soil mineralogy trends in california landscapes california s diverse environmental gradients serve as natural experiments for examining controls on soil mineral distribution in landscapes in this paper we use example soilscapes from throughout california to examine how lithology climate topography and duration of pedogenesis interact to produce distinctive weathering environments and characteristic suites of soil minerals seven soil geomorphic sequences were assembled from the literature to illustrate major soil mineralogical trends 1 granitic terrain of the peninsular ranges 2 granitic terrain of the central sierra nevada 3 andesitic terrain of the northern sierra nevada 4 fluvial terraces on the east side of the great valley 5 marine terraces of the central coast 6 ultramafic terrain of the klamath mountains and 7 an alluvial fan in the mojave desert results of this analysis show that kaolin is present in virtually all pedons irrespective of climate parent material age or topographic position kaolin does not form in ultramafic soils due to insufficient aluminum many secondary clay minerals reflect the state s strong climatic influence with palygorskite smectite and vermiculite in the dry hot environments hydroxy interlayered minerals gibbsite and short range ordered minerals in the cool moist environments in arid and semi arid regions the distribution of calcite gypsum and soluble salts is strongly related to patterns of eolian dust deposition and water infiltration and leaching

low to medium pressure variscan metamorphism in galicia nw spain evolution of a kyanite bearing synform and associated bounding antiformal domains

alignment of stone pavement clasts by unconcentrated overland flow implications of numerical and physical modelling the crucial role of stone pavements in arid environments for aeolian or alluvial processes and as numerical dating tools is increasingly acknowledged this role is based on the assumption that stone pavements are stable landforms formed gradually over time and predominantly by vertical processes however this is challenged by evidence of stone pavement clast reworking or burial bimodal mostly slope aspect symmetrical clast orientation is a frequent phenomenon in various study areas it implies that stone pavements may be influenced by unidirectional lateral processes besides vertical ones here the finding of lateral processes contributing to stone pavement evolution is supported by numerical modelling and physical experiments these unequivocally show that unconcentrated overland flow can transport clasts to form a closely packed stone mosaic with characteristics similar to those of natural stone pavements the commonly observed length axes orientation angle of 40 14 for natural stone pavement clasts is consistently reproduced by angle dependent force equilibrium monte carlo runs confirm the natural scatter and allow characterization of the control parameters of clast orientation the model explains up to 70 of the natural variance it is further validated by flume experiments which confirm model predictions of single object orientation angles experiments with multiple objects yield artificial stone pavements with properties similar to those found in the field the unidirectional lateral process acting on natural stone pavements requires the presence of a vesicular horizon this underlines the tight genetic coupling of this common epipedon feature and the clast cover the presented findings highlight the role of stone pavements as process and environment proxies however stone pavements represent information since the last surface disturbance only this has to be considered when using them as age indicators copyright 2013 john wiley sons ltd

cracks in desert pavement rocks further insights into mechanical weathering by directional insolation the formation of cracks is a fundamental first step in the physical weathering of rocks in desert environments in this study we combine new field data from the mojave u s gobi mongolia and strzelecki australia deserts that collectively support the hypothesis that meridional cracks cracks with orientations not readily attributable to rock anisotropies or shape in boulders or cobbles form due to tensile stresses caused by directional heating and cooling during the sun s daily transit the new studies indicate that rock size surface age and latitude play important roles with respect to their influence on rock fracture rock size and pavement surface age exert an influence on the development of rock cracks as the average clast size of mature desert pavements may be at or below the threshold clast size for thermal cracking of rocks latitude controlled seasonal temperature variations play a key role as demonstrated by 1 tightly clustered mean resultant orientations that differ by latitude as predicted in mcfadden et al 2005 and 2 very cold wintertime temperatures and strong diurnal gradients that may favor crack development in wintertime given the likelihood for strong clast heating during early morning hours the consistent evidence for meridional cracks in surfaces of diverse age and desert environments climate vegetation and distance of clast transport indicate that directional insolation may play the key role in initially generating and propagating rock fractures rather than a secondary role as implied in recent field and modeling studies of physical weathering in deserts

links between orogenic wedge deformation and erosional exhumation evidence from illite age analysis of fault rock and detrital thermochronology of syn tectonic conglomerates in the spanish pyrenees abstract we present new geochronologic data illuminating the tectonic and erosional history of the orogenic wedge exposed in the south central pyrenees spain we interpret illite age analyses from four fault gouges that record thrust belt development and document the importance of out of sequence thrusting fault activity occurs in pulses with slip occurring contemporaneously on multiple faults throughout the wedge new apatite fission track data from syn orogenic sediments of the sis conglomerate body reveal ages of 48 to 42 ma with no consistent variation upsection in strata deposited from 41 to 30 ma we interpret these data as well as thermal modeling of track length distributions to imply rapid cooling in the interior of the pyrenean wedge during the middle eocene the record of fault activity and erosion suggests that orogenic wedges may not evolve in a steady fashion but generally exhibit significant changes in rates of deformation and exhumation the observed correlation in the timing of tectonism and erosional exhumation provides evidence for links between tectonic and surface processes

hercynian structure of the axial zone of the pyrenees the aran valley cross section spain france abstract in the axial zone of the pyrenees palaeozoic rocks crop out with superimposed hercynian and alpine deformation the hercynian structures present two domains infrastructure and suprastructure a complete crosssection of the central part of the axial zone of the pyrenees is presented in which hercynian structures of both domains can be observed in the aran valley cross section the infrastucture corresponds to the garona dome domain where the main hercynian structures are north vergent recumbent folds with an associated mainly flatlying foliation s 2 in the garona dome domain the peak of the medium to high grade metamorphism is simultaneous with the main hercynian structures the aran valley and noguera ribagorca areas correspond to the suprastructure in these domains the main hercynian structures are upright folds with an associated mainly steep lying foliation s 3 in these suprastructural domains low grade syn s 3 metamorphism is present this crosssection relates the structures of the infra and the suprastructure suggesting a deformational sequence for the axial zone of the pyrenees this sequence is deduced from the fact that the north vergent folds observed in the garona dome are earlier than the main structures of the suprastructure the boundary of the garona dome and the suprastructure is a decollement level observed at the base of the silurian the deformational sequence proposed here provides some constraints for models of hercynian evolution and the emplacement of the alpine structures of the pyrenees

sequential syndeformational porphyroblast growth during hercynian low pressure high temperature metamorphism in the canigou massif pyrenees the sequence of growth of garnet staurolite and aluminosilicate in fe rich metapelitic rocks from the canigou massif pyrenees is established using evidence of inclusion reaction and pseudomorphing textures between the different minerals compositional zoning patterns in garnet and staurolite that can be related to the kfmash reaction grid and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures the evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases even where all three are in textural equilibrium interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the kfmash reaction grid indicates the importance of continuous reactions in the development of these phases some garnet and staurolite porphyroblasts underwent renewed growth during breakdown producing rims enriched in mn and zn respectively the presence of aluminosilicate in these assemblages i e the absence of a clear andalusite absent zone in the field is attributed to a strong pressure dependence for the aluminosilicate producing reactions porphyroblast matrix microstructural relations indicate that hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation s3

the main variscan deformation event in the pyrenees new data from the structural study of the bielsa granite abstract a structural and magnetic fabric study of the bielsa granite axial zone of the pyrenees provides new data indicating that this pluton was emplaced during the main variscan event recognized in the pyrenees we argue that the later post triassic deformation was localized along mylonitic bands reactivating earlier variscan shear bands the bielsa granite intrudes cambro ordovician metasediments at its northern border and the pluton is unconformably overlain by sediments triassic in age to the south and east the main structures in the igneous body are magmatic foliations and lineations defined by feldspar phenocrysts and biotite and dextral shear bands with dominant wnw ese trends the wnw ese striking magmatic foliations have variable dips and the magmatic lineation a subhorizontal wnw ese trend the zonation of low field magnetic susceptibility coarsely related to mineral content indicates a layer arrangement of rock types with more basic compositions at the northern border the magnetic ellipsoids are prolate along a wnw ese central band and oblate at the pluton borders where flattening is more important all these data allow us to interpret the bielsa intrusion as being caused by coeval nne shortening and dextral shear the presence of shear bands indicates that strain localization under dextral transpression continued during late variscan times

surface control of desert pavement pedologic process and landscape function cima volcanic field mojave desert california desert pavement is a distinctive feature widespread across arid lands of the world it plays a dynamic role in geomorphic hydrologic and ecologic processes where desert pavement predominates infiltration is limited and rainfall is delivered as runoff to nearby bare ground areas where shrubs cluster desert pavement surfaces may appear monotonously flat and barren but we have found instead that they are a complex association of landscape and hydrologic elements governed by their surface characteristics previously we identified six unique surface mosaic types that accurately capture the subtle but distinct variations in surface clast arrangements for a desert pavement landscape formed on a single aged basalt flow in the mojave desert we now report that these surface mosaics predict the spatial distribution of fundamental desert vegetation and soil characteristics characteristics of soil morphology and texture the leaching depth of soluble salts percent plant cover and shrub species diversity are remarkably consistent for each mosaic type across a 580 000 year old basalt flow even when measured 1 km apart hydrologic character is distinctly different between desert pavement and bare ground regions and vegetation distributions reflect the spatially heterogeneous soil moisture where desert shrubs cluster on the three bare ground surface mosaics leaching is deep removing most soluble salts to below the 50 cm depth where shrubs are absent or few on the three desert pavement mosaics leaching depths are shallow with soluble salt depth

