagnetic pole lower than 45 seems to be a reasonable criterion to be categorized as a transition the mean dipole moment for the pole latitude of 30 n 50 n band is larger than that for 30 s 50 s indicating that there might have been a persistent asymmetry of palaeointensity between normal and reversed states or some kind of geometrical asymmetry between the northern and southern hemispheres

mesoscale fluvial erosion parameters deduced from modeling the mediterranean sea level drop during the messinian late miocene 1 after a base level drop rivers are the first components of the landscape to respond by incising into topography a base level drop results in a knickpoint in the downstream part of river longitudinal profiles whether knickpoints are preserved or erased during the upstream propagation of incision is still debated preservation and erasure of knickpoints are two end member processes that work in natural systems at different timescales different length scales and different places the huge 1500 m and fast tens of kiloyears sea level drop in the mediterranean during the messinian resulted in the fast propagation of incision far inland this is especially the case in the rhone valley southern france where the knickpoint is 300 km from the mediterranean coast numerical modeling of this event has been performed using the ros model which simulates both erosional and depositional processes in rivers the best fit between numerical results and geological data is obtained for a nonlinear relation between incision and drainage area and for a small transport length of sediment this small transport length at least 2 orders of magnitude lower than the length of the rhone suggests that the longitudinal profile relaxed in a diffusive way so that the initial knickpoint was not preserved finally after a base level fall the propagation of fluvial incision is very fast at geological timescales hundreds of kiloyears despite this the diffusive response implies that the time required for restoration of an equilibrium profile is very long

microwave paleointensities indicate a low paleomagnetic dipole moment at the permo triassic boundary the quantity of igneous material comprising the siberian traps provides a uniquely excellent opportunity to constrain earth s paleomagnetic field intensity at the permo triassic boundary there remains however a contradiction about the strength of the magnetic field that is exacerbated by the limited number of measurement data to clarify the geomagnetic field behavior during this time period for the first time a microwave paleointensity study has been carried out on the permo triassic flood basalts in order to complement existing datasets obtained using conventional thermal techniques samples which have been dated at 250 ma of the permo triassic trap basalts from the northern extrusive maymecha kotuy region and the southeastern intrusive areas of the sytikanskaya and yubileinaya kimberlite pipes localities on the siberian platform are investigated these units have already demonstrated reliable paleomagnetic directions consistent with the retention of a primary remanence furthermore scanning electron microscope analysis confirms the presence of iron oxides likely of primary origin microwave thellier type paleointensity experiments izzi protocol with partial thermoremanent magnetization checks are performed on 50 samples from 11 sites of which 28 samples from 7 sites provide satisfactory paleointensity data the samples display corresponding distinct directional components positive ptrm checks and little or no zig zagging of the arai or zijderveld plot providing evidence to support that the samples are not influenced by lab induced alteration or multi domain behavior the accepted microwave paleointensity results from this study are combined with thermal thellier type results from previously published studies to obtain overall estimates for different regions of the siberian traps the mean geomagnetic field intensity obtained from the samples of the northern part is 13 4 12 7 t maymecha kotuy region whereas from the southeastern part is 17 3 16 5 t sytikanskaya kimberlite pipe and 48 5 7 3 t yubileinaya kimberlite pipe suggesting that the regional discrepancy is probably due to the insufficient sampling of geomagnetic secular variation and thus multiple localities need to be considered to obtain an accurate paleomagnetic dipole moment for this time period it demonstrates that the overall mean paleointensity of the siberian traps is 19 5 13 0 t which corresponds to a mean virtual dipole moment of 3 2 1 8 1022 am2 results indicate that the average magnetic field intensity during permo triassic boundary is significantly lower by approximately 50 than the present geomagnetic field intensity and thus it implies that the mesozoic dipole low might extend 50 myr further back in time than previously recognized

geomagnetic palaeointensity just prior to the cretaceous normal superchron abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on a basalt lava sequence at zhuanchengzi in liaoning province northeastern china the new k ar age obtained from these lava flows is 120 93 0 88 ma marine anomaly m0 detailed investigations were conducted on two samples from each lava flow to determine their rock magnetic properties the temperature dependences of the saturation magnetization j t and the initial susceptibility t show evidence of low titanium magnetite and a curie temperature of 560 585 c with generally good reversibility of heating and cooling curves hysteretic properties ratios of mrs ms and hcr hc indicate that pseudo single domain psd grains of magnetite are dominant in these lavas stepwise thermal demagnetization isolated well defined characteristic magnetization chrm in all the 20 lava flows with a mean direction of d i 174 9 60 1 with 95 2 3 the modified version of the thellier and thellier palaeointensity method coe 1967a with systematic partial thermoremanent magnetization ptrm checks was used forty two of 71 samples 59 yielded reliable palaeointensity results virtual dipole moment vdm values range from 1 71 1022 to 5 27 1022 a m2 with an average of 3 96 0 07 1022 a m2 this low dipole field intensity value is approximately half the present day vdm consistent with older results obtained from the caribbean submarine basalt glasses for the period of 125 1 ma pick and tauxe 1993 this suggested that a weak geomagnetic field probably occurred just prior to the cretaceous normal superchron cns

