upper mantle reflector structure and origin beneath the scottish caledonides the mantle reflector pattern beneath the scottish and irish caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle previous investigations interpreted the reflector as a thrust related to the caledonian orogeny silurian early devonian or as a normal fault or shear zone active during post caledonian e g devonian or permo triassic extension this study uses all available deep seismic reflection data to map mantle reflector structure along the caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology the distribution of the mantle reflector shows that it is not spatially related to either caledonian compressional or subsequent basin extensional structures in a simple way a structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform synform pair beneath the west orkney basin north of the scottish mainland and the faeroe shetland basin west of shetland respectively separated by an inferred zone of structural disruption a working hypothesis for this study is that these two surfaces may have once been part of a continuous more or less planar surface that has been offset and deformed by either or perhaps a combination of two processes 1 differential extension of crust that was at least partially coupled to the uppermost mantle beneath the west orkney and faeroe shetland basins 2 a broad approximately orthogonal i e east west trending zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust our results also suggest that the mantle reflector structure may be inherited from neoproterozoic events associated with rifting of the laurentia baltica supercontinent

prediction of aeolian features on planets application to titan paleoclimatology we examine the conditions necessary to form aeolian features on planets such as ripples and dunes specifically we compute saltation and suspension speeds for the terrestrial planets triton titan and two possible primordial titan atmospheres and compare these with theoretical estimates of friction speeds we examine the availability of sediments and the mass transport rates needed to form aeolian features on titan and compare likely feature dimensions with the resolution of instruments on the cassini mission to evaluate the detectability of such features a novel element is the consideration of a weibull wind speed distribution adding a probabilistic element to the formation and destruction of dunes we find dunes unlikely to form in any titan atmospheres suggesting that dunelike bedforms are only likely to be found in areas that were once submerged by liquids

longevity of aeolian megaripples abstract megaripples are distinguished from regular ripples by their larger dimensions and bimodal grain size distributions the interplay between wind grain size and ripple morphology height and wavelength controls their development two main mechanisms limit megaripple height the first megaripple flattening due to winds that are above the fluid threshold of the coarse grains destroys the armoring layer of the megaripple the second is megaripple erosion by the impacts of fast moving fine saltating grains that propel the coarse grains constituting the armoring layer for any given wind regime and grain size distribution the potential megaripple dimensions are limited by these two mechanisms here we study the first mechanism and estimate the duration of strong winds sustained above the fluid threshold needed to flatten megaripples strong gusts of wind in contrast cannot destroy the megaripples but can cause ripple migration based on data from previous works on megaripples we find a scaling law between the ripple morphology and the coarse mode of grains at the crest using this scaling relation allows us to calculate the wind velocity and duration needed for megaripple flattening in general the coarser the particles at the megaripple crest the stronger the wind needed to flatten the megaripples moreover the greater the strength of the wind required to flatten the megaripples the lower the recurrence probability taken together these findings increase the longevity of megaripples we apply the results for a megaripple field in the southern arava valley israel

megaripple flattening due to strong winds abstract megaripples in nahal kasuy in the southern negev desert of israel are characterized by a mean wavelength of about 70 cm and by a bimodal distribution of coarse and fine particle sizes the latter property being a pre requisite for their formation in our three year project megaripple development was monitored using a digital elevation model dem constructed from above ground stereo digital photographs temporal dynamics of wind power drift potential dp were measured and grain size analyses were performed on samples taken from different parts of the megaripple the coarse grains that protect the crest enable the ripple to grow but when ripple height becomes too high the bed shear stress increases thus allowing the wind to move the armoring layer when this happens during strong wind storms the megaripple will flatten and even disappear as was observed in our field study we present measurements that for the first time directly show the megaripple to normal ripple transition and we suggest two possible physical processes as potential causes of this phenomenon megaripple flattening can occur either when the wind exceeds the fluid threshold for a sufficient length of time or after a sequence of storms with winds blowing from the same direction