a strontium neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the trois seigneurs massif pyrenees france nd sr and o isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310 340 ma hercynian age metamorphic terrane in the pyrenees france lower paleozoic shales and phyllites have 87sr 86sr values of 0 707 0 717 at 310 ma but model values at 310 ma of 0 709 0 736 based on assumed depositional age of 450 ma and an initial 87sr 86sr 0 707 on a regional scale 87sr 86sr was homogenized to about 0 713 to 0 717 in the higher grade pelitic schists during metamorphism much of this 87sr 86sr exchange occurred at very low grades below the biotite isograd but significant changes also accompanied the 18o lowering of the phyllites 13 to 16 during their transformation to andalusite and sillimanite grade schists 18o 11 to 12 all of these effects are attributed to pervasive interactions with hydrothermal fluids wickham and taylor 1985 the data also show that a syn metamorphic plutonic complex dominated by a biotite granite body was derived by mixing of a relatively mafic magmatic end member 87sr 86sr 0 7025 0 7050 and 18o 7 5 to 8 0 with two metasedimentary sources both having 87sr 86sr 0 715 and 18o 10 0 to 12 0 but with one being more homogeneous than the other the more homogeneous component and the mantle derived magmatic end member dominate at low structural levels within the complex the less homogeneous end member that dominates at high levels is clearly derived from the local paleozoic pelitic schists a rb sr age of 330 20 ma was obtained on hornblende from a deep level within the complex which fixes this age for the regional metamorphism as well although a post metamorphic granodiorite magma body at trois seigneurs also displays heterogeneities in 18o and 87sr 86sr and thus does not give a clear cut rb sr isochron the data are consistent with an emplacement age between 260 and 310 ma similar to ages of other late granodiorites in the pyrenees 143nd 144nd is very uniform within the hercynian crust both at trois seigneurs nd 3 to 7 and elsewhere in the pyrenees almost all igneous lithologies have depleted mantle mid proterozoic model ages consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 ma or earlier rb sr mineral ages exhibit a complex cooling history reflecting late hercynian and mesozoic thermal events our results show that profound homogenization of the 87sr 86sr and 18o 16o ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis particularly in regions undergoing extensional tectonics such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas this may explain why many peraluminous hercynian granitoids of western europe have anomalously low 87sr 86sr initial values compared to their probable sedimentary parent rocks

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

soil horizon development and the tempo and modes of vegetation change during the holocene in a sonoran desert basin usa silt rich and clay rich vesicular horizons v horizons that develop beneath desert pavements limit infiltration and plant available soil moisture a radiocarbon date from buried charcoal constrains the timing of emplacement of a sonoran desert alluvial fan deposit to the very end of the pleistocene approximately 12 7 ka the surface of the fan is dotted with 1 5 3 m light colored plant scars that mark places once occupied by larger perennial plants a few very large ring like clones of larrea tridentata have persisted on the surface and represent plants that may have initially established as long ago as the early holocene the gravelly fan deposit initially would have possessed high infiltration capacities and fostered the establishment of vegetation two principal sets of factors have driven vegetation change over the course of the holocene during the first part of the holocene differences in life history attributes of various species dispersal and colonization vs competitive abilities would have contributed to a succession of perennial plant occupants culminating with the predominance of the slow growing long lived but competitively dominant l tridentata during the latter part of the holocene and continuing to the present continued development of the pavement and associated v horizon has increasingly diminished the capacity of the surface to absorb precipitation and is responsible for the more recent phase involving progressive elimination of vegetation development of soil features that markedly influence soil hydrology is a major component of vegetation change during the late quaternary in the more arid portions of the sonoran and mojave deserts

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

on the reliability of ams in ilmenite type granites an insight from the marimanha pluton central pyrenees this study has been financed by the research project cgl2009 08969 spanish ministry of science and innovation bou acknowledges financial support from the geotransfer group university of zaragoza dga and the jaedoc postdoctoral program csic reviews from d bilardello b almquist and editor e appel are warmly acknowledged for their help to improve the manuscript info eu repo grantagreement ec fp7

microtopographic evolution of lava flows at cima volcanic field mojave desert california microtopographic profiles were measured and power spectra calculated for dated lava flow surfaces at cima volcanic field in the eastern mojave desert of california in order to quantify changes in centimeter to meter scale roughness as a function of age for lava flows younger than about 0 8 m y roughness over all spatial scales decreases with age with meter scale roughness decreasing slightly more than centimeter scales flows older than about 0 8 m y show a reversal of this trend becoming as rough as young flows at these scales modeling indicates that eolian deposition can explain most of the change observed in the offset or roughness amplitude of power spectra of flow surface profiles up to 0 8 m y other processes such as rubbling and stone pavement development appear to have a minor effect in this age range changes in power spectra of surfaces older than about 0 8 m y are consistent with roughening due to fluvial dissection these results agree qualitatively with a process response model that attributes systematic changes in flow surface morphology to cyclic changes in the rates of eolian soil formation and fluvial processes identification of active surficial processes and estimation of the extent of their effects or stage of surficial evolution through measurement of surface roughness will help put the correlation of surficial units on a quantitative basis this may form the basis for the use of radar remote sensing data to help in regional correlations of surficial units

relation between infiltration and stone cover on a semiarid hillslope southern arizona abstract a knowledge of the spatial distribution of infiltration rates is essential to the application of realistic distributed rainfall runoff models in semiarid areas one method of determining this distribution is to develop a predictive equation based on a readily observable surface property such as stone cover previous studies of the relation between infiltration and stone cover on shrub covered semiarid hillslopes have yielded both positive and negative correlations it is suggested that positive correlations result where infiltration measurements are confined to areas between large shrubs negative correlations on the other hand reflect pronounced shrub intershrub differences in infiltration and stone cover and are found where both shrub and intershrub areas are sampled in this study of a semiarid hillslope in southern arizona both shrub and intershrub areas are sampled and a negative correlation is obtained between infiltration and stone cover this correlation arises from the fact that under shrubs fine sediments have accumulated primarily as a result of differential splash whereas between shrubs such sediments have been selectively removed by a combination of rainsplash and overland flow leaving behind a gravel lag consequently infiltration rates are higher under shrubs than between them owing to the higher percentage of sand in the surface soil the larger quantity of organic matter both within the soil and on its surface and the greater digging and burrowing by animals in addition simulated rainfall experiments imply that shrubs with moderately dense canopies are more effective than adjacent surface gravels in dissipating the kinetic energy of raindrops and thereby reducing surface sealing and promoting infiltration inasmuch as the field site is typical of gentle shrub covered piedmont hillslopes over much of the sonoran desert inverse relations between infiltration rate and stone cover are probably characteristic of such hillslopes and may be used to predict the spatial distribution of infiltration rates for inclusion in rainfall runoff models

desert pavement development and landscape stability on the eastern libyan plateau egypt abstract desert pavement surfaces of the eastern libyan plateau in central egypt represent a stable landscape preserving middle and upper paleolithic artifacts detailed measurements of pavement clasts indicate significant variability in clast size density lithology and orientation between pavements but no spatial relationship among any of these pavement variables over the study area pavement characteristics are unrelated to local geomorphic features including slope gradient and aspect suggesting a desert pavement surface that has developed without significant influence from transporting mechanisms such as overland flow and slope failure meridional vertical cracks in surface clasts implicate thermal stresses due to diurnal solar variation as a mechanical weathering process whereas the presence of a clast free silty layer within all soil profiles indicates that these are accretionary pavement surfaces that have grown upward over time the desert pavement in this region has likely developed in situ through mechanical breakdown of surface clasts and desert pedogenesis indicating long term stability for this region and minimal taphonomic effects on artifacts 2 cm in diameter deposited on this surface over the last ca 100 ka

soil geomorphic significance of land surface characteristics in an arid mountain range mojave desert usa abstract mountains comprise an extensive and visually prominent portion of the landscape in the mojave desert california landform surface properties influence the role these mountains have in geomorphic processes such as dust flux and surface hydrology across the region the primary goal of this study was to describe and quantify land surface properties of arid mountain landforms as a step toward unraveling the role these properties have in soil geomorphic processes as part of a larger soil geomorphic study four major landform types were identified within the southern fry mountains in the southwestern mojave desert on the basis of topography and landscape position mountaintop mountainflank mountainflat intra range low relief surface and mountainbase a suite of rock vegetation and morphometric land surface characteristic variables was measured at each of 65 locations across the study area which included an associated piedmont and playa our findings show that despite the variation within types landforms have distinct land surface properties that likely control soil geomorphic processes we hypothesize that surface expression influences a feedback process at this site where water transports sediment to low lying areas on the landscape and wind carries dust and soluble salts to the mountains where they are washed between rocks incorporated into the soil and retained as relatively long term storage recent land based video and satellite photographs of the dust cloud emanating from the sierra cucapa mountains in response to the 7 2 magnitude earthquake near mexicali mexico support the hypothesis that these landforms are massive repositories of dust