phanerozoic hot spot traces and paleogeographic reconstructions of the siberian continent based on interaction with the african large low shear velocity province abstract we review intraplate magmatism in siberia and its folded surroundings from 480 ma to the present we describe several large igneous provinces lips and the intervals in which they were continuously formed within the limits of the siberian continent the altay sayan early paleozoic magmatic area 598 446 ma the altay sayan lip 408 393 ma the viluy lip 380 350 ma the barguzin vitim lip 310 275 ma the late paleozoic rift system of central asia 318 242 ma the siberian traps and west siberian rift system 250 249 ma the east mongolian and west trans baikalian lip 228 195 ma and a number of various aged late mesozoic and cenozoic rift zones and magmatic areas from 160 ma to the present day following lawver and muller 1994 kharin 2000 lawver et al 2002 and chernysheva et al 2005 we accept the position of the icelandic hot spot under the siberian trap area at the permo triassic boundary that enables us to estimate the geographic coordinates of the siberian trap location at 250 ma they are the same as iceland today presently the icelandic hot spot is situated above the african large low shear velocity province llsvp that indicates a hot mantle plume we suggest a set of paleogeographic reconstructions of the siberian continent for which we evaluate the paleolatitude based on the paleomagnetic data and estimate the paleolongitude position by placing siberia above the african llsvp furthermore we estimate the geographic coordinates for other ancient hot spots in the framework of the african llsvp that we consider to be responsible for the intraplate magmatism during different time periods in the phanerozoic eon available rare element and isotopic characteristics of the intraplate magmatic rocks of siberia enable us to determine three primary sources moderately depleted mantle prema enriched mantle em i and em ii of the mantle origin magma we propose that the model explains the interaction of the hot mantle plume including hot spots plume tails with the siberian intraplate magmatism areas throughout the phanerozoic eon

interaction between normal fault slip and erosion on relief evolution insights from experimental modelling the growth of relief in active tectonic areas is mainly controlled by the interactions between tectonics and surface processes erosion and sedimentation the study of long lived morphologic markers formed by these interactions can help in quantifying the competing effects of tectonics erosion and sedimentation in regions experiencing active extension river long profiles and faceted spurs triangular facets can help in understanding the development of mountainous topography along normal fault scarps in this study we developed analogue experiments that simulate the morphologic evolution of a mountain range bounded by a normal fault this paper focuses on the effect of the fault slip rate on the morphologic evolution of the footwall by performing three analogue experiments with different fault slip rates under a constant rainfall rate a morphometric analysis of the modelled catchments allows comparing with a natural case tunka half graben siberia after a certain amount of fault slip the modelled footwall topographies of our models reaches a dynamic equilibrium i e erosion balances tectonic uplift relative to the base level close to the fault whereas the topography farther from the fault is still being dissected due to regressive erosion we show that the rates of vertical erosion in the area where dynamic equilibrium is reached and the rate of regressive erosion are linearly correlated with the fault throw rate facet morphology seems to depend on the fault slip rate except for the fastest experiment where faceted spurs are degraded due to mass wasting a stream power law is computed for the area wherein rivers reach a topographic equilibrium we show that the erosional capacity of the system depends on the fault slip rate finally our results demonstrate the possibility of preserving convex river long profiles on the long term under steady external tectonic uplift and rainfall conditions

an improved methodology for thellier type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence we propose a new methodology for thellier type paleointensity experiments in standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alteration of magnetic mineralogy begins and subsequent data have to be discarded we emphasize that when alteration begins at low temperature and continues throughout the experiment which then fails in standard analyses the resulting unblocking temperature tub spectrum of the alteration product does not necessarily overlap with the whole tub spectrum of natural remanent magnetization nrm because tub depends on physical characteristics of the new grains not the temperatures at which they were formed nrm may survive in an uncontaminated higher tub window if alteration remanence only has low tub then partial thermoremanent magnetization ptrm checks at temperature step tj 1 can be used to correct for alteration that has occurred at tj and furthermore thermal demagnetization of a full trm acquired in the laboratory at the end of the progressive thellier thellier experiment should reveal the true uncontaminated tub spectrum of the nrm in higher temperature intervals we propose a new experimental sequence which allows monitoring and correction of alteration and provides two semi independent estimates of palaeointensity in the circumstances described above we illustrate the issues involved through new experimental work on igneous rocks from a paleozoic charnockitic syenite where we unravel trm from multidomain isothermal remanence despite massive alteration during laboratory heating finally we point out the value of these experiments for verifying primary trm in igneous rocks indicating properties that distinguish trm from other remanence

