temperature dependence of shear wave attenuation in partially molten gabbronorite at seismic frequencies summary torsion oscillatory deformation experiments have been performed at high temperatures 600 1170 c and over a wide range of low frequencies 20 2 10 3 hz on fine grained gabbronorite samples from the oman ophiolite in order to determine the shear wave attenuation as a function of temperature and melt fraction the specimens have a small and uniform grain size 0 25 0 3 mm and do not contain secondary hydrated minerals measurements of internal friction q 1 w ere performed using a forced oscillatory torsion apparatus at small strains 10 7 and with increasing small temperature steps to reduce thermal microcracking the general dependence of q 1 to frequency is q 1 w here is the angular velocity of forced oscillations and is an empirical exponent below the melting temperature 1050 c has average values of 0 15 at low frequency 0 5 hz and 0 06 at higher frequency above the melting temperature has average values of 0 22 at low frequency and 0 02 at higher frequency this frequency dependence of q 1 is attributed to a viscoelastic behaviour due to the diffusion controlled grain boundary sliding and partially to the squirt flow of the meltphase wetting grain boundaries the onset of melting is associated with a markedly higher q 1 and a stronger dependence of q 1 on temperature the melt related mechanical dissipation process could be a melt squirt flow the characteristic frequency for the melt squirt flow is m 0 15 300 hz when the melt pocket aspect ratio is 10 3 10 2 around the melting temperature the internal friction can be approximated by an experimental power law q 1 a 1 d 1 exp ea rt with 0 08 a 34 72 s m and ea 873 kj mol 1

trace fossils from interdune deposits an example from the lower triassic aeolian tumlin sandstone central poland abstract the tumlin sandstone up to 105 m thick is the deposit of a midcontinent extensive at least 1500 km2 dune field it consists of dune deposits and of subordinate intercalations of interdune deposits the latter are represented by horizontally bedded sandstone accumulated in frequently changing conditions on dry damp and wet surface dry surface deposits predominate subordinate but relatively common are damp surface deposits with characteristic adhesion structures and wet surface deposits represented by very thin mudstone layers and horizons of wave ripples which originated in ephemeral ponds the interdune deposits contain numerous trace fossils whose occurrence is almost entirely connected with wet surface and damp surface deposits the dune deposits are devoid of trace fossils the following forms have been recognized arenicolites isp cruziana problematica diplocraterion isp gordia marina planolites montanus planolites isp palaeophycus tubularis tetrapod footprints radial structures shallow double furrows and various oval depressions in places intensive but shallow bioturbation of sediment is observable the trace fossil assemblage is similar to the scoyenia ichnofacies but lacks scoyenia and ancorichnus the assemblage is more comparable to the mixed arenicolites cruziana ichnofacies sensu bromley and asgaard 1979 the invertebrate burrows show an r selected strategy in sediment colonization around the palaeozoic mesozoic transition the highly diversified ichnofauna compared to that of the permian interdune deposits shows increased colonisation of interdunes by an invertebrate mainly arthropod fauna

the geometry of folded tectonic shear sense indicators the geometry of refolding of early stretching lineations and associated sense of shear indicators is often not fully considered and can potentially lead to grossly incorrect tectonic interpretations of orogens and orogenic events sense of shear is not inverted or flipped in all cases of refolding by folds with closure angle 90 the geometry of inversion of flipping of shear sense is determined entirely by the angle between the stretching lineation and the latter fold axes folding around an axis orthogonal to the stretching lineation 90 does not result in shear sense inversion this is a special case if there is any degree of obliquity 90 between fold axis and stretching lineation there is a change in the orientation of the stretching lineation on alternate limbs and if 45 the sense of shear is inverted in alternate limbs

multiple foliation development during thrusting and synchronous formation of vertical shear zones abstract three orthogonal foliations developed during large scale n s thrusting in the central portion of the mount isa block australia two of these foliations which are well preserved in their original orientations in porphyroblasts of pelitic schists but considerably affected and commonly destroyed in the schist matrix due to the effects of subsequent deformation and metamorphism formed synchronously and have a common stretching lineation which is near horizontal and trends n s one foliation formed near horizontal and the other near vertical striking n s both developed during n s directed thrusting with the n s vertical foliation accommodating sinistral differential displacement between the thrust belt to the west and that to the east the third foliation which dips steeply to the north and strikes e w formed during n s shortening of the thrust belt and tends to overprint the other foliations formed in the same orogeny the n s vertical foliation is well preserved within a narrow belt of gneissic rocks called the wonga duchess belt that also locally contains the subsequently folded remains of the foliation that formed near horizontal this belt separates multiply thrust repeated beds to the west from large scale apparently less repetitive thrusts to the east the direction of thrusting appears to change across this belt from n to s on the west to s to n on the east the differential deformation to either side of the belt was taken up by sinistral shear along this belt numerous related but smaller scale vertical shear zones occur up to 3 km away to either side of this narrow zone the primary vertical foliation within the wonga duchess belt appears to have formed during transform like faulting by progressive inhomogeneous simple shear with no component of orthogonal shortening

seismic frequency laboratory measurements of shear mode viscoelasticity in crustal rocks ii thermally stressed quartzite and granite forced torsional oscillation techniques have been used to explore the seismic frequency shear mode viscoelasticity of specimens of two crustal rocks cape sorell quartzite and delegate aplite cycled between room temperature and 700 c under conditions of moderate confining pressure the anisotropy and intergranular inhomogeneity of thermal expansivity in these materials give rise to large deviatoric stresses resulting in thermal cracking at temperatures above a pressure dependent threshold temperature associated with the onset of very pronounced temperature sensitivity of the shear modulus in general accord with the predictions of fracture mechanics models for delegate aplite in particular the shear modulus behaves reproducibly during multiple thermal cycles at different confining pressures consistent with the notion that the thermal cracks are of low aspect ratio minimum maximum dimension and are therefore readily closed by the prevailing confining pressure once the thermal stresses are removed marked frequency dependent dissipation of shear strain energy is observed on heating each rock to temperatures 500 c although the attenuation varies significantly with prior thermal history probably as a result of progressive dehydration and relaxation of deviatoric stresses temperature and pressure dependent crack densities for delegate aplite have been estimated by comparison of the observed shear moduli with those expected for a crack free aggregate in parallel with the forced oscillation tests measurements have been made of the rate at which argon pore pressure equilibrium is re established following a perturbation combination of these results which provide a proxy for permeability with the inferred crack densities indicates that the variation of permeability with crack density is well described by a percolation model with a threshold crack density of 0 2

anatomy of a mixed marine non marine lowstand wedge in a ramp setting the record of a barremian aptian complex relative sea level fall in the central neuquen basin argentina

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

tracks and trackways of myotragus balearicus bate artiodactyla caprinae in pleistocene aeolianites from mallorca balearic islands western mediterranean tracks and trackways of the ruminant goat myotragus balearicus bate 1909 are abundant in the late pleistocene aeolianites of the south coast of mallorca spain the carbonate aeolianites form impressive dunes and related sand ramps that were initiated as echo dunes in front of a 20 40 m high paleocliff of miocene reefal limestone where the dunes built up to the cliff top they covered it as an aeolian ramp the dunes and sand ramps were active during the dry summer months but were passive during the wet winter period the small goat like animals were abundant on the island and for unknown reasons crossed the aeolian accumulations in large numbers this study deals with the tracks formed in the dune deposits all preserved trackways indicate impression into moist sand special features of the tracks include the structure produced by the withdrawal of the foot here called the axis and a disturbance zone of plastic deformation around this on dune crests the disturbance zone surrounds the axis more or less symmetrically however in addition a pressure pad of dislocated slightly rotated sediment bound by curved microfaults is commonly produced posterior to the axis by propulsive pressure of the foot on steep stoss and lee slopes the pressure pad becomes oriented in a down slope position as a result of gravitational slip of the walking animal combination of disturbance of the sediment in this way by manus followed by overprinting of similar disturbance by pes produces highly complicated track structure this structure may be characteristic of artiodactylous mammals in soft sand particularly aeolian deposits the structure is designated as bifidipes aeolis isp nov

key ring structure gradients and sheath folds in the goantagab domain of nw namibia abstract the concept of deformation phases is one of the corner stones of structural geology but despite its simplicity there are situations where the concept breaks down in the goantagab domain of nw namibia structures in an area of complex deformation can be subdivided into at least four sets attributed to four deformation phases on the basis of overprinting relations three of these sets of structures however formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions these sets of structures show gradational transitions in space that can be understood by a concept of key ring structure gradients where older d a structures are reoriented and overprinted by new structures d a 1 that have similar orientation and seem to grade into d a structures outside the overprinted area this kind of behaviour may be common in inhomogeneous non coaxial flow in the core of the goantagab domain d 2 structures are thus reoriented and overprinted by local d 2b folds and foliations that have the same orientation and style as d 2 structures outside the domain core