the rate of granule ripple movement on earth and mars abstract the rate of movement for 3 and 10 cm high granule ripples was documented in september of 2006 at great sand dunes national park and preserve during a particularly strong wind event impact creep induced by saltating sand caused 24 granules min 1 to cross each cm of crest length during wind that averaged 9 m s 1 at a height well above 1 m which is substantially larger than the threshold for saltation of sand extension of this documented granule movement rate to mars suggests that a 25 cm high granule ripple should require from hundreds to thousands of earth years to move 1 cm under present atmospheric conditions

proposal for a terrane based nomenclature for the lewisian gneiss complex of nw scotland the current nomenclature for the lewisian gneiss complex has evolved from lithological and structural correlations made prior to any dating initial flawed geochronological studies gave some names an apparent chronological standing but as work advanced fitting events into a coherent regional framework became increasingly difficult modern dating studies have shown that the lewisian gneiss complex was progressively assembled from disparate blocks of archaean continental crust and juvenile proterozoic arcs which satisfy the definition of terranes each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies this paper proposes a new systematic terminology for the lewisian gneiss complex that is more applicable to this new tectonic framework

sedimentology of cold climate aeolian sandsheet deposits in the askja region of northeast iceland active aeolian sandsheets cover an area of 270 km2 in the askja region of ne iceland the sandsheet is divided into i a deflationary upwind part that is characterised by deflation lags ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles ii an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars and iii a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet these sinuous crested oblique bedforms attain heights of 0 35 m and downwind spacings of 6 m they are composed principally of low density pebble grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby askja volcanic centre the distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial ice margin fluvial aeolian and volcaniclastic processes under the influence of a distinctive climatic regime sediment supply at the upwind sandsheet margin adjacent to the dyngjujokull glacier increases during summer periods when rates of glacial ablation are at a maximum longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat and by periodic eruptions of volcaniclastic debris from the askja volcanic centre the availability of sediment for aeolian transport is controlled by the level of the groundwater table the depth and continuity of the permafrost layer in the proglacial area the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months the transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969 the suite of aeolian landforms developed within the askja region as defined by sediment texture and composition and bedform scale morphology and spatial distribution reflects a special set of controlling circumstances which mean that the askja sandsheet deposits differ significantly from those described from elsewhere

geochemistry and tectonic setting of lewisian clastic metasediments from the early proterozoic loch maree group of gairloch nw scotland abstract the early proterozoic 2 0 ga loch maree group lmg of the lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the archaean basement the metasediments comprise three main mapped units the flowerdale schists kerrysdale schists and charlestown schists together with numerous narrower bands the dominant clastic metasediments within the schist units semipelites and psammites are chemically distinguished as a series of greywackes and lithic sandstones with variable lil and transition element abundances fractionated ree patterns and overall compositions similar to paas three chemical groups are recognized whose range of composition largely reflects the mixing between two main source components lewisian gneissose basement with an intermediate acid continental crust composition and local rift related amphibolites with a primitive tholeiitic composition which also incorporates components not preserved within the lmg the geochemistry of the clastic metasediments confirms a previous tectonic model for the lmg as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement as well as from internal volcanogenic sources

