factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

accurate stratospheric particle size distributions from a flat plate collection surface the first representative chemical structural and morphological analysis of the solid particles from a single collection surface has been performed this collection surface sampled the stratosphere between 17 and 19 km in altitude in the summer of 1981 and therefore before the 1982 eruptions of el chichon a particle collection surface was washed free of all particles with rinses of freon and hexane and the resulting wash was directed through a series of vertically stacked nucleopore filters the size cutoff for the solid particle collection process in the stratosphere is found to be considerably less than 1 m the total stratospheric number density of solid particles larger than 1 m in diameter at the collection time is calculated to be about 2 7 10 1 particles per cubic meter of which approximately 95 are smaller than 5 m in diameter previous classification schemes are expanded to explicitly recognize low atomic number material with the single exception of the calcium aluminum silicate cas spheres all solid particle types show a logarithmic increase in number concentration with decreasing diameter the aluminum rich particles are unique in showing bimodal size distributions in addition spheres constitute only a minor fraction of the aluminum rich material about 2 3 of the particles examined were found to be shards of rhyolitic glass this abundant volcanic material could not be correlated with any eruption plume known to have vented directly to the stratosphere the micrometeorite number density calculated from this data set is 5 10 2 micrometeorites per cubic meter of air an order of magnitude greater than the best previous estimate at the collection altitude the maximum collision frequency of solid particles 5 m in average diameter is calculated to be 6 91 10 16 collisions per second which indicates negligible contamination of extraterrestrial particles in the stratosphere by solid anthropogenic particles

the 1982 el chichon volcano eruptions a satellite perspective abstract the march 28 april 3 and april 4 1982 eruptions of el chichon volcano were detected and monitored using the goes and noaa 6 environmental satellites the satellite data provided information about the height and dispersal of the eruption plume and subsequent ash cloud on each of these dates based on the satellite data it was concluded that the march 28 eruption the second of two eruptions on april 3 and the april 4 eruption injected material into the stratosphere at heights between 17 and 31 km

el chichon volcanic ash in the stratosphere particle abundances and size distributions after the 1982 eruption volcanic ash particles collected from the stratosphere after the march april 1982 explosive eruption of el chichon volcano mexico were mostly 2 40 m vesicular shards of silicic volcanic glass that varied in abundance at 16 8 19 2 km altitude from 200 m 3 30 49 n lat in may to 1 3 m 3 45 75 n in october at the minimum the ash cloud covered latitudes 10 60 n in july and 10 s 75 n in october in may and july ash particles were mostly free individual shards and clusters of shards but by october were intimately associated with liquid droplets presumably sulfuric acid in may 1982 the total stratospheric burden of ash was at least 240 tons 2 2 108 g although the total ash injected into the stratosphere by the eruption was probably 480 8400 tons

fission track dating of zircon zircons from mont blanc granite french italian alps zircon population in granite rocks are most often heterogeneous with respect to uranium concentrations which can vary from a few hundred ppm to 1 and the typology of the grains for example in the type of dominant prism 100 or 110 making them difficult to date using fission track ft dating the mont blanc zircons which vary greatly in both uranium content and zircon type are typically difficult to date by the fission track method to obtain information about fission track retention characteristics of these minerals experimental isothermal annealing was performed on grains free of alpha damage i e annealing of freshly induced tracks uranium poor grains with a dominant 100 prism were found to be more stable a step etching method sem was used to date the zircons uranium poor zircons with a dominant 100 prism yielded higher fission track ages than did uranium rich ones with a 110 dominant prism thus etching conditions temperature and etching time and type of prism chosen

high resolution 3d numerical modeling of thrust wedges influence of decollement strength on transfer zones 1 the mechanics and dynamics of thin skinned compressible thrust wedges with prescribed offsets in the backstop i e transfer zones are investigated using a three dimensional finite difference numerical model with a visco brittle plastic rheology the main questions addressed are as follows i what is the influence of the initial length of the backstop offset and ii what is the effect of the frictional strength of the main decollement on the structural evolution of the brittle wedges along such transfer zones results show that the shorter the backstop offset the earlier these two thrust planes connect forming a curved frontal thrust along the entire width of the model younger in sequence thrusts are formed parallel to this curved shape long backstop offsets produce strongly curved thrust faults around the indenting corner simulations with a weak basal friction evolve toward almost linear frontal thrusts orthogonal to the bulk shortening direction increased basal drag in models with a strong decollement favors propagation of the backstop offset into a transfer zone up to the frontal thrust these simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts whereas the tapers on the other side of the transfer zone are smaller than analytical values this difference is amplified with increasing length of the backstop offset and or strength of the decollement modeled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the minab zendan transform transfer fault between the zagros and makran

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

mechanical modelling of folding versus faulting in brittle ductile wedges abstract the question of whether layered sequences of sedimentary rock subjected to layer parallel compression will fold or fault is investigated by means of mechanical modelling using the finite element method the model consists of a rigid basement a weak visco elasto plastic decollement horizon and a relatively competent elastic plastic cover sequence syntectonic erosion and sedimentation on the surface are included using a diffusion model numerical results show that folding is the dominant mode of deformation when the decollement horizon is relatively thick and has a low viscosity when the upper layer has a relatively high elastic shear modulus and when the total thickness of the sequence is relatively small if any one of these conditions is not satisfied faulting dominates other parameters such as the angle of internal friction and cohesion influence the style of deformation but have little influence on the boundary between folding and faulting results are interpreted in terms of competing deformation instabilities if fold amplification rates are large folding dominates over faulting if on the other hand fold amplification rates are small faulting dominates because stresses intersect the failure surface before significant fold amplification can occur

a review of brittle compressional wedge models abstract fold and thrust belts and accretionary wedges form by compression of sedimentary sequences and basement rocks into forward and backward thrusts folds nappes and duplexes for over a century models have been used to investigate the essential characteristics of such brittle wedges here i review model studies of brittle thrust wedges in orthogonal compression focussing on critical taper theory analogue and numerical techniques for modelling brittle behaviour and the most commonly investigated variations in wedge model studies those in basal dip basal strength internal strength and surface processes many model results can be placed in the context of critical taper theory which provides analytical solutions for wedge taper angle and slip line orientations for a homogeneous material on the verge of failure throughout dynamic forward models have confirmed critical taper predictions of decreasing surface dip for increasing basal dip wider wedges with steeper forward thrusts and shallower backward thrusts for decreasing basal strength narrower wedges and enhanced exhumation for surface erosion and decreasing surface dip for increasing internal strength but analogue and numerical models have been able to take these results a step further by investigating the evolution of non critical thrust wedges and the effects of non homogeneous materials with decollement layers and strain weakening shear zones these results have highlighted the strong impact of heterogeneous materials on the internal structures of thrust wedges and raise the question whether the complexity of structures that are observed in many natural fold and thrust belts requires that lithological layering inherited faults or a wide range of rheologies need to be included in forward models of brittle thrust wedges