viscoelastic behaviour of basaltic lavas the rheological properties of basaltic lavas from etna hawai i and vesuvius have been investigated at temperatures between 500 and 1150 c using a small strain oscillatory shear the viscoelastic response of the lavas to small forced sinusoidal torques 109 pa s could be measured at strain rates less than 10 2 10 1 s 1 at 800 c temporal variations in complex shear modulus and internal friction suggest that over durations of up to 120 h structural adjustments were occurring within some of the samples this time varying behaviour of lava samples may be attributed to the slow closing healing of microcracks and small pore spaces resulting in the apparent stiffening of lava samples under annealing thus those parts of lava flows that undergo slow cooling will have more elastic properties regions that cool faster possess smaller shear moduli and higher internal friction due to thermal microcracking and less cohesion between crystals and the bulk glassy matrix

frequency dependent rheology of vesicular rhyolite frequency dependent rheology of magmas may result from the presence of inclusions bubbles crystals in the melt and or from viscoelastic behavior of the melt itself with the addition of deformable inclusions to a melt possessing viscoelastic properties one might expect changes in the relaxation spectrum of the shear stresses of the material e g broadening of the relaxation spectrum resulting from the viscously deformable geometry of the second phase we have begun to investigate the effect of bubbles on the frequency dependent rheology of rhyolite melt the present study deals with the rheology of bubble free and vesicular rhyolite melts containing spherical voids of 10 and 30 vol we used a sinusoidal torsion deformation device vesicular rhyolite melts were generated by the melting at 1 bar of an armenian obsidian dry fountain erevan armenia and little glass mountain obsidian california the real and imaginary parts of shear viscosity and shear modulus have been determined in a frequency range of 0 005 10 hz and temperature range of 600 900 c the relaxed shear viscosities of samples obtained at low frequencies and high temperatures compare well with data previously obtained by parallel plate viscometry the relaxed shear viscosity of vesicular rhyolites decreases progressively with increasing bubble content the relaxation spectrum for rhyolite melt without bubbles has an asymmetric form and fits an extended exponent relaxation the presence of deformable bubbles results in an imaginary component of the shear modulus that becomes more symmetrical and extends into the low frequency high temperature range the internal friction q 1 is unaffected in the high frequency low temperature range by the presence of bubbles and depends on the bubble content in the high temperature low frequency range the present work in combination with the previous study of stein and spera 1992 illustrates that magma viscosity can either increase or decrease with bubble content depending upon the rate of style of strain during magmatic flow

dinosaur tectonics a structural analysis of theropod undertracks with a reconstruction of theropod walking dynamics abstract a dinosaur trackway in the middle jurassic eolian entrada sandstone of southern utah u s a exposes three undertracks that we have modeled as isolated tectonic regimes showing the development of fold thrust ramp systems induced by the dinosaur s feet the faulted and folded sequence is comparable to crustal scale tectonics associated with plate tectonics and foreland fold thrust belts a structural analysis of the dinosaur tracks shows the timing and direction of the forces exercised on the substrate by the animal s foot during the stride based on the structural analysis we establish a scenario for foot movements and weight distribution in the feet during the end of the weight bearing phase of the stride the weight of the animal was transferred to the front of the digits creating a rotated disk below the foot that was bounded by an extensional fault at the front and a thrust ramp toward the back as the body accelerated the foot was forced backward the rotated disk was forced backward al

petrogenesis of cogenetic nepheline and quartz syenites and granites northern damara orogen namibia enriched mantle versus crustal contamination abstract geochemical and nd sr o whole rock isotope data and pb isotope data from leached feldspars for a suite of synorogenic ca 540 ma syenites and granites from the northern part of the proterozoic damara orogen namibia constrain their sources and petrogenesis major rock types are mildly potassic nepheline syenites and quartz syenites that were primarily generated by fractional crystallization from a mantle derived alkaline magma even the most primitive samples show pronounced depletion in nb ti sr and p on a primitive mantle normalized diagram indicating the involvement of a recycled crustal component in the source extrapolation of the sr nd pb o isotope composition of the syenites back to a hypothetical parental melt with 8 wt 10 wt mgo suggests derivation from a moderately enriched lithospheric upper mantle documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage t

scale invariant sheath folds in salt sediments and shear zones abstract sheath folds are developed in a broad spectrum of geological environments in which material flow occurs including gravity driven surficial deformation in ignimbrites unconsolidated sediments and salt together with deeper level ductile shear zones in metamorphic rocks this study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales elliptical closures defining eye folds represent y z cross sections through highly curvilinear sheath folds our analysis of the published literature coupled with field observations reveals remarkably similar ellipticities r yz for sheath folds in metamorphic shear zones r yz 4 23 salt r yz 4 29 sediment slumps r yz 4 34 glaciotectonites r yz 4 48 and ignimbrites r yz 4 34 nested eye folds across this range of materials n 1800 reveal distinct and consistent differences in ellipticity from the outer r yz to the inner most r y z elliptical rings of individual sheath folds the variation in ratios from outer to inner rings r r yz r y z in gravity driven surficial flows typically displays a relative increase in ellipticity to define cats eye folds r r yz that are remarkably constant r 2 0 99 across 9 orders of magnitude sheath y axes range from 0 1 mm to 75 km our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain with r also reflecting the rheological significance of layering associated with original buckle fold mechanisms the scaling pattern of sheath folds reflects the length scales of the precursor buckle folds and width of deformation zones across a broad range of materials and environments with continued deformation the layering marking the original folds may become increasingly passive to define sheath folds these empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials strain rates and scales encompassing a variety of deformation settings

flame foliation evidence for a schistosity formed normal to the extension direction foliations are normally thought to develop approximately parallel to the xy plane of the finite strain ellipsoid i e perpendicular to the main shortening direction we present a new type of schistosity named flame foliation that forms orthogonal to the main extension direction approximately parallel to the yz plane of finite strain flame foliation consists of anastomosing biotite rich selvedges overprinting s1 in pelitic layers of metaturbitites in nw namibia the biotite crystals in the selvedges are peculiar because they are oriented oblique or orthogonal to the flame foliation itself and parallel to the previous s1 cleavage a feature no other foliation shows in some cases biotite flames flank minor quartz veinlets implying that the flame foliation developed by infiltration of fluids and biotite growth along extensional fractures perpendicular to the main d1 extension direction where overprinting ductile d2 deformation is strong the flame foliation is transformed into a normal crenulation cleavage by rotation of biotite in the flames flame foliation is found preferentially close to syntectonic syenite granite plutons and their formation in metapelitic rocks may be enhanced by fluid overpressure due to devolatilisation reactions 2006 elsevier ltd all rights reserved

a damara orogen perspective on the assembly of southwestern gondwana the pan african damara orogenic system records gondwana amalgamation involving serial suturing of the congo sao francisco and ro o de la plata cratons north gondwana from 580 to 550 ma before amalgamation with the kalahari antarctic cratons south gondwana as part of the 530 ma kuunga damara orogeny closure of the adamastor ocean was diachronous from the aracuao belt southwards with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580 550 ma in the kaoko belt and at 545 530 ma in the gariep belt peak deformation metamorphism in the damara belt was at 530 500 ma with thrusting onto the kalahari craton from 495 ma through to 480 ma coupling of the congo and ro o de la plata cratons occurred before final closure of the mozambique and khomas damara belt oceans with the consequence that the kuunga suture extends into africa as the damara belt and the lufilian arc and zambezi belt of zambia palaeomagnetic data indicate that the gondwana cratonic components were in close proximity by c 550 ma so the last stages of the damara kuunga orogeny were intracratonic and led to eventual out stepping of deformation metamorphism to the ross delamerian orogen c 520 500 ma along the leading edge of the gondwana supercontinental margin