new absolute paleointensity results from the parana magmatic province uruguay and the early cretaceous geomagnetic paleofield 1 thellier paleointensity experiments are carried out on the early cretaceous arapey formation which belongs to the parana magmatic province pmp in northern uruguay twenty seven samples out of 225 analyzed yield reliable absolute intensity determinations for these samples the nrm fraction f used for determination ranges between 0 33 to 0 83 and the quality factor q ranges from 4 5 to 14 1 being generally greater than 5 the flow mean paleointensity values obtained in this study range from 16 3 2 3 to 41 5 3 8 t with corresponding virtual dipole moments vdms from 3 4 0 6 to 8 2 0 8 1022 am2 this corresponds to mean vdm value of 6 4 2 3 1022 am2 40ar 39ar dating on the pmp sequence by feraud et al 1999 gave a mean age of 132 2 0 5 ma our results in conjunction with previous studies kosterov et al 1998 goguitchaichvili et al 2002 suggest that the paleostrength during the early cretaceous might be as variable and strong as brunhes data and not anomalously low as previously suggested the scatter observed in our study is similar to recent findings by granot et al 2007 for the troodos ophiolite

absolute paleointensity from hawaiian lavas younger than 35 ka abstract paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3 3 and 28 2 ka from the mauna loa volcano in the big island of hawaii determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in hawaii although the overall average field intensity appears to be lower since the present results were determined at higher temperatures than in the previous studies this discrepancy raises questions regarding the selection of low versus high temperature segments that are usually made for absolute paleointensity the virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows when averaged within finite time intervals the worldwide values match nicely the variations of the sint 200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period the convergence between the existing records at hawaii and the rest of the world does not favour the presence of persistent strong non dipole components beneath hawaii for this period

experiments on trm intensity dependence on cooling rate palaeointensity studies have an important role to play in understanding the origin and time variation of the geomagnetic field however theoretical considerations indicate that the intensity of magnetization recorded by a rock or archaeological sample will be dependent on the rate at which it cooled as well as field magnitude experiments on synthetic magnetite and titanomagnetites of varying grain sizes indicate that for acicular magnetite at low concentration slower cooled trm is stronger than faster cooled trm but for all other samples slower cooled trm is weaker this is the predicted behaviour however the magnitude of the effect is much greater than the predicted kinetic effects and indicates that the magnetization of sub microscopic magnetite rich inclusions in the titanomagnetites strongly controls the trm intensity of the matrix

acid drainage from sulfides hosting gold mineralization furtei sardinia the past mining activity for kaolin minerals left several abandoned quarries in the area of furtei southern sardinia recently gold mineralization has been found in association with intensely altered andesitic rocks and an open pit gold mine is expected to open in the near future twenty one water samples were analyzed for a wide range of major and trace elements in order to define the present hydrochemical situation before exploitation starts waters interacting with the altered andesitic rocks had higher salinity and metal concentrations than waters in contact with nonaltered rocks acid drainage and high concentrations of metals al fe mn zn cu as in solution were observed in waters leaching the mineral deposit an environmental impact study is urgently called for since the waste material derived from the exploitation is expected to enhance acidification and metal releasing processes due to the increase in the specific yield which favors the oxidation of sufide minerals

nonlinear thermoremanence acquisition and implications for paleointensity data abstract in paleointensity studies thermoremanence is generally regarded as a linear function of ambient magnetic field at low fields comparable to that of the present day earth we find pronounced nonlinearity at low fields for a class of materials with silicate hosted magnetite that otherwise perform well in paleointensity experiments we model this nonlinearity with narrow size ranges of large acicular single domain grains which are most likely in a vortex state i e nonuniformly magnetized sometimes labeled pseudosingle domain simple trm theory predicts that even certain single domain particles will also exhibit a nonlinear response saturating in fields as low as the earth s such behavior although likely to be rare may bias some paleointensity estimates the bias is especially pronounced when the laboratory field is higher than the ancient field fortunately the fundamental assumption that thermoremanence is proportional to applied field can and should be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps

the role of secondary minerals in controlling the migration of arsenic and metals from high sulfide wastes berikul gold mine siberia abstract the role of secondary minerals in controlling the migration of as cu zn pb and cd has been investigated in piles of high sulfide waste at the berikul au mine kemerovo region russia these wastes contain 40 45 wt sulfides and have been stored for approximately 50 a near the mokry berikul river sulfide oxidation generates acid pore solutions ph 1 7 with high concentrations of so42 190 g l fe 57 g l as 22 g l zn 2 g l cu 0 4 g l pb 0 04 g l and cd 0 03 g l from these solutions as is precipitated as amorphous non stoichiometric fe sulfoarsenates in the lower horizons of the waste piles during precipitation of the fe sulfoarsenates the concentration of fe in these phases decreases from 34 to 21 wt that of as increases from 11 to 22 wt while the s content remains approximately constant 5 4 5 8 wt arsenic is also accumulated in jarosite beudantite solid solutions up to 8 4 wt as which occur as inclusions in the amorphous fe sulfoarsenates in efflorescent crusts on the surface of the waste pile as co precipitates with the fe iii sulfates copiapite 0 27 wt as and rhomboclase 0 87 wt as zinc and cu are incorporated primarily into fe ii sulfates i e melanterite in the interior of the waste pile and rozenite in the efflorescent crust the zn mineral dietrichite is also formed at the surface of the waste pile as a result of evaporation of pore solutions and is the only fe ii sulfate containing detectable amounts of as 0 64 wt lead is mainly co precipitated with minerals of the jarosite group where the pb content may reach 4 3 wt co precipitation of toxic elements with sulfates and sulfoarsenates of fe is shown to be a significant mechanism in controlling the concentration of heavy metals in pore solutions of high sulfide mine wastes precipitation of secondary phases causes the formation of a hardpan layer with low permeability at a depth of 1 1 5 m below the surface of the waste pile rainwater accumulates above the hardpan horizons and slowly drains along these aquicludes to the bottom of the pile most of the rainwater evaporates during infiltration this leads to formation of the described efflorescent sulfate crusts dissolution of these crusts during the next rain storm produces highly acidic surface waters ph 1 1 rich in so42 30 g l fe 18 g l as 0 24 g l zn 0 12 g l cu 0 04 g l pb and cd 0 002 g l during the warm t 0 c period of the year which lasts about 7 months these surface waters transport a total of a few tens of kilograms of as and zn several kilograms of cu and a few hundred grams of pb and cd from the waste pile into the mokry berikul river as a result the concentrations of these metals in the river water increase by an order of magnitude thus reaching levels close to or exceeding the maximum values permissible for drinking water

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

hydrochemistry of superficial waters in the adoria mine area northern portugal environmental implications the purpose of this work is to characterize the hydrochemical behavior of acid mine drainages amd and superficial waters from the adoria mine area northern portugal samples of superficial and mine drainage water were collected for one year bi monthly with ph temperature eh conductivity and hco3 determined in situ with chemical analyses of so4 ca k mg na cl ag as bi co cu fe mn ni pb zn and cd in the mine there are acidic waters with low ph and significant concentrations of so4 and metals fe mn zn cu pb cd and ni while in the superficial natural stream waters outside the mine the ph is close to neutral with low conductivity and lower metal concentrations the stream waters inside the mine influence are intermediate in composition between amd and natural stream waters outside the mine influence principal component analysis pca shows a clear separation between amd galleries and amd tailings with tailings having a greater level of contamination

a reconnaissance 40ar39ar geochronologic study of ore bearing and related rocks siberian russia abstract 40 ar 39 ar age spectra of biotite from a mineralized vein in the ore bearing noril sk i intrusion and from picritic like gabbrodolerite from the weakly mineralized lower talnakh intrusion show that these bodies were emplaced at 249 2 ma which is not significantly different from the age of the permian triassic boundary the ore bearing intrusions postdate the lower third of the flood basalt sequence in the noril sk area and on the basis of geochemistry can best be correlated with lavas slightly younger than those which they cut thus flood basalt was erupted at the time of the permian triassic mass extinction event although its role in this event is as yet ill defined additional new 40 ar 39 ar age data for a group of intrusive and extrusive rocks on the western margin of the siberian craton indicate that mafic magmatism extended over a period of several tens of million years whereas paleomagnetic data suggest that the bulk of the siberian flood basalt sequence near noril sk has been erupted in only a million years or so 40 ar 39 ar ages of plagioclase from early flood basalt flows are about 2 younger than those obtained for biotite from the crosscutting noril sk i intrusion probably because of slight alteration and argon loss from the plagioclase

