long term atmospheric deposition as the source of nitrate and other salts in the atacama desert chile new evidence from mass independent oxygen isotopic compositions abstract isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the atacama desert in northern chile has shown anomalous 17 o enrichments in both minerals 17 o values of 14 21 in nitrate and 0 4 to 4 in sulfate are the most positive found in terrestrial minerals to date modeling of atmospheric processes indicates that the 17 o signatures are the result of photochemical reactions in the troposphere and stratosphere we conclude that the bulk of the nitrate sulfate and other soluble salts in some parts of the atacama desert must be the result of atmospheric deposition of particles produced by gas to particle conversion with minor but varying amounts from sea spray and local terrestrial sources flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200 000 to 2 0 m years during hyper arid conditions similar to those currently found in the atacama desert correlations between 17 o and 18 o in nitrate salts from the atacama desert and mojave desert california indicate varying fractions of microbial and photochemical end member sources the photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture biologic activity and nitrogen cycling these isotopic signatures have important implications for paleoclimate astrobiology and n cycling studies

flow and fabric development during the diapiric rise of magma scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere and new numerical results are presented for the incremental strain pattern developed in spherical diapirs the results predict a time dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain finite strains are discussed in terms of the different sub fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin and magmatic or even isotropic fabrics in the i

importance of pressure solution and coble creep in the deformation of polymineralic rocks the theoretical framework for modeling pressure solution in polyphase rocks is presented it is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere it is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts and therefore the importance of this deformation mechanism importance is perhaps underestimated this behavior contrasts with that of a polymineralic rock undergoing dislocation creep where existing models show that the rheology will be some average of the rheologies of individual phases for instance the rheology of a dunite in pressure solution is governed by the diffusion of both mgo and sio2 whichever is the slowest will control the viscosity of the rock conversely in a harzburgite with a specific texture flow will be governed by the fastest diffusing species thus its strength may be as low as that of a quartz rock of comparable grain size given the comparative strength of many phases undergoing dislocation creep relative to quartz this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously predictions on textural evolution of polymineralic aggregates are also made and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation even though the rock as a a whole shortens in this direction

algorithm for error adjustment of potential field data along a survey network gravity and magnetic surveys on sea land or air invariably exhibit discrepancies at intersections of survey lines these misties have to be removed before any meaningful contour map can be prepared the misties are caused by many errors errors in location fixing instrumental drift diurnal magnetic variations inaccuracies in eotvos corrections because of errors in heading and speed of ship or aircraft play a major role foster 1970 proposed a method for statistical estimation of the misties at line intersections with special reference to aeromagnetic data the method can be extended to all gravity and magnetic surveys on land sea or air however it requires an approximately rectangular grid of survey lines in case the network is irregular and there are lines in various directions crossing each other foster s method would not be applicable

integration of 3d geological modeling and gravity surveys for geothermal prospection in an alpine region abstract thermal sources are common manifestations of geothermal energy resources in alpine regions the up flow of the fluid is well known to be often linked to cross cutting fault zones providing a significant volume of fractures since conventional exploration methods are challenging in such areas of high topography and complicated logistics 3d geological modeling based on structural investigation becomes a useful tool for assessing the overall geology of the investigated sites geological modeling alone is however less effective if not integrated with deep subsurface investigations that could provide a first order information on geological boundaries and an imaging of geological structures with this aim in the present paper the combined use of 3d geological modeling and gravity surveys for geothermal prospection of a hydrothermal area in the western alps was carried out on two sites located in the argentera massif nw italy the geothermal activity of the area is revealed by thermal anomalies with surface evidences such as hot springs at temperatures up to 70 c integration of gravity measurements and 3d modeling investigates the potential of this approach in the context of geothermal exploration in alpine regions where a very complex geological and structural setting is expected the approach used in the present work is based on the comparison between the observed gravity and the gravity effect of the 3d geological models in order to enhance local effects related to the geothermal system it is shown that a correct integration of 3d modeling and detailed geophysical survey could allow a better characterization of geological structures involved in geothermal fluids circulation particularly gravity inversions have successfully delineated the continuity in depth of low density structures such as faults and fractured bands observed at the surface and have been of great help in improving the overall geological model

evolution and geochemical signatures in a neogene forearc evaporitic basin the salar grande central andes of chile abstract the salar grande is a 45 km long n s axis 4 5 km wide fault bounded pull apart neogene forearc evaporitic basin located in the cordillera de la costa of northern chile whose sedimentary record is composed almost exclusively of a massive halite salt body underground waters coming from the east now the precordillera and altiplano were progressively enriched in solutes by interaction with volcanic rocks these fluids drained into a lacustrine system located in the depresion central in the llamara quillagua area and thick sequences of diatomites carbonates evaporites and clastic sediments were deposited these lacustrine areas eventually dried out and became a salar with anhydrite and halite during this stage late miocene pliocene evolved brines of the llamara quillagua area migrated eastwards through structural paths reaching the salar grande basin which acted as a final sink a proof of this connection is the anhydrite level that links the western llamara and southeastern salar grande areas the accessory minerals and geochemical signatures in the rock salt are a record of the inputs arriving to the former lake and reflect variations in climate and in tectonic and volcanic activity the arrangement of halite textures along the sequence shows an evolution from a very shallow ephemeral saline lake some cm dm deep on average to a salar where interstitial processes controlled by phreatic brines dominate this evolution is coherent with a slight increase in the terrigenous content of the rock salt and documents the increasing aridity of the climate as a response to the tectonic uplift of the andean ranges in pliocene times pore brines were definitively lost as a response to the downfall of the water table generated by the beginning of the exorheism due to the opening of the loa river canyon towards the sea the aim of this paper is to describe the salar grande through the study of its geological setting and its salt record a detailed description of the salar will enable us to reach valuable conclusions about environmental conditions in the area during the neogene prior to the opening of the hydrologic system to the pacific ocean