geochemistry and genesis of geothermal well water from a carbonate evaporite aquifer in chongqing sw china thermal water is an important natural resource the hydrogeochemistry and geothermometry of thermal water from the qianyi well twqw which is associated with the tongjing warm springs located in the eastern sichuan fold belts in chongqing sw china was investigated the reservoir of this region consists of upper and middle triassic carbonate and evaporite rocks the twqw was of ca mg so4 type with water temperature of 47 3 c plotting on the giggenbach na k mg diagram indicates that the twqw was immature due to a mixing of shallow karst groundwater and thermal water the mixing model indicates that the twqw was composed of 49 deep thermal water and 51 shallow karst groundwater geothermometers suggest a reservoir temperature of about 79 c locating this aquifer at a depth of 2 5 km the d and 18o values plot near the local meteoric water line suggesting that the twqw originates from local meteoric water at recharge elevations of 780 1160 m a s l a conceptual model of the genesis for tongjing warm springs was developed rainfall infiltrates via karst outcrops in the elevated areas of the northern tongluoxia anticline extracts heat from reservoir rocks at depth dissolves minerals and becomes thermal water the thermal water is driven by gravity flows through the carbonate evaporite aquifer and follows the tongluoxia anticline of the eastern sichuan fold belts to the southwest the thermal water ascends to the surface along the incision created by the wentang river and mixes with shallow karst groundwater to create warm springs this study may be relevant to other karst geothermal reservoirs in china

stages and rate of the gravel shattering process by salts in desert reg soils abstract gravel shattering by salts is a common process in gravelly young to mature desert reg soils the shattered gravel is found on the surface and in the b and c horizons down to a maximum depth of 50 cm field studies on the process of salt weathering in deserts are scarce therefore the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the negev desert the studied reg soils were developed on two alluvial fan chronosequences in the negev desert the fan of nahal ze elim and the fan of nahal shehoret the development of the shattering process can be divided into five stages a e based on chemical physical and micromorphological features of the reg soils stages a d very young yo young reg soils represent the early holocene to the present some 14 000 years salinity values are between 0 27 16 mmho cm and the percentage of the shattered gravel is between 20 and 70 the shattered gravel however still maintains its original shape stage e mature reg soil represents the middle pleistocene to early holocene or 500 000 years salinity values at this stage reach 62 mmho cm although part of the gravel does not keep its original shape and starts to disperse in the profile the percentage of shattered gravel is only 75 80 it was found that the process of salt shattering in reg soils does not depend on concentrations or mineralogy of the salts but on the micro environmental conditions of the soil profile the process requires extreme and rapid variations of temperatures and moisture conditions such as found close to the soil surface to cause salt crystallization in a confined system with time the sealing of the soil surface through the development of desert pavement and a gravel free b horizon results in a change in the soil micro environment this change impedes the process irrespective of the amounts concentrations and types of salt minerals which exist in the profile the fact that in extreme arid regions the gravel shattering process develops primarily close to the reg soil surface suggests that it can be used to identify buried reg soils and to reconstruct climatic and micro environmental conditions of paleosols

alluvial sediment or playas what is the dominant source of sand and silt in desert soil vesicular a horizons southwest usa 1 vesicular a av soil horizons form beneath desert pavements from the accretion of aeolian sediment dust commonly thought to be derived primarily from desiccating pluvial lakes and playas with contributions from ephemeral washes and alluvial fans particle size distributions of av horizons are typically bimodal with primary modes of very fine silt and fine sand suggesting that the horizon matrix is derived from multiple sources here we conduct detailed chemical and physical analysis of both av horizon soil samples and potential sources of aeolian sediment to better constrain the relative contributions of dust associated with the development of av horizons geochemical data from both sand 125 250 m and silt 2 32 m fractions in av horizons and potential dust sources in the eastern mojave desert and western sonora desert usa point to large contributions from nearby sources including distal alluvial fans and washes and comparably lower contributions from regional sources such as playas the silt mode is derived from suspension transport of dust and the fine sand mode is derived from saltating sand the desiccation of pluvial lakes in the mojave desert is commonly believed to have driven episodes of aeolian activity contributing to sand dunes and av horizon formation we propose that alluvial fans and washes are underappreciated as desert dust sources and that pulses of dust from late pleistocene and holocene alluvial fans dwarfed pulses of dust from desiccating pluvial lakes in the eastern mojave desert

a geomorphic classification of ephemeral channels in a mountainous arid region southwestern arizona usa article i nfo despite the global abundance of arid region ephemeral streams hydrologic and geomorphic data for these systems are limited compared to their perennial counterparts high spatial and temporal variability in flow makes hydrologic and geomorphic aspects of dryland ephemeral channels difficult to characterize perennial stream classifications have been extended to dryland ephemeral streams but do not adequately describe ob served differences in channel geometry and characteristics of ephemeral channels in desert environments we present a geomorphic classification for ephemeral streams in mountainous regions based on planform degree of confinement and composition of confining material five stream types were identified in the sonoran desert ofsouthwestern arizona 1 piedmontheadwater 2 bedrock 3 bedrockwith alluvium 4 incisedalluvium and 5 braided channels nonparametric permutational multivariate analysis of variance for 101 surveyed reaches indicated differences p b 0 001 in channel geometry and hydraulics among the five stream types nonmetric multidimensional scaling ordination identified the strongest channel geometry and hydraulic vari ables capable of distinguishing the five channel types and a classification tree determined relative importance of these variables in the following order width to depth ratio w d stream gradient s stream power and shear stress a classification tree and discriminant analysis used w d s and for 86 study reaches on the u s army yuma proving ground 77 and 77 internal validation hit rate respectively to predict stream type of 15 separate study reaches on barry goldwater air force range with 67 and 73 external validation hit rates respectively differences in channel geometry among the five stream types reflect likely differences in hydrology hydraulics and sediment transport with implications for disturbance regime channel adjustment to disturbance and ecological sensitivity

desert pavement dynamics numerical modeling and field based calibration desert pavements are widely used as a relative surface dating tool because they are progressively better developed on surfaces ranging from thousands to hundreds of thousands of years in age recent work however has highlighted the dynamic nature of pavements and undermined their use as surface age indicators quade 2001 proposed that latest pleistocene vegetation advances destroyed all mojave desert pavements above 400 m elevation making all such pavements holocene in age in an effort to reconcile young pavement evidence with their widespread use as pleistocene surface age indicators we developed a numerical model based on the classic conceptual model in which pavements co evolve with their underlying eolian epipedons over millennial timescales in this co evolutionary process fine grained eolian deposition and av horizon development within the eolian epipedon promotes surface clast motion and pavement development enhancing the eolian sediment trapping ability of the pavement in a positive feedback model results illustrate the multi scale nature of pavement dynamics pavements may require tens of thousands of years to fully develop from a newly abandoned alluvial surface but may heal over timescales of decades to centuries if a mature eolian epipedon is present as such there is no inconsistency between rapid pavement healing and a pleistocene age for the underlying alluvial surface to calibrate the model we conducted surficial geologic mapping and pavement sedimentological analysis on two desert piedmonts our study areas include both proximal and distal fan environments illustrating the role of parent material texture in controlling the mode of pavement formation using available geochronology our work provides a rigorous calibration of pavement formation rates in our study areas and provides evidence supporting the use of pavements as local relative surface age indicators over holocene to late pleistocene timescales copyright 2007 john wiley sons ltd

geomorphic controls on biological soil crust distribution a conceptual model from the mojave desert usa abstract biological soil crusts bscs are bio sedimentary features that play critical geomorphic and ecological roles in arid environments extensive mapping surface characterization gis overlays and statistical analyses explored relationships among bscs geomorphology and soil characteristics in a portion of the mojave desert usa these results were used to develop a conceptual model that explains the spatial distribution of bscs in this model geologic and geomorphic processes control the ratio of fine sand to rocks which constrains the development of three surface cover types and biogeomorphic feedbacks across intermontane basins 1 cyanobacteria crusts grow where abundant fine sand and negligible rocks form saltating sand sheets cyanobacteria facilitate moderate sand sheet activity that reduces growth potential of mosses and lichens 2 extensive tall moss lichen pinnacled crusts are favored on early to late holocene surfaces composed of mixed rock and fine sand moss lichen crusts induce a dust capture feedback mechanism that promotes further crust propagation and forms biologically mediated vesicular av horizons the presence of thick biogenic vesicular horizons supports the interpretation that bscs are long lived surface features 3 low to moderate density moss lichen crusts grow on early holocene and older geomorphic surfaces that display high rock cover and negligible surficial fine sand desert pavement processes and abiotic vesicular horizon formation dominate these surfaces and minimize bioturbation potential the biogeomorphic interactions that sustain these three surface cover trajectories support unique biological communities and soil conditions thereby sustaining ecological stability the proposed conceptual model helps predict bsc distribution within intermontane basins to identify biologically sensitive areas set reference conditions for ecological restoration and potentially enhance arid landscape models as scientists address impacts of climate change and anthropogenic disturbances

post eruptive sediment transport and surface processes on unvegetated volcanic hillslopes a case study of black tank scoria cone cima volcanic field california abstract conical volcanic edifices that are made up from lapilli to block bomb pyroclastic successions such as scoria cones are widespread in terrestrial and extraterrestrial settings eruptive processes responsible for establishing the final facies architecture of a scoria cone are not well linked to numerical simulations of their post eruptive sediment transport using sedimentological geomorphic and 2d fragment morphology data from a 15 ky old scoria cone from the cima volcanic field california this study provides field evidence of the various post eruptive sediment transport and degradation processes of scoria cones located in arid to semi arid environments this study has revealed that pyroclast morphologies vary downslope due to syn eruptive granular flows along with post eruptive modification by rolling bouncing and sliding of individual particles down a slope and overland flow processes the variability of sediment transport rates on hillslopes are not directly controlled by local slope angle variability and the flank length but rather by grain size and morphological characteristics of particles such as shape irregularity of pyroclast fragments and block lapilli ratio due to the abundance of hillslopes degrading in unvegetated regions such as those found in the southwestern usa granulometric influences should be accounted for in the formulation of sediment transport laws for geomorphic modification of volcanic terrains over long geologic time