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

complex vein systems as a data source in tectonics an example from the ugab valley nw namibia abstract neoproterozoic metaturbidites in the lower ugab domain namibia contain a complex network of four sets of quartz calcite veins overprinted by km scale folds associated with four regional foliations the veins formed by fluid overpressure predating the main deformation deformation structures developed at the junction of two mobile belts during the assembly of gondwana the ns kaoko belt and the ew trending damara belt km scale ns trending folds were initiated during ew constriction in the kaoko belt while their further development and all subsequent events are related to constriction in the ew damara belt with coeval sinistral strike slip in the kaoko belt deformation of the veins and development of four orthogonal foliations are due to gradual changes in the bulk tectonic framework rather than separate orogenic events the veins are deformed in a complex manner allowing a full 3d reconstruction of regional sequence of events the local complex tectonics could be reconstructed because of the perfect local exposure and the multitude of veins it illustrates the potential complexity of tectonic events and structural evolution in apparently simple slate belts

reconstructing the shape of large synformal folds based on the solution of the 2d non linear magnetic inverse problem abstract this paper deals with a new approach to reconstructing the shape of a poorly exposed synform that folds strongly magnetic layers the approach is based on the solution of the 2d non linear magnetic inverse problem in areas best suited for 2d modeling the interpretation model consists of two parts i a geometric model imitating a synform cross section and ii a set of elementary layers of a fold characterized by anisotropic magnetic susceptibility the main task is to determine the deepest point of magnetic layers and the direction and amount of their dip it can be solved by a selection of optimal model parameters from a set of random numbers with a priory model constraints the solution is an iterative process which defines intervals of geometrical model parameters that correspond to minimal values of the misfit between observed and modeled magnetic fields the proposed approach is characterized by a stability of the determination of reference fold points coordinates which is sufficient to confidently answer a number of questions important for the geologist these include whether a synformal fold is upright inclined or overturned determining the depth of its trough line which combined with the fold outline on the ground surface allows an evaluation of the hinge line curvature as an example the large paleoproterozoic serpovidny isoclinal synformal anticline in the keivy terrane of the northeastern fennoscandian shield has been chosen according to field observations this synform is a giant sheath fold consisting of the poorly exposed strongly magnetic core and weakly magnetic limbs the 3d morphology of the serpovidny fold reconstructed using the new approach is shown to be very close to the geometry determined from field observations thereby this study favors the conclusion based the sheath geometry of the serpovidny isoclinal synformal anticline that an alpine type tectonics operated in the paleoproterozoic

archean to recent aeolian sand systems and their sedimentary record current understanding and future prospects the sedimentary record of aeolian sand systems extends from the archean to the quaternary yet current understanding of aeolian sedimentary processes and product remains limited most preserved aeolian successions represent inland sand sea or dunefield erg deposits whereas coastal systems are primarily known from the cenozoic the complexity of aeolian sedimentary processes and facies variability are under represented and excessively simplified in current facies models which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes archean and neoproterozoic aeolian successions remain poorly constrained palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts during the triassic widespread desert conditions prevailed across much of europe during the jurassic extensive ergs developed in north america and gave rise to anomalously thick aeolian successions cretaceous aeolian successions are widespread in south america africa asia and locally in europe spain and the usa several eocene to pliocene successions represent the direct precursors to the present day systems quaternary systems include major sand seas ergs in low lattitude and mid latitude arid regions pleistocene carbonate and holocene modern siliciclastic coastal systems the sedimentary record of most modern aeolian systems remains largely unknown the majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes whereas successions representing linear and star dunes remain under recognized research questions that remain to be answered include i what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology ii what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation iii can sophisticated four dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation and iv is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of earth history when large scale aeolian systems were active or has the evidence to support the existence of other major desert basins been lost for many periods throughout earth history

530 ma syntectonic syenites and granites in nw namibia their relation with collision along the junction of the damara and kaoko belts abstract the lower ugab and goantagab structural domains are located at the junction between the n s trending kaoko and the e w trending damara belts nw namibia where neoproterozoic metavolcano sedimentary sequences were intruded by several syenitic granitic plutons we present here new u pb ages on zircon grains from the voetspoor and doros plutons together with petrological geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of gondwana amalgamation the plutons are composed of three main rock types hornblende quartz syenite syenodiorite and biotite granite the two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures the biotite granite occurs in the sw parts of the intrusions and clearly cuts the syenitic rocks although the plutons are mainly isotropic the structures around them demonstrate that their intrusion occurred during a second deformation phase d2 with a component of sinistral solid state rotation with respect to the wall rocks in response to d2 transpression four samples were dated using u pb shrimp methodology in single zircon grains a hornblende monzodiorite from the voetspoor pluton yielded an age of 534 4 5 ma a hornblende monzonite from the doros pluton produced an age of 528 5 ma the biotite granite facies was sampled in the doros intrusion and yielded an age of 530 4 5 ma in addition a granitic vein folded by d2 close to the northeastern contact of the doros pluton with the encasing phyllites amis river formation was also dated yielding an age of 533 6 ma the data show that all granite syenite from doros and voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522 ma within the errors d1 d2 deformational phases took place under greenschist facies biotite zone conditions and during d3 the metamorphic grade was slightly lower we interpret that the plutons are coeval to peak metamorphism of the region 530 520 ma and that d2 and d3 sinistral transpressional phases are due to collision in the damara belt the e w compressional event and second metamorphic episode in the kaoko belt occurred between 580 and 560 ma and are apparently unrelated to the thermo tectonic evolution described here although d1 might be partially related to this event the sinistral transpressional d2 phase resulted probably from the position of the area considered at the junction between the belts and not in the frontal damara collision further to the east this new interpretation is consistent with the ar ar ages for the region about 500 ma interpreted to reflect cooling of the orogen the enrichment in lree k rb ba and sr and depletion in nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination we interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab breakoff during collision between kalahari and congo cratons

extremely fluid behavior of hydrous peralkaline rhyolites the viscosities of a series of water bearing peralkaline rhyolitic melts have been experimentally determined the dry melt compositions are composed of a series of additions of na2o to a metaluminous base composition the melts initially hydrated at high pressures and quenched isobarically have been prepared by cutting and polishing then reheating across the glass transition at 1 atm where they are annealed to a relaxed metastable state and then investigated dilatometrically using micropenetration methods the measurements have been performed in the viscosity range of 108 5 1011 5 pa s which corresponds to temperatures in the range of 675 220 c for these compositions despite the relatively low viscosities of dry peralkaline melts in comparison with metaluminous melts of similar sio2 content the viscosities of peralkaline rhyolitic melts also decrease strongly and non linearly with the addition of water the resulting viscosity temperature relationships for water bearing peralkaline rhyolitic melts are shifted to much lower temperatures such that glass transition temperatures for moderate cooling rates correspond to extraordinarily low temperatures a model is presented for the calculation of melt viscosities in the range of 108 5 1011 5 pa s for peralkaline rhyolites with up to 7 wt h2o the very fluid nature of these peralkaline rhyolites over a wide range of water contents may facilitate a very efficient degassing history of glassy peralkaline rhyolites in nature efficient degassing might explain the apparent contradiction of the presence of common water rich melt inclusions in phenocryst phases hosted in water free glassy rhyolites versus the absence of vesicular layers or textural evidence for a vesicular past for the glassy rocks

viscoelastic behavior of partially molten granites effects of partial melting on complex shear modulus g and internal friction q 1 of granites have been investigated using a torsion deformation apparatus in the frequency range of 2 mhz 20 hz at 650 1250oc granite samples from kirchberg w erzgebirge germany og1 and fine grained quartz feldspar porphyry a

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

eolian deposits of the northern margin of the south china jianghan basin reconstruction of the late cretaceous east asian landscape in central china abstract the late cretaceous honghuatao formation includes a succession of thick clastic sedimentary sequences which are widely exposed in northwestern jianghan basin central china here detailed sedimentological and facies architectural analyses are performed allowing to re interpret and identify the controversy for sedimentary environment and to investigate the paleoclimatic condition the honghuatao stratigraphic sequence includes eolian dunes and dry interdunes deposits with fluvial channel deposits at the bottom and top the vertical change between fluvial and eolian systems indicates alternating wet and dry climate at the same time the provenance analysis combined with the paleocurrents from the w nw and ne indicates that huangling dome south qinling belt and dabie orogen supply the main detrital source for the honghuatao formation the late cretaceous first rifting subsidence in the jianghan basin provided accommodation space for the preservation of the aeolian accumulation moreover the existence of the mountain belts around the basin that is the orographic effect precluded the winds from transporting humidity to the interior of the jianghan basin generating the desert system this study shows that the formation of eolian deposits in the honghuatao formation and coeval strata in numerous adjacent basins was a sedimentary response to the prevailing desert climate in the mid to low latitude areas of the northern hemisphere during the late cretaceous which was predominately controlled by the subtropical high pressure system on a planetary scale