finite deformation in and around a fluid sphere moving through a viscous medium implications for diapiric ascent abstract different types of geodynamically active regions like mountain belts hot spots or areas of salt tectonics are characterized by diapirism one key for the reconstruction of the dynamic history of such structures is the progressive strain which can sometimes be determined from field observations successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir a constant viscosity fluid sphere of radius r rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere regions of very high strains are found in a tube with a radius 1 2 r behind the sphere and in a shell of thickness of 0 1r around the sphere the following three dimensional strain regimes can be identified in the half space above the sphere down to its equator finite strains are oblate behind the sphere they progressively change to plane strain in the diapiric source region they are prolate viscous drag at the sphere s surface leads to an internal circulation with one overturn after 10r of rise if the sphere has a relatively low viscosity finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining after several body radii of rise the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the sphere s surface where strong prolate and oblate strains are observed respectively finite strain determinations in a falling ball experiment cruden 1988 are compared with the theoretical results at horizontal distances of a few body radii from the fall axis the effect of confining container walls is clearly seen in the experimental strain data the results are compared briefly with available strain data from the field which seem to be significantly lower than predicted proposed explanations include a short memory of the rock fabric and a lack of recognition of the strain concentration which would be expected for a temperature dependent rheology it might also be possible that few natural diapirs rise more than a few radii in the solid state

kinematic gps as a source for airborne gravity reduction in the airborne gravity survey of switzerland an airborne gravity survey of switzerland was performed in a joint project between the swiss federal institute of technology zurich and lacoste and romberg gravity meters inc the survey was flown in a twin otter aircraft equipped by the swiss federal directorate of cadastral surveying the aircraft was outfitted with three global positioning system receivers one for navigational purpose and two as sources of positioning velocity and vertical acceleration for airborne gravity reduction four receivers were installed on the ground as reference stations the gravity data were recorded with a modified lacoste and romberg marine gravimeter at a sampling rate of 1 s modifications included anti alias filters and an absolute encoder for the measuring screw the flights were performed at a barometric altitude of 5100 m above sea level an airborne bouguer anomaly map computed at flight altitude with topographic corrections of up to 167 km and a density of 2670 kg m3 is presented bouguer anomaly values at crossing points between lines flown at comparable altitudes h 10 m show differences varying between 1 1 and 2 7 mgal 10 5 m s2 comparison of the ground bouguer anomaly map upward continued to 5100 m and the one computed from airborne data shows a strong similarity both in amplitude and wavelength

ascent of a granitoid diapir in a temperature varying medium a relationship for the ascent velocity of a granitoid diapir is derived and used in conjunction with a crustal heat flow model which incorporates a depth dependent temperature to predict emplacement histories ascent rate is strongly dependent on the width of a deformation layer of variable viscosity that is formed by the transfer of heat to the country rock and on the buoyancy force which propels the diapir through it ascent velocity is described by the expression u 2 gd3z 0r where is the density contrast between diapir and country rock g is gravitational acceleration d is the deformation zone width z is a parameter which describes the increase in viscosity across the zone 0 is the country rock viscosity at the contact and r is the diapir radius the width of the deformation zone as dictated by the temperature distribution and rheological function is the point at which the viscosity increases by a factor f from the contact value although ascent rates vary with choice of this parameter ascent distances are not strongly dependent on f results of simulations which incorporate a spectrum of input parameters reveal that ascent distance is strongly dependent on both the rheology and ambient temperature of the encompassing rock but only weakly dependent on the initial diapir temperature ascent velocity varies with the diapir size with larger bodies moving more slowly and ascending for longer periods relative to smaller plutons as a result of the proportionately larger amount of thermal energy available for ascent larger bodies will rise greater distances than smaller diapirs however the additional ascent appears nominal relative to the contrast in available energy modeling diapirs using a no slip boundary condition decreases ascent velocity by a factor of about 3 relative to results incorporating a stress free boundary condition the time scale of granitoid diapirism over crustal distances cannot be less than 104 years due to the rate limiting effect of thermal conduction above the body and not greater than 105 years reflecting solidification times of kilometer sized plutons a consequence of ascent by a deformation zone mechanism is the generation following emplacement of a mechanically weakened crust which extends from beneath the diapir to near the source perhaps explaining the relatively common occurrence of synmagmatic to postmagmatic mafic dikes within granites believed to be derived from interaction of basalt with the lower crust

the strength of the continental crust detachment zones and the development of plastic instabilities abstract the maximum strength of the continental crust is constrained by the geothermal gradient the lithological make up of the crust and whether or not the byerlee relation holds to the base of the continental crust if this linear byerlee relation holds then maximum shear stress levels sustained at low geothermal gradients 10 20 c km 1 could be as high as 300 mpa in the upper part of the mantle for strike slip regimes and up to 700 mpa in thrust regimes towards the base of the crust however if byerlee s relation breaks down at moderate temperatures and pressures and if such breakdown is associated with the transition from unstable to stable sliding on pre existing faults then the maximum stress levels in the crust being set by the shear stress at the breakdown depth are much lower ca 300 mpa for low geothermal gradients and thrust regimes the lithosphere is quite weak below the crust for geothermal gradients greater than 20 c km 1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned for a geothermal gradient of 10 c km 1 the entire continental crust and part of the upper mantle behave as an elastic brittle slab at higher geothermal gradients the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make up large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries at these higher geothermal gradients such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle detachment at or near the moho is to be expected only for low geothermal gradients lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high ca 100 200 mpa for geothermal gradients ranging from 10 to 50 c km 1 and for quartz rich felsic mafic and peridotitic rock types thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ca 10 km

geochemical isotopic and mineralogical constraints on atmospheric deposition in the hyper arid atacama desert chile abstract modern atmospheric deposition across the atacama was collected by an array of dust traps that stretched from the pacific coast to the andean altiplano and the material was analyzed for its geochemical mass and isotopic composition the coastal trap had the second highest insoluble mineral particle and highest soluble salt deposition rates due to significant inputs from the morro mejillones range and the pacific ocean respectively the andean trap had the highest insoluble mineral particle deposition owing to transport of weathered material but the lowest deposition rate of soluble salts due to its distance from the ocean and anthropogenic sources the removal of oceanic material was effective by the coastal mountains while the westward transport of the andean material was determined to be minimal the atmospheric deposition in the inland traps was mainly from the local entrainment of surface material inland anthropogenic emissions and transport of marine aerosols the nitrate isotopes 15 n and 17 o suggested that no x sources and no 3 chemistry shifted along the west east transect and were greatly impacted by anthropogenic emissions with soil no 3 being a minor source of deposited nitrogen