the vesicular layer and carbonate collars of desert soils and pavements formation age and relation to climate change abstract the vesicular fine grained a horizon av is the widespread ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages now known to form mostly through accumulation of eolian dust recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation changes in the magnitude of eolian activity and effective leaching related to quaternary climatic changes are also hypothesized to have influenced the evolution of the av horizon numerical modeling geochronologic and field laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of av how the changing av horizon increasingly influences soil infiltration and carbonate translocation and accumulation and the control that clasts of the evolving pavement exert on pedogenic processes results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived eolian material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by av horizon formation the early development of a weakly to moderately developed av horizon directly over gravelly parent material in late and middle holocene soils moderately influences soil infiltration but significant leaching of very soluble materials and some carbonate in dust are permitted in older pleistocene soils however the texturally more mature av and underlying cumulic nongravelly horizons more strongly limit the rate and depth of leaching and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material other aspects of av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate referred to as carbonate collars development of a numerical model that integrates soil hydrology a co2 production diffusion model calcite kinetics and thermodynamic considerations composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation model results combined with results of 13c and 18o analyses of collar carbonate demonstrate how precipitation of calcite on pavement clasts and within the av is favored at a depth much shallower than that indicated by the classic carbonate depth climate relationship of jenny and leonard jenny h j leonard c d 1935 functional relationships between soil properties and rainfall soil science 38 363 381 and arkley arkley r j 1963 calculations of carbonate and water movement in soil from climatic data soil science 96 239 248 or simulated by numerical models of carbonate accumulation simultaneous development of thick carbonate collars and the av horizon requires the sustained pavement clast av horizon coupling for at least centuries to possibly millennia new thermoluminescence ages also indicate that much of the av horizon formed in the holocene and that it is certainly much younger than the older pleistocene pavements this supports the previously proposed hypothesis that increased dust flux during the pleistocene to holocene transition triggered and or greatly accelerated av horizon development an understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments but it also provides important clues into processes of pavement evolution and preservation of av horizons during long glacial periods the av horizon is not merely an insignificant surficial zone of recent dust accretion instead its development profoundly influences the genesis of desert soils and pavements

kinematics of orogenic collapse in the variscan pyrenees deduced from microstructures in porphyroblastic rocks from the lys caillaouas massif in the pyrenees a phase of bulk vertical shortening d3 is indicated by a horizontal crenulation cleavage s3 which is developed in a pre existing schistosity s2 porphyroblasts of the lys caillaouas massif regionally preserve 1 1 the initial pre d3 orientation of s2 in the form of a vertical and e w trending preferred orientation of their inclusion trails 2 2 a vertical l2 stretching lineation defined by the elongation of the inclusions these data are consistent with n s compression and crustal thickening during d2 the angle between the inclusion trails s2 i and the external schistosity s2 e records the subsequent rotation of s2 from vertical to gently dipping orientations at higher crustal levels s2 remained steep suprastructure and little d3 deformation is evidenced with depth d3 deformation progressively intensifies and zones can be distinguished with opposite dip directions of s2 opposite apparent porphyroblast rotation senses and opposite f3 crenulation asymmetries these micro structures on a n s section through the lys caillaouas and the garonne dome massifs record a regional partitioning of bulk coaxial vertical shortening this deformation partitioning was probably controlled by a not exposed gneiss dome which behaved relatively rigid as compared to its metasedimentary cover in zones of non coaxial d3 deformation s2 was extended reactivated and forms the dominant schistosity where d3 was locally more coaxial s2 was shortened isoclinally crenulated and s3 forms the macroscopic cleavage the fact that d3 extension in the lys caillaouas massif immediately followed d2 compression and that similar extension is recorded in large parts of the variscan foldbelt suggests gravitational collapse of a thickened orogen this is consistent with the flattening type of d3 strain recognized in a number of pyrenean variscan massifs peak high t low p metamorphic conditions coincide with the onset of orogenic extension when very little crustal thinning could have been accomplished yet this suggests that heat induced strain softening within the orogen may have triggered its collapse

multi scale strainfield analysis using geostatistics investigating the rheological behavior of the hot variscan crust of the pyrenees axial zone the present study provides a quantitative way to analyze and interpolate strainfields from ductile fabric field measurements using geostatistics based on variogram analysis and kriging interpolations the developed methodology allows computing automatic cleavage and lineation trajectory maps at different scales from a same field structural dataset for the example provided by the hot variscan crust of the pyrenees the analysis of cleavage stretching lineations orientation and kinematic data document pure shear conditions recorded during convergence strain was vertically partitioned between an upper crust submitted to homogeneous thickening responding to n s orogen normal shortening and a hot and partially molten mid lower crust affected by horizontal longitudinal orogen parallel flow the emplacement of plutons and gneiss domes generated heterogeneities in the upper crust and induced strain localization into steep anastomosed transpressional shear zones flanking those structures the results show that geostatistics applied to structural geology are powerful tools for retrieving tectonic regimes of large orogenic segments by identifying and quantifying perturbations in the strainfields induced by protracted and potentially complex tectonic histories the variogram analysis also gives first order clues to evaluate whether the considered structural domains underwent single continuous ductile or polyphased brittle ductile deformation

geomorphology on geologic timescales evolution of the late cenozoic pacific paleosurface in northern chile and southern peru abstract the atacama desert on the western margin of the central andes is one of the driest and oldest deserts in the world it is defined by a distinct and ancient surface known as the pacific paleosurface pps or atacama paleosurface the age of this surface is determined as the time at which sediment deposition ceased and the surface was effectively abandoned early studies suggested that this abandonment took place between 14 and 10 ma and was related to both the uplift of the andes and the onset of hyperaridity in the region here we provide a regional re examination of the pps compiling existing work on the underlying geology sedimentology surface exposure dating and seismic profiling we also present new multispectral satellite maps of the pps and 45 new cosmogenic 3he and 21ne surface exposure ages in order to constrain the formation age and the preservation and incision history of the paleosurface we conclude that the pps is not a single paleosurface but instead is a mosaic of smaller surfaces that were formed by aggradational and degradational processes over 19 million years or more and should be termed collectively as the pacific paleosurfaces the time at which individual paleosurfaces formed is related to regional climate where the location of each is controlled by regional tectonic activity cosmogenic surface exposure ages suggest that the surfaces are a record of regional scale climate events

the variscan geology of the pyrenees abstract the main outlines of the geology of the variscan part of the pyrenees are discussed rocks involved in this cycle are high grade basement gneisses palaeozoic sediments and their metamorphic equivalents late intrusive granodiorites and early pre variscan granites the main features of the stratigraphy of the palaeozoic are given structures fall into two domains a low grade suprastructure essentially with steep folds and cleavages and a high grade infrastructure with dominantly low dipping foliations an important phase of early pre cleavage folding occurs in low grade rocks mainly along the southern border of the axial zone in high grade rocks most structures and the metamorphism postdate the main cleavage phase in low grade rocks the influence of the alpine orogeny on the variscan structures consists mainly of faults steep reverse faults in the northern and south directed thrusts in the southern part of the pyrenees metamorphism took place under high geothermal gradients and low pressures as indicated by the abundant occurrence of andalusite and cordierite

biogeomorphology of a mojave desert landscape configurations and feedbacks of abiotic and biotic land surfaces during landform evolution abstract terrestrial ecosystems can be more holistically understood by investigating the morphology of landscape mosaics the assemblage of their ecological communities and the linkages and feedbacks between the mosaics and communities the overarching objectives of this study were to 1 study the abiotic and biotic configurations of landform units as mosaics within a mojave desert chronosequence and 2 elucidate their potential feedbacks interactions and dynamics during landform evolution seven landform units distributed over three geomorphic ages were identified including young bars and swales intermediate aged flattened bars flattened swales and bioturbation units and old desert pavements and shrub zones these landform units were characterized according to abiotic and biotic land surface properties landform units were statistically distinct and predictable based on a specific suite of abiotic and biotic properties vascular plant functional group and biological soil crust community diversity varied with geomorphology with greatest diversity associated with bars and shrub zones and lowest diversity associated with desert pavements biological soil crust communities were controlled by geomorphic age surface rock size and protruding rocks with young bar units having the highest abundance and diversity perennial forbs were observed in old shrub zones with small rocks and few protruding rocks a high clast density and a finer sized clast distribution were found particularly in desert pavements and flattened swales and generally inhibited biological soil crust and plant cover evolutionary trajectories for landforms of a lower piedmont landscape can be dominated by either abiotic and biotic landform processes these two trajectories are distinctly different and are associated with their own unique linkages feedbacks and dynamics of abiotic and biotic land surface properties producing a highly diverse desert landscape

large near surface nitrate pools in soils capped by desert pavement in the mojave desert california we found exceptionally high nitrate levels up to 12 750 kg ha 1 at shallow depths 1 m in soils mantled by desert pavement a common land surface feature in arid regions nearby soils without desert pavement had nitrate contents that were one to two orders of magnitude lower the soil conditions coincident with desert pavement i e stability antiquity and virtually no leaching favor the retention and accumulation of nitrate delivered by atmospheric deposition or in situ fixation the nitrate stored in soils under desert pavement is a previously unrecognized pool of nitrogen that has the potential to increase the global nitrogen inventory for near surface desert soils to five times previous estimates its near surface occurrence makes this labile nitrogen pool particularly susceptible to mobilization by climate change or human disturbance risking contamination of surface and groundwaters