low geomagnetic field intensity during the kiaman superchron thellier and microwave results from the great whin sill intrusive complex northern united kingdom 1 coe modified thellier cmt and microwave mw paleointensity experiments were conducted on samples from the multi phase late carboniferous ca 301 ma to early permian ca 270 ma great whin sill complex statistically comparable results were obtained with both techniques from high quality data forty three samples 25 with cmt and 18 with mw yielded mean paleointensity values of 12 0 5 8 t and 16 5 8 2 t for the early and late intrusive phases respectively corresponding to vdms of 1 6 0 8 1022 and 2 1 1 1 1022 am2 when combined with previous data the results suggest a generally low dipole moment during the krs reducing from 75 of the present value at 301 ma to 20 27 of the present value later in the superchron these data imply either a decoupling between processes controlling reversal rate and field strength or a more complex intriguing time lag relationship more absolute palaeointensity results are needed to further evaluate these alternatives

new constraints on sediment flux dependent river incision implications for extracting tectonic signals from river profiles we present new field data from rivers draining across active normal faults that incise across the same lithology at the fault have been subjected to similar climatic regimes and tectonic settings and were perturbed by a well documented increase in fault slip rate ca 1 ma in spite of these similarities the rivers exhibit markedly different long profiles and patterns of catchment incision we use channel slope and hydraulic geometry data for each river to calculate bed shear stresses b and show that there is no simple relationship between peak b and the relative uplift rates across the faults u which differ by a factor of four the long term average sediment supply to each channel q s estimated from time averaged catchment erosion rates can explain the b versus u data if bedload modulates bedrock incision rate e in a strongly nonlinear way together these field data allow us for the first time to evaluate theoretical predictions of the role of sediment on river profile evolution and to quantify the magnitude of the effect in natural systems

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

importance of cooling rate dependence of thermoremanence in paleointensity determination 1 the practical effect of cooling rate on the magnitude of thermal remanent magnetization trm is experimentally tested using both natural and synthetic magnetites for synthetic and natural single domain sd magnetites trm intensity increases as the cooling rate decreases because a longer exposure time for slower cooling offers more chances to achieve an equilibrium magnetization multidomain md magnetites have the opposite response trm decreases as the cooling rate increases because slower cooling allows more time to achieve lower magnetization because of self demagnetization for pseudo single domain psd magnetite the effect of cooling rate on the remanence intensity appears to be insignificant intermediate in behavior between sd and md for a sd trm differences are restricted to lower temperatures but diminish near the curie point strongly indicating a serious problem in a conventional treatment of cooling rate correction on the practical side merely applying trm differences as a correction can overcorrect the true paleointensity another important significance of the present study is that a cooling rate difference alone can cause a slight nonlinearity in an arai plot for a sd

geomorphic significance of postglacial bedrock scarps on normal fault footwalls the existence of well preserved holocene bedrock fault scarps along active normal faults in the mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate we test and quantify this interpretation using a case study in the italian central apennines holocene rates are derived from measurements of weathering pit depth along the magnola scarp where previous cosmogenic cl 36 analyses constrain exposure history to estimate the average hillslope erosion rate over similar to 10 5 years we introduce a simple geometric model of normal fault footwall slope evolution the model predicts that the gradient of a weathering limited footwall hillslope is set by fault dip angle and the ratio of slip rate to erosion rate if either slip or erosion rate is known the other can be derived applying this model to the magnola fault yields an estimated average weathering rate on the order of 0 2 0 4 mm yr more than 10 times higher than either the holocene scarp weathering rate or modern regional limestone weathering rates a numerical model of footwall growth and erosion in which erosion rate tracks the oxygen isotope curve reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles we conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking

low intensity of the geomagnetic field in early jurassic time from a large collection of jurassic continental tholeiites cropping out in europe and africa we selected 90 samples for paleointensity determinations the samples were carefully selected to avoid any secondary magnetizations especially viscous magnetization use of the thellier method reveals that magnetic property changes due to heating begin often at quite low temperatures but fortunately without modifying noticeably their natural remanent magnetization thermoremanent magnetization ratio twenty eight well clustered paleointensity estimates were obtained from two european dikes that were emplaced during early jurassic time the kerforne dike at brenterc h in brittany northwestern france and the messejana dike on the iberian peninsula spain and portugal virtual dipole moments calculated from both magmatic units are similar and only about one third of present day values these new data lend support to the recently postulated low dipole moment of the mesozoic geomagnetic field