3d visualization of sheath folds in ancient roman marble wall coverings from ephesos turkey abstract archaeological excavations and restoration of a palatial roman housing complex in ephesos turkey yielded 40 wall decorating plates of folded mylonitic marble cipollino verde derived from the internal hellenides near karystos greece cipollino verde was commonly used for decoration purposes in roman buildings the plates were serial sectioned from a single quarried block of 1 25 m3 and provided a research opportunity for detailed reconstruction of the 3d geometry of meterscale folds in mylonitized marble a gocad model is used to visualize the internal fold structures of the marble comprising curtain folds and multilayered sheath folds the sheath folds are unusual in that they have their intermediate axis normal to the parent layering this agrees with regional tectonic studies which suggest that cipollino verde structures formed by local constrictional non coaxial flow sheath fold cross section geometry exposed on the surface of a plate or outcrop is found to be independent of the intersection angle of the fold structure with the studied plane consequently a single surface cannot be used as an indicator of the three dimensional geometry of transected sheath folds

a rheological investigation of vesicular rhyolite abstract the rheology of vesiculating rhyolitic systems exerts a strong control on the transport of silicic magmas in the subvolcanic to volcanic environments we present here an investigation of vesiculating and vesiculated rhyolites using dilatometric methods this study examines the effect of vesicle content on the viscosity of a natural supercooled rhyolitic liquid with 0 70 vesicles the experimental samples of rhyolitic glass are derived from fusion of a natural obsidian from little glass butte oregon crystal free rhyolite glasses of varying porosity were prepared by fusing obsidian powder in a pt crucible differing porosities were obtained by varying the temperature 1300 1650 c and duration 0 5 6 h of the fusions cylindrical samples of the resulting vesiculated rhyolites were cored from the crucible using diamond tools and their ends were ground flat and parallel for dilatometry the porosity of each sample was determined from archimedean buoyancy density determinations and comparison with bubble free rhyolite 2 331 g cm 3 porosity 1 p p o the density of foamed samples was determined using their mass volume and regular geometry viscosities were determined in the parallel plate mode at stresses of 5 10 3 to 10 5 pa the viscosimeter was calibrated using nbs 711 glass the bubble contents were microscopically investigated using a video reflected light system and image analysis software distribution functions of the size orientation aspect ratio and surface porosity were obtained the viscosity of rhyolite decreases with increasing bubble content a general relationship of the form 0 1 c describes the effect of porosity in volume fraction on the viscosity where c is a dimensionless constant 22 4 2 9 and log 10 0 10 94 0 04 pa s at 850 c

sheath fold formation around slip surfaces terra nova 24 417 421 2012 abstract sheath folds are associated with shear zones and have been used as high strain indicators we present results from a three dimensional analytical study showing that the flow perturbation around a planar weakness acting as a slip surface in simple shear leads to the formation of sheath folds at low shear strain closed traces of marker layers are exhibited in cross sections cut perpendicular to the shear direction in the vicinity of the slip surface tip in cross sections cut parallel to the shear direction and normal to the shear plane flanking folds are observed our model captures the first order observations of natural sheath folds such as diversity of shapes and multiple eye structures natural equivalents of the planar weaknesses that we model include fractures veins and rheologically weak layers flow perturbation around these zones of weaknesses is proposed as an alternative mechanism of sheath fold formation which emphasizes the coeval operation of brittle and ductile processes in shear zones

flow and fracture maps for basaltic rock deformation at high temperatures understanding how the strength of basaltic rock varies with the extrinsic conditions of stress state pressure and temperature and the intrinsic rock physical properties is fundamental to understanding the dynamics of volcanic systems in particular it is essential to understand how rock strength at high temperatures is limited by fracture we have collated and analysed laboratory data for basaltic rocks from over 500 rock deformation experiments and plotted these on principal stress failure maps we have fitted an empirical flow law norton s law and a theoretical fracture criterion to these data the principal stress failure map is a graphical representation of ductile and brittle experimental data together with flow and fracture envelopes under varying strain rate temperature and pressure we have used these maps to re interpret the ductile brittle transition in basaltic rocks at high temperatures and show conceptually how these failure maps can be applied to volcanic systems using lava flows as an example c 2002 elsevier science b v all rights reserved

modelling of segment structures boudins bone boudins mullions and related single and multiphase deformation features finite element modelling has been used to simulate the development of segment structures deformed layer segments separated by veins such as boudins mullions and bone boudins a parameter sensitivity analysis is used to compare the influence of the nature of the flow the relative viscosities of veins in necks and the host rock and the initial geometry of the layer segments parameter fields have been determined for the relative viscosity of veins and layers and the kinematic vorticity number of flow reworked segment structures can have several shapes such as bone bulging shortened bone boudins and their asymmetric equivalents such as domino and shearband boudin geometry the model for asymmetric reworked segment structures is applied to such features from the lower ugab metaturbidites in nw namibia the model suggests that these structures form where the neck veins are stronger than the boudinaged layer with a significant simple shear component of the bulk flow the quartz filled necks in the lower ugab are therefore stronger than the quartz rich wall rock in greenschist facies where the progressive deformation occurred bone boudins are usually interpreted to form in transpressional flow but simulations of the rotation of tension gashes show that they can also form in simple shear or slightly transtensional shear flow

rheological properties of basaltic lavas at sub liquidus temperatures laboratory and field measurements on lavas from mount etna models of many magmatic processes require accurate data on the rheological properties of lava at sub liquidus temperatures laboratory measurements of the rheological properties of basalts erupted on mount etna in 1983 were made at various crystal concentrations in a specially designed furnace using a haake rotovisco viscometer attached to a spindle designed to eliminate slippage at the melt spindle interface measurements were made at strain rates between 0 3 and 5 s 1 over the range of eruption temperatures on mount etna 1084 1125 c at temperatures above 1120 c the 1983 lava behaves as a newtonian fluid at lower temperatures the lava is a thixotropic pseudoplastic fluid with a maximum yield strength of 78 pa at a temperature of 1087 c in view of its low yield strength over this temperature range the theological behaviour of the lava approximates to that of a power law fluid apparent viscosities at unit shear strain rates increase from 150 pa s at 1125 c to 3000 pa s at 1084 c unit strain rate apparent viscosities measured in the field using a rotating vane viscometer range from 1385 pa s to 1630 pa s these values are in close agreement with those measured at the same temperature in the laboratory and with those calculated theoretically from the physico chemical properties of the lava

monoclinic model shear zones abstract although many ductile shear zones are supposed to have developed by approximately simple shear flow some must have formed under different conditions a few types of such non simple shear zones have been proposed in the literature such as transpression transtension and stretching shear zones this paper presents a full three dimensional kinematic model of shear zones with monoclinic flow geometry monoclinic shear zone types can be classified according to flow parameters and according to the geometry and orientation of accumulating finite strain modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones these include stretching lineations normal to the displacement direction foliations normal to the shear zone boundary switching of the position of principal finite strain axes x y and z during progressive deformation and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes

preservation and erosion of theropod tracks in eolian deposits examples from the middle jurassic entrada sandstone utah u s a abstract the middle jurassic entrada sandstone exposed near the town of escalante southern utah consists of large scale cross bedded eolian deposits that are interbedded with horizontally laminated sand sheets and thin sets of eolian cross strata representing periods with a moister climate the flat bedded units contain numerous tracks and trackways from small to large sized theropod dinosaurs these tracks are today exposed in several distinct erosional states allowing detailed studies of track and undertrack formation in eolian deposits tracks that originally were emplaced on sloping surfaces show in their present day erosional state a morphology distinct from those originally emplaced on horizontal surfaces further the range of eroded track morphologies can help identify badly eroded tracks from nonbiogenic structures in similar deposits

aeolian sediments in lower to middle triassic rocks of central wyoming abstract the lower to middle triassic red peak and crow mountain formations of central wyoming have commonly been interpreted as being of shallow marine or shelf origin this paper presents conclusive evidence of an aeolian origin for parts of the two formations the interpretation is based on highly indicative sedimentary structures including wind ripple grainfall and grainflow strata lag deposits and granule ripples in the sediments of these formations facies in the upper red peak and lower crow mountain formations represent aeolian dune sandsheet wet interdune and sabkha environments emphasis is placed on the recognition and interpretation of correlative surfaces of regional extent which not only allow subdivision of the aeolian sediments into different genetic sequences but also aid in the reconstruction of the depositional history the recognition of an aeolian deposystem and of regional bounding surfaces adds valuable information to the efforts of reconstructing the triassic paleogeography of the western interior

comment on eye and sheath folds in turbidite convolute lamination aberystwyth grits group wales by h l o mcclelland n h woodcock c gladstone journal of structural geology 33 2011 1140 1147