oblique diapirism the criffel granodiorite granite zoned pluton southwest scotland abstract strain trajectories strain gradients and quartz c fabrics have been used to propose an emplacement history of the criffel zoned pluton strain has been estimated from the microdioritic xenoliths included in the outer granodiorite textural criteria have been used to complete the pattern of strain variations throughout the pluton strain gradients are found to be related to the granodiorite granite boundary zone with highest strain towards the northeast side of the pluton giving an asymmetric horseshoe pattern quartz c fabrics allow two strain histories to be distinguished within the pluton an early coaxial flattening in the granodiorite followed by a non coaxial deformation in the granite and the adjacent granodiorite these features are interpreted as the result of an oblique diapiric intrusion of the inner granite into the partially emplaced and crystallized granodiorite

airborne gravimetry from a small twin engine aircraft over the long island sound in january 1990 a test of the feasibility of airborne gravimetry from a small geophysical survey aircraft a cessna 404 was conducted over the long island sound using a bell aerospace bgm 3 sea gravity meter gravity has been measured from large aircraft and specially modified de havilland twin otters but never from small standard survey aircraft the gravity field of the long island sound is dominated by an asymmetric positive 30 rngal anomaly which is well constrained by both marine and land gravity measurements using a trimble 4000 gps receiver to record the aircraft s horizontal position and radar altimeter elevations to recover the vertical accelerations gravity anomalies along a total of 65 km were successfully measured the root mean square rms difference between the airborne results and marine measurements within 2 km of the flight path was 2 6 mgai for 15 measured values the anomalies recovered from airborne gravimetry can also be compared with the gridded regional free air gravity field calculated using all available marine and land gravity measurements the rms difference between 458 airborne gravity measurements and the regional gravity field is 2 7 mgai this preliminary experiment demonstrates that gravity anomalies with wavelengthsas short as 5 km can be measured from small aircraft with accuracies of 2 7 mgal or better the gravity measurements could be improved by higher quality vertical and horizontal positioning and tuning the gravimeter s stabilized platform for aircraft use

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

the alpine swiss french airborne gravity survey summary in february 1998 a regional scale airborne gravity survey was carried out over the french occidental alps within the framework of the geofrance 3 d research program the survey consisted of 18 ns and 16 ew oriented lines with a spacing of 10 and 20 km respectively covering the whole of the western french alps total area 50 000 km2 total distance of lines flown 10 000 km the equipment was mounted in a medium size aircraft dehavilland twin otter flowing at a constant altitude of 5100 m a s l and at a mean ground speed of about 280 km h 1 gravity was measured using a lacoste romberg relative air sea gravimeter type sa mounted on a laser gyro stabilized platform data from 5 gps antennae located on fuselage and wings and 7 ground based gps reference stations were used to determine position and aircraft induced accelerations the gravimeter passband was derived by comparing the vertical accelerations provided by the gravimeter with those estimated from the gps positions this comparison showed that the gravimeter is not sensitive to very short wavelength aircraft accelerations and therefore a simplified formulation for computing airborne gravity measurements was developed the intermediate and short wavelength non gravitational accelerations were eliminated by means of digital exponential low pass filters cut off wavelength 16 km an important issue in airborne gravimetry is the reliability of the airborne gravity surveys when compared to ground surveys in our studied area the differences between the airborne acquired bouguer anomaly and the ground upward continued bouguer anomaly of the alps shows a good agreement the rms of these differences is equal to 7 68 mgal for a spatial resolution of 8 km however in some areas with rugged topography the amplitudes of those differences have a striking correlation with the topography we then argue that the choice of an appropriate density reduction by a factor of 10 per cent for computing the ground topographic corrections over the highest mountains results in significantly reducing the differences between airborne and ground upward continued bouguer anomalies which shows that some of the misfit stems from errors in the ground data

problems in the extrapolation of laboratory rheological data abstract the many types of variables and deformation regimes that need to be taken into account in extrapolating rheological behaviour from the laboratory to the earth are reviewed the problems of extrapolation are then illustrated with two particular cases in the case of divine rich rocks recent experimental work indicates that within present uncertainties of extrapolation the flow in the upper mantle could be either grain size dependent and near newtonian or grain size independent and distinctly non newtonian both types of behaviour would be influenced by the present of trace amounts of water in the case of quartz rich rocks the uncertainties are even greater and it is still premature to attempt any extrapolation to geological conditions except as an upper bound the fugacity and the scale of dispersion of the water are probably two important variables but the quantitative laws governing their influence are not yet clear

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

an airborne gravity study of eastern north carolina the naval research laboratory nrl has developed a prototype airborne gravity measurement system the core of the system is a lacoste and romberg air sea gravity meter mounted on a three axis stable platform corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters the original prototype system was designed for use over oceanic areas we recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained the radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces vertical accelerations due to horizontal velocity over a curved rotating earth the eotvos correction and precise two dimensional positions are determined from a texas instrument p code global positioning system the updated system was tested

deformation around a rising diapir modeled by creeping flow past a sphere a new experimental method for determining the pattern of flow and three dimensional progressive finite deformation of fluid elements in low reynolds number re viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder this is intended as a model analogue of natural rock deformation associated with magmatic and solid state diapirs rising through ductile crust the kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures the experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent newtonian test fluid used sgm36 are discussed in detail the progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis the progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere the results compare favorably with the external strain pattern associated with post tectonic mesozonal diapiric intrusions

the effects of shape on crystal settling and on the rheology of magmas the effects of the shape of phenocrysts on their settling velocity and on the rheology of magmas are discussed spherical crystals fall at a velocity given by stokes law and need to be present in large concentrations before they have a marked effect on the viscosity of a suspension elongate crystals fall at a much smaller velocity which may be determined from the formulas and graphs given herein such elongate crystals can have a dramatic effect on the viscosity of a suspension even at low volume fractions suspended crystals and bubbles will not impart a yield stress to the magma unless they form a touching network across the entire suspension it is suggested therefore that most previous inferences of a yield stress in subliquidus magmas are erroneous measurements of the subliquidus rheology of magmas need to be accompanied by careful characterization of the microstructure and distribution of the solid phase on which such rheologies critically depend