structural inheritance in the central pyrenees the variscan to alpine tectonometamorphic evolution of the axial zone estimating structural inheritance in orogens is critical to understanding the manner in which plate convergence is accommodated the pyrenean belt which developed in late cretaceous to paleogene times was affected by cretaceous rifting and variscan orogeny here we combine a structural and petrological study of the axial zone in the central pyrenees to discuss structural inheritance low grade paleozoic metasedimentary rocks were affected by a variscan transpressional event that produced successively 1 regional scale folds 2 isoclinal folding steep pervasive cleavage and vertical stretching synchronous with peak metamorphism 3 strain localization into ductile reverse shear zones the persistence of a relatively flat envelope for the paleozoic sedimentary pile and variscan isograds and the absence of alpine crustal scale faults in the core of the axial zone suggests that the axial zone constitutes a large variscan structural unit preserved during pyrenean orogeny this configuration seems to be inherited from cretaceous rifting which led to the individualization of a large continental block future axial zone against a hyper extended domain along the north pyrenean fault zone this study places the currently prevailing model of pyrenean belt deformation in a new perspective and has important implications for crustal evolution and inheritance in mountain belts more generally supplementary materials raman spectroscopy of carbonaceous materials data and a figure illustrating peak fitting of the raman spectrum of carbonaceous material and raman spectra from the various samples of the pallaresa cross section are available at https doi org 10 6084 m9 figshare c 3906247

clast size controls and longevity of pleistocene desert pavements at lathrop wells and red cone volcanoes southern nevada formation of desert pavement and accretionary soils are intimately linked in arid environments well sorted fallout scoria lapilli at lathrop wells 75 80 ka and red cone ca 1 ma volcanoes southern nevada formed an excellent parent material for pavement allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular a av horizons variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation pavement becomes ineffective when clasts can saltate under the strongest winds while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation necessary for av development contrary to predictions that all pavements above altitudes of 400 m would have been reset in their development after late pleistocene vegetation advances the soils and pavements show clear differences in maturity between the two volcanoes this indicates that either the pavements and or soils develop slowly over many tens of thousands of years and then are very stable or if they are disrupted by vegetation advances subsequent pavements are re established with successively more mature characteristics

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

thermal evolution of a hyperextended rift basin mauleon basin western pyrenees onshore and offshore geological and geophysical observations and numerical modeling have greatly improved the conceptual understanding of magma poor rifted margins however critical questions remain concerning the thermal evolution of the prerift to synrift phases of thinning ending with the formation of hyperextended crust and mantle exhumation in the western pyrenees the mauleon basin preserves the structural and stratigraphic record of cretaceous extension exhumation and sedimentation of the proximal to distal margin development pyrenean shortening uplifted basement and overlying sedimentary basins without pervasive shortening or reheating making the mauleon basin an ideal locality to study the temporal and thermal evolution of magma poor hyperextended rift systems through coupling bedrock and detrital zircon u th he thermochronometric data from transects characterizing different structural rifting domains these new data indicate the basin was heated during early rifting to 180 c with geothermal gradients of 80 100 c km the proximal margin recorded rift related exhumation cooling at ca 98 ma whereas the distal margin remained 180 c until the onset of paleocene pyrenean shortening lithospheric scale numerical modeling shows high geothermal gradients 80 c km and synrift sediments 180 c can be reached early in rift evolution via heat advection by lithospheric depth dependent thinning and blanketing caused by the lower thermal conductivity of synrift sediments mauleon basin thermochronometric data and numerical modeling illustrate that reheating of basement and synrift strata might play an important role and should be considered in the future development of conceptual and numerical models for hyperextended magma poor continental rifted margins

two stage deformation of the mont louis andorra granite pluton variscan pyrenees inferred from magnetic susceptibility anisotropy the mont louis andorra granodiorite pluton eastern pyrenees france andorra and spain was the subject of a thorough study of its internal structures principally the measurement of the anisotropy of its magnetic susceptibility and the classification of its microstructures regularly spaced sampling 281 sites allowed the precise mapping of the susceptibility magnitudes consistent with mapped rock types the magnetic structures fall into two distinct families which occur in separate parts of the pluton and have different microstructures family i is observed almost everywhere and has close to horizontal lineations remarkably constant in ne sw azimuth magmatic to submagmatic microstructures characterize these rocks in contrast family ii is confined within nw se trending corridors where moderate to steep nw plunges of the lineations are recorded and subsolidus microstructures are observed magnetic structures of family i are argued to equate with the kinematic framework of the deforming magma during its emplacement family ii structures represent a late to post consolidation reworking and therefore record a second stage of deformation in the pluton it is concluded that magma emplacement was related to a ne sw trending strike slip and or thrust fault system coeval with the sw verging thrust episode documented in the pyrenees the second stage corresponds to dextral shear movements with a thrust component on nw se striking planes corridors containing family ii structures are shear zones responsible for dissection of the massif into three blocks the eastern block representing the former base and the western block the top of the pluton restoration of the pluton9s shape along these corridors demonstrates the importance of the early variscan ne sw kinematic direction because the family ii structures that form shear zones affecting the pluton have similar orientation and kinematics as the compressive steep main phase of deformation ubiquitous in the pyrenees the pluton of mont louis andorra was probably emplaced before this main phase of deformation

implications of the seismic structure for the orogenic evolution of the pyrenean range abstract the dynamic evolution of the pyrenees is discussed in the light of geophysical data recent deep seismic sounding have revealed the crustal structure of the pyrenees which is used to test the different evolutionary models proposed until now the crustal thickness of the paleozoic axial zone paz and the north pyrenean zone npz differ by more than 10 km ranging from about 30 km in the npz to 40 50 km in the paz the transition from paz to the npz identified at the surface as the north pyrenean fault npf is sharp at depth and marked by a vertical step at least in the eastern half of the range the npz is characterized by additional throws and dips of the moho in the east whereas in the west a heterogeneous middle to lower crust is encountered with high velocity anomalies the seismic results suggest that the paz and the npz belong to different plates the npf being the plate boundary these results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following nne ssw directions they rather suggest that after a period of extension two main orogenic events took place a phase involving shearing and thinning which affected mainly the present day npz and a later compressive phase which explains the building up of the chain the thickening of the crust and the enhancement of a pre existing difference in crustal thickness between the european and iberian plates

spatial and temporal variability of water percolation and movement in a system of longitudinal dunes western negev israel subsurface lateral flow has previously been identified in dune areas where the average annual rainfall exceeds 200 mm very little is known about subsurface flow in sandy deserts with less than 100 mm of rainfall per year the present study deals with the water regime in a system of longitudinal dunes in the negev desert sixteen boreholes were dug down to a depth of 6 m across a sandy ridge and the adjacent corridor soil moisture and water movement were monitored with a neutron probe during two consecutive years the first year had been relatively wet and in the following year the rainfall was slightly lower than the long term average the data obtained show that in an average year water percolation is limited to 60 cm with no lateral water movement deeper percolation in the range 180 400 cm occurs in response to rainy spells with about 100 mm of cumulative rain within two months the process involves subsurface lateral flow and water movement by the piston effect the lateral flow on the flanks of the dune is explained by differences in hydraulic conductivities within the sloping layers 1997 by john wiley sons ltd

evaluating the life expectancy of a desert pavement abstract this paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the sonoran desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements this synthesis identifies vital factors key factors and site specific factors promoting pavement stability hyperaridity is the vital factor in pavements surviving for 106 years or more aided by minimal bioturbation and clast size reduction three key factors aid in pavements surviving for 104 to 105 years accumulation of allochthonous dust underneath pavement cobbles a flat topography and a lack of headward retreating swales or gullies a unified explanation for pavement longevity however did not emerge from a literature review because a variety of site specific factors can also promote pavement antiquity including resistant bedrock beneath the pavement disk shaped cobbles to promote dust accumulation and microclimatological and ecological reasons for minimal bioturbation both key and site specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the sonoran desert central arizona the buildup of cosmogenic 10be and in situ 14c optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between 30 and 332 ka with conditions for longevity including flat surface topography pavements underlain by consolidated granitic bedrock a lack of headward retreating gullies and swales 87sr 86sr analyses indicating the infiltration of allochthonous dust floating disk shaped pavement cobbles and a quartzite lithology resistant to disintegration however 10be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate these weaker materials allowed the growth of headward retreating swales that in turn promoted exposure of newer gravels by surface erosion

rock varnish microlamination dating of late quaternary geomorphic features in the drylands of western usa abstract varnish microlamination vml dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts in this study we establish a generalized late quaternary i e 0 300 ka varnish layering sequence for the drylands of western usa and tentatively correlate it with the specmap oxygen isotope record we then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late quaternary geomorphic features in the study region vml dating of alluvial fan deposits in death valley of eastern california indicates that during the mid to late pleistocene 5 15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation during the holocene interglacial time however 0 5 1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate vml dating of alluvial desert pavements in death valley and the mojave desert reveals that pavements can be developed rapidly