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

paleomagnetic constraints on the structure of the deccan traps abstract new paleomagnetic results from two widely separated areas of the deccan traps allow one to test the simple normal reversed normal nrn magnetostratigraphic model previously proposed the new data from the mandla region to the northeast of the main outcrop of the deccan traps together with earlier apparently conflicting results are all compatible with the nrn stratigraphy provided the flows are not strictly horizontal but have a synform antiform structure with 500 m amplitude and 150 km wavelength the structure parallels the paleozoic narmada son rift system with slopes never exceeding 0 5 sampling in rajahmundry a remote southeastern outlier of the deccan with well constrained age allowed identification of the rn boundary in an area where only normal polarities had been reported it has also been possible to correlate the detailed geochemical stratigraphy in the western ghats with the nrn magnetic stratigraphy a compilation of 35 magnetic studies together with the above findings confirms the general applicability of the nrn model and reveals the large scale topography of the rn boundary which is an isochron over most of the deccan the rn isochron surface displays a boomerang shape with topography ranging from sea level to above 1600 m this observed topography is consistent with a simple model in which doming and renewed rifting and dyke intrusion resulted from impingement 70 65 ma ago of the reunion plume under the indian lithosphere the narmada son rift was rejuvenated and a continental rift system with minor extension with a triple junction in the cambay graben resulted and was subsequently covered by the deccan flows although in places tectonic faulting and volcanism continued to interfere the rajahmundry outcrops constrain the original extent and southeastward dip away from the triple junction of much of the traps a significant part of which has now been removed by erosion and rifting away of the seychelles

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

testing determinations of absolute paleointensity from the 1955 and 1960 hawaiian flows abstract a new exhaustive study of the flows erupted in 1955 and 1960 from the kilauea volcano big island of hawaii has been conducted in order to constrain the dispersion inherent to the data of absolute paleointensity reported so far and ultimately to determine what are the most critical parameters to obtain suitable determinations the mean directions derived from both flows agree with the expected field within less than 2 paleointensity experiments have been performed using a double heating thellier procedure completed by systematic ptrm checks almost all samples display a linear nrm trm straight line but only 20 of the samples have provided suitable results less than 10 away from the expected field in the other cases the deviation from the expected paleointensity is related to the quality of the ptrm checks which thus provides a threshold value for acceptance of ptrm checks all successful determinations less than 10 away from the expected value have a maximum drat selkin and tauxe 2000 that never exceeds 5 while the mean value of all deviations is lower than 3 5 another interesting feature is the existence of a link between the unblocking temperature spectra and the accuracy of the paleofield which points out the origin of difficulties in the presence of a wide distribution of grain sizes and or the existence of several magnetic phases the results also indicate that magnetite with a narrow range of high unblocking temperatures and little variability in grain size is a favorable situation to obtain suitable determinations with a high success rate these observations may well explain the dispersion inherent to the previous studies of the hawaiian flows

macroscale dynamics of experimental landscapes abstract we review results from laboratory scale modelling of erosion and relief dynamics under variable uplift and rainfall rates under constant values of these forcing parameters an experimental landscape typically evolves towards a steady state between uplift and erosion and we show how the geometry of the steady state landscape adjusts to the rates of uplift and rainfall the comparison between these laboratory scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions laboratory scale modelling in geomorphology is therefore only experimental not analogue despite these limitations experimental models may be used to provide physical tests for numerical models and they give insights into first order behaviours and directions for future research

a selective procedure for absolute paleointensity in lava flows 1 we present a compilation of experiments of absolute paleointensity using double heating protocols on very recent lava flows from hawaii la reunion the canary islands and santorini the existence of a sharp distribution of grain sizes carried by a single mineralogical phase always yielded successful determinations of paleointensity that deviate by less than 10 from the actual field value thus a rapid decrease of at least 70 of the initial magnetization over a narrow range of temperatures prior to the curie point combined with a unique mineralogical phase define an optimal situation for obtaining reliable estimates of absolute paleointensity consequently we suggest that stepwise standard thermal demagnetization of companion specimens should be routinely performed prior to paleointensity experiments not only do these measurements provide important information about the characteristic magnetization but they indicate which samples are appropriate for paleointensity experiments which increases the success rate to almost 100