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

the role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts abstract soils in similar geomorphic settings in hyperarid deserts 1 should have similar characteristics because a negative moisture balance controls their development however reg soils in the hyperarid southern negev and namib deserts are distinctly different soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics annual rainfall amount can be similar 15 30 mm but storm duration can drastically alter reg soil properties in deserts the cooler fall winter and dry hot summers of the southern negev desert with a predominance brief 1 day rainstorms result in gypsic saline soils without any calcic soil horizon although the namib desert receives only 50 60 of the southern negev annual rainfall its rainstorm duration is commonly 2 4 days this improves leaching of the top soil under even lower annual rainfall amount and results in weeks long grass cover the long term cumulative effect of these rare rain grass relationships produces a calcic gypsic saline soil the development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil moisture regime in deserts and has important implications for the use of key desert soil properties as proxies in paleoclimatology

diapiric ascent of magmas through power law crust and mantle there has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust we argue here that this is mainly because the country rocks have previously been assumed to be newtonian and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten the ascent velocity of diapirs is calculated through an equation with the form of the hadamard rybczynski equation for the rise of spheres through newtonian ambient fluids this well known equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid these correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of newtonian drops through power law fluids the equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks the velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth the depth of solidification of the diapir is systematically studied as a function of the geothermal gradient buoyancy of the body solidus temperature of the magma and rheological parameters of the wall rock the results show that when the wall rock behaves as a power law fluid the diapir s ascent rate increases without a similar increase in the rate of heat loss in this way diapirs rising at 10 to 102 m yr can ascend into the middle or upper crust before solidification strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates the thermal energy of the diapir is used to soften the country rock only at late stages of ascent the transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

mechanics of active salt diapirism abstract an active diapir forcefully intrudes its overburden driven by diapir pressure that overcomes the resistance of the overburden strength possible causes for the driving pressure are differential loading of the source layer and a density contrast with the overburden possible resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion for a typical density contrast between salt and a sedimentary overburden a simple force balance demonstrates that the diapir height must be more than two thirds to three quarters the thickness of the surrounding overburden to initiate substantial active diapirism narrow diapirs must be even taller physical and numerical modeling show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward curved reverse faults can separate the flaps from the regional overburden normal faults in the crestal graben propagate downward with new faults created farther outward as the roof arches over the rising diapir thin wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps in cases leading to emergence the piercing diapir evolves from a pointed crest to a progressively rounded and widening crest as the flaps rotate outward the numerical models show that overburden flexure contributes to early formation of active diapir structures but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness active diapirism becomes progressively more difficult for diapir crests ranging from a rectangle a round cornered rectangle a semicircle to a triangle with a pointed crest

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

anomalous 17o compositions in massive sulphate deposits on the earth the variation of 18o that results from nearly all physical biological and chemical processes on the earth is approximately twice as large as the variation of 17o this so called mass dependent fractionation is well documented in terrestrial minerals1 2 evidence for mass independent fractionation 17o 17o 0 52 18o where deviation from this tight relationship occurs has so far been found only in meteoritic material and a few terrestrial atmospheric substances3 in the rock record it is thought that oxygen isotopes have followed a mass dependent relationship for at least the past 3 7 billion years ref 4 and no exception to this has been encountered for terrestrial solids5 here however we report oxygen isotope values of two massive sulphate mineral deposits which formed in surface environments on the earth but show large isotopic anomalies 17o up to 4 6 these massive sulphate deposits are gypcretes from the central namib desert and the sulphate bearing miocene volcanic ash beds in north america the source of this isotope anomaly might be related to sulphur oxidation reactions in the atmosphere and therefore enable tracing of such oxidation these findings also support the possibility of a chemical origin of variable isotope anomalies on other planets such as mars6

emplacement kinematics of a granite diapir the chindamora batholith zimbabwe the geometric features of a large 50 40 km granitic batholith and its surrounding greenstone belt envelope are described the batholith is made up of several intrusive components and an intrusion sequence based on contact relations and xenoliths has been established the igneous rocks show a local foliation s fabric and preferred orientation of minerals the intensity of this fabric accords with the finite strain determined from xenolith shape analysis when the strain variations are analysed in detail it appears that the batholith was built up by a succession of magmatic pulses each of which led to a stretching of the previously consolidated igneous material so called balloon tectonics evidence is presented that xenolith deformation commences only when the surrounding igneous material has crystallized and a method is suggested whereby the position of the surface separating crystal supported phase plastic behaviour from melt supported phase liquid behaviour at different stages in the plutonic history can be determined ductile shear zones are present and their geometry accords well with the shape changes necessary to allow the outer parts of the pluton to stretch

oxygen 17 anomaly in soil nitrate a new precipitation proxy for desert landscapes abstract the nitrogen cycle in desert soil ecosystems is particularly sensitive to changes in precipitation even of relatively small magnitude and short duration because it is already under water stress this suggests that desert soils may have preserved past evidence of small variations in continental precipitation we have measured nitrate no 3 concentrations in soils from the atacama chile kumtag china mojave us and thar india deserts and stable nitrogen and oxygen isotope 15 n 17 o and 18 o abundances of the soil no 3 17 o anomalies 17 o the deviations from the mass independent isotopic fractionation were detected in soil no 3 from almost all sites of these four deserts there was a strong negative correlation between the mean annual precipitation map and soil no 3 17 o values o no 3 soil 17 this map o no 3 soil 17 correlation advocated o no 3 soil 17 as a new precipitation proxy and was then used to assess precipitation changes in southwestern us at the pliocene pleistocene boundary in south america during the miocene and the sahara desert in the past 10 kyr using no 3 17 o in paleosols or ancient aquifers global and the us maps of surface o no 3 soil 17 were also projected with available map datasets based on the map o no 3 soil 17 model