internal friction in tholeiitic basalts internal friction in two tholeiitic basalts from deccan traps india was measured at 1 3 hz in the temperature range 500 900 c the results gave two attenuation peaks one at approximately 620 c and the other near 830 c the peaks are attributed to bimodal grain size distribution in contrast poly crystalline alumina with a single grain size distribution gave one attenuation peak at 810 c grain boundary sliding in the presence of an intergranular fluid phase is suggested as the possible mechanism of anelasticity in the basalts with this assumption we estimate the viscosity of the palagonite glass in the basalts to be 1 7 1013 p in the temperature range 600 640 c and 2 6 5 0 109 p in the temperature range 820 840 c these estimates are in general agreement with direct measurement of viscosity in tholeiite basalt by scarfe 1977

shear modulus and internal friction of calcite rocks at seismic frequencies pressure frequency and grain size dependence abstract the shear modulus g and associated internal friction q 1 have been measured at seismic frequencies for a suite of four essentially monomineralic calcite rocks with grain sizes ranging from 4 to 1000 m the measurements are based on a previously described technique for the observation of forced torsional oscillations of a series combination of a rock cylinder and a steel standard under high confining pressure at room temperature the behaviour of each rock has been studied as a function of strain amplitude 10 8 10 6 maximum at surface pressure 0 300 mpa and oscillation period 1 300 s no evidence is found of any departure from linearity in the range of strain amplitudes studied corrections which are significant only at the shortest periods 1 3 s have been applied to the raw data to account for the influence of torques exerted on the specimen assembly by the gas pressure medium up to about 100 mpa increase in pressure results in a marked increase in g and decrease in q 1 attributable to the closure of crack porosity except in the fine grained solnhofen limestone in which both g and q 1 are insensitive to change in pressure indicative of the virtual absence of crack porosity at higher pressures g is much less pressure sensitive and q 1 is essentially pressure independent with values less than 0 001 in all four rocks since the rock specimens were jacketted under conditions of normal humidity these relatively low values of q 1 indicate a diminished role for adsorbed water in the anelasticity of rocks at high effective pressures within the pressure independent regime q 1 is essentially independent of frequency for periods of 3 100 s and increases by only 30 as the grain size decreases from 1000 to 4 m thus within this regime it is concluded that the observed value of q 1 results to a large degree from an intracrystalline mechanism with a broad range of relaxation times the possibility of significant dislocation damping will be discussed

episodicity of seismicity accompanying melt intrusion into the crust 1 microseismicity from a dike intruding the mid crust of iceland occurs episodically on fractures orientated parallel to the dike plane we interpret it as caused by fragments of chilled magma being broken and pushed along the conduit by melt flow laboratory experiments on basalt samples under shear strains of 10 4show that the shear strength of a sample cooled through the glass transition initially increases to a maximum at 0 7 homologous temperature but subsequently decreases until failure at 0 58 homologous temperature we interpret the failure as due to the connection of microcracks episodicity in microseismicity on timescales of hours to days can arise from a cycle in which magma in 0 1 0 5 meter thick dikes first cools and becomes stronger but then weakens along the dike margins with continued cooling against the country rock continued pressure of magma from below may then cause failure along dike parallel fractures

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

dynamics and viscosity of a a and pahoehoe lava flows of the 2012 2013 eruption of tolbachik volcano kamchatka russia the 2012 2013 flank eruption of tolbachik volcano kamchatka lasted 9 months and produced 0 54 km3 of basaltic trachyandesite lava thus becoming one of the most voluminous historical lava effusions of basic composition in subduction related environments globally from march to july 2013 the volcano monotonously erupted lava of constant composition sio2 52 wt with a nearly stable effusion rate of 18 m3 s despite the uniform eruptive and emplacement conditions the dominant style of lava propagation throughout that time gradually changed from a a to pahoehoe we report results of instrumental field measurements of the a a and pahoehoe flow dynamics documented with time lapse cameras as well as the lava viscosity determined by flow rate and shear stress using penetrometer methods maximal propagation velocities of the a a fronts ranged from 2 to 25 mm s and those of the pahoehoe from 0 5 to 6 mm s the flow front velocities of both lava types experienced short period fluctuations that were caused by complex flow mechanics of the advancing flow lobes minimal viscosities of lava of the a a lobes ranged from 1 3 105 to 3 3 107 pa s flow rate method and those of the pahoehoe from to 5 103 to 5 104 pa s shear stress method our data include the first ever measured profiles of viscosity through the entire thickness of actively advancing pahoehoe lava lobes we have found that both the a a and pahoehoe flows were fed by identical parental lava which then developed contrasting rheological properties owing to differences in the process of lava transport over the ground surface the observed transition from the dominant a a to the dominant pahoehoe propagation styles occurred due to gradual elongation and branching of the lava tube system throughout the course of the eruption such evolution became possible because the growing lava field composed of semisolidified flows provided an environment for shallow subsurface intrusions and internal migrations of lava that with time developed into branches of the lava tube system based on our data we propose phenomenological models of the a a and pahoehoe flow mechanics

intrusion mechanisms in a turbidite sequence the voetspoor and doros plutons in nw namibia abstract two syntectonic plutons of cambrian age intruded neoproterozoic metaturbidites in namibia at the junction of the ns trending kaoko and ew trending damara belts sinistral transpression in the kaoko belt produced km scale upright d1 folds overprinted by minor d2 folds d3 is associated with n s shortening in the damara belt the plutons show two main pulses of intrusion hornblende syenite intruded late during d1 or during d2 and biotite granite during d3 each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons the internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50 of the intrusion mass suggest that the plutons have a disk or wedge shape a marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor the biotite granite intruded into mega strain shadows and tension gashes alongside and in the syenite during d3 the plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to d1 d2 transpression

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

fragility and an extremely low shear modulus of high porosity silicic magma abstract the rheology and strength of bubbly magma govern eruption dynamics by determining the possibility of fragmentation of ascending magmas they are also required parameters for understanding seismic monitoring we measured the rheology and strength of high porosity rhyolitic magma at 500 950 c the measured shear modulus and strength are several orders of magnitude lower than bubble free rhyolite melt implying that high porosity magma cannot avoid fracturing during magma ascent the occurrence of fractures is observed in the low temperature magma 800 c in this temperature range the measured attenuation is low that is the elastic energy originated by deformations avoids attenuation and is stored in the bubbly magma until released by fracturing q 1 the newly found porosity dependent strength based on our measurements comprehensively explains three different fragmentation criteria that have been previously proposed independently our measurements also show that the shear modulus becomes lower by increasing porosity which can slow the shear wave velocity these results suggest that knowing the attenuation of the seismic wave is useful to evaluate magma temperature and the possibility of a fragmentation event that may determine subsequent volcanic activities

late jurassic fluvial eolian deposits from the tianchihe formation ningwu jingle basin shanxi province china abstract eolian systems were widespread in china in the cretaceous however the reports of pre cretaceous eolian and its related systems are scarce which prohibits studying the construction accumulation and preservation of eolian systems as well as its allogenic and autogenic controlling factors in this paper we report on fluvial eolian successions from the late jurassic tianchihe formation in the ningwu jingle basin these successions represent the oldest reported inland fluvial eolian systems in china fourteen lithofacies types are recognized in the tianchihe formation and have been grouped into six facies associations including meandering fluvial channel levee and overbank in the lower part and eolian sandsheet eolian dune ephemeral fluvial channel and floodplain in the upper part the fluvial system comprises a series of wetting upward cycles which indicate humid climate culminating in high precipitation and shallow water table level eolian dune deposits composed of grainflow grainfall wind ripple strata and pin stripe laminations currently implies that all deposits types are internally bounded by reactivation surface and superimposition surface suggesting compound dunes or draas eolian sandsheet deposits consist dominantly of subcritical climbing translatent strata and pin stripe lamination the association of eolian dune and eolian sandsheet is development of a dry eolian system twelve drying upward cycles mainly composed of eolian sandsheet eolian dune successions and four wetting upward cycles consisting of eolian sandsheet dune ephemeral fluvial channel floodplain successions occur in the eolian system these repeated cycles likely reflect an evolving erg system directly controlled by sediment supply sediment availability and relative water table the vertical transition from fluvial to eolian systems indicate a local climatic oscillation from relatively humid to arid conditions which may be related with the uplift of the east china plateau in the late jurassic