controls on the height and spacing of eolian ripples and transverse dunes a numerical modeling investigation abstract ripples and transverse dunes in areas of abundant sand supply increase in height and spacing as a function of time grain size and excess shear velocity how and why each of these factors influence ripple and transverse dune size however is not precisely known in this paper the controls on the height and spacing of ripples and transverse dunes in areas of abundant sand supply are investigated using a numerical model for the formation of eolian bedforms from an initially flat surface this bedform evolution model combines the basic elements of werner s werner b t 1995 eolian dunes computer simulations and attractor interpretation geology 23 1107 1110 cellular automaton model of dune formation with a model for boundary layer flow over complex topography particular attention is paid to the relationship between bed shear stress and slope on the windward stoss side of evolving bedforms nonlinear boundary layer model results indicate that bed shear stresses on stoss slopes increase with increasing slope angle up to approximately 20 then decrease with increasing slope angle as backpressure effects become limiting in the bedform evolution model the linear boundary layer flow model of jackson and hunt jackson p s hunt j c r 1975 turbulent wind flow over a low hill quarterly journal of the royal meteorological society 101 929 955 generalized to 3d is modified to include the nonlinear relationship between bed shear stress and slope bed shear stresses predicted by the modified jackson and hunt flow model are then used to predict rates of erosion and deposition iteratively through time within a mass conservative framework similar to werner werner b t 1995 eolian dunes computer simulations and attractor interpretation geology 23 1107 1110 beginning with a flat bed the model forms ripples that grow in height and spacing until a dynamic steady state condition is achieved in which bedforms migrate downwind without further growth the steady state ripple spacing predicted by this model is approximately 3000 times greater than the aerodynamic roughness length of the initially flat surface which is a function of grain size and excess shear velocity once steady state ripples form they become the dominant roughness element of the surface the increase in roughness associated with ripple formation triggers the same bedform instability that created ripples causing dunes to form at a larger scale in this way the numerical model of this paper suggests that ripples and dunes are genetically linked transverse dunes in this model have a steady state height and spacing that is controlled by the effective roughness length of the rippled surface which is shown to be on the order of 500 times greater than the original roughness length but varies significantly with the details of ripple morphology the model predictions for ripple and dune spacing and their controlling variables are consistent with field measurements from the published literature the model of this paper provides a preliminary process based understanding of the granulometric control of ripples and dunes in areas of abundant sand supply and unidirectional prevailing winds and it argues for a genetic linkage between ripples and dunes via a scaling relationship between eolian bedform size and the aerodynamic roughness length

the largest wind ripples on earth reply we thank de silva 2010 1 for the opportunity to discuss the origin of the unique megaripples mr found on the puna plateau forman and krohling were invited as co authors of this reply because we produced new data supplied here de silva suggests that the genetic relationship inferred by

petrogenesis and geochemical halos of the amphibolite facies lower proterozoic kerry road volcanogenic massive sulfide deposit loch maree group gairloch nw scotland abstract the palaeoproterozoic kerry road deposit is one of the oldest examples of volcanogenic massive sulfide vms mineralization this small vms deposit 500 000 tons grading at 1 2 cu 3 5 zn is hosted in amphibolite facies mafic siliciclastic units of the c 2 0 ga loch maree group scotland sulfide mineralization consists of pyrite and pyrrhotite with subordinate chalcopyrite and sphalerite occurring in disseminated vein and semi massive to massive textures the deposit was highly deformed and metamorphosed during the c 1 8 1 7 ga laxfordian orogeny textural relationships of deformed sulfide minerals related to early laxfordian deformation d1 d2 indicate initial high pressure low temperature 100 mpa 150 c conditions before reaching peak amphibolite facies metamorphism as evident from pyrrhotite crossing the brittle ductile transition prior to chalcopyrite late laxfordian deformation d3 d4 is marked by local retrograde greenschist facies at low pressure and temperature microprobe analyses of amphiboles record evidence of the original alteration halo associated with the kerry road deposit with a systematic mg and si enrichment from ferrotschermakite 150 m to mg hornblende 90 m to actinolite 0 m on approach to the vms deposit furthermore whole rock geochemistry records a progressive enrichment in si cu co and s and depletion in al ti v cr y and zr with proximity to the vms system these elemental trends together with amphibole geochemistry are potentially useful exploration vectors to vms mineralization in the loch maree group and in similar highly deformed and metamorphosed terranes elsewhere

timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion reply u pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the gruinard bay badcall bay and lochinver areas of the mainland lewisian gneiss complex of north west scotland zircons from the gneisses contain complex zoning patterns making them amenable to shrimp ion microprobe analysis with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation from gruinard bay a tonalitic granulite yielded a protolith age of 2 825 8 ma for oscillatory zoned zircon cores with recrystallised low u rims preserving an age of 2 733 12 ma an inherited 2 905 15 ma age component was recognised in a sample of trondhjemitic gneiss the protolith of which is estimated to have been emplaced at 2 858 11 ma low u rims yielding an age of 2 729 20 ma are also present which together with the rim age from the granulite are considered to date granulite facies metamorphism in the gruinard region for comparison with the gruinard bay gneisses a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at badcall bay in the assynt terrane age data do not show definitive relationships with zoning patterns in the zircons and show a spread along concordia typical of previous ion microprobe age data from the scourie gneisses both samples do however show a significant isotopic disturbance at 2 480 ma the most recent estimate for the badcallian metamorphism protolith ages for components of the assynt terrane range between ca 2 900 and 3 030 ma and appear to be older than the 2 860 2 825 ma gneiss protoliths from the gruinard bay area the timing of granulite facies metamorphism also appears to be different between the two areas suggesting that the two granulite regions of the mainland may not be related as previously thought instead they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the palaeoproterozoic the boundary between the two terranes is thought to lie to the south of lochinver corresponding with the strathan line because of the observed 2 480 ma age signature to the north and 2 730 ma signature to the south by defining a new mainland terrane the gruinard terrane the number of mainland blocks which includes the assynt central rhiconich northern and southern terranes has been expanded

variable wind ripple migration at great sand dunes national park and preserve observed by timelapse imaging abstract granule ripples at great sand dunes national park and preserve gsdnpp were observed using inexpensive digital timelapse cameras over a 70 day period in winter 2010 2011 the ripples migrated during a handful of discrete events visible ripple movement occurred on only 11 days during the observation period the movement conditions are documented with hourly and 15 minute records from two nearby weather stations and by a cup anemometer at the site itself during the most prominent movement episode when local winds averaged 10 m s ripples of several sizes were observed simultaneously and a reciprocal relationship of ripple size and propagation speed was seen with small 10 cm ripples moving at 1 4 cm min and larger 80 cm ripples at 0 15 cm min ripple sizes and morphologies evolve throughout the observation period

multiple post svecofennian 1750 1560 ma pegmatite dykes in archaean palaeoproterozoic rocks of the west troms basement complex north norway geological significance and regional implications abstract the west troms basement complex north norway consist of meosarchaean neoarchaean and palaeoproterozoic crust which likely represent the continuation of the karelian craton of the fennoscandian shield beneath and across the caledonian nappes the complex is transected by several meta sedimentary volcanic belts with ages from ca 2 7 to 1 9 ga and linear ductile shear zones that record polyphase deformation after emplacement of several late svecofennian plutonic complexes 1 80 1 79 ga the astridal supracrustal belt of the senja shear zone was intruded by various generations of granite pegmatite dykes which yield zircon and titanite u pb ages ranging between 1758 and 1562 ma with evidence of a weak caledonian overprint at about 400 ma that formed or reset some but not all titanites the age range indicates that formation of the pegmatites occurred after termination of the main svecofennian orogeny 1 92 1 79 ga of the karelian craton comparison within the larger scale context of the fennoscandian shield suggests that emplacement of these pegmatites may have been caused by late stage reactivation of intracratonic shear zones possibly reflecting distal effects of deeper magmatic processes such as intrusion of coeval aged rapakivi granite suites ca 1 65 1 5 ga in finland and or the mesoproterozoic gothian orogeny and magmatism affecting the southwestern parts of fennoscandia the mesoproterozoic ages however also correspond to late laxfordian ca 1 7 1 6 ga tectonic events in the lewisian basement on the laurentian margin of scotland suggesting a potential early link between these basement complexes

the geochemistry and geochronology of the gneisses and pegmatites of the tollie antiform in the lewisian complex of northwestern scotland the tollie antiform occupies a tectonic break between an area to the north where remnants of archaean structures are preserved and an area to the south characterized by substantial proterozoic r