on granitoid emplacement and related structures a review granitoid magmas are emplaced at the upper crust under a complex interaction of gravitational processes and horizontal tectonics natural intrusive bodies show a wide variety of structural patterns that must be strongly related with their dynamic history of ascent and emplacement comparisons of natural structural patterns with experimental and numerical models are necessary to interpret in a coherent fashion the emplacement mechanism involved from natural experimental numerical and theoretical considerations one can reach important conclusions on the problem of ascent and final emplacement of granitoid magmas in the earth s crust the present paper reviews the principal emplacement mechanisms referred to in the literature that is doming diapirism ballooning stoping cauldron subsidence and dike propagation dike propagation is the most effektive process in magma transport from deep zones in the crust and upper mantle extensional fractures for dike propagation and magma ascent can be developed at depth in the crust according to the modern dike propagation theory in orogenic domains magmas reach the upper crust through narrow channels or dikes and are accumulated in a final reservoir in the form of a discordant supracrustal pluton or a ballooning pluton if during emplacement any regional deformation acts simultaneously

using pluton ages to date regional deformations problems with commonly used criteria studies of pretectonic plutons in the foothills terrane central sierra nevada california along with a review of studies on syntectonic and post tectonic plutons indicate that no single criterion can establish the relative timing of pluton emplacement and regional deformation criteria most often used are whether a pluton is deformed cuts regional structures or is associated with porphyroblasts that postdate regional structures however all three types of plutons may 1 appear deformed or nondeformed 2 have contacts that appear to cut structures in the wall rocks and 3 have porphyroblasts in their aureoles that display varied timing relations the ambiguity of these criteria emphasize the need for careful studies of the structures present throughout the pluton and surrounding wall rock with particular attention paid to regions where the pluton contact is at high angles to regional structures porphyroblast cleavage relations are an important tool in this regard but also must be used with caution

the genesis ascent and emplacement of the northern arran granite scotland implications for granitic diapirism integrated structural and petrological data are used to trace the evolution of the northern arran coarse granite during the latter stages of its ascent and its emplacement structural analysis of the aureole of the granite suggests that it rose as a single body of magma which is confirmed by petrological data during its ascent the pluton intersected the downward dipping limb of an existing synform refolding and displacing it upward to form a concentric or rim synform this direct evidence of vertical displacement of the country rocks confirms that the pluton is a diapir diapiric ascent was permitted by a reduction in wall rock viscosity and hence viscous drag forces on the pluton aided by heat loss due to crystallization during ascent most of the deformation within the aureole can be attributed to ascent of the pluton the early rim synform is overprinted by flattening strains and faulting caused by in situ radial expansion which represents the ultimate evolution of a diapiric body the final shape of the pluton is determined by inhomogeneous deformation of the aureole which is partly controlled by pre existing faults

chlorine 36 dating of very old groundwater 2 milk river aquifer alberta canada the milk river aquifer in southern alberta canada consists of sandstone interbedded between thick shale units the groundwater is confined and discharges by both upward and downward leakage through the shales the concentration of cl increases by about 2 orders of magnitude downgradient through the aquifer this cl increase complicates 36cl dating of the system climatic changes flushing of connate water introduction of cl from older water below the aquifer and ion filtration have been proposed as possible explanations for the increase the 36cl data show a consistent decrease of the 36cl cl ratio downgradient but an increase in the 36cl concentration downgradient in certain parts of the aquifer dates calculated from the 36cl cl ratio show a reasonable distribution in light of the hydraulic controls on the system whereas dates calculated from the 36cl concentration give negative ages this suggests that ion filtration is responsible for the cl increase and that the 36cl cl dates are to be preferred we estimate water ages to be more than 2 m y near the distal end of the aquifer

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

geodetic and geophysical results from a taiwan airborne gravity survey data reduction and accuracy assessment 1 an airborne gravity survey was conducted over taiwan using a lacoste and romberg lcr system ii air sea gravimeter with gravity and global positioning system gps data sampled at 1 hz the aircraft trajectories were determined using a gps network kinematic adjustment relative to eight gps tracking stations long wavelength errors in position are reduced when doing numerical differentiations for velocity and acceleration a procedure for computing resolvable wavelength of error free airborne gravimetry is derived the accuracy requirements of position velocity and accelerations for a 1 mgal accuracy in gravity anomaly are derived gps will fulfill these requirements except for vertical acceleration an iterative gaussian filter is used to reduce errors in vertical acceleration a compromising filter width for noise reduction and gravity detail is 150 s the airborne gravity anomalies are compared with surface values and large differences are found over high mountains where the gravity field is rough and surface data density is low the root mean square rms crossover differences before and after a bias only adjustment are 4 92 and 2 88 mgal the latter corresponding to a 2 mgal standard error in gravity anomaly repeatability analyses at two survey lines suggest that gps is the dominating factor affecting the repeatability fourier transform and least squares collocation are used for downward continuation and the latter produces a better result two geoid models are computed one using airborne and surface gravity data and the other using surface data only and the former yields a better agreement with the gps derived geoidal heights bouguer anomalies derived from airborne gravity by a rigorous numerical integration reveal important tectonic features

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

biological soil crust and surface soil properties in different vegetation types of horqin sand land china abstract physical and chemical properties including coverage thickness hardness moisture particle size distribution organic matter and nutrient contents etc of biological soil crust and 0 5 0 cm surface soil under the crust in three types of vegetation semi shrub artemisia frigida shrub salix gordejevii and tree populus simonii were surveyed in 2005 and 2006 in horqin sand land to understand the effects of different vegetation types on development of soil crust and surface soil properties under crust the results showed that 1 no physical soil crust or biological soil crust bsc formed on mobile dunes without vegetation though most ground surface in originally mobile dunes had been covered by bsc fifteen years after planted vegetation was established 2 bsc development was best in shrub s gordejevii sites second in semi shrub a frigida sites and weakest in tree p simonii sites 3 development of bsc was better in moss crust distributed mainly beneath or near plant canopies than in lichen crust distributed mainly between plant canopies in all three vegetation types 4 surface soil properties 0 5 0 cm under bsc were improved significantly in all three vegetation types compared to those in mobile sand land the magnitude of improvement was greater in s gordejevii sites than in a frigida and p simonii sites and greater in moss crust sites than lichen crust sites but the magnitude of improvement decreased significantly with increasing surface soil depth 5 based on the above mentioned results we suggest that s gordejevii should be favored in future planting in horqin sand land