a physical explanation for an observed area slope elevation relationship for catchments with declining relief a general relationship between the contributing area slope and mean elevation of catchments with relief declining after a tectonic uplift event is presented this relationship is based on the continuity equation for runoff and erosion processes in the catchment the key hypothesis underlying this relationship is that as a catchment declines the nondimensionalized catchment approaches a constant form this hypothesis is verified for computer simulated catchments the area slope elevation relationship covers several cases catchments declining toward a peneplain catchments declining from a high slope dynamic equilibrium resulting from a high rate of tectonic uplift to a low slope one resulting from a lower rate of tectonic uplift and catchments declining from an elevated initial condition as for example in the decline of a mine spoil heap a previously published relationship between slope and area for catchments in dynamic equilibrium and based on runoff and erosion physics is shown to be a special case of this general relationship the new area slope elevation relationship is compared with data from simulated catchments and a field catchment it is thus shown that for declining catchments the area slope elevation relationship is a good predictor of catchment form for catchments with declining relief it is argued that the slope area elevation relationship is sufficient with the planiform drainage pattern to completely define the elevation properties of the catchment such as for instance the hypsometric curve

palaeointensity results from the permo carboniferous kiaman reversed superchron the great whin and midland valley sills of the northern united kingdom summary fifty thellier palaeointensity determinations were made from the great whin sill gws of northern england and the midland valley sill mvs of central scotland which form the major part of a late carboniferous quartz dolerite complex palaeodirectional analysis confirms that the reversed polarity characteristic remanence carried by samples from both sills is consistent with the permo carboniferous reversed superchron palaeomagnetic results suggest that the two sills may not be exactly contemporaneous the mvs was intruded rapidly some time during the emplacement of the gws which was itself emplaced over a considerable time period the mvs may well represent the northernmost expression of the instrusive activity a mean palaeointensity of 22 9 2 6 t was calculated for the great whin sill with a corresponding vdm value of 5 9 1022 a m2 the palaeointensity value calculated from the midland valley sill was much lower 13 0 0 5 t with a corresponding vdm value of 3 3 1022 a m2 detailed rock magnetic analyses and stepwise thermal demagnetization studies reveal that the remanence carrier in both sills is magnetite of mixed domain state multidomain grains are less important in the midland valley sill and thus greater confidence is attached to palaeointensity results from this sill vdm values from this and other studies of the permo carboniferous reversed superchron p crs indicate that the dipole strength showed similar variations to that witnessed for the past 5 ma there is no conclusive support in favour of either the weak or strong field models that have been suggested for superchrons more palaeointensity results are needed from the termination of the p crs and the late permian early triassic in order to understand how the dipole field evolved during the p crs and its relationship to the so called mesozoic dipole low

height of faceted spurs a proxy for determining long term throw rates on normal faults evidence from the north baikal rift system siberia 1 we present new results on the long term throw rates of active normal faults in the north baikal rift nbr eastern siberia based on a statistical analysis of triangular faceted scarps fault bounded ridges in the nbr display typical morphologies with several contiguous facets separated by fault perpendicular catchments over a range of 20 fault segments analyzed triangular facet heights vary from 200 to 900 m as fault scarps have been developing under similar long term climatic conditions we infer that the scatter in mean facet height arises from long term differences in fault throw rate we compare the morphology of nbr facets with results obtained in a previously published numerical model of facet growth using facet height as an input model results provide estimates of the long term fault throw rate nbr throw rates vary between 0 2 and 1 2 mm yr 1 the throw rates are then compared with the cumulated throw which has been constrained by geophysical and stratigraphic data in the basins this provides an estimate of the age of fault and basin initiation we show that the modern stage of basin development started circa 3 myr ago except for the north baikal basin 8 ma our results also suggest that a proportion of the observed throw is inherited from an earlier tectonic stage

faceted spurs at normal fault scarps insights from numerical modeling 1 we present a combined surface processes and tectonic model which allows us to determine the climatic and tectonic parameters that control the development of faceted spurs at normal fault scarps sensitivity tests to climatic parameter values are performed for a given precipitation rate when hillslope diffusion is high and channel bedrock is highly resistant to erosion the scarp is smooth and undissected when instead the bedrock is easily eroded and diffusion is limited numerous channels develop and the scarp becomes deeply incised between these two end member states diffusion and incision compete to produce a range of scarp morphologies including faceted spurs the sensitivity tests allow us to determine a dimensionless ratio of erosion f for which faceted spurs can develop this study evidences a strong dependence of facet slope angle on throw rate for throw rates between 0 4 and 0 7 mm a facet height is also shown to be a linear function of fault throw rate model performance is tested on the wasatch fault utah using topographic geologic and seismologic data a monte carlo inversion on the topography of a portion of the weber segment shows that the 5 ma long development of this scarp has been dominated by a low effective precipitation rate 1 1 m a and a moderate diffusion coefficient 0 13 m2 a results demonstrate the ability of our model to estimate normal fault throw rates from the height of triangular facets and to retrieve the average long term diffusion and incision parameters that prevailed during scarp evolution using an accurate 2 d misfit criterion