viscosity measurements of subliquidus magmas alkali olivine basalt from the higashi matsuura district southwest japan abstract we carried out viscosity measurements and sampling of a crystal suspension derived from alkali olivine basalt from the matsuura district sw japan at subliquidus temperatures from 1230 c to 1140 c under 1 atm with nno oxygen buffered conditions viscosity increased from 31 to 1235 pa s with a decrease in temperature from 1230 to 1140 c on cooling olivine first appeared at 1210 c followed by plagioclase at 1170 c the crystal content of the sample attained 31 vol at 1140 c plagioclase 22 olivine 9 non newtonian behaviors including thixotropy and shear thinning were pronounced in the presence of tabular plagioclase crystals the cause of such behavior is discussed in relation to shear induced changes in melt crystal textures relative viscosities r s m where s and m are the viscosities of the suspension and the melt respectively were obtained by calculating melt viscosities from the melt composition and temperature at 1 atm using the equation proposed by giordano and dingwell giordano d dingwell d b 2003 non arrhenian multicomponent melt viscosity a model earth and planetary science letters 208 337 349 the obtained relative viscosities are generally consistent with the einstein roscoe relation which represents r for suspensions that contain equant and equigranular crystals even though the crystal suspension analyzed in the present experiments contained tabular plagioclase and granular olivine of various grain sizes this consistency is attributed to the fact that the effect of crystal shape was counterbalanced by the effect of the dispersion of crystal size the applicability of the einstein roscoe equation with respect to crystal shape is discussed on the basis of the present experimental results our experiments and those of sato sato h 2005 viscosity measurement of subliquidus magmas 1707 basalt of fuji volcano journal of mineralogical and petrological sciences 100 133 142 show that the relationship between relative viscosity and crystal fraction is consistent with the einstein roscoe relationship for axial ratios that are smaller than the critical value of 4 6 5 but discrepancies occur for higher ratios

significance of the early foliation at bermagui n s w australia the first generation mesoscopic folds f1 in the ordovician turbidites at bermagui n s w australia are tight to locally isoclinal and have a convergently fanning axial surface foliation s1 which is a differentiated crenulation cleavage s1 overprints an earlier foliation herein called s in psammites s is a strong alignment of mineral grains and rough micaceous folia and in pelites s is a microbanding of quartzose and micaceous layers depending on rock type and position in the f1 fold profile s may be 1 parallel to bedding 2 rotated out of parallelism with bedding by the s1 cleavage and 3 truly cross cutting bedding in many places s cuts across bedding demonstrating that it is a superimposed tectonic foliation and not simply an inherited depositional fabric in outcrops south of bermagui s changes vergence with apparent congruence with f1 folds in other outcrops near bermagui however s maintains a constant vergence around f1 folds the geometry indicating a pre f1 anticline to the east since there is no evidence for large scale pre f1 folds the various relations of s to s1 and f1 are best explained by the rarely described process of late stage hinge nucleation during a single protracted f1 folding sequence other possibilities such as fold hinge migration and pre fold imbricate stacking are discussed but considered less likely

instabilities in the summit region of mount etna during the 1999 eruption during the 1999 eruption of mount etna sicily the summit of the volcano changed dramatically lavas erupted from a fissure on the southern flank of se cone formed a large compound flow field that buried a substantial part of the northern wall of valle del bove and the ground between there and se cone hot mass flow deposits formed on the eastern and western flanks of the chasm following a period of intense strombolian activity at the same time a new vent opened on the southern flank of se cone and there was a major rockfall from its summit seven weeks later part of the outer western rim of bocca nuova crater collapsed during another period of intense strombolian activity and lava emerged at a high effusion rate through the breached crater wall following a subsequent collapse of the bocca nuova crater rim a hot avalanche flowed a few hundred metres on top of the previous lava flow field similar deposits have been described on other volcanoes but their importance in the evolution of the summit of etna has not previously been recognised

high resolution compositional mapping of matrix phases implications for mass transfer during crenulation cleavage development in the moretown formation western massachusetts abstract high resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation the moretown formation of western massachusetts contains a well developed crenulation cleavage with alternating mica rich crenulation limbs and mica poor crenulation hinges compositional mapping shows two generations of plagioclase the second of which was synchronous with the crenulation cleavage a significant amount of the syn crenulation plagioclase 10 20 modally grew in hinge domains a small amount of syn crenulation plagioclase 1 and a large amount of phengitic muscovite grew in limb domains the maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages and thus cannot be used as undeformed reference domains for comparison with crenulated regions compositional mapping facilitates a new degree of integration between petrologic and structural analysis knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation in the moretown formation plagioclase and phengite producing reactions play a large role in controlling the nature and magnitude of mass transfer but microstructures control the location of reactants and products within the evolving fabric

extrusional tectonics in the core of a transpressional orogen the kaoko belt namibia the pan african kaoko belt of nw namibia provides a wellexposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis the kaoko belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time 575 550ma it is divided into a barrovian style escape zone with inclined nappes and thrusts that verge outward towards the foreland a steep high grade orogen core that experienced intense wrench style deformation and in the hinterland an older 650ma exotic coastal terrane that was reworked at moderate t low p conditions during transpressional orogenesis the kinematic lineation array defined by stretching lineations and shear sense indicators shows variation in degree and polarity of lineation obliquity to the grain of the belt lineation plunge and different polarity of vergence along the plunge with respect to the overall sinistral shear sense the gross regional lineation pattern defines an arcuate array from near orogen parallel in the orogen core to higher angle obliquity across the escape zone reflecting lateral escape towards the orogen margin two highgrade lobes within the orogen core are coincident with obliqueinclined upward flow trajectories based on the kinematic lineation array and represent oblique extrusion of lower crustal material into middle crustal levels an intervening low grade metamorphic trough within the orogen core and the coastal terrane coincides with acute oblique downward and outboard directed apparent flow trajectories these kinematic patterns are consistent with barometric constraints for peak metamorphism of 5 2kbar and 4 5kbar in the low grade trough and coastal terrane and 8 0kbar in the two high grade lobes integration of the kinematic and metamorphic datasets shows that oblique shallowly inclined extrusion trajectories occurred within the orogen core of this classic transpressional orogen and that stretching lineations approximately reflect the particle paths experienced across orogen metamorphic gradients indicate a marked contrast between the escape zone and orogen core the escape zone experienced an inverted barrovian style high p moderate t metamorphism and low thermal gradients of 20 c km the orogen core experienced moderate p high t metamorphism and high thermal gradients of 30 40 c km clockwise p t paths were experienced in all parts of the kaoko belt tight and shallow conduction dominated p t paths were experienced in all orogen core domains involving heating dominated prograde paths followed by decompression consistent with extrusional tectonics steeper and more open advection dominated p t paths were experienced in the crustal over thrust escape zone which experienced low thermal gradients

crystallization history of the 1984 mauna loa lava flow during a 3 week eruption in 1984 mauna loa produced vent lavas that increased in crystallinity from 1 to 30 and 27 km long flows that increased in crystallinity as they moved downstream we examined the crystallization history of these lavas using crystal size distribution csd analysis to study the rates of crystallization viscosity increase and latent heating typical average growth and nucleation rates were 5 10 9 cm s 1 and 5 cm 3 s 1 for microphenocrysts 20 to 500 m size crystals nucleated in the rift zone and 5 10 8 cm s 1 and 5 104 cm 3 s 1 for microlites 1 to 20 m size crystals nucleated in the channel these crystallization rates are high compared with those found in other csd studies of igneous rocks probably due to highly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel growth and nucleation rates decreased with time at the vent and with distance downstream the maximum downstream total crystallinity measured is 39 25 microlites 14 microphenocrysts in a quenched sample 14 km from the vent growth and nucleation rates cannot be calculated for postemplacement samples but they place upper limits of 53 58 on the amount of crystallization in the channel 9 20 km from the vent crystallization could have been mostly responsible for the 105 fold downstream increase in apparent viscosity although degassing and increasing incorporation of solid lava fragments also contributed another effect of crystallization on the lava flow was the sizeable latent heating 0 01 j g 1 s 1 over the first half of the flow length if the crystallinity of downstream quench samples is representative of the hot fluid core which may have been counteracted by entrainment of cooler material measurements of crystallization are shown to be crucial in the study of lava flow emplacement dynamics

water and the viscosity of andesite melts the viscosity of a synthetic andesite like melt has been measured between 1010 and 1014 p for water contents in the range 0 3 5 wt the very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision after a steep viscosity decrease of 5 orders of magnitude for 1 wt h2o an additional 2 5 wt h2o causes a further viscosity decrease of only 2 orders of magnitude these viscosity decreases are qualitatively similar to those observed previously for more silica rich compositions the new data join smoothly with available high temperature measurements made at high pressures on water bearing andesite melts because the intrinsic effects of pressure are as small for water bearing as for water free samples the depressing effect of water on the viscosity of natural andesite melts can be estimated changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity although quantitative applications are not yet possible the configurational entropy theory accounts qualitatively for these features

gelatins as rock analogs a systematic study of their rheological and physical properties abstract in a laboratory model the behavior of the selected analog material must be properly scaled to nature here we systematically investigate both rheological and physical properties of a wide range of gelatins as functions of temperature composition concentration ageing and applied strain rate results show that gelatins behavior changes gradually from purely elastic to visco elasto brittle to purely viscous nonlinear rheology going from the gel to the sol state the rheological variability of gelatins appears promising for the potential use of these analog materials to simulate crustal and lithospheric rheological behavior in particular we found that pig skin 2 5 wt at 10 c has the required rheological properties for a suitable experimental set up to model crustal deformation

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

evidence for climate shifts in the 2 0ga upper makgabeng formation erg south africa abstract eolian ergs and associated environs are sensitive to short and long term climatic changes in one of the oldest erg deposits the 2 0 ga makgabeng formation vertical changes in facies associations reflect one of the earliest records of short term climatic shifts in a continental setting the makgabeng formation is separated into lower and upper erg deposits by a playa or saline pan deposit the lower erg deposit consists of dune sets with thin lenses of dry and deflationary interdune strata that transition vertically to thicker damp to wet interdune strata a laterally persistent playa deposit in the middle of the section consists of mudstone with deep and shallow penetrating mud cracks and subordinate siltstone and sandstone interbeds above this lower mudstone interval the playa strata are sandy overlying the playa deposit the upper erg deposit consists of thick eolian sets with thin lenses of dry interdune deposits grain size change near the top of the upper erg deposit corresponds with the appearance of fluvial sheet flood eolian cross beds sculpted by mass flows and thin playa deposits vertical stacking of facies associations demonstrates temporal shifts in precipitation and fluctuating water tables fluvial and playa deposits record high water tables whereas low water tables are reflected in the core erg the transition from a lower to higher water table is recorded by wet interdune interspersed within the dune strata toward the top of this interval rapid climatic amelioration occurred near the termination of the makgabeng erg resulting in impingement of ephemeral river systems development of playas and generation of massive sand flows this 2 0 ga erg demonstrates the impact of climate change on erg development resulting from shifts in monsoonal impingement through time

non newtonian viscosity of basaltic magma 1 basaltic melt drives most of earth s volcanism understanding its rheology is crucial for any model of magma transport and volcanic eruption basaltic magma is generally treated as a quasi newtonian liquid but there are observations of non newtonian behaviour with a method that allows measurement of non newtonian viscosity of a representative melt molten basaltic rock we found a strong shear rate dependency of viscosity in a wide range of temperatures the temperature viscosity dependency indicates properties of the molten phase as the cause the viscosity data are in good agreement with a power law model

stratigraphic evolution of an aeolian erg margin system the permian cedar mesa sandstone se utah usa the permian cedar mesa sandstone of south east utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which in terms of palaeogeographic setting reflects a transition from a dry erg centre through a water table controlled aeolian dominated erg margin to an outer erg margin subject to periodic fluvial incursion the erg margin succession represents a wet aeolian system accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes and a range of dry damp and wet flooded interdune elements is recognized variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre locally interconnected damp and or wet interdune ponds in the aeolian dominated erg margin and fully interconnected fluvially flooded interdune corridors in the outer erg margin relationships between aeolian dune and interdune units indicate that dry damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration temporal variation in the rates of water table rise and bedform migration determined the angle of climb of the erg margin succession such that accumulation rates increased during periods of rapidly rising water table whereas sediment bypassing zero angle of climb occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table during these periods in the outer erg margin the expansion of fluvially flooded interdunes in front of non climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass flood supersurfaces a spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the cedar mesa erg margin succession

transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials realistic lava flow models require a comprehensive understanding of the rheological properties of lava under a range of stress conditions previous measurements have shown that at typical eruption temperatures lavas are non newtonian this is commonly attributed to the formation and destruction of crystal networks in the present study the effects of bubbles on the time dependent non newtonian properties of vesicular melts are investigated experimentally using analogue materials the shear thinning behaviour of bubbly liquids is shown to be dependent on the previous shearing history this thixotropic behaviour which was investigated using a rotational vane viscometer is caused by delayed bubble deformation and recovery when subjected to changes in shear stress the viscoelastic transition and the transient flow behaviour of analogue fluids were studied using both a rotational vane viscometer and oscillatory shear apparatus these experiments have shown that vesicular suspensions are viscoelastic fluids with a yield strength power law rheology and a non zero shear modulus these properties are also found in polymer fluids commonly used as analogue materials for lava such as gum rosin we show that when materials with this rheology are accelerated in channels they may be fragmented and when they flow through a narrowing conduit pulsating flow can develop as a consequence of a transition from slip to non slip conditions at the conduit wall this has important implications both for effusive and explosive volcanic eruptions

morphology and dynamics of inflated subaqueous basaltic lava flows during eruptions onto low slopes basaltic pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments such as at contemporary subaerial hawaiian type volcanoes in hawaii la reunion and iceland in continental environments states of oregon idaho washington and in the deep sea at juan de fuca ridge the galapagos spreading center and at the east pacific rise this study these lava have all undergone inflation processes yet they display highly contrasting morphologies that correlate with their depositional environment the most striking difference being the presence of water lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea lakes and rivers their height is 2 3 times smaller and their length being 10 15 times shorter based on heat diffusion equation we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker by 25 cooled layer at the flow surface which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments thus limiting lava breakouts to form new lobes hence promoting inflation buoyancy also increases the ability of a lava to inflate by 60 together these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows

long period seismicity and very long period infrasound driven by shallow magmatic degassing at mount pagan mariana islands long period lp seismicity and very long period infrasound ivlp were recorded during continuous degassing from mount pagan mariana islands in july 2013 to january 2014 the frequency content of the lp and ivlp events and delay times between the two arrivals were remarkably stable and indicate nearly co located sources using phase weighted stacking over similar events to dampen noise we find that the lp source centroid is located 60 m below and 180 m west of the summit vent the moment tensor reveals a volumetric source modeled as resonance of a subhorizontal sill intersecting a dike we model the seismoacoustic wavefields with a coupled earth air 3 d finite difference code the ratios of pressure to velocity measured at the infrasound arrays are an order of magnitude larger than the synthetic ratios so the ivlp is not the result of lp energy transmitting into the atmosphere at its epicenter based on crater shape and dimensions determined by structure from motion we model the ivlp as acoustic resonance of an exponential horn the source of the continuous plume from gas analysis is shallow magmatic degassing which repeatedly pressurized the dike sill portion of the conduit over the 7 months of observation periodic gas release caused the geologically controlled sill to partially collapse and resonate while venting of gas at the surface triggered resonance in the crater lp degassing only accounts for 12 of total degassing indicating that most degassing is relatively aseismic and that multiple active pathways exist beneath the vent

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

development of highly non cylindrical folds around rigid ellipsoidal inclusions in bulk simple shear regimes natural examples and experimental modelling abstract the three dimensional development of strongly non cylindrical folds around competent ellipsoidal inclusions in bulk simple shear regimes is studied in experiments and natural examples examples from the continental allochthonous terrane of the braganca nappe complex ne portugal show rim folds and sheath folds associated with different parts of rigid ellipsoidal boudins experimental work has been carried out with models made from analogue materials silicone putty and plasticine and deformed in a simple shear machine we have considered three different models to simulate natural examples and the results show that fold morphology depends on the shape of the inclusion and position around the inclusion although the bulk deformation regime is layer parallel homogeneous simple shear we can distinguish local deformation regimes responsible for the folding associated with different parts of the rigid body flow and strain patterns must therefore be complex around inclusions or populations of inclusions prolate to oblate strain ellipsoids can be expected in different positions close to the rigid inclusion in our experiments the competent inclusions do not rotate synthetically with the applied bulk simple shear e g clockwise rotation of inclusion in dextral shear instead they back rotate early in the shearing history and keep this position throughout the experiment this is the result of the size relationship between the inclusion and the finite width of the shear zone the fold pattern around rigid inclusions may be used as a shear sense criterion