airborne gravimetry using a strapped down lacoste and romberg air sea gravity meter system a feasibility study marine gravimeters mounted on stabilized platforms are commonly used in aircraft to perform airborne gravity measurements the role of the stabilized platform is to level the sensor mechanically whatever the aircraft attitude however this compensation is generally insufficient due to the sensitivity of modern gravity sensors correcting the offlevel error requires that an offlevel correction calculated from positioning data be added to gravimeter measurements which complicates not only the processing but also the assessment of precision and resolution this paper is a feasibility study describing the levelling of a completely strapped down lacoste and romberg gravimeter for airborne gravimetry operation by means of gps positioning data it focuses on the calculation of the sensor offlevel correction needed for the complete gravity data processing the precision of the offlevel correction that can be achieved in terms of gps data precision and gravity wavelengths is theoretically studied and estimated using the gravity and gps data acquired during the alpine swiss french airborne gravity survey carried out in 1998 over the french western alps while a 1 cm precision of gps determined baseline coordinates is sufficient to achieve a 5 mgal precision of the offlevel correction we maintain that this precision has to reach 1 mm to ensure a 1 mgal precision of the offlevel correction at any wavelength without a stabilized platform the onboard instrumentation becomes significantly lighter furthermore the correction for the offlevel error is straightforward and calculated only from gps data thus the precision and the resolution of airborne gravity surveys should be estimated with a better accuracy

beryllium 10 concentrations in the hyper arid soils in the atacama desert chile implications for arid soil formation rates and el nino driven changes in pliocene precipitation abstract meteoric 10be concentrations in soil were measured to understand the mechanism timescale and climatic dependence of soil formation in the hyper arid atacama desert the observed systematic decline of soil 10be concentrations with depth has been reproduced using a simple model that assumes soil matrix including 10be builds up as layers over time while 10be decays in situ this suggests a mechanism of soil accumulation via atmospheric deposition which is in agreement with stable isotopic evidence the model estimates an age of 6 6 0 4 ma for the total soil profile small discrepancies between the model and observations are likely mainly due to changes in precipitation rates that can impact 10be delivery rates and 10be movement within the profile interpreted in this way the 10be data suggest drying in the atacama after 4 7 ma and returning to an insignificant wet period at 1 ma which was possibly connected to el nino or la nina like climate change

the rise of solid state diapirs when magma inside a diapir solidifies it increases in density and becomes a stiff power law fluid if the diapir is still positively or negatively buoyant after solidification it will continue to rise or start to sink the results presented here suggest that solidified diapirs may rise sink a distance of up to a few kilometres in geologically reasonable times rocks and bubble and or crystal rich magmas behave as power law fluids this paper compares numerical results of viscosity and strain rate profiles across power law and newtonian diapirs and wall rocks the comparison shows that the core of power law diapirs deforms more slowly and the margins much faster than in newtonian diapirs this pattern of strain rate distribution leads to the often observed isotropic or weakly deformed core surrounded by a strongly sheared margin furthermore the results suggest that the strain rate of the ambient fluid wall rock is imposed across the contact into a power law diapir resulting in similar strains on either side of the contact and that power law wall rocks in contrast to newtonian wall rocks deform into a rim synform as the diapir rises

evidence for active landscape evolution in the hyperarid atacama from multiple terrestrial cosmogenic nuclides the atacama desert is one of the driest places on earth multiple lines of evidence show that the atacama has been hyperarid since at least the late miocene among these are cosmic ray exposure ages indicating that individual clasts on some surfaces have been preserved for 9 ma and possibly since the oligocene although these remarkably old ages indicate slow landscape evolution it is not clear whether this pace is characteristic of the entire atacama or only of specific regions landforms or landscape elements to address this question we measured cosmogenic 10be 26al and 21ne from a wide variety of landscape elements in a transect across the central atacama where modern precipitation is at an extreme minimum but where the concentration of cosmogenic nuclides in stable landscape elements has not previously been recorded we find that the hyperarid core of the central atacama has substantially slower erosion rates than its eastern and western margins however even the driest part of this transect has erosion rates comparable to those of other deserts ranging from 0 2 0 4 m ma the most stable landscape elements are boulder fields with exposure ages of 1 5 2 6 ma the vast majority of samples in the central atacama desert however have cosmogenic nuclide concentrations corresponding to ages 5 ma documented elsewhere in the atacama were not found in our area and appear to be limited to exceptionally stable boulders or cobbles in either the northern or southern extremes of the atacama desert we suggest that the central atacama has been subject to episodic pliocene and pleistocene rainfall and geomorphic activity perhaps due to intrusion of pacific moisture

airborne gravity and precise positioning for geologic applications airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade airborne gravity is especially useful when measured in conjunction with other geophysical data such as magnetics radar and laser altimetry the aerogeophysical survey over the west antarctic ice sheet described in this paper is one such interdisciplinary study this paper outlines in detail the instrumentation survey and data processing methodology employed to perform airborne gravimetry from the multi instrumented twin otter aircraft precise positioning from carrier phase global positioning sys tem gps observations are combined with measurements of acceleration made by the gravity me ter in the aircraft to obtain the free air gravity anomaly measurement at aircraft altitude gps data are processed using the kinematic and rapid static kars software program and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the gps position solu tion accuracies for the free air anomaly are determined from crossover analysis after significant editing 2 98 mgal rms and from a repeat track 1 39 mgal rms the aerogeophysical survey covered a 300 000 km 2 region in west antarctica over the course of five field seasons the grav ity data from the west antarctic survey reveal the major geologic structures of the west antarctic rift system including the whitmore mountains the byrd subglacial basin the sinuous ridge the ross embayment and siple dome these measurements in conjunction with magnetics and ice penetrating radar provide the information required to reveal the tectonic fabric and history of this important region