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

mineralogy of chondritic interplanetary dust particles from a mineralogical survey of approximately 30 chondritic micrometeorites collected from the lower stratosphere and studied in detail using current electron microscopy techniques it is concluded that these particles represent a unique group of extraterrestrial materials these micrometeorites differ significantly in form and texture from components of carbonaceous chondrites and contain some mineral assemblages which do not occur in any meteorite class electron microscope investigations of chondritic micrometeorites have established that these materials 1 are extraterrestrial in origin 2 existed in space as small objects 3 endured minimal alteration by planetary processes since formation and 4 can suffer minimal pulse heating 600 c on entering earth s atmosphere the probable sources for chondritic interplanetary dust particles idps are cometary and asteroidal debris and perhaps to a lesser extent interstellar regions these sources have not been conclusively linked to any specific mineralogical subset of idp although the chondritic porous cp aggregate is considered of likely cometary origin chondritic idps occur in two predominant mineral assemblages 1 carbonaceous phases and phyllosilicates and 2 carbonaceous phases and nesosilicates or inosilicates although particles with both types of silicate assemblages are observed olivines pyroxenes layer silicates and carbon rich phases are the most commonly occurring minerals in many chondritic idps other phases often observed in variable proportions include sulphides spinels metals metal carbides carbonates and minor amounts of sulphates and phosphates individual mineral grain sizes range from micrometers primarily pyroxenes and olivines to nanometers with the predominant size for all phases less than 100 nm specific mineral characteristics for particular chondritic idps provide an indication of processes which may have occurred prior to collection in the earth s stratosphere for example pyroxene mineralogy in some chondritic aggregates is consistent with condensation from a vapor phase and we consider with condensation in a turbulent solar nebula at relatively low temperatures 1000 c carbonaceous phases present in other cp aggregates have been used to imply low temperature formation processes such as fischer tropsch synthesis 530 c or carbonization and graphitization 315 c alteration processes have been implicated in the formation of some layer silicates in cp aggregates and may have involved hydrocryogenic alteration at 0 c in general interpretations of transformation processes on submicrometer size minerals in chondritic idps are consistent with formation at a radius equivalent to the asteroid belt or greater during the later stages of solar nebula evolution using currently available models

effect of basal friction on surface and volumetric strain in models of convergent settings measured by laser scanner abstract this paper uses measurements by a high accuracy laser scanner to investigate the role of basal friction on surface and volumetric strains in sandbox models simulating fold thrust belts and accretionary wedges we monitor progressive deformation wedge growth and strain distribution in three models with similar initial boundary conditions but with different basal frictions our analyses show that in addition to influencing the kinematics and geometry of model wedges basal friction also governs both the surface and volumetric strains of the wedge after 16 3 bulk shortening the volume decreased 5 0 5 9 5 0 5 and 12 5 0 5 in the models shortened above low intermediate and high friction decollements respectively applied to nature our model results suggest that more compaction and penetrative strain is expected in convergent settings with a high friction decollement than those shortened above a low friction decollement or a weak basal bed like the salt formation under parts of the zagros fold thrust belt this volume decrease probably reduces the porosity in the deformed lithologies

injection of gases into the stratosphere by explosive volcanic eruptions 1 explosive eruptions can inject large amounts of volcanic gases into the stratosphere these gases may be scavenged by hydrometeors within the eruption column and high uncertainties remain regarding the proportion of volcanic gases which eventually reach the stratosphere these are caused by the difficulties of directly sampling explosive volcanic eruption columns and by the lack of laboratory studies in the extreme parameter regime characterizing them using the nonhydrostatic nonsteady state plume model active tracer high resolution atmospheric model atham we simulated an explosive volcanic eruption we examined the scavenging efficiency for the climatically relevant gases within the eruption column the low concentration of water in the plume results in the formation of relatively dry aggregates more than 99 of these are frozen because of their fast ascent to low temperature regions consideration of the salinity effect increases the amount of liquid water by one order of magnitude but the ice phase is still highly dominant consequently the scavenging efficiency for hcl is very low and only 1 is dissolved in liquid water however scavenging by ice particles via direct gas incorporation during diffusional growth is a significant process the salinity effect increases the total scavenging efficiency for hcl from about 50 to about 90 the sulfur containing gases so2 and h2s are only slightly soluble in liquid water however these gases are incorporated into ice particles with an efficiency of 10 to 30 despite scavenging more than 25 of the hcl and 80 of the sulfur gases reach the stratosphere because most of the particles containing these species are lifted there sedimentation of the particles would remove the volcanic gases from the stratosphere hence the final quantity of volcanic gases injected in a particular eruption depends on the fate of the particles containing them which is in turn dependent on the volcanic and environmental conditions

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

new constraints for the timing of the alpine metamorphism in the internal ophiolitic nappes from the western alps as inferred from fission track data abstract fission track ages performed on zircons from different units of the internal ophiolites of the western alps lie between 135 8 ma and 90 8 ma these data imply that the piemonte nappe consists of different ophiolitic nappes which followed distinctive pressure temperature time p t t paths during the alpine orogeny 1 1 the zircon fission track clock of the internal i e eastern ophiolitic units has been significantly reset during the cretaceous eo alpine eclogitic metamorphism in contrast with zircon from external i e western units 2 2 these internal ophiolites situated south of val d aoste have not been heated above 250 c since late cretaceous times this means that the post eclogitic greenschist stage 350 450 c which occurred in the studied samples should be of cretaceous age that is much older than the lepontine stage the duration of this post eclogitic greenschist stage appears to have been variable according to the unit considered 3 3 the lepontine stage occurred 35 40 ma ago late eocene early oligocene only locally resetting the apatite fission track clock

the zagros folded belt fars iran constraints from topography and critical wedge modelling summary the late miocene tectonics of the zagros folded belt fars province has for long been related solely to folding of the cover controlled by a ductile decollement between basement and the sedimentary cover however geological constraints topography analysis and seismotectonic studies reveal that basement thrusting may produce locally significant deformation in the cover to determine how the deep seated deformation in the basement may contribute to the overall topography we first examine the filtered large and short wavelengths of the topography we find that the short wavelength component of the topography 20 25 km including the zagros folds is superimposed on the differential uplift at the regional scale in other words the regional base level of folded marker horizons remains parallel to the regional topography of interest modelling reveals that the salt based wedge model alone is not able to reproduce the large wavelength component of the topography of the zagros folded belt this reveals that when a thick relatively to its overburden layer of salt forms the basal decollement it is generally too weak and cannot support the growth of significant topography we then test an alternative thick skinned crustal wedge model involving the crust of the arabian margin which is decoupled above a viscous lower crust this model satisfactorily reproduces the observed topography and is consistent with present day basement thrusting topography analyses and geological constraints we conclude that basement involved thickening and shortening is mechanically required to produce the shape of the zagros folded belt since at least 10 ma finally the involvement of the basement provides mechanical and kinematic constraints that should be accounted for cross sections balancing and further assessing the evolution of zagros at crustal or lithospheric scales

satellite limb scanning thermal infrared observations of the el chichon stratospheric aerosol first results the infrared radiometer experiment on the solar mesosphere explorer satellite has been continuously measuring the 6 8 m thermal emission from the stratospheric aerosol from the el chichon volcano since the time of eruption in early april 1982 inversion results for the zonally averaged infrared extinction coefficient in height latitude and time show that the aerosol increased in mass to a maximum of 8 tg 8 1012gm about 15 weeks after the april 4 eruption it descended in height with an average speed consistent with the gravitational settling time of particles with a diameter of about 1 4 m

dynamic constraints on the crustal scale rheology of the zagros fold belt iran thin skinned fold and thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer the resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner it is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales here we use the zagros mountains of iran as a case study as it is one of the most spectacular well studied thin skinned fold and thrust belts in the world both analytical and numerical models are employed to study what con trols fold spacing and under what conditions folding dominates over thrusting the models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover deformation is dominated by thrusting which is inconsistent with the data of the zagros fold belt if we instead take into account additional decollement layers that have been documented in the fi eld a switch in deformation mode occurs and crustal scale folding is obtained with the correct spacing and time scales we show that fold spacing can be used to constrain the friction angle of the crust which is 5 the zagros fold belt this implies that on geological time scales the upper crust is signifi cantly weaker than previously thought possibly due to the effect of fl

geochemistry of basic dikes in the lanzo massif western alps petrogenetic and geodynamic implications abstract the alpine peridotite massif of lanzo italy contains three generations of basic dikes gabbros and basalts the older gabbros are plagioclase rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes they both were derived from the n type mid ocean ridge basalt morb magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir the basaltic dikes are the n type morb and closely resemble the alpine apennine ophiolitic basalts they were derived from a different upper mantle source than the parental magmas of the gabbros the source of the basalts was less depleted in light ree the presence of basic magmas with n type morb affinities in the lanzo massif is consistent with the close genetic relationship between the alpine peridotite body and the ophiolites of the liguro piemontese basin