polyphase magmatic pulses along the northern gondwana margin u pb zircon geochronology from gneiss domes of the pyrenees abstract alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the aston and hospitalet domes of the axial zone of the pyrenees are subjects of a laser ablation icp ms u pb zircon geochronology study the age spectra recorded by detrital magmatic xenocrystic and inherited zircons reveal a more complex nearly continuous paleozoic magmatic history of the variscan basement of the pyrenees than previously known inherited and detrital zircons of mesoarchean paleoproterozoic to ediacaran ages attest to the peri gondwana location of the cambrian sediments that later form the metamorphic core of the variscan pyrenees the youngest magmatic zircon ages fall into the late carboniferous and earliest permian ranging from ca 306 297 ma and represent the emplacement ages of the dikes and small granite intrusions the age spectra of magmatic xenocrystic zircons contain several maxima middle 475 465 ma and late ordovician 455 445 ma early 415 402 ma and late devonian 385 383 ma early 356 351 ma and middle carboniferous ca 328 ma middle ordovician and middle carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded the aston and hospitalet orthogneisses and the soulcem granite the presence of early mid carboniferous magmatic zircons in several samples lends further support to a wide spread early variscan magmatic activity in the central pyrenees the other age peaks do not have equivalent igneous or metaigneous rocks in the central axial zone but are thought to be present in the pyrenean crust not exposed and yet to be identified the diversity of ordovician devonian and carboniferous up to permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre variscan and variscan orogenies the source of the heat for the devonian to early mid carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone lithospheric scale shearing or a mantle plume tuzo

dynamical processes on desert pavements and the healing of surficial disturbances abstract mature desert pavements are traditionally regarded as hallmarks of stability but their stability is dynamic not static in a study aimed at documenting this dynamic stability and its role in healing surface disturbances experiments were performed over a 5 yr period on small cleared patches or plats on pavement surfaces in panamint valley california these experiments show that stones from plat edges begin to resurface the clearing at rates of about 1 per year on 40 cm square plats and 10 per year on 10 cm square plats stones contributing to the regenerated pavement have smaller average diameters than stones on the surrounding pavement cavities 5 10 cm deep formed in mature pavement by removal of embedded boulders fill by ravel and slope failure after five years cavity depth has been reduced by as much as 60 forty year old boulder cavities are nearly completely refilled and have been repaved by smaller than average pavement stones gaps caused by removal of small stones 2 3 cm have completely healed in 5 yr displacement of surface stones by small animals is a major component of the healing process

influence of relative surface age on hydraulic properties and infiltration on soils associated with desert pavements abstract av horizons found in desert pavement environments are known to evolve pedogenically over geologic time this study was conducted to determine whether increased pedogenic development of the av vesicular horizon over relative time impacts the hydraulic properties of individual soil peds and the mechanism of infiltration as inferred by dye patterns we examined peds from the av horizons associated with desert pavements that mantled three different alluvial deposits with different relative surface ages qf5 10 ka qf3 50 100 ka and qf2 10 50 ka and included an additional surface qf6 4 ka for the dye studies we hypothesized that increases in the development of the av over time would lead to a more structured soil surface with greater potential flow between soil peds and lower hydraulic conductivity of the soil peds themselves results showed that average k s and of the qf5 peds were significantly greater than estimated for the qf2 and qf3 peds although k s was greater for the qf5 peds the steady state infiltration rate was equal for the qf3 qf2 and qf5 surfaces perhaps indicating a reduction in matrix flow through soil peds and an increase in interped flow between soil peds studies were also conducted using dyed water and a tension disc infiltrometer set to saturation following the tests the soil was excavated in 2 cm increments and photographed the dye patterns for the qf6 indicated that the water moved rapidly through the soil matrix into deeper soil and without any preferential flow around peds the qf5 exhibited more dye stained soil at depth than seen in the qf2 and qf3 perhaps due to higher ped conductivity qualitative observations on the qf2 and qf3 suggest that water flowed primarily along the ped faces and then toward the ped interiors the results suggest that infiltration through av horizons evolves from a matrix dominated process on the younger soils to a preferential flow dominated process on older surfaces

feasibility of afc models for the petrogenesis of calc alkaline magma series one of the most popular petrogenetic concepts is that of simultaneous assimilation and fractional crystallisation afc the equations governing these processes are well known and widely applied the power and attraction of the model are that with the minimum of constraints petrogenetic schemes can be erected for essentially any group of rocks these models are usually based on the observed variations in a small number of trace elements and or isotopic systems in this paper we have generated well constrained afc models for the variscan calc alkaline querigut complex in the french pyrenees our models are based on geological constraints on the identities of the parent magma and candidate assimilants and the sm nd and rb sr isotope systematics of the rocks the modelling is successful in mimicking the isotope array for the actual rocks however the results are massively inconsistent with those produced using the major and trace element concentrations our major conclusion is that afc modelling should be undertaken only with robust initial constraints on the compositions of the parent magma and the proposed assimilant with due respect for crystal liquid partition coefficients in so far as they are known the results should then be accepted only if the isotope based model parameters are consistent with those derived from major and trace element variations

40ar 39ar age of mylonites along the merens fault central pyrenees seven mylonitic samples and two coarse muscovites from the central pyrenees have been dated by the 40ar 39ar method whole rock specimens of mylonite were cut out of thin section chips allowing complete characterisation of mineralogy and texture several specimens showed rising staircase patterns in the range 50 90 ma with much higher ages in the highest temperature steps this is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures one biotite quartz mylonite gave a plateau age of 93 2 ma other inferred mica ages are about 60 73 ma for biotite and 50 60 ma for muscovite it is probable that biotite contains excess argon and that 50 ma approximates to the cooling age in the mylonites one coarse muscovite collected immediately below the major merens shear zone gave a hercynian plateau age while another collected within the merens zone gave a partially reset hercynian age taken together the data indicate that the shear zones were active in alpine times 100 ma and probably about 50 ma ago they are believed to have formed during the early stages of eocene compression in the pyrenees deformation and resultant uplift probably terminated an important thermal event in this part of the pyrenean basement which may have begun at the time of the mid cretaceous north pyrenean metamorphism 90 100 ma

cosmogenic 36cl ages of quaternary basalt flows in the mojave desert california usa abstract basalt flows provide excellent opportunities for calibration and intercomparison of quaternary dating methods remote sensing methods and rates of geomorphic processes the immediate motivation for this study was to provide chronology for a blind test of the utility of rock varnish microstratigraphy as an indicator of the age of flow emplacement five basaltic eruptive centers in the mojave desert of california were sampled for cosmogenic 36cl analysis multiple samples were taken from most centers and with one exception produced good agreement assuming a surficial erosion rate of 1 mm kyr 1 the flows yielded the following ages amboy crater 79 5 ka pisgah crater 22 5 1 3 ka cima field i cone 27 1 3 ka cima field a cone 21 1 6 ka and 11 5 1 5 ka cima field flow of unidentified origin 46 2 ka the ages from the cima i and a cones are in good agreement with previous cosmogenic 3he dating ages from the three previously undated flows are significantly older than previous estimates based on flow appearance tanzhou liu performed varnish microstratigraphic analysis on samples collected from the same sites his results were submitted for publication without knowledge of the 36cl ages his age estimates agree well with the 36cl ages for the three previously undated flows strongly supporting the validity of varnish microstratigraphy as a chronological correlation tool

development of high thermal gradients by coeval transpression and magmatism during the variscan orogeny insights from the cap de creus eastern pyrenees abstract the cap de creus massif in the eastern pyrenees is a fragment of the variscan mid crust geological mapping and meso and micro scale analysis of the relationships between fold structures metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes deformation metamorphism and magmatism low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of 5 km zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid sillimanite schists and small migmatite pods are distributed around the small intrusions of basic intermediate composition the inferred maximum thermal gradient is 80 c km this thermal pattern is similar to patterns observed in other variscan massifs in the pyrenees where high metamorphic gradients are interpreted in different ways in the ne cap de creus the type and distribution of prograde metamorphism was caused or at least enhanced by the intrusion of diorites and granitoids alternatively deformation developed at prograde conditions and synchronously with magmatism displays a highly heterogeneous distribution producing a sub vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity this zone of high strain developed in a broad transpressive regime could have favoured the injection of magmatic sheets from deep to mid crustal levels progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones

shear wave splitting in the appalachians and the pyrenees importance of the inherited tectonic fabric of the lithosphere splitting of teleseismic shear waves has been measured in the appalachians eastern usa and the pyrenees western europe using data recorded by permanent and portable stations from a comparison of the results it appears that an interpretation of the recorded seismic anisotropy in terms of geodynamics is not straightforward successive geodynamic events have generated structures that may have resulted in a similar pattern of mantle flow and that therefore may have contributed in the development of the recorded anisotropy combining geological and geophysical arguments it appears that the mantle anisotropy measured across the appalachians and the pyrenees may not be systematically appalachian or pyrenean in age but may be mainly due to a lithospheric structure formed during earlier major tectonic events i e the grenvillian and the hercynian orogenies respectively we suggest that during major episodes of continent assembly a pervasive tectonic fabric is developed in the lithospheric mantle in the subsequent evolution of the continent this fabric may induce a significant mechanical anisotropy that will drastically influence the mechanical behaviour of the iithosphere when submitted to new tectonic events

relation between sediment yield and gradient on debris covered hillslopes walnut gulch arizona the relation between overland flow erosion and gradient on debris covered semiarid hillslopes in walnut gulch experimental watershed arizona is investigated by conducting three sets of field experiments under simulated rainfall on two different substrates each set of experiments yields a convex upward sediment yield gradient relation with a vertex at about 12 the key to understanding this relation is the relation between runoff and gradient on slopes less than 12 runoff appears to increase very slowly with gradient and so sediment yield increases with gradient mainly in response to the increase in the downslope component of gravity on slopes steeper than 12 runoff decreases rapidly as gradient increases this decrease in runoff outweighs the increase in the down slope component of gravity as gradient increases and causes sediment yield to decrease in all three sets of experiments runoff is controlled by either stone cover or stone size and stone size increases with gradient inasmuch as stone cover and stone size are indicators of mantle stoniness these findings suggest that curvilinear relations between runoff and gradient and hence convex upward relations between sediment yield and gradient occur only on hillslopes with coarse weathering mantles in which stoniness increases with gradient different combinations of substrate and climate may produce such hillslopes they are especially likely to develop in arid and semiarid climates but they may also form in alpine periglacial and humid ones where substrates are favorable hillslopes on which mass movement processes are the dominant denudational agents will not have convex upward relations

thermobarometric constraints on the conditions of metamorphism in the canigou massif pyrenees implications for hercynian geothermal gradients quantitative thermobarometry of garnet biotite sillimanite f cordierite pelitic assemblages from the canigou massif and constraints based on consideration of high grade metapelitic reaction equilibria indicate that peak temperatures during hercynian metamorphism reached 725 25 c at depths of 16 2 km 4 5 0 5 kbar this corresponds to an average peak geothermal gradient of 45 50 c km 1 which is in broad agreement with results from studies of granulite facies assemblages in the pyrenees but is significantly lower than gradients proposed for some other amphibolite facies areas in the terrane it is suggested that the claims by numerous authors for peak hereynian geothermal gradients in the pyrenees in excess of 70 100 c km 1 fail to consider the effects of post metamorphic attenuation of metamorphic sequences and the unreliable nature of p estimates made in the absence of suitable assemblages for quantitative thermobarometry

zebra stripes in the atacama desert fossil evidence of overland flow abstract some hillslopes in the hyperarid region of the atacama desert in northern chile have surface clasts organized into distinct contour parallel bands separated by bare soil we call the bands zebra stripes due to the contrast between the darkly varnished clasts and the light colored salt rich soil gravel that comprises the zebra stripes is sorted such that the coarsest clasts are at the downslope front and fine progressively upslope how and when the zebra stripes formed are perplexing questions particularly in a region experiencing prolonged hyperaridity using googleearth satellite imagery and field observations we report the first quantitative and qualitative observations of zebra stripes in order to test hypotheses of the mechanisms and timing of their formation we consider soil shrink swell seismic shaking and overland flow as possible formation mechanisms and find that overland flow is the most likely based on cosmogenic 10 be concentrations in surface clasts salt deposition rates from the atmosphere and content in the soils we propose that the salt rich soils began accumulating 10 6 y ago and the zebra stripes formed 10 3 10 4 y at the latest the zebra stripe pattern has been preserved due to the self stabilization of the clasts within the stripes and the continued absence of life which would disturb the surface as seen at a wetter site to the south we conclude that the occurrence of zebra stripes is diagnostic of a set of distinct characteristics of local and or regional precipitation soil hillslope form and bedrock type

polymetamorphism and ductile deformation of staurolite cordierite schist of the bossost dome indication for variscan extension in the axial zone of the central pyrenees the bossost dome is an e w trending elongated structural and metamorphic dome developed in cambro ordovician metasedimentary rocks in the variscan axial zone of the central pyrenees a steep fault separates a northern half dome cored by massif granite from an e w trending doubly plunging antiform with granitic sills and dykes in the core to the south the main foliation is a flat lying s 1 2 schistosity that grades into a steeper dipping slaty cleavage at the dome margins three major deformational and two metamorphic phases can be differentiated s 1 2 schistosity is an axial planar cleavage to w vergent recumbent folding that probably occurred in mid westphalian time peak regional metamorphism m 1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust strong non coaxial deformation recording uniform top to the se extension during d 2 a is preserved in staurolite garnet schists in a 1 5 km thick shallowly se dipping zone in the southeastern dome a 500 m thick contact aureole m 2 was imprinted on the regionally metamorphosed rocks following the intrusion the bossost granite during d 2 b more coaxial deformation prevailed during synkinematic growth of m 2 phases in the inner part of the contact aureole around the northern part of the dome where it obliterated d 2 a fabrics progressive non coaxial deformation continued in the southeastern antiform and is recorded by late synkinematic growth of cordierite successive overprinting of the m 1 staurolite garnet assemblage by andalusite and cordierite of m 2 is preserved in the southern part the assemblage muscovite cordierite staurolite biotite is considered metastable given the low mn and zn contents of staurolite and cordierite and interpreted as the result of prograde metamorphism during decompression p t conditions during m 2 were approximately 3 kbar and 600 c pervasive crenulations and mesoscopic to regional southerly verging folds are the result of d 3 nne ssw compression post dating ductile deformation and contact metamorphism polymetamorphic assemblages of the bossost dome preserve a regionally confined zone of ese directed extensional shearing within an overall n s compressional setting exact timing of extensional shearing is not known but can be constrained by recumbent folding during the mid westphalian and granitic intrusions which confine it to late carboniferous time c 305 ma crustal scale flat lying extensional shear zones with similar orientation and time frame are observed in the hospitalet massif of the eastern axial zone this suggests that crustal extension though probably restricted by regional strain partitioning over orthogneiss or intruding granitic bodies within an overall compressive setting was not uncommon in late carboniferous time in the axial zone of the pyrenees

the role of lateral strength contrasts in orogenesis a 2d numerical study in this paper we present a series of thermo mechanical experiments on continent continent collision zones to investigate the role of lateral strength contrasts in terms of collision dynamics and orogen geometries our results show that differences in crustal rheology may lead to a variety of different patterns of deformation and crustal geometries upper plate indentation forms a sequence of foreland propagating thrust units made of brittle upper crust on the lower plate the strong deformation of the lower plate stands in stark contrast to the undeformed upper plate in contrast subduction of strong lithosphere beneath a weak upper plate forms a complex pattern of deformation deformation initiates on the lower plate and forms an antiformal stack made of brittle upper crust before it indents the upper plate hence deformation is present on both the lower and upper plate despite differences in initial strength the outcome of this study has direct implications to natural collision zones where differences in strength between the upper and lower plate exist for example along strike variability of upper vs lower plate shortening in the alps are likely to be controlled by contrasts in crustal strength furthermore we suggest that the antiformal stack in the axial zone of the pyrenees might indicate a strong lower plate at the onset of collision our results may therefore provide important constraints for the rheological state of continents during collision

characterization of lava flow degradation in the pisgah and cima volcanic fields california using landsat thematic mapper and airsar data digital landsat thematic mapper tm observations multi frequency polarimetric airsar radar data microtopographic profiles derived from helicopter borne photography and field measurements were analyzed over the pisgah and cima volcanic fields and lavic lake playa mojave desert california surface radiance factors were extracted from tm data using radiative transfer procedures to model the atmosphere and surface simultaneously airsar data were calibrated to backscatter specific cross sections using corner reflectors deployed at a number of locations analyses focused on the use of key field sites to understand compositional and textural controls on tm band 5 1 55 to 1 75 m radiance factors and c 5 6 cm l 24 cm and p 68 cm band specific cross sections for the pisgah and lavic lake sites the tm data are explained using a linear mixing model of basalt and fine grained sediment clay to sand sized grains of clay minerals quartz feldspar carbonate and lithic fragments radiance factors with the addition of shadows for the rough a9a flow on the other hand airsar data are shown to be primarily controlled by surface roughness as there is a strong correlation between specific cross section and surface height standard deviation for the elongate flows at the cima volcanic field which exhibit a mix of pahoehoe and a9a properties we find a positive linear relationship between tm band 5 radiance factors and flow ages for the 3 m y age range represented specific cross sections remain constant until about 0 14 ma and then decrease with age up to about 0 56 ma on the other hand data for the two older flows 0 70 and 0 85 ma show increasing specific cross sections with increasing age we interpret the increase in tm band 5 radiance factors from 0 05 to 0 25 with increasing flow age and the decrease in specific cross sections 8 db change at 35 incidence angle for l band for flows up to 0 56 ma in age as due to smoothing by mass wasting and infill by fine grained aeolian deposits the increased specific cross sections for older flows are interpreted to be a consequence of surface roughness generated by fluvial dissection

syntectonic emplacement of the maladeta granite pyrenees deduced from relationships between hercynian deformation and contact metamorphism the relationships between hercynian deformation and contact metamorphism in the aureole of the aneto unit a constituent pluton of the maladeta granite complex spanish pyrenees provide new data relevant to the controversy between the syn and post tectonic models of emplacement for the pyrenean granites the deformation in the aneto aureole is related to two superimposed hercynian episodes the first was a top to the sw thrusting which induced sw verging recumbent folds the second which was essentially compressive in the aneto area is represented by e w trending subvertical cleavage and folds with subhorizontal axes this second episode can be assigned to the d2 main hercynian tectonic event of the pyrenees the contact metamorphic assemblages grew during the d2 event and were deformed by it close to the contact the conclusion that the pluton was emplaced during d2 is strengthened by the structural and kinematic similarities between the magmatic deformation in the southern rim of the pluton and the d2 deformation in the country rocks this syntectonic timing is supported by new geochronological data that conflict with an existing rb sr post tectonic permian whole rock age for the maladeta granite more generally in accordance with some very recent structural and u pb zircon studies our results contradict the geodynamical models of the pyrenees which relate the granite intrusions to a late extensional event

paleomagnetism of siluro devonian sequences ne spain the paleogeography of northeastern spain is addressed in a detailed paleomagnetic study of siluro devonian boundary sediments from the catalan coastal ranges ccr at santa creu d olorda barcelona and the eastern pyrenees at baen the sequences studied are all well dated biostratigraphically and have undergone only very minor deformation mainly during the late carboniferous and extremely low grades of metamorphism anchizone or less after stepwise thermal demagnetisation of samples and principal component analysis of the data up to two paleomagnetic directions can be clearly identified after removal of a present day magnetic overprint the first component labeled b is identified only in samples from the ccr with intermediate unblocking temperature spectra 300 400 c an overall mean direction of 187 03 semi angle of cone of 95 confidence 95 9 0 and precision parameter k 43 1 in situ and 195 03 95 24 4 and k 7 1 51 samples at seven sites after bedding correction is obtained for b the exact age of this secondary component is uncertain but when compared to published paleomagnetic data a late carboniferous to early triassic age could be inferred a stable endpoint direction labeled c can be identified between 400 and 600 c in samples from both regions for the ccr it is southwesterly directed with intermediate inclination values in situ and yields a mean direction of 208 48 95 12 8 and k 28 4 six sites after bedding correction it passes the fold test is considered to be primary in origin and is thus indicative of the siluro devonian paleofield direction in the ccr in the pyrenean samples the c direction is northwesterly directed in situ with a mean direction of 240 51 95 6 5 and k 86 5 seven sites after bedding correction although there is a clear improvement in grouping of these site mean directions this is not significant at the 95 level of confidence combining the site mean directions obtained for component c from both regions yields an overall mean direction of 224 51 95 8 5 and k 24 6 13 sites after bedding correction the improvement in grouping is significant at the 99 level of confidence at 90 unfolding reinforcing the interpretation that this is a primary magnetisation adopting the reversed polarity option the inclination value obtained translates into paleolatitudes of 30 s placing ne spain in the same latitudinal belt as the armorican massif and the barrandian terrane of the bohemian massif i e the armorican terrane assemblage in latest silurian earliest devonian times

cosmogenic 3he surface exposure dating of stone pavements implications for landscape evolution in deserts the formation of stone pavements a ubiquitous gravel armor mantling landforms in arid regions of the world has been previously attributed to erosion by wind and water or alternating shrinking and swelling of soil horizons implying that gravel is concentrated at the land surface in a time transgressive manner a newly proposed model for pavement evolution differs from these models in that pavement clasts are continuously maintained atthelandsurfaceinresponsetodepositionandpedogenicmodificationofwindblowndust in situ cosmogenic 3 he surface exposure ages on volcanic and alluvial landforms in the mojave desert of california are used to understand pavement evolution over geologic time scales and to test this new model these exposure ages are stratigraphically consistent show internal consistency at each site and for stone pavements adjacent to pristine continuously exposed volcanic bedrock are indistinguishable at the 1 level we conclude that stone pavements are born at the surface and that pavements may provide one of the longest term records of geologic hydrologic and climatic processes operating on desert surfaces

blind testing of rock varnish microstratigraphy as a chronometric indicator results on late quaternary lava flows in the mojave desert california rock varnish is a manganiferous dark coating ubiquitous in desert landscapes to test the validity of varnish microstratigraphy as a chronometric indicator varnish samples were collected from radiometrically dated and undated late quaternary lava flows in amboy cima and pisgah volcanic fields avf cvf pvf in the mojave desert of california western united states varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the younger dryas and heinrich events in the north atlantic region microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western us sites with available radiometric age constraints based on this regional chronology varnishes from the a flow h flow and a stone pavement surface in the cima volcanic field were estimated to be 16 5 24 74 85 and 74 85 ka respectively these ages are consistent with previously published cosmogenic 3 he ages of 18 20 72 74 and 80 85 ka for these geomorphic surfaces varnishes from the i flow at cima yielded a puzzling age estimate of 39 ka which is consistent with an older 3 he age of 37f6 ka reported for the i flow but inconsistent with a younger 3 he age of 31f7 ka and a cosmogenic 36 cl age of 27f1 3 ka for the same flow reinterpretation of the original varnish age data with knowledge of then available field mapping results of the i flow suggests that the i cone is polycyclic and different flow units were probably unintentionally sampled in the field the revised varnish ages of 30 and 39 ka for the i flow thus may be in good agreement with their corresponding 3 he and 36 cl ages in a blind test of the method varnishes from the phase 1 flow at pisgah an unnamed flow called here the ivflow at cima and the amboy flow were estimated to be 24 30 46 60 and 74 85 ka respectively these ages agree well with 36 cl ages of 22 5f1 3 46f2 and 79f5 ka reported for the same flows by phillips geomorphology 2002 these test results provide convincing evidence that varnish microstratigraphy once radiometrically calibrated can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western us drylands d 2002 elsevier science b v all rights reserved

the origin of pyrenean hercynian volcanic rocks france spain ree and smnd isotope constraints reply abstract the hercynian orogeny of the pyrenees generated a suite of basalt to rhyolite calc alkaline volcanic rocks that were erupted subaerially these melts may either have been generated above a subduction zone or by lower crustal melting after input of mantle derived magmas in an extensional environment sm nd garnet whole rock mineral isochrons date a rhyolitic ignimbrite from the coll de oli ignimbrite member at 313 14 ma 2 mswd 0 7 and a rhyolitic lava of the coll de pi region at 320 2 ma 2 mswd 0 2 these ages suggest that some of the volcanism was synchronous with high temperature low pressure metamorphism and partial melting deep in the hercynian crust as seen in rocks now exposed in the axial and northern pyrenean zones and dated by u pb on zircon at between 309 5 and 315 5 ma the silicic volcanic rocks are peraluminous and have highly fractionated ree patterns with low abundances of the hree and la yb n 70 mineral and whole rock ree abundances suggest derivation of the silicic rocks by partial melting of a metasedimentary source in equilibrium with garnet and or garnet fractionation in melts derived from peraluminous sources nd isotope compositions of the entire volcanic rock suite mafic silicic compositions are within the same range as those of sediments metamorphosed during the hercynian orogeny and hercynian basement rocks they have mid proterozoic depleted mantle model sm nd ages and do not exhibit mixing trends with mantle sources even the most mafic of the volcanic rocks appear to have interacted extensively with the crust analysis of the chemistry of the volcanic rocks does not unequivocally constrain the hercynian tectonic setting of the pyrenees

environmental history recorded in aeolian deposits under stone pavements mojave desert usa abstract reconstructing the evolution of arid landscapes is challenged by limited availability of appropriate environmental archives a widespread surface feature stone pavement traps aeolian fines and forms a special accretionary archive seven stone pavement covered sections on basalt flows in the eastern mojave desert are condensed into a composite section comprising five sedimentological units supported by an osl based chronology three of the units are of accretionary nature and each is covered by a stone pavement they were deposited 50 9 36 6 ka

formation mechanisms and control factors of vesicular soil structure abstract vesicular structure is a widespread feature of soils in arid environments typically associated with stone pavements it has a major control function for dust and water fluxes and hence for site ecology previous models of vesicular structure formation and stability are inconsistent and mostly focused on the investigation of isolated control parameters to test existing models to infer plausible mechanisms of vesicular structure formation and to unveil controlling environmental and sedimentological parameters we present quantitative data of vesicle properties of artificial and natural vesicles vesicles are not unstable but their changes in size are non linear with time they form due to surface sealing by puddling and by a wetting front which advances downward thereby elevating gas pressure within the sediment matrix translocation of clay or precipitation of calcium carbonate support vesicle stabilisation vesicular structure forms within a wide range of environmental and sedimentological settings at least 6 3 15 7 mm of water per wetting cycle 0 70 sand content 0 8 5 6 ms cm electric conductivity and 1 28 calcium carbonate content high sand contents favour larger and rounder vesicles whereas increasing calcium carbonate content has the opposite effect the concentration of dissolved ions does not considerably influence vesicle formation under an artificial cover stone vesicles could not be created at all whereas they were concentrated around stone margins the main limiting factor for vesicle formation in natural environments is root activity the type and density of surface cover determine fine material fluxes into the aggregates of vesicular horizons the shape of vesicles may be related to dust influx intensity accordingly horizons with spherical vesicles may indicate constant low dust influx on poorly vegetated surfaces and be therefore used as proxies of environmental conditions

k ar dating of the cima volcanic field eastern mojave desert california late cenozoic volcanic history and landscape evolution thirty four k ar ages and supporting paleomagnetic measurements from flows of the cima volcanic field provide a detailed volcanic history and a temporal basis for analysis of evolving landforms the cima field has undergone three periods of volcanic activity spanning late miocene through latest pleistocene time 1 7 6 to 6 5 2 4 5 to 3 6 and 3 1 0 to at least 0 015 m y ago these precisely dated sequences afford a unique opportunity to quantify the long term effects of pedogenesis and erosion on volcanic landforms in an arid environment morphologic and pedologtc data indicate that the most stable geomorphic surfaces in the field occur on flows between 0 25 and 0 75 m y old younger flows are dominated by eolian aggradation and outcrop rubbling and older flows are dominated by surface runoff and fluvial dissection a process response model involving progressive pedogenesis and a subsequent shift from infiltration to surface runoff is proposed to explain this temporal variation in land surface stability