re examining pluton emplacement processes discussion in a recent review paterson fowler 1993 showed decreasing away from the pluton as discussed by gughow estimated volumes of displaced wall rock are lielmo 1993b smaller than pluton volumes and how the widths of laboratory experiments indicate that rocks behave as strain aureoles around natural plutons are considerably power law fluids with exponents n between 2 5 and 5 narrower than in models of diapirs where spheres rise hansen carter 1982 kirby 1983 wilks carter through newtonian fluids cruden 1988 schmeling et 1990 the power law exponent n of a viscous fluid is a al 1988 they then argued that mechanisms other than measure of the sensitivity of its viscosity stress or ductile flow of the wall rocks were necessary to account strain role for n 1 newtonian fluid the viscosity is for these observations although i accept that several constant for any strain rate for n 1 the fluid softens mechanisms may act simultaneously and change in imwith increased strain rate the strain rate dependency of portance as plutons are emplaced i argue here that i the viscosity of power law fluids has important effects on the displaced volumes of wall rocks may be underestithe velocity and flow patterns of the ambient fluid mated because of difficulties in assessing the shape and around rising spheres or diapirs and therefore the size of the three dimensional flow cell in the wall rocks width of their strain aureoles crochet et al 1984 caused by pluton emplacement and more importantly kawase moo young 1986 weinberg podladchikov ii the narrow strain aureoles may result from diapiric in press in the following discussion the difficulties in rise of plutons through power law rocks and or result determining the volumes of rock displaced by diapir from the effects of thermal softening of the wall rocks emplacement are first considered the effect which several field studies have measured the width of the power law rocks might have on the width of the strain strain aureole caused by pluton emplacement see gugaureole is then discussed and finally the possible effects lielmo 1993a paterson fowler 1993 and references of thermal softening of the wall rocks by the diapir are therein the measured width depends on three main examined factors i the finite strain of the wall rocks which depends on the velocity field imposed by the diapir this in turn depends on the diapir s mechanism and depth of rock volumes displaced by diapirs emplacement and on the rheology and physical boundaries of its wall rocks the longer a rock volume is paterson fowler 1993 studied horizontal shortensubmitted to a particular strain rate velocity field the ing in strain aureoles around plutons and then integrated larger is its strain the velocity of flow decreases with shortening to three dimensions around plutons of differdistance from the contact of rising spheres so that ent shapes the amount of shortening they obtained for strains have to accumulate a long time to be distinguishseveral natural examples only accounted for about 30 able from any background strain ii late deformation of the volume of the plutons they argued that this will mask the strain aureoles of pre tectonic plutons volume difference implies that other mechanisms such while interference with regional tectonic strains will as doming of the roof stoping assimilation and rigid affect the width of the aureoles of syn tectonic plutons translation of wall rock must have operated at the same guglielmo 1993b iii the measurable width of the time as viscous flow of the wall rock aureole depends on distinguishing the strain due to the however this discrepancy may largely be due to pluton from regional strains of any age paterson difficulties in measuring small pluton related strains fowler 1993 this distinction becomes increasingly beyond the obvious strain aureole and difficulties in more difficult as strains are spread over larger volumes determining the shape of the flow cell around the pluton the width of the strain aureole around a rising sphere depends on the power law exponent n of the country current address research school of earth sciences anu act rocks it is widest in newtonian fluids and narrows as n 6200 canberra australia increases however even in the case of rocks with high n

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

role of biological soil crusts in affecting soil evolution and salt geochemistry in hyper arid atacama desert chile abstract eight soil profiles either barren or covered with biological soil crusts bscs were sampled at multiple depths in a remote valley in the atacama desert chile to explore the impact of bscs on soil evolution and salt geochemistry bsc sites had thicker soil profiles compared to non bsc sites which could be a result of past geological processes and also partially accounted for by the establishment of bscs that can help limit erosion and promote significant soil accumulation the greater abundances of fine particles below bscs could essentially be attributed to the ability of bscs to trap more fine grains the generally higher salt ion inventories but smaller salt ion concentrations in soil profiles beneath bscs were probably due to that bscs can promote the retention of salt ions and insoluble dust simultaneously but the latter to a greater extent nitrate oxygen isotopes indicated the predominant existence of atmospheric nitrate in the soils suggesting biological processes of bscs may not significantly impact the ion accumulation or distribution in the underlying soils a mechanism of consistent soil accumulation via the retention of atmospheric deposition and occasional interruption of subsurface collapsing due to erosion processes was proposed to explain the development of soil profiles and different landscape features as well as the role of bscs in soil evolution in this study area the soil ages were estimated to be around 440 ky based on the proposed soil accumulation mechanism

gravity measurements in an airplane using state of the art navigation and altimetry in 1976 the u s naval oceanographic office made experimental gravity measurements over the atlantic in an airplane using a lacoste and romberg shipboard gravity meter a honeywell electrostatic gyro inertial system a rosemont pressure port altimeter a honeywell radar altimeter loran c and other electronic navigational aids preliminary results were reported at the 47th annual international seg meeting in calgary canada in 1977 these preliminary results were compared with sea gravity data furnished by the naval oceanographic office and were found to be in general agreement however there were some unexplained systematic errors of about 15 mgal and the airplane gravity profiles were considerably more noisy than helicopter gravity profiles reported by william gumert 1977 personal communication after the paper was given we found that corrections for schuler oscillations in the inertial system significantly reduced the systematic errors we also found that by making full use of a half second samp

airborne gravity measurements over mountainous areas by using a lacoste romberg air sea gravity meter this paper introduces a new approach to airborne gravity data reduction well suited for surveys flown at high altitude with respect to gravity sources mountainous areas classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western french alps by using a lacoste romberg air sea gravity meter the part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft this finding is theoretically argued by solving the equation of the gravimetric system gravity meter and stabilized platform the transfer function of the system is derived and a new formulation of airborne gravity data reduction which takes care of the sensitive response of spring tension to observable gravity field wavelengths is given the resulting gravity signal exhibits a residual noise caused by electronic devices and short wavelength eotvos effects the use of dedicated exponential filters gives us a way to eliminate these high frequency effects examples of the resulting free air anomaly at 5100 m altitude along one particular profile are given and compared with free air anomaly deduced from the classical method for processing airborne gravity data and with upward continued ground gravity data the well known trade off between accuracy and resolution is discussed in the context of a mountainous area