aerodynamic roughness lengths over movable beds comparison of wind tunnel and field data 1 we evaluate the increase in apparent roughness length during aeolian saltation using a data set comprising 291 wind profiles measured in wind tunnels or in the field we tested the charnock 1955 modified charnock sherman 1992 and raupach 1991 relationships for the entire data set and for segregated wind tunnel and field data sets we assessed the abilities of the models to predict enhanced roughness and investigated scaling differences between wind tunnels and field environments for the entire data set the charnock relationship worked best with an r2 of 0 25 for the segregated data sets the charnock relationship also worked best with an average r2 of 0 675 as opposed to 0 66 for the modified charnock and 0 61 for the raupach model for conditions with shear velocity 1 0 ms 1 the comparable average values of r2 are 0 54 0 625 and 0 645 respectively there was an order of magnitude difference in the roughness length responses for wind tunnel and field data for each model evaluated convincing evidence of a fundamental scaling constraint in the wind tunnel data the scaling differences are attributed to conceptual and bias errors that include the use of the law of the wall versus the wake defect law or differences in turbulence characteristics especially regarding the role of large coherent structures raupach s 1991 a was found to be an extremely volatile constant making the modified charnock relationship most suitable for predicting aerodynamic roughness lengths over movable sand beds

strain analysis and strain path modelling in the loch tollie gneisses gairloch nw scotland abstract a quantitative structural analysis is presented for the loch tullie gneisses of the lewisian complex outcropping at gairloch the gneisses and the dykes they contain are folded into a large antiformal structure known as the tollic antiform quartz aggregates in quartzo feldspathic gneisses have been used as finite strain markers in eleven specimens across the antiform two models using rotational strain simple shear and irrotational strain pure shear are used to reconstruct the strain path results show that only the rotational strain model satisfies the strain data and the field evidence and indicates a steeply northeast 75 dipping shear plane and moderately northwest 55 plunging shear direction with a southwest side down sense of shear a strain profile is constructed for the tollie gneisses using the model and the attitude of gneissose layering this shows increasing shear strain to the southwest to a maximum gamma value of approximately 8 the strain profile indicates a horizontal dextral displacement of 4 7 km and a vertical displacement of 6 8 km for the tollie gneisses the tollie antiform thus lies on the northeast margin of a large scale shear zone the main zone of deformation of which can be traced southwestwards some 4 km such a shear zone presents a major tectonic boundary within the lewisian of northwest scotland

granule ripples in the kumtagh desert china morphology grain size and influencing factors granule ripples are found mainly in four regions of the kumtagh desert in china they are characterized by an asymmetrical shape with gentle lower slopes on both sides and abrupt crests the ripples tend to be oriented perpendicular to the prevailing winds except when they form near obstacles such as yardangs the wavelengths range between 0 31 m and 26 m and heights h range from 0 015 m to 1 m the relationship between wavelength and height can be described by a simple linear function and the mean ripple index h is about 20 4 for the study sites the sediments are poorly sorted with negative to very negative skewness at lee and stoss slopes and between ripple troughs which confirms the poured in and shadow appearance described by previous researchers the bimodal or trimodal distributions of grains with modes of 1 16 0 5 and 3 16 and the enrichment of coarse particles at the ripple surface with coarse granule contents ranging between 5 2 and 62 1 indicate that the underlying layer is the original sediment source and that the granule ripples resist erosional processes although the impact of saltating particles and consequently the creep and reptation of coarse grains are responsible for granule ripple initiation at a micro scale however the characteristics of local sediments wind regimes and topographical obstacles as well as the feedbacks among bedform and airflow more strongly affect the development and alignment of granule ripples at a macro scale