palaeomagnetism of east siberian traps and kimberlites two new poles and palaeogeographic reconstructions at about 360 and 250 ma summary we studied basalt sills and dykes and kimberlite pipes from the eastern part of the siberian platform these are spread out over a few hundred kilometres and are divided into two age groups late permian early triassic mir aikhal and olenyek regions and late devonian early carboniferous viluy and markha basins aikhal region the palaeomagnetic poles determined for each sill or dyke are statistically different from each other but averaging all corresponding directions from these sills and dykes with those from kimberlites of different generations yields results with acceptable statistics this is probably due to the averaging out of secular variation and or changes in magnetic polarity the resulting palaeomagnetic poles allow one to reconstruct the palaeopositions of siberia during the two large flood basalt events at about 360 and 250 ma a significant rotation of the siberian platform during the devonian and carboniferous is suggested the new data are in general agreement with some earlier palaeomagnetic poles for the late permian early triassic traps of the tungusska basin and significantly reduce uncertainties for previous late devonian early carboniferous siberian poles which were very scattered together with geological and absolute age data the palaeomagnetic data support the hypothesis that kimberlite magmatism and trap intrusions were both rapid and relatively close in time the 360 ma event is emerging as a magmatic event with considerable extent over the russian and siberian platforms related to incipient continental break up and triple junction formation possibly at or near the time of the frasnian famennian mass extinction

a linear field dependence of thermoremanence in low magnetic fields abstract we tested a linear field dependence of thermoremanent magnetization trm to saturation isothermal remanent magnetization sirm ratio for magnetite containing natural samples the trm sirm shows a linear field dependence to very low field ranges

dating bedrock gorge incision in the french western alps ecrins pelvoux massif using cosmogenic 10be terra nova 22 18 25 2010 abstract we report in situ produced 10be data from the gorge du diable french western alps to date and quantify bedrock gorge incision into a glacial hanging valley we sampled gorge sidewalls and the active channel bed to derive both long term and present day incision rates 10be ages of sidewall profiles reveal rapid incision through the late holocene ca 5 ka at rates ranging from 6 5 to 13 mm yr 1 present day incision rates are significantly lower and vary from 0 5 to 3 mm yr 1 within the gorge our data imply either delayed initiation of gorge incision after final ice retreat from internal alpine valleys at ca 12 ka or post glacial surface reburial of the gorge our results suggest that fluvial incision rates 1 cm yr 1 into crystalline bedrock may be encountered in transient landscape features induced by glacial interglacial transitions

a comparison of experimental and natural drainage basin morphology across a range of scales 1 numerous quantitative models have been developed to simulate the long term evolution of river basin topography most of these models reach an equilibrium state when the base level is lowered at a constant rate and the substrate and climate are constant in time at equilibrium the erosion balances the base level lowering so the topography remains frozen in time however some recent laboratory experiments contradict this behavior in these experiments the space time mean erosion rate balances the base level lowering rate but the actual erosion rates are highly variable in space and time consequently the channel networks are substantially mobile the experimental flow conditions are approximately similar but not dynamically equivalent to those in nature so it is unclear whether the observed dynamics are representative this paper examines whether the processes occurring in the experiments produce basins that are geometrically similar to typical natural basins to make this comparison the conceptual framework of scaling invariance is used many similarities are observed but we find that the experimental basins are slightly more elongated across a range of scales than the natural basins we consider and the relationship between channel slope and drainage area exhibits a break that is not found in the natural basins we analyze in the experiments the slopes of small channels also depend on the proximity of large channels this dependency is more pronounced for experiments with more mobile channel networks and it is not observed in most of the natural basins we analyze

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

a palaeomagnetic and palaeointensity study of the mauchline lavas south west scotland the mauchline lavas of sw scotland erupted close to the carboniferous permian boundary and near the middle of the permo carboniferous reversed superchron pcrs rock magnetic studies have identified mixed magnetite hematite assemblages both minerals record reversed components of magnetisation a fold test is not diagnostic but the mean remanence recovered in 23 stable samples at five sites di 192 5 26 1 95 14 3 accords with the regional field at the time of eruption and the pole position 346 8 e 47 2 s is coincident with late carboniferous early permian poles a palaeointensity study of 15 samples employing the modified thellier thellier method yields nine acceptable estimates of ancient field intensity with a mean value of 13 6 4 5 t from four lava flows the corresponding virtual dipole moment of 3 3 1022 am2 is approximately 40 of the present day value this result supports recent studies from western europe which suggest that the geomagnetic field strength was low during the pcrs but is in conflict with russian data which suggest that the field strength was close to or slightly higher than the present day value