inter annual variability of southerly winds in a coastal area of the atacama desert implications for the export of aeolian sediments to the adjacent marine environment the analysis of the aeolian content of marine cores collected off the coast of the atacama desert mejillones bay chile suggests that marine sediments can record inter annual to inter decadal variations in the regional southerly winds responsible for particle entrainment at the surface of the nearby desert however the establishment of a simple and direct correlation between the sediment and wind records is complicated by the difference of time scales between the erosion and accumulation processes the aim of this work is to i assess the inter annual variability of the surface winds responsible for the sand movements and ii determine whether the integration over periods of several months completely smoothes the rapid changes in characteristics of the transported and deposited aeolian material to accomplish this aim 14 years of 10 m hourly wind speed measured at the cerro moreno antofagasta airport between 1991 and 2003 and at the orica station between 2000 and 2004 were analyzed for each year the wind speed statistical distribution can be represented by a combination of two to three weibull functions winds of the lowest weibull mode are too weak to move the sand grains at the surface of the pampa this is not the case for the intermediate mode and especially for the highest speed mode which are able to erode the arid surface and transport particles to the bay in each individual year of the period of study the highest speed mode only accounted for a limited number of strong erosion events quantitative analysis of the distribution of the friction velocities and of their impact on erosion using a saltation model suggests that although all wind speeds above threshold produce erosion events values around 0 45 m sec 1 contribute less to the erosion flux this gap allows separation of the erosion events into low and high saltation modes the correlation r 0 997 between the importance of the third weibull mode and the extent of higher rate saltation indicates that the inter annual variability of the erosion at the surface of the pampa as well as the transport of coarse particles 100 m are directly related to inter annual variations in the prevalence of the strongest winds finally a transport and deposition model is used to assess the possible impact of the wind inter annual variability on the deposition flux of mineral particles in the bay the results suggest that inter annual differences in the wind speed distributions have a quantifiable effect on the intensity and size distribution of this deposition flux this observation suggests that a detailed analysis of the sediment cores collected from the bay could be used for reconstructing the inter annual variability of past winds

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

cannibal creek granite post tectonic ballooning pluton or pre tectonic piercement diapir the cannibal creek granite in northern queensland australia previously has been interpreted to be a post tectonic pluton which underwent solid state deformation during in situ ballooning a reinterpretation of structures in and around the pluton indicates that this pluton is more appropriately viewed as a pierce ment diapir emplaced by magmatic flow significant deformation took place after pluton emplacement in fact the cannibal creek granite has some characteristics similar to pre tectonic granites examined elsewhere therefore it is somewhat misleading to view the granite as a post tectonic pluton

polydiapirs multiwavelength gravity structures polydiapirs domes in domes result from the evolution of a multiwavelength rayleigh taylor gravitational instability the present work investigates the initiation and development of polydiapirs by means of two dimensional finite difference models of gravitationally unstable triple layered sequences of newtonian fluids the values of viscosities thicknesses and densities were systematically varied for selected examples in triple layered sequences only two types of density stratification are likely to result in multiwavelength instabilities that will eventually develop into polydiapiric structures the limiting factors and evolution of such structures were studied for both these types of density stratifications finite difference calculations on models with physical parameters appropriate to evaporitic sequences and covered by a viscous clastic overburden showed that a very small downward density decrease in the evaporitic source may cause an internal overturn that will compete with the main overturn between source and overburden this may lead to the development of sequential or simultaneous polydiapirs the mature polydiapirs in the models exhibit spines curtain folds pendant repetitions of the source layers inside the large diapirs and anomalous interdiapiric distances all these features are known in natural salt diapirs suggesting that polydiapirs may be far more common in nature than hitherto recognized

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

the potential roles of biological soil crusts in dryland hydrologic cycles biological soil crusts bscs are the dominant living cover in many drylands of the world they possess many features that can influence different aspects of local hydrologic cycles including soil porosity absorptivity roughness aggregate stability texture pore formation and water retention the influence of biological soil crusts on these factors depends on their internal and external structure which varies with climate soil and disturbance history this paper presents the different types of biological soil crusts discusses how crust type likely influences various aspects of the hydrologic cycle and reviews what is known and not known about the influence of biological crusts on sediment production and water infiltration versus runoff in various drylands around the world most studies examining the effect of biological soil crusts on local hydrology are done by comparing undisturbed sites with those recently disturbed by the researchers unfortunately this greatly complicates interpretation of the results applied disturbances alter many soil features such as soil texture roughness aggregate stability physical crusting porosity and bulk density in ways that would not necessarily be the same if crusts were not naturally present combined these studies show little agreement on how biological crusts affect water infiltration or runoff however when studies are separated by biological crust type and utilize naturally occurring differences among these types results indicate that biological crusts in hyperarid regions reduce infiltration and increase runoff have mixed effects in arid regions and increase infiltration and reduce runoff in semiarid cool and cold drylands however more studies are needed before broad generalizations can be made on how biological crusts affect infiltration and runoff we especially need studies that control for sub surface soil features such as bulk density micro and macropores and biological crust structure unlike the mixed effects of biological crusts on infiltration and runoff among regions almost all studies show that biological crusts reduce sediment production regardless of crust or dryland type copyright 2006 john wiley sons ltd

influence of cryptogamic crust disturbance to wind erosion on sand and loam rangeland soils a portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the mallee region of southeastern australia three levels of crust disturbance nil moderate and severe simulating stock trampling were investigated the results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed on the loamy soil the crust helped maintain sediment flux rates below the erosion control target to 5 g m 1 s 1 for a 65 km h 1 wind measured at 10 m height once the crust was severely disturbed sediment fluxes increased to 1 6 times the erosion target on the sandy soil even with no crust disturbance the sediment flux was 1 6 times the erosion control target disturbing the crust increased sediment fluxes to a maximum of 6 7 times the erosion control target removal of the crust also decreased the threshold wind velocity that resulted in an increase to the ris k of erosion from 5 per cent to 20 per cent 1998 john wiley sons ltd