variation in metamorphic style along the northern margin of the damara orogen namibia the northern margin of the inland branch of the pan african damara orogen in namibia shows dramatic along strike variation in metamorphic character during convergence between the congo and kalahari cratons m3 metamorphic cycle low p contact metamorphism with anticlockwise p t paths dominates in the western domains ugab zone and western northern zone and high p barrovian metamorphism with a clockwise p t path is documented from the easternmost domain eastern northern zone the sequence of m3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite chlorite muscovite at the same time as an andalusite biotite muscovite transposed foliation was developed in the matrix the peak t metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite muscovite the kfmash p t pseudosection for metapelites in the ugab zone and western northern zone contact aureoles indicates tight anticlockwise p t loops through peak metamorphic conditions of 540 570 c and 2 5 3 2 kbar these semi quantitative p t loops are consistent with average p t calculations using thermocalc which give a pooled mean of 556 26 c and 3 2 0 6 kbar indicating a high average thermal gradient of 50 c km in contrast the eastern northern zone experienced deep burial high p moderate t barrovian m3 metamorphism with an average thermal gradient of 21 c km and peak metamorphic conditions of c 635 c and 8 7 kbar the calculated p t pseudosection and garnet compositional isopleths in kfmash appropriate for the metapelite sample from this region document a clockwisep t path early plagioclase kyanite biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing xfe xmg and xca and decreasing xmn compositions indicating steep burial with heating the developed kyanite garnet biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats indicating decompression which was followed by retrogressive cooling and chlorite muscovite growth the clockwise p t loop is consistent with the foreland vergent fold thrust belt geometry in this part of the northern margin earlier formed 580 570ma pervasive matrix foliations m2 were overprinted by contact metamorphic parageneses m3 in the aureoles of 530 3ma granites in the ugab zone and 553 514ma granites in the western northern zone available geochronological data suggest that convergence between the congo and kalahari cratons was essentially coeval in all parts of the northern margin with similar ages of 535 530ma for the main phase of deformation in the eastern northern zone and northern platform and 538 505ma highgrade metamorphism of the central zone immediately to the south consequently nne ssw directed convergent deformation and associated m3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the inland branch in the west heat transfer was dominated by conduction and externally driven by granites whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production the ultimate cause was along orogen variation in crustal architecture including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession

the interaction of aeolian and fluvial processes during deposition of the upper cretaceous capping sandstone member wahweap formation kaiparowits basin utah u s a abstract detailed examination of the upper cretaceous capping sandstone member of the wahweap formation in the kaiparowits basin utah reveals the presence of aeolian stratification deposition by aeolian processes is recognized and distinguished from subaqueous deposition by the presence of centimeter scale inversely graded wind ripple stratification the aeolian stratification in the wahweap formation is the first occurrence reported from upper cretaceous strata on the colorado plateau of the western u s a aeolian stratification occurs as thin wind reworked caps of fluvial bar sandstones deposited within low sinuosity braided streams and as more extensive deposits of small scale dunes that developed in geographically restricted dune fields aeolian reworking of bars took place during low stage flow and was possibly controlled by intermittent seasonal discharge variations prolonged aridity led to increased sand supply entering the aeolian system dunes nucleated and grew rapidly between the braided stream systems these small scale dunes rarely developed extensive or tall slip faces as evidenced by the rare preservation of grain flow strata and the dominance of wind ripple stratification the dune field was characterized by sinuous crested dunes probably barchans amid and linked to the extensive braided stream systems based on modern analogs the resulting change in sand storage supply probably is related to a short term shift to a more semi arid arid climate possibly seasonally however a longer term climate change to semi arid arid may be indicated for the dune complex near the boundary with the overlying kaiparowits formation

numerical experiments on the dynamics of channelised lava flows at mount cameroon volcano with the flowgo thermo rheological model abstract as for many other effusive volcanoes mount cameroon mc is a volcano for which only limited information exists on lava flow properties and emplacement dynamics for recent eruptions this study provides new quantitative constraints for rheological and dynamic characteristics of lava flow effusion for the 1982 and 2000 eruptive events used to calibrate the flowgo thermo rheological model for these lava flows the flowgo 1 d physical model is used to simulate down flow evolution of the geometry and rheology of channel contained cooling limited lava flows morphometric data from historical lava flows were acquired from the field e g channel geometry levee and background slope in order to estimate lava yield strength velocity and effusion rate lava density and viscosity were also estimated from compositional data and laboratory methods to account for uncertainty in the input rheological and geometrical data three end member scenarios were used to bracket the potential range in lava channel initial dimension initial lava temperature and phenocryst content for each of these scenarios two crustal growth models were used one assuming strong insulation due to lava flow surface crusting the other a much lower crusting rate twelve numerical simulations were made per flow and the results were compared against the channel geometry microlite content yield strength and viscosity estimates from field and laboratory investigations best fit models where obtained for both the 1982 and 2000 lava flows using a low rate of surface crusting high initial temperature and low phenocryst content model predicted lengths were within 5 of the actual lengths modelled mean effusion rates for the 1982 52 64 m 3 s 1 and 2000 10 m 3 s 1 flows closely matched field data derived estimations 26 68 and 9 5 m 3 s 1 respectively flowgo model results are highly sensitive to initial channel dimensions phenocryst content and the flowgo model is unable to reproduce the observed rapid near vent increase in microlite content near vent rapid cooling induced by lava degassing is yet to be fully documented and integrated into numerical lava flow models such as flowgo but obtained results already provide valuable insights for lava flow hazard assessment at mount cameroon

controls on lineation development in low to medium grade shear zones a study from the cap de creus peninsula ne spain lineations composed of similarly oriented elongate mineral aggregates or grains are a common feature in deformed rocks but it is unclear which factors control the development of such lineations field observations and microstructural analysis of samples which were taken from discrete greenschist to lower amphibolite facies shear zones of the easternmost variscan pyrenees show that strain is only one of several factors that control the strength and type of a lineation dynamic recrystallization metamorphic reactions and rigid body rotation are also important controlling factors for the development of lineations the most important of these is dynamic recrystallization the way in which dynamic recrystallization influences lineation development is largely a function of the initial fabric in a specific rock type different lithologies with different initial fabrics produce distinctly different types and strengths of lineations even if deformed to the same finite strain in the same shear zone an initial fine grain size and a monomineralic composition of the parent rock type commonly hinder the development of lineations in contrast in initially coarse grained polymineralic rocks well developed lineations commonly develop therefore the ratio of initial to dynamically recrystallized grain size determines to a large extend the development of such lineations

relations between dune generations in the gran desierto of mexico multiple generations or genetic units of dunes characterized by different morphologies sand grain size and sorting and degree of post depositional alteration of sediments occur in the gran desierto sand sea of mexico dune generations are separated by super bounding surfaces episodic accumulation of dunes is a result of a complex response of different sediment sources to regional climatic and eustatic changes based on examples from three parts of the sand sea relations between different generations of dunes suggest that there are two end member modes of sand sea accumulation 1 stacking or superposition of dune generations and 2 accumulating mosaics of dunes of different morphological units the mode of accumulation is determined by sand supply and accumulation rate stacking occurs where sand supply is high and or the sand sea area is restricted by topography accumulation of mosaics is the preferred mode of accumulation where sand supply is low

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

significance of thin sets of eolian cross strata abstract thin cosets composed of thick sets of trough cross strata record episodes when ergs were sand saturated and erg buildup exceeded the rate of subsidence and water table rise interdunes were enclosed and dry thin sets with irregular set boundaries accumulated when ergs were unsaturated as at white sands dune field in southern new mexico interdunes were flat extensive and moist thin lenses of cross strata with irregular upper surfaces are the erosional remnants of lowermost portions of dunes that were within the zone of capillary rise thin flat bedded to massive interdune deposits were also preserved in this setting due to low sand supply upbuilding of the erg surface was limited to the rate of subsidence water table rise like sediments deposited in shallow subaqueous settings bu unlike most colian sands bed by bed accumulation of the thin sets was limited by short term accommodation space the allogenic interpretation presented above contrasts strongly with the widely accepted climbing interdune autogenie interpretation of the entrada we question this gradualistic interpretation not only on the basis of stratal relationships between dune and interdune deposits but on theoretical grounds as well to maintain moist wet interdunes when large bedforms are climbing to preserve thick tabular sand bodies deposition must not exceed the rate of groundwater rise we have seen very few irregular erosion surfaces between thick sets of eolian cross strata they are extremely common between thin sets widespread moist interdunes in modern ergs and evidence of high water tables in ancient eolian sequences are signals of low sand supply thin eolian sets with irregular upper b unding surfaces record the birth and demise of ergs as well as the fluctuations of erg margins the absence of animal tracks in the thick jurassic cross strata and their great abundance in and adjacent to the thin sets may be a result of an erg margin origin of the thin sets or of ameliorated climatic conditions during their deposition because they accumulate in zones and episodes of low sand supply cosets composed of thin sets are akin to deflation surfaces in for example the permian cedar mesa sandstone the movement of the groundwater table after the initial deflationary phase slow rise in the jurassic rapid drop in the permian may reflect the higher amplitude of late paleozoie sea level fluctuations