crustal xenoliths from tertiary volcanics of the northern hessian depression a suite of crustal xenoliths from tertiary basaltic tuffs of the northern hessian depression nhd volcanic field comprises abundant meta igneous pyroxene granulites of mafic noritic to anorthositic iat and tonalitic composition less abundant are granitic tonalitic and leucogranitic gneisses and metasedimentary xenoliths a total of 49 samples were analyzed for modal compositions for major and trace elements including li rb sr ba cs v sc cr co ni y zr nb ta hf th and ree and oxygen isotopes two pyroxene thermometry yields temperatures between 700 and 900 c for mafic and noritic granulites feldspar thermometry indicates temperatures of 660 710 c for tonalitic granulites and 470 520 c for granitic and tonalitic gneisses one highly depleted sillimanite rich metasediment contains cordierite and garnet which have equilibrated at temperatures of 780 c the general lack of garnet in the mafic and noritic granulites and the presence of sillimanite in felsic xenoliths indicates that metamorphic pressures have not exceeded 10 kb major and trace element data and oxygen isotope compositions of the mafic granulites are compatible with an origin from spilitized enriched type morb rocks enrichment in 18o to 11 and in li to 34 ppm at average sio2 contents of 44 1 wt these low t spilites were transformed into amphibolites and then pyroxene granulites during subsequent high temperature metamorphic events low si al k and rb concentrations along with an contents in plagioclase ranging from near 50 to 98 mole percent suggest that amphibolite facies protoliths have generated tonalitic melts during partial melting at temperatures above 700 c the mafic granulite xenoliths are interpreted as restites whereas the tonalitic samples probably represent the extracted partial melts derived by 20 to 30 percent degree of melting metasedimentary xenoliths strongly depleted in granitic component could represent restites from which granitic s type partial melts have been extracted tonalitic and leucogranitic gneisses including one trondhjemite xenolith have many chemical characteristics e g ree distribution in common with tonalite trondhjemite granodiorite suites of the north atlantic region but cannot be accounted for a more specific origin estimated elastic properties of the main types of nhd xenoliths yield p wave velocities of 6 0 6 4 km 1 for granitic tonalitic and trondhjemite gneisses including tonalitic granulites and 6 5 7 0 for the more mafic xenoliths when compared with two seismic depths vp profiles these data are in accordance with a model where the mafic andesitic noritic and tonalitic granulites comprise abundant rock types at depths between 29 km moho and 20 km which mainly consists of old oceanic crust including subduction related volcanic products the more felsic xenoliths probably represent material from depths between 12 and 20 km

u pb zircon systematics at gruinard bay northwest scotland implications for the early orogenic evolution of the lewisian complex the gruinard bay area of the mainland lewisian complex comprises a metamorphosed suite of archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves the u pb zircon dating yields ages of 2731 14 ma and 2728 2 ma for two trondhjemite and 2732 4 ma for one granite sample also revealing the presence of large amounts of inherited xenocrystic zircons although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies the textural relations between biotite hornblende quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high ti amphibole formed in hornblende granulite facies structural relations link this metamorphism to a steep northeast trending fabric coeval with the intrusion of the trondhjemites dated at 2730 ma dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 ma this age range reflects new growth during the 2730 ma metamorphic metasomatic events superimposed on older zircon phases which include combinations of xenocrystic cores and magmatic and or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging techniques e g cathodoluminescence alone a pegmatitic vein that escaped the d3 strain and related isotopic disturbances yields a precise age of 2792 2 ma which constrains to some degree the earliest orogenic events in the area age relationships displayed in the central block at scourie badcall and in the gruinard bay area indicate that petrogenetic events in both areas were comparable about 2800 ma and that both areas underwent trondhjemitic magmatism about 2730 ma in contrast at gruinard bay there is no isotopic evidence for a period of high grade metamorphism and magmatism at 2490 2480 ma that drastically affected the scourie block indicating that at this stage the two regions occupied different levels of the crust

transverse instability of megaripples as a result of their inherent differences in stability sand ripples and megaripples exhibit variations in terms of their wavelengths and grain size distributions unimodal for sand ripples and bimodal for megaripples while sand ripples form almost straight lines megaripples have greater sinuosity due to their transverse instability a property that causes small megaripple undulations to grow with time the origin of the instability is due to variations in megaripple height variations that do not diminish over time and due to the inverse dependence of ripple drift velocity on the height thus the taller regions of ripples will move more slowly than the adjacent shorter portions an outcome that promotes further perturbation growth we show an example based on fi eld work of the transverse instability of megaripples the instability growth rate depends on the difference between the heights of the different segments of the megaripple in contrast to the underlying instability of megaripples normal sand ripples are essentially stable and are not affected by transverse perturbations instead reacting quickly to the wind which tends to smooth ripple height irregularities the transverse instability of megaripples derives from the composition of their crests which comprise coarse particles that allow initial perturbations in ripple height to grow further the results suggest a physical mechanism for the transverse instability of megaripples and new insight into the spatial patterns of sand ripples