the dinevar transtensional pull apart basin nw zagros mountains iran a geological study and comparison to 2d finite element elastic models the dinevar transtensional pull apart basin is a nw se trending basin located in the northwest part of the zagros mountains sanandaj sirjan tectonic zone iran this basin was formed by oblique extension in a strike slip fault system which was controlled by the pre existing sahneh and the mianrahan faults in addition to a field study we constructed a 2d finite element elastic model based on the major fault segments according to the field study various geological structures are formed around the pre existing fault segments including pencil structures with 13 aspect ratio 19 tight to open asymmetric folds oblique normal faults synthetic and antithetic faults normal and reverse fault drags and negative and hybrid type flower structure fe model results show that around the fault step is a zone of displacement gradient that indicates extension transtension within the overlapping zone consequently subsidence is predicted over the releasing fault step the modelled stress distributions within the fault step and around the fault segment tips are asymmetric elastic modelling shows that the mean normal stress within the step is tensional which would impede plastic yield while the region outside the step is characterised by compressive mean normal stress the mean normal stress pattern is characterised by a regional minimum localised inside the extensional step transtension zone and two local maximum and minimum couples formed at the advancing and retreating sides respectively in the fault segment tips the results show that two main subsidence regions depocentres or sub basins form along and in the proximity of the fault segment tips the imposed relative motion is entirely accommodated by creep on the faults which causes plastic strain to accumulate outside the faults mostly localised within the fault step this localised strain between fault segments could initiate connecting faults or reactivate earlier faults generally at high angles relative to the master fault segments as suggested by the stress and strain patterns the rotation and magnitude variation in the stress and strain trajectories reflects heterogeneous deformation in general the dinevar transtensional pull apart basin is characterised by an elongate rhomboidal to trapezoidal shaped geometry the modelled stress distribution and strain localisations within the basin are in general agreement with the locations of fault segments within the dinevar transtensional pull apart basin

tertiary sequence of deformation in a thin skinned thick skinned collision belt the zagros folded belt fars iran 1 we describe how thin skinned thick skinned deformation in the zagros folded belt interacted in time and space homogeneous fold wavelengths 15 8 5 3 km tectono sedimentary evidence for simultaneous fold growth in the past 5 5 2 5 ma drainage network organization and homogeneous peak differential stresses 40 15 mpa together point to buckling as the dominant process responsible for cover folding basin analysis reveals that basement inversion occurred 20 ma ago as the arabia eurasian plate convergence reduced and accumulation of neogene siliciclastics in foreland basin started by 10 ma ongoing contraction occurred by underplating of arabian crustal units beneath the iranian plate this process represents 75 of the total shortening it is not before 5 ma that the zagros foreland was incorporated into the southward propagating basement thrust wedge folds rejuvenated by 3 2 ma because of uplift driven by basement shortening and erosion since then folds grew at 0 3 0 6 mm yr and forced the rivers to flow axially a total shortening of 65 78 km 16 19 is estimated across the zagros this corresponds to shortening rates of 6 5 8 km ma consistent with current geodetic surveys we point out that although thin skinned deformation in the sedimentary cover may be important basement involved shortening should not be neglected as it requires far less shortening moreover for such foreland folded belts involving basement shortening underplating may be an efficient process accommodating a significant part of the plate convergence

a reappraisal of earthquake focal mechanisms and active shortening in the zagros mountains of iran summary the zagros mountains of sw iran are one of the most seismically active intra continental fold and thrust belts on earth and an important element in the active tectonics of the middle east surface faulting associated with earthquakes is extremely rare and so most information about the active faulting comes from earthquakes we use long period teleseismic p and sh body waves to determine the orientation and depth of faulting in 16 new earthquakes and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the zagros we use this information to investigate the style and distribution of active faulting in the zagros and how it contributes to the n s shortening of the arabia eurasia collision when the data are ranked in quality and carefully evaluated simple patterns are seen that are not apparent when routine catalogue data are taken at face value an important change in the fault configuration occurs along strike of the belt in the nw overall convergence is oblique to the trend of the belt and the surface anticlines and is achieved by a spatial separation partitioning of the orthogonal strike slip and shortening components on separate parallel fault systems by contrast in the se overall convergence is orthogonal to the regional strike and achieved purely by thrusting in the central zagros between these two structural regimes deformation involves parallel strike slip faults that rotate about vertical axes allowing extension along the strike of the belt the overall configuration is similar to that seen in other curved shortening belts such as the himalaya and the java sumatra trench all the zagros earthquakes we have been able to check have centroids shallower than 20 km and are confined to the upper crust many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover which is active even beneath areas of known shallow structural decollement such as the dezful embayment the dominant style of shortening is high angle reverse faulting with dips 30 though some lower angle thrusting occurs in places active thrust and reverse faulting is relatively confined to the lower topography on the sw edge of the belt today and only strike slip faulting affects the higher topography profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the simple folded belt but has progressively migrated sw over the last 5 ma there is no evidence for a seismically active structural decollement such as a low angle thrust beneath the zagros nor is there any seismic evidence for active subduction either beneath the zagros or beneath central iran instead the arabian margin seems to have shortened by distributed thickening of the basement only in the syntaxis of the oman line at the se end of the zagros is there any evidence for a low angle thrust of regional extent here earthquakes continue 50 km north of the zagros thrust line the geological suture between the arabian margin and central iran reaching depths of 30 km and may represent thrusting of arabian basement beneath central iran to this extent

three dimensional numerical modeling of upper crustal extensional systems 1 we focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models to date few 3 d models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones we use an arbitrary lagrangian eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions plastic materials weaken with accumulating strain to localize deformation a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions controls on the geometry and spacing of three dimensional plastic shear zones are investigated the sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested the model results indicate the primary controls of strain dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction the amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other resulting in a trade off effect between the amount of offset between the initial grabens and the strain weakening ratio the three dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions 1 small offset grabens with a single relay zone 2 intermediate offset grabens with one or more secondary step over graben segments and 3 large offset grabens with limited to no significant segment interaction

effect of the eruption of el chichon on stratospheric aerosol size and composition dominant effects of the el chichon eruption on stratospheric aerosols at 19 8 to 20 7 km are 1 vapor depositional growth of the small aerosol background mode 2 development of a large particle mode by sedimentation from the highest altitudes in the cloud 3 a change in the large particle mode from sulfate coated silicates to sulfate aerosols some with silicate cores 4 a 100 fold increase in sulfate mass in the large particle mode terminal velocities of large silicate particles maximum r 2 3 m sampled 1 month after eruption and calibrated with the aid of lidar data indicate initial injection to 26 to 27 km smaller velocities of sulfate aerosols median r 0 5 m are compatible with major growth in 2 to 3 months at 27 to 28 km aerosol settling accounts for the descent of the main lidar return to 26 5 km in august and to 20 to 21 km in december additional growth by condensation and or coalescence during descent is required to explain the largest aerosols r 0 5 to 0 9 m although sedimentation is the dominant cause of bimodality mixing of air from low and high latitudes can produce bimodal or trimodal distributions

morphology of ash aggregates from wet pyroclastic surges of the 1982 eruption of el chichon volcano mexico the detailed stratigraphic study of the pyroclastic surge units s1 iu and s3 produced during the most violent phases of the 1982 eruption of el chichon volcano contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates based on their morphology and internal structure four different types of aggregates were distinguished a accretionary lapilli b armored lapilli c irregular aggregates and d cylindrical aggregates the first three types have been described in the volcanological literature field and experimental studies cylindrical forms are reported here for the first time these hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer grained matrix they formed around millimeter size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges sem analyses suggest different mechanisms of formation for the four types of aggregates irregular aggregates and armored lapilli formed nearly instantaneously whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud

numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements 1 thin skinned fold and thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal decollement often formed by water saturated shale layers or low viscosity salt horizons a two dimensional finite element model with a viscoelastoplastic rheology is used to investigate the structural evolution of fold and thrust belts overlying different types of decollements in addition the influence of multiple weak layers in the stratigraphic column is studied model shale decollements are frictional with lower friction angles as the cover sequence model salt layers behave linear viscous due to a lower viscosity as the cover sequence or with a power law rheology single viscous decollement simulations have been compared to an analytical solution concerning faulting versus folding results show that fold and thrust belts with a single frictional basal decollement generate thrust systems ramping from the decollement to the surface spacing between thrust ramps depends on the thickness of the cover sequence the structural evolution of simulations with an additional low frictional layer depends on the strength relationship between the basal and the intersequential decollement tectonic underplating and antiformal stacking occur if the within sequence decollement is weaker in the frontal part of models deformation is restricted to the upper part and imbrication occurs with a wavelength depending on the depth of the intermediate weak layer salt decollement with a viscosity of 10 18 pa s leads to isolated box folds detachment folds multiple salt layers 10 18 pa s result in long wavelength folding our results for both frictional and viscous decollements are in bulk agreement with the mohr coulomb type critical wedge theory citation ruh j b b j p kaus and j p burg 2012 numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements tectonics 31 tc3005 doi 10 1029 2011tc003047

fission track evidence for quaternary uplift of the nanga parbat region pakistan the north striking nanga parbat haramosh massif protrudes into the northwestern himalaya along the axis of a great syntaxis1 2 fig 1 where the hindu kush karakorum and himalayan ranges converge as the indus suture zone3 enters this region from the east it bifurcates into two branches encircling what may be a docked island arc terrane4 the southern branch the main mantle thrust crops out on both flanks of the nanga parbat massif forming a tight structural loop5 this massif and the adjacent terrane contain some of the highest peaks in the himalaya nanga parbat and the indus river located just 20km away define the world s greatest continental relief 6 930 m we report here the discovery of unexpectedly young sphene zircon and apatite fission track dates from the nanga parbat haramosh massif these dates as low as 1 3 myr for zircon and 0 4 myr for apatite imply that during the pleistocene the nanga parbat region was uplifted and eroded at nearly 1 cm yr 1

3d effects of strain vs velocity weakening on deformation patterns in accretionary wedges abstract shear zones are weaker than surrounding rocks whether weakening depends on accumulated strain or strain rate is debated we used a three dimensional numerical code with a visco plastic brittle rheology to investigate the influence of strain and strain rate weakening often referred to as velocity weakening on the evolution of thin skinned fold and thrust belts two model setups are used i a uniform setup to recognize the effects of each weakening mode on the structural evolution and dynamics of fold and thrust belts and ii a dual setup with two adjacent domains of decollement strength to investigate the structural response of laterally varying systems strain weakening and velocity weakening lead to remarkably different structural patterns in particular strain weakening favours out of sequence shear zones whilst some amount of finite plastic strain has to be achieved before the structural development initiates weakening in contrast velocity weakening starts with the onset of high strain rate bands and does not favour out of sequence deformation we also tested separately the influences of shortening velocity cover sequence thickness and weakening style and amount on the structural evolution of the dual systems the shortening velocity has a minor effect in strain weakened models velocity weakening and a thinner cover sequence enhance the occurrence of strike slip shear zones we conclude that the weakening mode strongly influences the fault patterns and the dynamics of thrust wedges inversely natural fault patterns are keys to discriminate weakening processes

a comparison of numerical surface topography calculations in geodynamic modelling an evaluation of the sticky air method summary calculating surface topography in geodynamic models is a common numerical problem besides other approaches the so called sticky air approach has gained interest as a free surface proxy at the top boundary the often used free slip condition is thereby vertically extended by introducing a low density low viscosityfluid layer this allows the air crust interface to behave in a similar manner to a true free surface we present here a theoretical analysis that provides the physical conditions under which the sticky air approach is a valid approximation of a true free surface two cases are evaluated that characterize the evolution of topography on different timescales 1 isostatic relaxation of a cosine perturbation and 2 topography changes above a rising plume we quantitatively compare topographies calculated by six different numerical codes using finite difference and finite element techniques using three different topography calculation methods i direct calculation of topography from normal stress ii body fitting methods allowing for meshing the topography and iii lagrangian tracking of the topography on an eulerian grid it is found that the sticky air approach works well as long as the term st ch hst l 3 is sufficiently small where st and hst are the viscosity and thickness of the sticky air layer and ch and l are the characteristic viscosity and length scale of the model respectively spurious lateral fluctuations of topography as observed in some marker based sticky air approaches may effectively be damped by an anisotropic distribution of markers with a higher number of markers per element in the vertical than in the horizontal direction

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

dynamical instability produces transform faults at mid ocean ridges transform faults at mid ocean ridges one of the most striking yet enigmatic features of terrestrial plate tectonics are considered to be the inherited product of preexisting fault structures ridge offsets along these faults therefore should remain constant with time here numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries irrespective of previous structure boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections the resultant curved ridges become transform faults within a few million years fracture related rheological weakening stabilizes ridge parallel detachment faults offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps

mechanical evolution of transpression zones affected by fault interactions insights from 3d elasto plastic finite element models abstract the mechanical evolution of transpression zones affected by fault interactions is investigated by a 3d elasto plastic mechanical model solved with the finite element method ductile transpression between non rigid walls implies an upward and lateral extrusion the model results demonstrate that a transpression zone evolves in a 3d strain field along non coaxial strain paths distributed plastic strain slip transfer and maximum plastic strain occur within the transpression zone outside the transpression zone fault slip is reduced because deformation is accommodated by distributed plastic shear with progressive deformation the 3 axis the minimum compressive stress rotates within the transpression zone to form an oblique angle to the regional transport direction 9 10 the magnitude of displacement increases faster within the transpression zone than outside it rotation of the displacement vectors of oblique convergence with time suggests that transpression zone evolves toward an overall non plane strain deformation the slip decreases along fault segments and with increasing depth this can be attributed to the accommodation of bulk shortening over adjacent fault segments the model result shows an almost symmetrical domal uplift due to off fault deformation generating a doubly plunging fold and a positive flower structure outside the overlap zone expanding asymmetric basins subside to negative flower structures on both sides of the transpression zone and are called transpressional basins deflection at fault segments causes the fault dip fall to less than 90 86 89 near the surface 1 5 km this results in a pure shear dominated triclinic and discontinuous heterogeneous flow of the transpression zone

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

experimental modelling of orogenic wedges a review abstract experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for more than 30 years although the technique dates back as far as the early xix th century on one hand several first order parameters controlling the structural evolution of mountain belts have been intensively investigated using the classic tectonic sandbox models the main parameters are the properties of the basal decollement the deforming material the backstop and fluxes kinematics and surface processes on the other hand the morphological evolution of a mountain relief subjected to changing tectonic or climatic forcing has been addressed using another kind of approach called geomorphic models nowadays the literature is extremely rich particularly for the sandbox technique so that it becomes difficult to have an exhaustive view of the effects of the above parameters on mountain evolution in this article we propose a detailed review of the main results obtained using both tectonic and geomorphic approaches our goal is to provide an almost complete state of the art in the experimental study of relief dynamics to guide present and future researchers in their understanding of mountain belt evolution

on the role of incompetent strata in the structural evolution of the zagros fold thrust belt dezful embayment iran abstract the dezful embayment is the most important fertile oil province of the zagros fold thrust belt it includes several incompetent strata as basal and intermediate decollement levels that play a significant role on the structural styles and hydrocarbon preservation based on the interpretation of seismic profiles the influence of the gachsaran formation and the evaporitic kalhur member of the asmari formation on the geometry of deformation was investigated in different parts of the dezful embayment obtained results revealed that the thickness of the incompetent strata plays a crucial role in the formation and geometry of different types of fold structures e g rounded box chevron detachment fold in the dezful embayment there is a sharp difference between the geometry of surface and deep seated structures due to the existence of thick intermediate decollements e g gachsaran and kalhur in the dezful embayment therefore fault geometry and fold styles in upper and lower parts of these decollements are totally different in addition these incompetent strata act as a barrier level against the propagation of deep seated faults into the overlying layers therefore it seems that most of the faults exposed on the surface have originated from the upper decollement levels in the study area

structural styles and decoupling in stratigraphic sequences with double decollements during thin skinned contractional tectonics insights from numerical modelling abstract six series of particle based numerical experiments were performed to simulate thin skinned contractional tectonics in stratigraphic sequences with double decollements during horizontal shortening the models were assigned with varying rock competence depth and thickness of the upper decollement which resulted in significantly different styles of deformation and decoupling characteristics above and below the upper decollement the models composed of the least competent material produced distributed sinusoidal detachment folds with many shallow structures profoundly decoupled from the deep seated folds the models composed of a more competent material are dominated by faulted box folds with minor disharmonic folds developed in their limbs in contrast the results of models composed of the most competent material are characterised by localised piggyback thrusts fault bend folds and pop up structures with tensile fractures developed in fold hinges depth of the upper decollements also plays an important role in controlling structural decoupling i e the shallower the upper decollements the higher the degree of decoupling becomes thicker upper decollements can provide sufficient mobile materials to fill fold cores and contribute to the formation of secondary disharmonic folds helping enhance structural decoupling our modelling results are comparable to the structural features exhibited in the dezful embayment of the zagros fold and thrust belt with the miocene gachsaran formation acting as the shallow upper decollement and the fars with the triassic dashtak formation as its intermediate decollement this study demonstrates that rock competence depth and thickness of the upper decollements can jointly affect the structural styles and decoupling our modelling results are instructive for structural interpretation of deep zones in fold and thrust belts that exhibit distinct structural decoupling features

structural variation along the zagros and the nature of the dezful embayment structure varies along strike in the zagros fold and thrust belt of iran which is a principal element in the arabia eurasia continental collision pre collision late cretaceous ophiolite nappes kermanshah neyriz and related nappes of deep marine sediments radiolarite series were emplaced next to two regions pusht e kuh arc fars which later developed a consistent structural style across the range from the high zagros fault to the foreland limit of deformation the intervening area has a zone of highly imbricated arabian plate strata the bakhtyari culmination thrust southwest towards and over a low relief low elevation region the dezful embayment there are no ophiolite nappes northeast of the bakhtyari culmination isopachs reflect these different structural patterns from late cretaceous time but not earlier in late cretaceous time the dezful embayment recorded less deposition than adjacent areas to the northwest and southeast in the palaeogene there was little net difference between the dezful embayment and its margins the dezful embayment has been a depocentre since roughly 35 ma which is the likely time of initial collision between arabia and eurasia we propose that the syn collision structure and stratigraphy of the zagros is therefore strongly influenced by the variation in late cretaceous ophiolite emplacement but the original cause of this variation is not clear

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

high pressure metamorphism in the nappes of the valle dell orco traverse western alps collisional belt a detailed field and petrological study of rocks from nappes cut by the valle dell orco italian western alps in particular the sesia lanzo composite unit has revealed geological and metamorphic histories which started in pre alpine times and lasted up to the alpine subduction collisional processes during these processes the nappes sustained an early high p low t stage and a later low p greenschist facies stage but followed partly distinctive p t time trajectories this paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction underthrust to the later exhumation stage the metamorphic crystallization is often governed by incomplete and or local equilibrium and the pervasive syn metamorphic deformation and the composition of the syn metamorphic fluid phase if present have exerted an effective local control on reaction kinetics

stratospheric dust loading from early 1981 to september 1985 based on the twilight sounding method and stratospheric dust collections stratospheric aerosol loading from early 1981 to late 1985 was investigated by remote optical measurements using the twilight sounding method and by in situ mineral dust collections both experiments tracked the decay of aerosol abundances after the el chichon eruption a comparison between the remote optical observations and dust samplings suggests that aerosol maxima in 1985 were probably associated with a minor eruption of the bezymianny volcano considering the different dynamical behavior of volcanic ash and condensed sulfuric acid aerosol we traced the origin of collected dust to a minor eruption of una una volcano this collected dust that could not be detected by remote sensing techniques against the high background level due to condensed aerosol from el chichon highlights the complimentary nature of stratospheric dust collections and the twilight sounding method

enhanced residence of submicron si rich volcanic particles in the lower stratosphere abstract abundant submicron si rich grains were collected in the lower stratosphere between may 1981 and january 1982 analytical electron microscopy shows that the morphology surface features and chemical compositions of these platey grains are consistent with keatite keatite forms by devitrification of amorphous si rich grains that condensed in ascending volcanic eruption clouds low settling rates for platey grains should cause concentration of si rich grains in the stratosphere during extended periods of volcanic activity such a concentration is not observed probably because the grains were present as small aggregates with greater settling rates aggregation did not occur in the atmosphere because collision frequencies are too low thus aggregation took place in the volcanic cloud the faster settling rates for the aggregates is still sufficiently slow to increase the amount of small volcanic particles in the stratosphere thereby enhancing the amount of sites for heterogeneous aerosol nucleation

evidence for cretaceous paleogene boundary bolide impact winter conditions from new jersey usa abrupt and short lived impact winter conditions have commonly been implicated as the main mechanism leading to the mass extinction at the cretaceous paleogene k pg boundary ca 66 ma marking the end of the reign of the non avian dinosaurs however so far only limited evidence has been available for such a climatic perturbation here we perform high resolution tex86 organic paleothermometry on three shallow cores from the new jersey paleoshelf northeastern usa to assess the impact provoked climatic perturbations immediately following the k pg impact and to place these short term events in the context of long term climate evolution we provide evidence of impact provoked severe climatic cooling immediately following the k pg impact this so called impact winter occurred superimposed on a long term cooling trend that followed a warm phase in the latest cretaceous

the 1982 eruptions of el chichon volcano mexico 2 observations and numerical modelling of tephra fall distribution tephra fallout from the a 1 march 29 0532 ut b april 4 0135 ut and c april 4 1122 ut 1982 explosive eruptions of el chichon produced three tephra fall deposits over southeastern mexico bidirectional spreading of eruption plumes as documented by satellite images was due to a combination of tropospheric and stratospheric transport with heaviest deposition of tephra from the ene tropospheric lobes maximum column heights for the eruptions of 27 32 and 29 km respectively have been determined by comparing maximum lithic clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns these column heights suggest peak mass eruption rates of 1 1 108 1 9 108 and 1 3 108 kg s maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration new estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2 19 km3 1 09 km3 dre dense rock equivalent or roughly twice the amount of the deposit mapped on the ground up to one half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra

collection of microparticles at high balloon altitudes in the stratosphere we report the collection of stratospheric particles at 34 36 km using balloon borne collectors the concentration of particles on the collection surfaces and element concentrations were measured on the majority of the particles using scanning electron microscope sem and proton induced x ray emission pixe analysis respectively particle morphologies elemental composition and electron diffraction data were obtained on a small number of the collected particles using transmission electron microscope tem techniques the concentration of particles between 0 045 lower imaging limit and 1 m in radius is several orders of magnitude above the blank levels on the collection surfaces while the concentration of particles above 1 m is near blank levels more than 106 submicron particles were collected the concentration of submicron particles is between 10 and 50 times the concentration expected at the sampling altitude based on models using the measured flux of extraterrestrial particles to the atmosphere the higher concentration of submicron particles may be due to contributions from volcanic particles inaccuracies in the influx of particles of this size to the earth or breakup of larger particles analysis of the elemental composition using pixe showed cl s ti fe br ni zr zn sr and cu in decreasing order of concentration elements below s could not be detected in the analysis cl s and br are believed to be present due to reaction of the collection surfaces and particles with atmospheric gases while the other elements are present in the particles the elemental composition of the particles does not allow an unequivocal origin to be assigned because particles from several of the possible sources listed above contain the measured elements tem analysis of 23 of the submicron particles showed 16 to be non graphitic the particles ranged from al rich silicates to almost pure fe to one containing almost exclusively ba and s none were definitely chondritic in composition this collection of particles complements existing collections obtained by aircraft they were colleted at much higher altitude and are predominantly submicron in size collection and analysis of particles at high altitudes using balloons presents unique challenges and opportunities in understanding the particle population in the stratosphere

coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro western alps north italy abstract the objective of this work is to study the most significant static and syn tectonic transformations which accompanied the eclogite metamorphism 450 50 c 12 15 kbar of an ophiolite metagabbro from the western alps in partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or incomplete pseudomorphic reactions the disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate limiting factors which prevented a large scale eclogite equilibration the undeformed volumes are bounded by syn eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric foliation develops the enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation the bulk rock chemistry also played an important role in determining the kind of eclogite assemblage in particular only sheared gabbros rich in fe can produce bimineralic eclogites

chloritoid bearing assemblages in eclogitised metagabbros of the lanzo peridotite body western italian alps abstract dykes of metagabbro from the lanzo peridotite body in the western italian alps contain mg chloritoidtalc garnet na pyroxenes chlorite or fe chloritoid glaucophane garnet na pyroxenes eclogitic assemblages of cretaceous age in these metagabbros the metamorphic equilibration is incomplete and controlled by local equilibrium conditions here the chemical gradients which document the magmatic microstructural domains and the diffusion patterns during metamorphism survive the eclogitic crystallisation concentration of elements like al mg and fe near the boundaries between the magmatic domains favours the crystallisation in reaction coronas of mg rich chloritoid in mg metagabbro and of fe rich chloritoid in fe ti metagabbro phase compatibilities in the al2o3 feo mgo sio2 h2o system suggest temperatures around 400 500 c at approximate pressures of 15 18 kbar for the eclogitic metamorphism of the lanzo rocks in spite of the difficulties and of the inaccuracies of these thermobarometric estimates the inferred p t values are in good agreement with determinations involving na pyroxenes and garnets

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

creep of rocksalt abstract a review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates included in the summary are discussions of a microstructures induced during steady state creep b creep rupture of rocksalt c experiments associated with project salt vault and more recent field studies and d brine migration a representative steady state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics while the mechanical behavior of rocksalt is probably better understood than for all other rock types further investigations especially on load path stress history and creep rupture are clearly mandated furthermore additional investigations of brine migration and of bench and field scale deformations are needed the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock mass response

the el chichon stratospheric cloud solid particulates and settling rates sampling of the el chichon stratospheric cloud in early may and in late july 1982 showed that a significant proportion of the cloud consisted of solid particles between 2 m and 40 m size in addition many particles may have been part of larger aggregates or clusters that ranged in size from 50 m the majority of individual grains were angular aluminosilicate glass shards with various amounts of smaller adhering particles surface features on individual grains include sulfuric acid droplets and larger 0 5 m to 1 m sulfate gel droplets with various amounts of na mg ca and fe the sulfate gels probably formed by the interaction of sulfur rich gases and solid particles within the cloud soon after eruption ca sulfate laths may have formed by condensation within the plume during eruption or alternatively at a later stage by the reaction of sulfuric acid aerosols with ash fragments within the stratospheric cloud a wilson huang formulation for the settling rate of individual particles qualitatively agrees with the observed particle size distribution for a period at least four months after injection of material into the stratosphere this result emphasizes the importance of particle shape in controlling the settling rate of volcanic ash from the stratosphere

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

thrust wedges with decollement levels and syntectonic erosion a view from analog models abstract analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with one and two decollement levels different accretion mechanisms are activated depending on interactions between surface processes and wedge mechanics frontal accretion backthrusting underthrusting and underplating due to decollement induced duplex formation at depth these mechanisms may function simultaneously being located at different parts across the wedge for all the experiments a high friction is imposed at the base of models and the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during the shortening erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation erosion promotes development of major backthrusts in the thrust wedges without or with one decollement but no backthrusts was formed in the wedges with two decollements slow erosion allows lower extent of basal underthrusting in comparison with regular rate erosion variations in the erosion taper lead to changes in duplex geometry and exhumation rate in thrust wedges with one or two decollements the 6 erosion taper promotes formation of antiformal stack at the rear part of thrust wedge high rate of basal underthrusting and high extent of erosional removal the cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of thrust wedges the 8 erosion taper allows development of individual ramp anticlines and active forward thrusting of cover layers above the decollement and low rate of basal underplating below it with consequent low extent of erosional removal the results of our experiments support the observations on structural evolution and erosion in the alberta foothills of the canadian rockies

early evolution of a stratospheric volcanic eruption cloud as observed with toms and avhrr this paper is a detailed study of remote sensing data from the total ozone mapping spectrometer toms and the advanced very high resolution radiometer avhrr satellite detectors of the 1982 eruption of el chichon mexico the volcanic cloud atmosphere interactions in the first four days of this eruption were investigated by combining ultraviolet retrievals to estimate the mass of sulfur dioxide in the volcanic cloud krueger et al 1995 with thermal infrared retrievals of the size optical depth and mass of fine grained 1 10 m radius volcanic ash wen and rose 1994 our study provides the first direct evidence of gravitational separation of ash from a stratospheric gas rich plinian eruption column and documents the marked differences in residence times of volcanic ash and sulfur dioxide in volcanic clouds the eruption column reached as high as 32 km carey and sigurdsson 1986 and was injected into an atmosphere with a strong wind shear which allowed for an observation of the separation of sulfur dioxide and volcanic ash the upper more sulfur dioxide rich part of the cloud was transported to the west in the stratosphere while the fine grained ash traveled to the south in the troposphere the mass of sulfur dioxide released was estimated at 7 1 109 kg with the mass decreasing by approximately 4 1 day after the peak the mass of fine grained volcanic ash detected was estimated at 6 5 109 kg amounting to about 0 7 of the estimated mass of the ash which fell out in the mapped ash blanket close to the volcano over the following days 98 of this remaining fine ash was removed from the volcanic cloud and the effective radius of ash in the volcanic cloud decreased from about 8 m to about 4 m

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

3d numerical modeling of forward folding and reverse unfolding of a viscous single layer implications for the formation of folds and fold patterns abstract the main aims of this study are to show i that non cylindrical three dimensional 3d fold shapes and patterns can form during a single unidirectional shortening event and ii that numerical reverse modeling of 3d folding is a feasible method to reconstruct the formation of 3d buckle folds 3d viscous newtonian single layer folding is numerically simulated with the finite element method to investigate the formation of fold shapes during one shortening event an initially flat layer rests on a matrix with smaller viscosity and is shortened in one direction parallel to the layering forward modeling with different initial geometrical perturbations on the flat layer and different lateral boundary conditions generates non cylindrical 3d fold shapes and patterns the simulations show that in reality the initial layer geometry and the boundary conditions strongly control the final fold geometry fold geometries produced from the forward folding models are used as initial setting in numerical reverse folding models with parameters identical to those of forward models these reverse models accurately reconstruct the initial geometry of forward models with also only one extension event parallel to the previous shortening direction the starting geometry of the forward models is inaccurately reconstructed by the reverse models if a significantly different viscosity ratio than in the forward models is used this work demonstrates that reverse modeling has a high potential for reconstructing the deformation history of folded regions and rheological constraints such as viscosity ratio reverse models may be applied to natural 3d fold shapes and patterns in order to determine if they formed i during a single or multiple deformation events and ii as active buckle folds with a viscosity ratio 1 or as passive kinematic folds without buckling this approach may find much application to fold interference patterns in particular

viscosity estimates of salt in the hormuz and namakdan salt diapirs persian gulf the parabolic surface profiles of the hormuz and namakdan salt diapirs in the persian gulf suggest that they have been extruding with newtonian viscous rheologies for the last 10 4 years we derive velocity profiles for these diapirs neglecting gravitational spreading and erosion dissolution while assuming incompressible newtonian rheology of the salt fitting known rates of extrusion at specific points in its elliptical cross section the dynamic viscosity of the salt of the hormuz diapir is found to range between 10 18 and 10 21 pa s approximating its sub circular cross section to a perfect circle the range of viscosity of the salt of the namakdan diapir is obtained as 10 17 10 21 pa s these calculated viscosities fall within the range for naturally flowing salts elsewhere and for other salt diapirs but are broader than those for salts with newtonian rheology deforming at room temperatures the salts of the hormuz and namakdan diapirs are expected to exhibit a broader range of grain size which matches the limited existing data

lateral fold growth and fold linkage we use a three dimensional finite element model for viscous flow to investigate the lateral linkage mode of two initially isolated fold segments depending on the separation distance between the fold hinge lines four different fold linkage scenarios are observed linear linkage yields a sub cylindrical fold with a saddle at the location where the two initial folds linked oblique linkage produces a fold resembling a type ii refold structure oblique no linkage results in two curved folds with fold axes plunging in opposite directions linear no linkage yields a fold train of two separate subcylindrical folds with fold axes plunging in opposite directions representative numerical models for detachment and matrix folding for both power law and linear viscous rheologies exhibit the same four fold linkage modes the transition from linkage to no linkage occurs when the fold separation between the initially isolated folds is slightly larger than one half of the low amplitude fold wavelength we demonstrate a natural example of lateral fold linkage from the kurdistan region of iraq and point out the economic interest of saddle points linking the initially isolated fold segments

fission track evidence for late triassic oceanic crust in the french occidental alps five zircons and one apatite from plutonic rocks of the ophiolitic sequence from the piemontese zone western alps southeast france have been dated by the fission track method the ages obtained indicate that the minimum age of the alpine oceanic crust in this zone is between 192 6 and 212 8 m y late triassic liassic consideration of the available data on fission track retention conditions in zircons and pressure temperature conditions allowed by the high pressure mineralogic assemblages of the rocks leads to the conclusion that in the external part of the piemontese zone the temperature duration conditions of the high pressure alpine metamorphism were inadequate to significantly reset the zircons9 fission track clock suggesting a short lived metamorphic event

forward propagation of the zagros simply folded belt constrained from magnetostratigraphy of growth strata the temporal evolution of the zagros simply folded belt is constrained by a magnetostratigraphic sequence containing a progressive unconformity on the southern limb of the kuh e ghol ghol anticline in the central fars the investigated similar to 1400m thick sequence exposes a regressive megacycle containing from bottom to top open and shallow marine marls and sandy limestones fine to coarse grained fluvial deposits and alluvial conglomerates correlating the magnetostratigraphic section with the geomagnetic polarity time scale constrains the transition from marine to fluvial sediment deposition at similar to 6ma this transition was accompanied by a change in the accumulation rate from similar to 15cm ka to similar to 40cm ka as measured on lithified sediments alluvial river deposits first occurred at similar to 3 2ma the kuh e ghol ghol anticline began to grow at similar to 3 8ma witnessing fastest limb rotation rates of 40 degrees ma at similar to 3 3ma reporting magnetostratigraphic sections and ages of growth strata on a map of ne fars reveal an similar to 1cm a southwestward migration rate of the deformation front during the middle and late miocene

present day crustal deformation and plate kinematics in the middle east constrained by gps measurements in iran and northern oman summary a network of 27 gps sites was implemented in iran and northern oman to measure displacements in this part of the alpine himalayan mountain belt we present and interpret the results of two surveys performed in 1999 september and 2001 october gps sites in oman show northward motion of the arabian plate relative to eurasia slower than the nuvel 1a estimates e g 22 2m m yr 1 at n8 5 e instead of 30 5 mm yr 1 at n6 e at bahrain longitude we define a gps arabia eurasia euler vector of 27 9 0 5 n 19 5 1 4 e 0 41 0 1 myr 1 the arabia eurasia convergence is accommodated differently in eastern and western iran east of 58 e most of the shortening is accommodated by the makran subduction zone 19 5 2m m yr 1 and less by the kopet dag 6 5 2m m yr 1 west of 58 e the deformation is distributed in separate fold and thrust belts at the longitude of tehran the zagros and the alborz mountain ranges accommodate 6 5 2m m yr 1 and 8 2m m yr 1 respectively the right lateral displacement along the main recent fault in the northern zagros is about 3 2m m yr 1 smaller than what was generally expected by contrast large rightlateral displacement takes place in northwestern iran up to 8 mm yr 1 the central iranian block is characterized by coherent plate motion internal deformation 2m m yr 1 sites east of 61 e show very low displacements relative to eurasia the kinematic contrast between eastern and western iran is accommodated by strike slip motions along the lut block to the south the transition zone between zagros and makran is under transpression with right lateral displacements of 11 2m m yr 1

structural evolution of a three dimensional finite width crustal wedge abstract we present the results of three dimensional numerical experiments designed to study the response of a layer of crustal material subjected to convergence through an imposed basal velocity discontinuity and to surface processes erosion sedimentation we focus on the three dimensional response of the system arising from the finite width of the imposed velocity discontinuity in particular we describe the complex structures that develop around the wedge and their interactions with the loading unloading produced by the surface processes we show that the pro and retro shear zones that develop in a doubly vergent two dimensional wedge curve around the end of the velocity line discontinuity to merge into the strike slip structure that naturally develop i e as a consequence of the imposed boundary conditions along the edge of the wedge along the retro shear zone the stress orientation rotates along a vertical axis which implies that the retro shear zone is a pure thrust along all of its curved length whereas along the pro shear zone stresses rotate along a horizontal axis which in turn implies that the pro shear zone progressively evolves towards an oblique thrust in its curved section furthermore the outward motion i e perpendicular to the direction of imposed shortening along the curved section of the retro shear zone is accommodated by oblique extension along a secondary kinked structure antithetic to the retro shear zone we also show the complex evolution of the wedge when ductile flow and ductile strain softening is activated by increasing the imposed basal temperature in this situation the wedge is broader as structures develop at finite distances on either side of the line discontinuity and its dynamics resembles more that of a vise like orogen at the surface a flat plateau forms that accumulates sediment from the surrounding actively deforming mountain ranges until a channel breaks through one of the sides and flushes the inward draining basin of its sedimentary content the complex numerical models provide a detailed insight into the geometry and complex three dimensional structures within and at the margins of an orogen and how they can be affected by erosional processes at the surface

effect of spatial distribution of hormuz salt on deformation style in the zagros fold and thrust belt an analogue modelling approach scaled analogue models of thin skinned simultaneous shortening above adjacent viscous and frictional decollements simulate the effect of hormuz salt on the shortening in the zagros fold and thrust belt the models consisted of sand layers that partly overlay a viscous layer of silicone and were shortened from one end spatial distribution of the viscous decollement varied along strike and dip as occurs in part of the zagros fold and thrust belt in this belt phanerozoic sedimentary cover was shortened partly above the hormuz salt lying on the precambrian crystalline basement behaving as a basal viscous decollement model results display how the nature of the decollement affects the evolution of an orogenic belt using model results we explain the development of deflection zones and discuss strain partitioning formation of different topographic wedges and differential sedimentation along the zagros fold and thrust belt model results suggest the formation of a gentle taper consisting of both foreward and backward thrusts above a viscous decollement and a relatively steeper taper consisting only of forward vergent imbricates above a frictional decollement however in our models the steepest wedge with the highest topography formed where the viscous substrate had a limited extent with a transitional boundary pinch out perpendicular to the shortening direction shortening of this boundary led to development of frontal ramps associated with significant uplift of the area behind the deformation front

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

kinematics and surface fracture pattern of the anaran basement fault zone in nw of the zagros fold thrust belt the preexisting north south trending basement faults and their reactivation played an important role during the evolution of the zagros fold thrust belt the anaran basement fault in the lurestan r

progressive unconformities within an evolving foreland fold thrust belt zagros mountains a major angular unconformity between the bakhtyari conglomerates and the underlying agha jari formation has long been interpreted as indicating that orogeny in the zagros simply folded zone took place in plio pleistocene times this study uses field evidence of unconformities between older units in conjunction with geological maps and cross sections to argue that the front of the zagros simply folded zone has propagated in time and space these unconformities indicate that deformation started as early as end eocene in the northeast of the simply folded zone and propagated progressively to the southwest where unconformable contacts are only seen between younger units as shortening continued the southwest migration of the deformation front drove the foreland basin in front of it to its present position along the persian gulf and mesopotamia the climax of orogeny took place at end pliocene time when the most extensive unconformity in the zagros simply folded zone developed between the upper bakhtyari formation and older units active seismicity and documented present uplift imply that the simply folded zone is still propagating southwestward

detachment folding in the central and eastern zagros fold belt iran salt mobility multiple detachments and late basement control abstract detachment folding has been studied on the basis of field and subsurface examples from the central and eastern zagros iran we discuss different aspects of detachment folding well illustrated in the studied area in particular we focus on salt mobility multiple decollements and late basement control salt mobility concerns the hormuz basal detachment and the gachsaran upper detachment for the latter it is shown that mobility results not only from folding related diapirism but also from early gravity driven migration from growing anticlines towards intervening synclines concentric folding between two detachment levels is directly observed in the izeh zone central zagros where it is shown that the wave length depends on the distance between the two active decollements and that the fold shape is a function of the level of erosion throughout the zagros detachment folds mainly developed during an initial thin skinned phase of deformation which was followed by the current thick skinned stage this succession is particularly well expressed in the eastern zagros where basement faults cut early detachment folds obliquely

new views on earthquake faulting in the zagros fold and thrust belt of iran the zagros fold and thrust belt is amongst the world s most seismically active mountain ranges and is influential in our understanding of continental collisions the presence of hormuz salt at the base of the 10 km thick folded sedimentary cover together with the general absence of coseismic surface rupturing have led to an assumption that earthquakes are concentrated within the basement here we reinterpret the seismicity of the zagros in light of insar studies that show some recent earthquakes ruptured the competent group of mechanically strong strata in the lower sedimentary cover local network data demonstrate that microseismicity occurs within the basement reaching depths of 20 km and in places 30 km centroid depths of larger mw 5 earthquakes are mostly 4 14 km with rare events up to 20 km and 28 km along the oman line within the simply folded belt most of these events are contained within the competent group and are limited to mw 6 5 earthquakes occur earthquakes within the competent group can account for much of the overall shortening at this level but the seismic strain rate in the basement is much lower it is unclear whether the basement deforms by aseismic creep or folding beneath the sfb or by ductile shortening beneath the high zagros or iranian plateau

tectonic interpretation of an inverse gradient of zircon fission track ages with respect to altitude alpine thermal history of the gran paradiso basement extensive sampling along vertical and north south cross sections in the gran paradiso basement internal crystalline massif western alps reveals an inverse gradient of zircon fission track ages with respect to altitude which is explained by an inverted metamorphism during eocene time subsequent to the obduction of the schistes lustres nappe high pressure eoalpine phase the gran paradiso massif cooled down 90 80 myrs ago the austro alpine nappe of monte emilius dent blanche overthrust the gran paradiso massif 38 40 myrs ago inducing in its basement an inverted metamorphism or more precisely a nappe emplacement metamorphism greenschist facies the last cooling below 100 c occurred as early as 35 40 myrs ago in the southern gran paradiso massif in the northern part this last cooling occurred 20 myrs ago accompanied by an important uplift phase 2 mm year

increase in late neogene denudation of the european alps confirmed by analysis of a fission track thermochronology database abstract a sharp increase in deposited sediment volume since pliocene times has been observed worldwide and in particular around the european alps this phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing observation of in situ cooling histories for orogens is critical to assess the reality of the inferred increase in denudation rates and to determine whether this phenomenon is widespread or localized at active tectonic structures we exploit the unique density of fission track ages in the western european alps to reconstruct cooling isoage surfaces and to estimate exhumation rates on the orogen scale between 13 5 and 2 5 ma our novel technique is based on the association of isoage contours with age elevation relationships it uses map view interpolation enabling a spatio temporal analysis of exhumation rates over the entire western alps the resulting exhumation histories reconstructed for eight areas of the western alps display strong similarities in timing and rates with orogen wide average denudation rates inferred from sediment volumes this consistency validates the use of both techniques for the study of an orogen characterized by strong relief and high recent exhumation rates we conclude that exhumation rates in the western alps have increased more than twofold since late miocene times this increase may have been locally modulated by the distinct response of different tectonic units

deformation mechanics in inclined brittle ductile transpression zones insights from 3d finite element modelling abstract most natural examples of transpression zones developed at oblique convergence regime are inherently 3d and have inclined boundaries a 3d finite element model with an elasto plastic rheology is used to investigate the structural and mechanical evolution of inclined transpression zones in a rock sequence above a frictional basal detachment inelastic constitutive relationships allow permanent strains to develop in response to the applied loads fe modelling results show that oblique convergence is accommodated by discrete deformation at the main pre existing inclined faults 70 and by distributed brittle and ductile deformation at active blocks oblique contraction at the active blocks resulted mainly in layer parallel shortening orthogonal to the model outer boundaries whereas thickening in the horizontal and vertical directions was accommodated via layer parallel fault strike parallel extension and up dip extrusion i e inclined extrusion lateral extrusion should have compensated the rest and or volume loss took place folding and thickening of the mobile backstop produced a non cylindrical asymmetric bi vergent anticline where permanent strains developed principally in the steep forelimb secondary conjugate fault zones also accommodate oblique slip and contribute to uplift displacement vectors within the transpression zone are rotated counter clockwise ca 20 30 with respect to vectors in the fixed backstop areas with higher rotation values seem to correlate with those showing higher ellipticity values the presence of pre existing faults favored strain partitioning from the onset of deformation fe modelling results compared with analytical natural example and analogue modelling results show that our mechanical modelling can overall match inclined transpression zones geometry that present different modes of strain partitioning and localisation

rapid short term cooling following the chicxulub impact at the cretaceous paleogene boundary the mass extinction at the cretaceous paleogene boundary 66 ma is thought to be caused by the impact of an asteroid at chicxulub present day mexico although the precise mechanisms that led to this mass extinction remain enigmatic most postulated scenarios involve a short lived global cooling a so called impact winter phase here we document a major decline in sea surface temperature during the first months to decades following the impact event using tex86 paleothermometry of sediments from the brazos river section texas we interpret this cold spell to reflect to our knowledge the first direct evidence for the effects of the formation of dust and aerosols by the impact and their injection in the stratosphere blocking incoming solar radiation this impact winter was likely a major driver of mass extinction because of the resulting global decimation of marine and continental photosynthesis

toward 4d modeling of orogenic belts example from the transpressive zagros fold belt abstract latest developments in computer power and computational solutions open new ways to envision complex natural systems advances in high resolution 3d geodynamic modeling in particular allow investigating tectonic processes such as transpression which involve normal and tangential kinematics with respect to the bulk orogenic trends we demonstrate that implementing the 4 dimensions space and time provides and constrains new answers to long lasting discussions as case study we have chosen the zagros simply folded belt in iran which exhibits spatial variations of structural patterns from the homogeneously shortened wide fold belt in the fars to the narrower higher elevated area in the izeh high resolution 4d numerical modeling shows how a low viscosity decollement becoming frictional toward the dezful embayment influences the exposed fold patterns results also emphasize the importance of an oblique backstop to produce en echelon arranged folds where convergence related and backstop controlled folds are mingled in the transpressional orogen

analogue modeling of the role of multi level decollement layers on the geometry of orogenic wedge an application to the zagros fold thrust belt sw iran the presence of evaporate and incompetent formations i e decollement horizons within the sedimentary sequence of fold thrust belts can control their structural style and deformation evolution in the present study the influence of the decollement layers e g basal and internal decollement layers on the deformation style of several segments of the zagros fold thrust belt zftb sw iran e g fars arc dezful embayment and izeh zone was investigated using a series of analogue models of accretionary wedges the study of seismic profiles to understand the structural evolution of these segments of the belt where several decollement intervals acted as basal and internal decollements is complemented by the analogue model results the experimental results reveal that the thickness of the internal decollement layers influences the creation of fold dominated or thrust dominated deformations respectively experimental models and seismic data highlight that incompetent layers act as barrier units against fault propagation in sequence and or out of sequence faults into overlying strata towards southwest by fore deformation and control the rate of deformation propagation in the zftb the presence of both the basal and internal decollement layers located at different stratigraphic levels is required to form disharmonic decollement folds in the foreland of the zftb in addition the geometry spacing activity and propagation of faults as well as the topographic height of the critical wedges are directly controlled by low frictional decollements geophys j int 165 1 336 356 2006 geochem geophys geosyst 14 1131 1155 2013 the seismic profiles of the zftb showed that in addition to lithological contrasts the existence and activity of deep seated and basement faults had a big impact on the structural styles of the region