eolian features produced by the december 1977 windstorm southern san joaquin valley california high wind velocities during a windstorm in december of 1977 in the southern san joaquin valley california created eolian features and effects of a magnitude not previously reported in the literature three principal mechanisms of sediment movement by wind were studied by examining the size distribution of deposits in telephone poles saltation and suspension feeder pens and abandoned buildings saltation and suspension and gravel ripples surface creep the maximum grain sizes of rocks lodged in telephone poles at heights of 240 160 and 80 cm are 7 mm x 11 mm x 6 mm 10 mm x 15 mm x 6 mm and 20 mm x 23 mm x 5 mm respectively in some areas with rocky soils lag materials formed gravel ripples

gravel mantled megaripples of the argentinean puna a model for their origin and growth with implications for mars gravel megaripples in the puna of argentina are the most extreme aeolian megaripples on earth and are useful analogs for aeolian processes on mars field observations supplemented by experimental and numerical constraints on wind characteristics and aeolian transport reveal their conditions of formation and growth to be an aeolian geomorphology perfect storm the bedforms are formed on a substrate of weakly indurated ignimbrite aeolian deflation of which yields a bimodal lag of lithics and pumice clasts onto an undulating surface under normal wind conditions in this region the lithics are organized into bedforms on local upslopes and highs through creep induced by the impact of saltating sand and pumice the gravel bedforms grow through shadowing and trap sand and silt that is gradually kinetically sieved down to lift the gravel mantle upwards to form the megaripples these observations connote that the largest features are not ripples in the sense of migrating bedforms but rather nucleation sites of wind transported sediment strong control by bedrock topography means that the largest bedform wavelengths are not a result of particle trajectories and this complicates their comparison with other ripples and may require a new classification of relevance to mars the puna megaripples are morphologically and contextually similar to small ripple like transverse aeolian ridges tars moreover the puna gravels have similar equivalent weight mg to those composing granule ripples at meridiani planum and their local origin may have implications for the origin of sediment in martian aeolian bedforms finally the stable yet dynamic character of the puna megaripples could help reconcile current models of tars with periodic bedrock ridges that may be produced by aeolian erosion

largest wind ripples on earth unique wind ripples attaining heights to 2 3 m wavelengths to 43 m and a crest maximum grain size of 19 mm occur on the argentine puna plateau at 4000 m altitude these are the largest ripples reported on earth comparable only to mars counterparts they form in the presence of high proportions of low density pumice clasts 0 91 g cm 3 although crests are exclusively composed of varnished normal density clasts 2 43 g cm 3 mature ripple profiles are partly excavated on bedrock so they form by a combination of deflation winnowing of finer grains minor wind drift of fine gravel and lagging of clasts 4 cm the large ripple size appears to be related to strong winds dense saltation layers and a long time for evolution ripple sizes are smaller on obstacles as compared to flat terrain there is a lack of correlation between clast size wavelength and the extreme ripple size in spite of the thin atmosphere all of which suggest that while small scale gravel ripples may form according to a reptation model their evolution into large scale types may relate to aerodynamic instabilities originating at the saltation curtain air interface

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

short paper evidence for a grenville event in the lewisian of the northern outer hebrides northern britain lay between two areas of major tectonic activity in the late proterozoic but evidence for geological events of this age within britain are sparse twenty five new rb sr biotite ages from a 50 km traverse through the northern outer hebrides show that a block bounded in the south by the langavat shear zone was generally uniformly affected by an event close to 1 1 ga consideration of the age pattern on either side of the langavat belt suggests that there were kilometre scale displacements on this shear zone at that time involving uplift and erosion of the northern outer hebrides as a whole it is thus possible that the northern outer hebrides acted as a source area for both laxfordian and grenville detritus found in the torridonian

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic