the late paleozoic el imperial formation western argentina glacial to post glacial transition and stratigraphic correlations with arc related basins in southwestern gondwana abstract the el imperial formation of the san rafael basin records a succession of depositional environments during the latest mississippian to earliest permian that span before during and after the glaciation of west central argentina at the base of the formation a restricted marine environment is recorded in mudstone containing marl and rippled and deformed sandstone beds this unit or sequence 1 is incised by a deltaic facies association composed of cross bedded sandstone and conglomerate that form at least 5 stacked gilbert deltas the deltaic facies association grades upward into the glacially influenced facies association made up of stratified diamictite mudstone with dropstones and massive deformed sandstone indicating deposition by wet based tidewater glaciers that calved icebergs into the basin with contributions from mass movement processes the glacially influenced facies association is overlain by mudstone and horizontally laminated and cross bedded sandstone of the post glacial open marine facies association recording post glacial transgression followed by relative sea level fall the deltaic glacially influenced and post glacial open marine facies associations comprise sequence 2 sequence 2 is incised by conglomerate of the upper fluvial member or sequence 3 the strata of the el imperial formation are correlated to those of the other arc related basins of western argentina rio blanco calingasta uspallata and tepuel a bashkirian transgression and fluvial incision in the el imperial formation correlate with events in the rio blanco and calingasta uspallata basins to the north whereas glaciation continues to the south in the tepuel basin through the early permian the deviating stratigraphic record of the tepuel basin may be the result of its higher latitudinal position during the pennsylvanian early permian and higher altitude due to either tectonic convergence of the patagonian microplate or convergence along the panthalassan margin of southwestern gondwana

strain localization due to a positive feedback of deformation and myrmekite forming reaction in granite and aplite mylonites along the hatagawa shear zone of ne japan abstract with increasing mylonitization of aplite veins and their adjacent granite in the hatagawa shear zone of ne japan the modal content and grain size of feldspar porphyroclasts decrease while the modal content of fine grained 50 m polymineralic aggregate of plagioclase k feldspar quartz and biotite increases up to 60 80 in the granite and aplite ultramylonites deformation induced myrmekite lobes are ubiquitously developed around k feldspar porphyroclasts the occurrence and chemical composition of plagioclase and k feldspar in fine grained polymineralic aggregates indicate that the plagioclase is mostly derived from myrmekite while the interstitial k feldspar originates from isolated porphyroclast rims between growing myrmekite lobes and or precipitations from solution modal and bulk chemical compositions of granite and aplite indicate that the mylonites are preferentially developed in aplite veins richer in k feldspar than the surrounding granite deformation induced k feldspar replacement by myrmekite is then more favored during deformation to produce more abundant fine grained polymineralic aggregates subsequent reaction softening leads to more enhanced deformation which promotes further myrmekite forming reaction such a positive feedback of deformation and myrmekite forming reaction in aplite veins and their adjacent granite may be responsible for strain localization in the granite and aplite mylonites along the hatagawa shear zone

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

measurement of eolian sand ripple cross sectional shapes assuming that the profiles of eolian sand ripples remain constant over a short distance it is possible to measure their shapes by casting a shadow perpendicular to the ripple crests we describe a method whereby this can be done accurately and easily tests of the method demonstrate that accuracies on the order of one sand grain diameter can be achieved

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

an assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz bearing mylonite zones abstract recrystallized grainsize has been measured in quartz and in some cases feldspar in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings 1 1 supracrustal low temperature shallow dipping thrusts 2 2 deeper level major 100 km thrusts in high grade metamorphic rocks and 3 3 steeply dipping fault zones in granite within a low grade palaeozoic fold belt the concept of a steady state dynamically recrystallized grainsize largely unaffected by post creep processes e g static grain growth receives strong support especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones however the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize the available theoretical and empirical stress grainsize relationships do not account for these effects the concept of a steady state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation predicted deviatoric stresses have been computed using twiss theoretical model for comparison with other crustal stress estimates and most samples fall in the range from 20 to 150 mpa in a few cases where discrete high strain mylonite zones occur within contemporaneously regionally deformed rocks the recrystallized grainsize indicates that stresses were 1 5 to 2 times higher within the zones there is a clear need for improved theoretical and or empirical stress grainsize relationships for quartz and other minerals before extensive further application of this technique

how do bedform patterns arise new views on the role of bedform interactions within a set of boundary conditions one explanation for bedform patterns is self organization in which the pattern emerges because of interactions among the bedforms themselves models remote images field studies and lab experiments have identified bedform interactions that involve whole bedforms only bedform defects or that are remote interactions between bedforms it is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development constructive interactions including merging lateral linking cannibalization and remote transfer of sediment push the system toward fewer larger more widely spaced bedforms regenerative interactions including bedform splitting defect creation and calving push the system back toward a more initial state other interactions including off center collision defect migration and bedform and defect repulsion cause pattern change but may not be strongly constructive or regenerative although bedform interactions are ubiquitous to any field of bedforms their dynamics flow field modification and impact upon measurable pattern parameters are yet poorly understood most bedform interactions span bedform types and fluids supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform flow and grain fluid interactions bedform interactions alone however cannot account for the rich diversity of bedform patterns in nature it is proposed that field diversity arises because of boundary conditions which are the environmental variables within which a field evolves conceptually boundary conditions modify the shape of the attractor toward which a field evolves possibly by altering the type and frequency of bedform interactions boundary conditions are broadly similar within system types but are unique for each bedform field so that no two are ever exactly alike although aeolian and fluvial systems share some types of boundary conditions flow depth is a unique boundary condition in shallow fluvial systems copyright 2009 john wiley sons ltd

controls on the large scale spatial variations of dune field properties in the barchanoid portion of white sands dune field new mexico previous studies have shown that sediment fluxes and dune sizes are a maximum near the upwind margin of the white sands dune field and decrease to first order with increasing distance downwind these patterns have alternatively been attributed to a shear stress overshoot associated with a roughness transition localized at the upwind margin and to the influence of long wavelength topography on the hydrology and hence erodibility of dune field sediments i point out an issue that compromises the shear stress overshoot model and further test the hypothesis that long wavelength topographic variations acting in concert with feedbacks among aerodynamic granulometric and geomorphic variables control dune field properties at white sands building upon the existing literature i document that the mean and variability of grain sizes sand dryness aerodynamic roughness lengths bed shear stresses sediment fluxes and ripple and dune heights all achieve local maxima at the crests of the two most prominent scarps in the dune field one coincident with the upwind margin and the other located 6 7 km downwind computational fluid dynamics cfd modeling predicts that bed shear stresses erosion rates and the supply of relatively coarse poorly sorted sediments are localized at the two scarps due to flow line convergence hydrology and the spatially distributed adjustment of the boundary layer to variations in dune size as a result the crests of the scarps have larger ripples due to the granulometric control of ripple size larger grain sizes and or larger ripples lead to larger dunes and hence larger values of bed shear stress in a positive feedback

zircon and whole rock nd pb isotopic evidence for a grenville age and a laurentian origin for the basement of the precordillera in argentina suggestions that the precordillera of argentina has an early paleozoic connection to the appalachian ouachita rifted margin of the southern u s have been based on cambro ordovician stratigraphy and faunal assemblages further support comes from comparing precambrian grenville appalachian basement with xenoliths brought up in precordil leran miocene volcanic rocks u pb zircon ages and whole rock pb and nd isotopic data from the xenoliths show that precordilleran basement along with some part of the westernmost sierras pampeanas pie de palo range to the east is essentially unique in south america and is best interpreted as being derived from laurentia a grenville age is substantiated by u pb zircon upper intersect ages of 1102 pm 6 ma for a mafic xenolith and near 1100 ma for acidic xenoliths both xenolith types contain pink doubly terminated prismatic igneous zircons with relatively high u and pb concentrations rounded colorless low u and pb zircons with overgrowths produced in a later pre

large wind ripples on mars a record of atmospheric evolution wind blowing over sand on earth produces decimeter wavelength ripples and hundred meter to kilometer wavelength dunes bedforms of two distinct size modes observations from the mars science laboratory curiosity rover and the mars reconnaissance orbiter reveal that mars hosts a third stable wind driven bedform with meter scale wavelengths these bedforms are spatially uniform in size and typically have asymmetric profiles with angle of repose lee slopes and sinuous crest lines making them unlike terrestrial wind ripples rather these structures resemble fluid drag ripples which on earth include water worked current ripples but on mars instead form by wind because of the higher kinematic viscosity of the low density atmosphere a reevaluation of the wind deposited strata in the burns formation about 3 7 billion years old or younger identifies potential wind drag ripple stratification formed under a thin atmosphere

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

the pattern of grainfall deposition in the lee of aeolian dunes a simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance the deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity the velocity being weighted by its probability density the resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink beyond which deposition rates fall off roughly exponentially an important length scale emerges that characterizes this decay with distance from the brink the length increasing with wind velocity and decreasing with grain diameter it is shown that this length scale is on the order of one metre for typical grain size and wind conditions that this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes the position of a pivot point on the lee slope may be predicted separating source regions from accumulation regions for grainflow avalanche deposits the calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits but they provide the initial geometry for grainflow avalanches the initial failures should coincide with the steepest gradient in grainfall deposition slightly downslope from the grainfall maximum

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

the impact process in aeolian saltation two dimensional simulations a critical event in the trajectory of a sand grain saltating in air is its interaction with the surface we examine the phenomenon of grain bed impacts in two dimensions using a combination of dynamical computer simulations analytical models and physical reasoning the results indicate that the grain bed collisions can be treated as two body collisions with the bed particle assuming an effective mass greater than its true mass also the presence of geometrical surface irregularities has a strong bearing on the interaction between saltating and surface grain populations as well as on the formation of small scale bedforms

numerical simulation of saltating particles in atmospheric boundary layer over flat bed and sand ripples 1 in this work we numerically simulated the saltating particles in a turbulent boundary layer over flat bed and sand ripples by using natural sand grains in a wind tunnel we obtained the initial conditions for the simulation and also verified the correctness of the numerical model we carefully analyzed the numerically simulated saltating particle movement over the two sand beds and we found the following 1 the aeolian sand transport is a dynamic equilibrium process on both sand beds and it took longer to reach equilibration on the sand ripples than on the flat bed 2 according to the mass flux profile at the trough of the sand ripples there is a maximum mass flux at about 4 cm height in the leeward section the mass flux increases with height below 4 cm and decreases with height above 4 cm 3 the wind profile near the surface is modified by saltating particles on the two different sand beds and the flow field characteristics of the sand ripples are more complex than that of the flat bed

viscoplasticity of polycrystalline olivine experimentally deformed at high pressure and 900 c abstract we have performed tri axial compression experiments on olivine aggregates at 900 c and at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus deformation at two different constant strain rates 1 1 10 5 s 1 and 3 4 10 4 s 1 yields continuous hardening reaching maximal differential stresses of 930 and 1076 mpa respectively before sample failure at ca 10 strain the deformed samples were characterized by electron backscatter diffraction ebsd and transmission electron microscopy tem crystallographic preferred orientations cpo are weak but reveal an alignment of the 010 axes around the compression direction and some concentration of 001 and 100 axes normal to it tem observations confirm plastic deformation by dislocation glide with both 001 and 100 burgers vectors electron tomography performed for the first time on deformed fined grained olivine shows that 001 dislocations glide extensively on 110 and 130 planes in complement to the 010 and 100 glide planes often reported at high temperatures 100 dislocations glide mostly on 010 but 011 and 041 planes are also activated the experimental cpo and relative activity of slip systems are well reproduced by viscoplastic self consistent simulations in which all 100 and 001 slip systems have similar strengths they are also consistent with olivine cpo from natural samples deformed at low temperature comparison of the present mechanical results to those obtained for olivine single crystals at similar conditions shows that at low temperature below 1100 c and high stress 300 mpa polycrystalline olivine has a more complex mechanical behavior than single crystals however both datasets advocate for a much weaker dry mantle lithosphere than previously thought

zircon and whole rock nd pb isotopic provenance of middle and upper ordovician siliciclastic rocks argentine precordillera graptolite bearing middle and upper ordovician siliciclastic facies of the argentine precordillera fold thrust belt record the disintegration of a longlived cambro mid ordovician carbonate platform into a series of tectonically partitioned basins a combination of stratigraphic petrographic u pb detrital zircon and nd pb whole rock isotopic data provide evidence for a variety of clastic sediment sources four upper ordovician quartzo lithic sandstones collected in the eastern and central precordillera yield complex u pb zircon age spectra dominated by 1ae05 1ae10 ga zircons secondary populations of 1ae22 1ae30 and 1ae46 ga rare 2ae2 and 1ae8 ga zircons and a minor population 2 of concordant zircons in the 600 700 ma range archaean age grains comprise 1 of all zircons analysed from these rocks in contrast a feldspathic arenite from the middle ordovician estancia san isidro formation of the central precordillera has two well defined peaks at 1ae41 and 1ae43 ga with no grains in the 600 1200 ma range and none older than 1ae70 ga the zircon age spectrum in this unit is similar to that of a middle cambrian quartz arenite from the la laja formation suggesting that local basement rocks were a regional source of ca 1ae4 ga detrital zircons in the precordillera terrane from the cambrian onwards the lack of grains younger than 600 ma in upper ordovician units reinforces petrographic data indicating that ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins nd isotopic data n 32 for middle and upper ordovician graptolitic shales from six localities define a poorly mixed signal end 450 ma 9ae 6t o 4ae5 that becomes more regionally homogenized in upper ordovician rocks 6ae 2 1 ae0 tdm 1ae51 0ae15 ga n 17 a trend reinforced by the u pb detrital zircon data it is concluded that proximal recycled orogenic sources dominated the siliciclastic sediment supply for these basins consistent with rapid unroofing of the precordillera terrane platform succession and basement starting in mid ordovician time common pb data for middle and upper ordovician shales from the western and eastern precordillera n 15 provide evidence for a minor 30 component that was likely derived from a high l u pb terrane

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

eolian ripples as examples of self organization in geomorphological systems abstract ripples arise spontaneoulsy from sandy beds subjected to winds sufficient to cause the saltation of surficial grains while the initially fastest growing wavelengths are 4 6 times the mean hop length of grains impelled downwind by long trajectory high energy saltating grains the ripple wavelengths developed over long periods can be several times this numerical simulations based upon techniques pioneered by p k haff and coworkers reveal that a complex process of ripple coalescence is responsible for this evolution of the ripple field small fast moving ripples catch up with and are absorbed by larger slower forms each such merger resulting in a growth of the mean wavelength and a decline in the relative dispersion of the wavelengths an heuristic argument is presented which accounts for the fast initial growth and subsequent slowing of growth of the mean wavelength as the ripple field evolves ripple cross sectional shape appears to be controlled in large part by a distrianment function which represents the declining likelihood of a grain stopping on a surface as it slopes more steeply downwind that the resulting asymptotic wavelength in the simulations depends strongly upon impact angle supports sharp s 1963 suggestion that the length of the shadow zone plays an important role in controlling the final ripple wavelength

a silurian devonian marine platform deltaic system in the san rafael block argentine precordillera cuyania terrane lithofacies and provenance fil manassero marcelo jorge consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet la plata centro de investigaciones geologicas universidad nacional de la plata facultad de ciencias naturales y museo centro de investigaciones geologicas argentina

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

kinematics nature of deformation and tectonic setting of the taxaquara shear zone a major transpressional zone of the ribeira belt se brazil abstract in this paper we have investigated the taxaquara shear zone tsz a major strike slip structure within the ribeira belt rb se brazil in order to assess its kinematics the nature of deformation of rocks affected by the tsz and its tectonic setting the tsz presents a vertical ne trending mylonitic foliation and ene to wsw stretching lineation with variable plunge granitic rocks are intensely deformed giving rise to mylonites with newformed matrix assemblage quartz oligoclase k feldspar biotite muscovite rutile magnetite this assemblage within a pseudosection couple with muscovite biotite thermometer helps to constrain a metamorphic condition during deformation which spans 513 525 c and 3 9 4 4 kbar with mean peak at 520 c and 4 0 kbar quartz accommodates deformation through dislocation creep achieving subgrain rotation with minor contribution of grain boundary migration at a mean strain rate of 10 13 10 12 s 1 feldspar porphyroclasts record brittle deformation microfaults and evidences of dislocation creep e g kink twinning the cpos for monomineralic quartz ribbons display monoclinic pattern and activation of basal rhomb slip systems during deformation however the cpos for quartz and feldspar from fine grained polyphase matrix show a switch in deformation mechanism from dislocation creep to diffusion creep the vorticity and three dimensional finite strain data results suggest that the tsz was developed under a moderate to high strain conditions and a transpressional sub simple shear environment with predominance of simple over pure shear regional correlation with other structures from the ribeira belt and african belts based on finite strain quantification support that a major transpressional system was mainly responsible for the shear zones development during late stages of braziliano pan african orogeny

cataclastic rocks of the san gabriel fault an expression of deformation at deeper crustal levels in the san andreas fault zone the san gabriel fault a deeply eroded late oligocene to middle pliocene precursor to the san andreas was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels cataclastic rocks exposed along the present trace of the san andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz feldspar biotite chlorite amphibole epidote and fe ti oxide mineralogy representing the milled down equivalent of the original rock anderson and osborne 1979 anderson et al 1980 likewise fault gouge and associated breccia are common along the san gabriel fault but only where the zone of cataclasis is several tens of meters wide at several localities the zone is extremely narrow several centimeters and the cataclastic rock type is cataclasite a dark aphanitic and highly comminuted and indurated rock the cataclastic rocks along the san gabriel fault exhibit more comminution than that observed for gouge along the san andreas the average grain diameter for the san andreas gouge ranges from 0 01 to 0 06 mm for the san gabriel cataclastic rocks it ranges from 0 0001 to 0 007 mm whereas the san andreas gouge remains particulate to the smallest grain size the ultra fine grain matrix of the san gabriel cataclasite is composed of a mosaic of equidimensional interlocking grains the cataclastic rocks along the san gabriel fault also show more mineralogiec changes compared to gouge from the san andreas fault at the expense of biotite amphibole and feldspar there is some growth of new albite chlorite sericite laumontite analcime mordenite and calcite the highest grade of metamorphism is laumontite chlorite zone zeolite facies mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 30 c and 2 5 km thus suggesting an approximate geothermal gradient of 50 c km such elevated temperatures imply a moderate to high stress regime for the san andreas which is consistent with experimental rock failure studies moreover these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith porphyroclasts even in the most deformed rocks consist of relict plagioclase oligoclase to andesine alkali feldspar quartz biotite amphibole epidote allanite and fe ti oxides ilmenite and magnetite we have found no significant development of any clay minerals illite kaolinite or montmorillonite for many sites the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform and that originally micaceous foliated or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material as previously found for the gouge along the san andreas chemical mobility is not a major factor in the formation of cataclastic rocks of the san gabriel fault we see only minor changes for si and alkalies however there is a marked mobility of li which is a probable result of the alteration and formation of new mica minerals the gouge of the san andreas and san gabriel faults probably formed by cataclastic flow there is some indication presently not well constrained that the fine grained matrix of the cataclasite of from the san gabriel fault formed in response to superplastic flow

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

slip and recrystallization of halite gouge in experimental shear zones abstract cylindrical specimens of tennessee sandstone with dry crushed halite between 35 precut surfaces are deformed at 70 mpa confining pressure at displacement rates from 10 2 to 10 6 cm s at 25 c and the shear strains of 16 0 under these conditions peak differential stress is attained at shear strains between 3 and 5 followed by quasi steady state behavior with subtle but possibly significant oscillations over the complete variation of conditions steady state stress levels range from 146 to 169 mpa at shear strains 6 0 the fabric of the halite gouge 0 2 to 1 0 mm thick changes with increasing shear strain from 1 a homogeneously elongated grains up to peak stress indicative of dislocation glide to 2 b a bimodal mosaic of recrystallized grains consisting of very fine grained neoblasts and relatively large euhedral porphyroblasts that is associated with a small stress drop to 3 c a mosaic of uniformly sized anhedral neoblasts produced from the coalescence of porphyroblasts grain size of the very fine grained neoblasts remain reasonably constant with increasing but that of the porphyroclasts and of the resulting mosaic decrease with increasing 100 of the porphyroblasts are strongly and conspicuously oriented at 45 to the shear zone boundary at 4 4 and progressively rotate with the finite strain axes so that at 9 4 100 are inclined at 70 80 and 10 20 to the shear zone it is argued that fabrics b and c are consistent with dynamic recrystallization by grain boundary migration from favorably oriented nuclei grains a ghost structure of aligned grain boundaries and impurities pervades all the shear zones and is oriented at progressively smaller angles to the shear zone boundary as shear strain increases there is a one to one correspondence between these angles and those calculated assuming simple shear this fact along with the constant thickness of the gouge with increasing shear strain indicates that the deformation is modeled by progressive homogeneous simple shear

las matras block central argentina 37 s 67 w the southernmost cuyania terrane and its relationship with the famatinian orogeny abstract the las matras block in central argentina constitutes the southernmost part of the cuyania terrane which was accreted to the southwestern margin of gondwana during the early to mid ordovician famatinian orogeny the grenville aged rocks of the las matras block are represented by the tonalitic to trondhjemitic las matras pluton a new u pb conventional zircon age of 1244 42 ma confirms previous sm nd and rb sr isochron ages of this pluton mineral composition data are consistent with the tonalitic trondhjemitic character of the pluton and constrain its emplacement level to 1 9 to 2 6 kb this shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement a regional comparison indicates that the igneous metamorphic evolution of the grenville aged basement rocks of the cuyania terrane occurred over a period of more than 200 million years with ages older than 1200 ma up to those close to 1000 ma the shallowest crustal level is found in las matras suggesting a southward shallowing of the exposed level of basement the deformation and metamorphism associated with the collisional famatinian orogeny affect both the cuyania terrane and the adjacent western margin of gondwana and the gondwana margin was also the locus of the related arc magmatism but the compressive effects of the collision decrease in intensity toward the south the famatinian metamorphism and magmatism continue even further south into the patagonia region but the southern continuity of the cuyania terrane into this region remains uncertain

chemical changes associated with deformation of granites under greenschist facies conditions the example of the zawidow granodiorite se lusatian granodiorite complex poland abstract small scale subvertical retrograde shear zones are developed in the zawidow granodiorite located in the eastern part of the lusatian massif southwest poland heterogeneous deformation under conditions of greenschist facies metamorphism has produced mylonite zones that show a broad range of microstructural development three fabric types in the granodiorite deformation have been distinguished a weakly deformed protolith b moderately deformed in lens shaped domains c strongly foliated in fabric types b and c whole rock analyses show a decrease in cao mno cr sr pb whereas na2o and sio2 increase slightly overall al2o3 tio2 and zr remain essentially constant this pattern of element variation is consistent with the observed feldspar breakdown and increase of sericite and also with the transformation of biotite into chlorite mass balance calculations based on major and trace element geochemistry indicate constant volume or less than 10 volume loss from this study it is apparent that a low grade metamorphic deformation event does not have to have a profound influence on whole rock geochemical composition

structural analysis of the central and southwestern sudbury structure southern province canadian shield the sudbury structure ss is an unusual crater structure which acquired its present oval surface shape during northwest directed ductile thrusting lower amphibolite facies metamorphism accompanied the thrusting which generated a major reverse shear zone at least 50 km long the south range shear zone srsz transects the south range of the sudbury structure and exhumes a low level of the sudbury igneous complex sic assuming heterogeneous simple shear in the northwest southeast vertical plane on northeasterly striking glide surfaces minimal estimates of net displacement across the srsz exceed 8 km this displacement magnitude and the map pattern of the sic require the southwest closure of the ss to be steeply plunging in accord with a hypothetical funnel shape of the sic the rocks of the metasedimentary core of the ss are deformed into a family of second order buckle folds the tangent surface of which forms an upright open flexure within the first order structure of the sudbury synclinorium the

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

new geochronological information by rb sr and u pb investigations from the pre alpine basement of grande kabylie algeria this study presents the first geochronological results apart from previously obtained mica ages from metamorphic basement complex occurring within the internal zone of the alpine chain of algeria the rb sr whole rock and u pb zircon data reveal the existence of an episode of anatectic magmatism around 510 ma ago that corresponds to late pan african orogenic events a deep crustal fault is thought to represent a major phase of shearing associated with hercynian blastomylonitic metamorphism and the production of granitic remelts at 271 12 ma reworking of an ancient basement is demonstrated by the presence of inherited radiogenic lead in zircons from lower palaeozoic orthogneisses and metarhyolites upper intercepts on concordia plots yield ages of 1950 100 and 1321 30 ma respectively apart from the dating results which may provide new insight into the geological history of the region this study also provides general implications for the interpretation of geochronological data in metamorphic and

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

shear heating associated with movement along the alpine fault new zealand abstract geothermometry based on the fe mg partitioning between biotite and garnet has revealed a temperature increase of circa 100 c associated with the development of schist derived mylonites that presently crop out within the alpine fault zone in new zealand calculations show that the temperature increase is compatible with a short shear heating event 1 5 10 5 years it has been tentatively associated with a possible high stress deformation 1 2 kbar which occurred when the then existent mylonites were unable to accommodate imposed strain rates because of the misorientation of their fabric elements during the change in the relative slip vectors between the pacific and australian plates in the late miocene early pliocene the high stress deformation resulted in the present mylonites which have their fabric elements orientated to accommodate the present oblique reverse slip movement between the two plates

see through deformation experiments on brittle viscous norcamphor at controlled temperature strain rate and applied confining pressure abstract an integrated experimental analytical deformational system allows the observation of evolving microstructures in rock analogues under an optical macroscope at controlled confining pressure p c 0 001 0 3 mpa temperature t 10 150 c and strain rate 10 2 10 6 s 1 this system is ideally suited for studying grain scale processes across the complete spectrum of deformational behaviour from brittle cataclastic to fully viscous mylonitic flow the experimental system comprises two main elements 1 a modified urai means deformation rig and 2 a steel pressure vessel that accommodates the deformation rig yet is small enough to fit under an optical microscope first results of controlled confining pressure experiments on fluid bearing polycrystalline norcamphor an organic quartz analogue indicate that deformation involves a combination of dislocation creep and intergranular hydrofracturing leading to the formation of discrete shear surfaces comparable to c planes in mylonite at applied confining pressure the relative activity of brittle and viscous deformation mechanisms varies across the sample this in turn affects the distribution of strain on the sample scale the pressure dependence of strain localization in these experiments is interpreted to reflect a combination of local variations in the effective stresses and strain incompatibilities associated with the experimental simple shear configuration

retrowedge related carboniferous units and coeval magmatism in the northwestern neuquen province argentina the studied carboniferous units comprise metasedimentary guaraco norte formation pyroclastic arroyo del torreon formation and sedimentary huaraco formation rocks that crop out in the northwestern neuquen province argentina they form part of the basement of the neuquen basin and are mostly coeval with the late paleozoic accretionary prism complex of the coastal cordillera south central chile u pb shrimp dating of detrital zircon yielded a maximum depositional age of 374 ma upper devonian for the guaraco norte formation and 389 ma for the arroyo del torreon formation detrital magmatic zircon from the guaraco norte formation are grouped into two main populations of devonian and ordovician famatinian ages in the arroyo del torreon formation zircon populations are also of devonian and ordovician famatinian as well as of late neoproterozoic and mesoproterozoic ages in both units there is a conspicuous population of devonian magmatic zircon grains from 406 4 ma to 369 5 ma indicative of active magmatism at that time range the ehf values of this population range between 2 84 and 0 7 and the tdm hf are mostly mesoproterozoic suggesting that the primary sources of the devonian magmatism contained small amounts of mesoproterozoic recycled crustal components the chemical composition of the guaraco norte formation corresponds to recycled mature polycyclic sediment of mature continental provenance pointing to a passive margin with minor inputs from continental margin magmatic rocks the chemical signature of the huaraco formation indicates that a magmatic arc was the main provenance for sediments of this unit which is consistent with the occurrence of tuff mostly in the arroyo del torreon formation and very scarcely in the huaraco formation with a volcanic arc signature jointly indicating the occurrence of a carboniferous active arc magmatism during the deposition of the two units the guaraco norte formation is interpreted to represent passive margin deposits of mostly lower carboniferous age younger than 374 ma and older than 326 ma that precede the onset of the accretionary prism in chile and extend into the earliest stage of the accretion in a retrowedge position the arroyo del torreon and huaraco formations are considered to be retrowedge basin deposits to the early frontal accretionary prism eastern series of chile the presence of volcanism with arc signature in the units provides evidence of a mississippian magmatic arc that can be correlated with limited exposures of the same age in the frontal cordillera argentina the arc would have migrated to the west coastal batholith during pennsylvanian permian times coevally with the later basal accretionary prism western series the source of a conspicuous population of devonian detrital zircon interpreted to be of magmatic origin in the studied units is discussed in various possible geotectonic scenarios the preferred model being a magmatic arc developed in the chilenia block related to a west dipping subduction beneath chilenia before and shortly after its collision against cuyania gondwana at around 390 ma and not linked to the independent devonian mississippian arc developed to the south in patagonia

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

the early paleozoic evolution of the argentine precordillera as a laurentian rifted drifted and collided terrane a geodynamic model paleontologic and stratigraphic evidence points to the early paleozoic precordilleran terrane of western argentina as being the conjugate rift pair of the appalachians stratigraphic similarities of the cambrian and early arenig carbonate series and very strong affinities among trilobite conodont and brachiopod faunas show their close relationship the most probable provenance areas are the appalachian ouachita rifted margin and the newfoundland appalachians although the former fits better with geometric and drifting paths suggested by faunal affinities increasing celtic and baltic brachiopod genera and divergent stratigraphy since the arenig indicate the drifting of the precordilleran terrane collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed the collision of the precordillera with western gondwana occurred during the mid llanvirn to llandeilo a magmatic arc related to eastward subduction present coordinates was active in the famatina range east of the precordillera this region of celtic affinity shows faunal exchange with the precordilleran terrane since the late arenig and may represent accreted intra iapetus volcanic island arc complexes the rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the precordilleran terrane from periequatorial to peripolar latitudes between the cambrian and latest ordovician the deep ocean to the west of the precordillera started to close by the late ordovician with the eastward drift of the chilenian terrane absence of volcanic or pyroclastic arc derived rocks in the precordillera indicate west dipping subduction as chilenia approached the continental margin a new forebulge was established on the former collided precordilleran terrane developing an erosional unconformity in central precordillera talacasto tambolar arch a gondwanic signature was fully developed by the middle silurian when the malvinokaffric clarkeia fauna flourished before then the late ordovician glacial record and associated hirnantia fauna were the first clear tie to gondwana during the silurian the marginal basin behaved as a foreland with lithosphere rheology and eustasy governing the sequence stratigraphy wrench faulting along its eastern boundary displaced the precordillera toward the south continued shortening during closure with the chilenian terrane in the mid devonian produced thrust loading of the basin and generated a thick graywacke succession final accretion of chilenia late devonian generated a regional angular unconformity between the lower and upper paleozoic new eastward subduction was initiated west of the accreted chilenian terrane during the late permian triassic as indicated by the choiyoi volcanic complex which presently outcrops in the frontal cordillera

early paleozoic structural and metamorphic evolution of western sierra de san luis argentina in relation to cuyania accretion abstract structural metamorphic and isotopic data obtained from the nogoli metamorphic complex of western sierra de san luis indicate that the early paleozoic famatinian orogeny overprinted an already structured and metamorphosed older basement the older geological features are relict nw trending fabric associated with high grade amphibolite facies regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses arc magmatism medium p barrovian type high t amphibolite to granulite facies regional metamorphism and penetrative nne to ne trending foliation are related to the building of the famatinian orogenic belt the p t conditions of the famatinian prograde metamorphism reached a pressure peak of ca 8 kb with a thermal peak from 750 c up to 820 c u pb conventional and chemical dating and ar ar plateau ages constrain the peak of the main orogenic phase related to the famatinian belt to 470 457 ma early to mid ordovician greenschist facies retrograde metamorphism closely associated with shear zones and secondary ar ar plateau and sm nd ages suggest that a late to post orogenic phase of the famatinian belt was active at least since 445 ma this phase continued during the silurian to late devonian times through multiple reactivation of early shear zones the famatinian orogeny reset a previous thermal history and therefore the timing of the relict fabric could not be constrained conclusively with radiometric dates despite this difficulty a range of 520 to 490 ma suggests some inheritance from pampean events registered by the older nw se fabric the early to mid ordovician regional metamorphism and ductile deformation of the western sierra de san luis is interpreted as the orogenic effects of the collision of the allochthonous cuyania terrane with the autochthonous proto pacific margin of gondwana during the famatinian orogeny

cross sectional profiles of sand ripples megaripples and dunes a method for discriminating between formational mechanisms cross sectional profiles of sand ripples megaripples and sand dunes provide a useful tool for discriminating between formation by ripple and dune processes feature width defined as the basal break in slope along the profile to either side of the crest represents a good standard for comparison of profile attributes across more than three orders of magnitude aspect ratio height width as a function of log width separates measurements into clusters representing differing mechanisms of formation scaling both height and distance for individual profiles by feature width facilitates comparison of profile shapes across three orders of magnitude in width the data presented here should prove useful for evaluating possible mechanisms of origin for aeolian features observed remotely including on planetary bodies copyright 2012 john wiley sons ltd

the palaeoenvironmental significance of dune size hierarchies the size of a dune is a function of many factors one of which is the length of the period of consistent wind regime under which it grew ephemeral dunes are defined as those that are adjusted to diurnal wind regimes meso dunes to annual wind regimes and mega dunes to wind regimes that persist for longer periods these longer periods could be major climatic cycles but there seem to be no dunes whose size reaches that allowed by the length of some of these periods there may therefore be a maximum size beyond which mega dunes cannot grow dictated by sand supply aerodynamic processes such as speed up or by the scale of the atmospheric boundary layer these concepts are tested on dunes in the wahiba sands in oman analysed from a landsat image it is hypothesised that a large set of megadunes here formed in the lgm that a smaller set of mega dunes may be of younger dryas age and that there is a meso dune series of holocene age

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

the proto andean margin of gondwana an introduction a basic background is presented for the discussion of the early palaeozoic geology of western argentina covered by this book this includes the definition and terminology of orogenic cycles on this part of the gondwana margin represented by the eastern sierras pampeanas the pampean orogeny early cambrian relates to an intense but short lived period of terrane collision predating the rifting of the precordillera terrane from laurentia the famatinian cycle is predominantly represented by intense subductionrelated magmatism of early middle ordovician age developed on the continental margin of gondwana during the rifting and drifting of the precordillera terrane the grenvillian basement of the latter is further exemplified by a new rb sr whole rock isochron age of 1021 12 ma for orthogneisses from the western sierras pampeanas a mid ordovician granite in this area dated at 481 6 ma by u pb ion microprobe data may be related to rifting while the precordillera terrane was still attached to laurentia a divergence of opinion is pointed out between some authors in this book who favour mid ordovician collision of the precordillera with gondwana and others who place it much latter in silurian or devonian times

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

local shear zone pattern and bulk deformation in the gran paradiso metagranite nw italian alps the gran paradiso nappe of the northwestern alps mostly consists of augen gneisses derived from the alpine deformation of permian granitoids the regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top to west sense of shear the granitoid protolith is preserved in the kilometre scale low strain domain of the piantonetto valley and mainly consists of a porphyritic metagranite including joints leucocratic dykes and biotite rich schlieren in this low strain domain the alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite the shear zones mostly dip towards s se in a shallow shear zones1 to steep inclination shear zones2 the shear zones show typical features that can be explained by reactivation of pre existing joints and planar compositional heterogeneities palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening the kinematics of individual shear zones depends on the orientation of the original heterogeneities acting as nucleation planes and by partitioning of strain components at the kilometre scale with concentration of the flattening component to the piantonetto low strain domain the strain geometry and the kinematics of individual shear zones within piantonetto are not directly connected to the top to west sense of tectonic transport observed elsewhere in the gran paradiso nappe however the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the gran paradiso massif we conclude that the shear zone network of the piantonetto valley is representative of the incipient stages of ductile deformation of a granite nappe even if its architecture is determined by the arrangement of pre existing structural and compositional heterogeneities aspects of the large scale bulk strain can be derived from this local shear zone pattern

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

grain size reduction and foliation development in a deformed granitoid batholith abstract the predominantly peraluminous wologorong batholith in eastern new south wales australia has undergone heterogeneous deformation it always has at least one gneissic foliation but in strongly deformed zones it has two foliations one of these is a gneissic foliation s 1 consisting of relatively discontinuous fine grained micaceous and feldspathic folia anastomosing between feldspar relics and elongate aggregates of recrystallized quartz the other foliation s 2 is more continuous than s 1 and the folia are fine grained and commonly laminated as in mylonites the two foliations could have formed siimultaneously or sequentially during a single deformation s 2 formed oblique up to 30 to s 1 in order to accommodate an imposed shear component of the strain in strongly deformed parts of the batholith s 2 folia appear to have initiated on suitably oriented parts of anastomosing s 1 folia continued deformation locally reduced the angle between s 1 and s 2 and resulted in the development of mylonite zones grain size reduction by recrystallization and or neocrystallization occurred in all minerals during the deformation but quartz appears to have attained a steady state grain size of around 0 15 mm which was not reduced during more intense deformation deformation mechanisms are difficult to determine but microfracturing associated with offsetting and microboudinage intragranular slip and solution transfer of material may have operated in the earlier stages of deformation and grain boundary sliding and or solution transfer of material may have dominated the later stages the material transfer may have been associated with chemical and mineralogical changes that accompanied neocrystallization

local variations in quartz c axis orientations in non coaxial regimes and their significance for the mechanics of s c fabrics abstract the quartz c axis orientations of quartz bearing s c mylonites vary on the mm scale two specimens from different tectonic settings the variscan schwarzwald west germany and the eastern alps austria are investigated in the first specimen prism glide along a predominates and in the second specimen basal glide along a predominates in both specimens the pattern of c axis variations is similar it is suggested to be characteristic of s c mylonites on the local scale there are two patterns of preferred orientation 1 a crossed girdle type i which is inclined in a sense opposite to the inferred overall sense of shear this occurs in moderately strained domains 2 a single girdle which is slightly inclined in the same sense as the inferred overall sense of shear this occurs in domains that suffered higher strain both types are considered to reflect stable orientations of the easy glide direction crystallographic a direction at different stages of the deformational history two kinematic interpretations are discussed quartz c axis fabric variations reflect a spatial and possibly also temporal partitioning in flow path between domains that underwent either 1 spinning coaxial flow and non spinning non coaxial flow or 2 spinning non coaxial flow and non spinning non coaxial flow associated with variations in parameters of the deformation such as magnitude of strain and strain rate and perhaps also rate of recrystallization and recovery in either interpretation in moderately strained domains oblique c axis girdles are generated that cannot be used as shear sense indicators

planet wide sand motion on mars prior to mars reconnaissance orbiter data images of mars showed no direct evidence for dune and ripple motion this was consistent with climate models and lander measurements indicating that winds of sufficient intensity to mobilize sand were rare in the low density atmosphere we show that many sand ripples and dunes across mars exhibit movement of as much as a few meters per year demonstrating that martian sand migrates under current conditions in diverse areas of the planet most motion is probably driven by wind gusts that are not resolved in global circulation models a past climate with a thicker atmosphere is only required to move large ripples that contain coarse grains

measurement and interpretation of strain in the syntectonic solis de mataojo granitic complex uruguay the neoproterozoic solis de mataojo granitic complex is an intrusive body elongate north south emplaced in the sarandi del yi arroyo solis grande shear zone in the present work a quantification of the magmatic strain has been made by analysis of fabric enclave geometry and minor structures structural evidence indicates that deformation began in the magmatic state and continued through the sub solidus stage the observed distribution of magmatic foliations and lineations the fabric the geometry of the enclaves and the late to post magmatic structures are related to a non coaxial flattening regime that took place after the emplacement and late in the magmatic history these structures reveal that magmatic strain involved a shortening of the order of 80 with a simple shear component between 3 and 4 5 the solid state deformation records an additional mean of 2 7

high temperature deformation in the neoproterozoic transpressional ribeira belt southeast brazil abstract the neoproterozoic ribeira belt is subdivided in two domains with contrasting tectonic characteristics the northern domain is dominated by shallowly dipping foliations and orogen normal thrust tectonics the southern domain is characterized by a 1000 km long network of anastomosing transcurrent shear zones parallel to the belt this contrast is interpreted as reflecting continent continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain the central transitional domain of the ribeira belt displays the northern termination of the transcurrent shear zone network the alem paraiba padua shear zone system appss the 250 km long alem paraiba padua system involves granulites facies mylonites deformed through transpression a detailed study of the microstructure and lattice preferred orientation lpo of the rock forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust plagioclase crystals are plastically deformed they display curved twins and cleavages mechanical twins and evidence of dynamic recrystallization lpo of plagioclase is consistent with activation of the 010 100 and 010 001 slip systems lpo of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the 100 001 slip system quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration the lpo of quartz is therefore the result of a static transformation of an initial syn kinematic lpo and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

dynamic recrystallization in deformed plagioclase during progressive shear deformation grain size reduction of plagioclase accompanied with progressive ductile deformation was investigated in a centimeter scale small shear zone collected from the hatagawa shear zone northeast japan the angle between the foliation and the shear zone boundary decreases from the margin to the center of the shear zone indicating an increase of shear strain toward the center the volume fraction of fine plagioclase grains xpl diameter 15 m increases with progressive shear strain the value of xpl increases steeply from 30 to 60 between the angles 23 and 20 and then it gradually increases from 60 to 90 toward 0 the fine plagioclase grains have a variable dislocation density and a crystallographic preferred orientation suggesting that recrystallization accommodated dislocation creep transmission electron microscopy tem observations revealed that strain induced grain boundary bulging occurred along the grain boundaries of plagioclase porphyroclasts thus the grain size of plagioclase was reduced by strain induced grain boundary bulging the kinetic theory of dynamic recrystallization approximately predicts the formation of the above mentioned xpl curve and suggests that the inflection point of xpl at 23 20 is probably due to site saturation in which all of the original grain boundaries are covered by the recrystallized grains the kinetic theory of dynamic recrystallization also gives a physical basis for the classification of mylonite on the basis of the volume fraction of fine grains

morphologic diversity of martian ripples implications for large ripple formation large ripples with meter scale wavelengths are ubiquitous across mars curiosity s traverse of the bagnold dune field revealed a morphologic diversity of large martian ripples that helps constrain their formative mechanism large ripples develop in isolated fields and on dunes they form transversely and obliquely to longitudinally to the net sand flux direction in unimodally and bimodally distributed very fine to very coarse sand they have either straight or sinuous crestlines inactive ripples are covered with dust whereas migrating ripples are dust free here we present a unifying view of ripples that form in near bed sediment transport conditions encompassing fluid drag and coarse grained ripples to explain the range of large martian ripple morphologies and expand the use of bedforms as environmental indicators

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

high temperature microstructures and rheology of deformed granite erzgebirge bohemian massif abstract three stages of granite deformation were recognized in porphyric granitoids sheared under amphibolite facies conditions in the erzgebirge mts 1 weakly deformed granite in lozenge shaped pods 2 s c orthogneiss and 3 thin zones of banded mylonites weakly deformed granite forms a load bearing framework lbf structure in which quartz and feldspar aggregates exhibit similar strain intensities and shapes k feldspar deforms by fracturing and onset of dynamic recrystallization along clast margins plagioclase recrystallizes completely and quartz shows effects of grain boundary migration recrystallization and activity of basal a slip s c orthogneiss is interpreted as an interconnected weak layer iwl structure with high viscosity contrast between quartz and both feldspars concentration of deformation into thin quartz layers is indicated by intense strain of quartz grain size variations of recrystallized quartz and dominant prism a slip feldspars show well annealed structures and slip on 010 001 banded mylonite is marked by similar strain intensity of quartz and feldspar aggregates and low viscosity contrast between constituent phases quartz shows evidence for grain size increase and rhomb a c slip fully recrystallized feldspars show evidence for 010 100 slip the whole microstructural sequence and the transitions from lbf structure to iwl structures show the dependence of microstructural evolution of different materials on the bulk strain intensity

study on finite strain and its relationship with magnetic fabric along ailao shan red river shear zone 1 the finite strain along ailao shan red river shear zone has been studied the axial ratios of strain markers and shear strains vary greatly arising principally from deformation intensity and from contrasting competencies ellipticity of chloritic amygdales in basalt ranges from 6 79 to 10 47 corresponding shear strain gamma xz 2 22 2 93 and ellipticity of garnets from 1 56 to 2 95 gamma xz 0 45 1 14 in core gneiss the s c angle of the sheared rocks varies from 2degrees to 28degrees gamma sc 1 35 28 60 mostly in a range 2degrees to 7degrees optical observation thermal demagnetization and hysteresis curves were conducted to determine the nature of minerals and the contribution of paramagnetic and ferromagnetic minerals to magnetic susceptibility the result shows that ferromagnetic minerals and hornblende dominate the magnetic fabric of samples with high to very high susceptibility and play an important role in the magnetic fabric of samples with low to intermediate susceptibility the ferromagnetic minerals are mainly pseudo single domain and multidomain ti poor magnetite most samples have their magnetic lineation lying between the c and s surfaces in the remaining samples deviations from this common alignment principally resulted from the magnetic minerals being deflected by porphyroclasts mineral cleavage and cracks the axial ratio correlation is very poor between the magnetic and actual or imaginary strain ellipsoid calculated from the s c fabric this poor correlation suggests that the shape of the finite strain ellipsoid cannot be evaluated using the magnetic fabric when the latter is a combined result from multiple minerals and especially when the shape effect of multidomain titanomagnetite is important

peraluminous grenvillian ttg in the sierra de pie de palo western sierras pampeanas argentina petrology geochronology geochemistry and petrogenetic implications abstract combined petrological geochemical isotopic and geochronological data shed light on the origin and evolution of a peraluminous garnet bearing two mica granitoid el tigre granitoid etg cropping out in southwestern sierra de pie de palo 31 31 30 s 68 15 12 w and to constrain the age and petrogenetic conditions of this intrusive event etg experienced amphibolite to greenschist facies metamorphism after igneous crystallization followed by strong deformation restricted to narrow mylonite zones etg shear zone and partial dynamic recrystallization under lower t conditions a dextral shear sense is compatible with kinematic observations registered along the nne striking regional las pirquitas overthrust active at 473 10 ma k ar on 1 1 normative corundum low cao values between 1 72 and 2 41 plotting mostly in the granite trondhjemite fields of the ab an or diagram the trace element contents show a relatively low abundance of rb hfs elements such as y nb ta ga and zr and high concentrations of ba sr and lree the chondrite normalized ree pattern has a high slope with la yb n 9 48 55 32 and a negative or absent europium anomaly relationships between trace elements suggest the classical setting of granitoids produced in a convergent plate setting a u pb shrimp crystallization age on zoned igneous zircon of 1105 5 4 1 ma suggests that the etg could be part of the magmatic complex forming the grenvillian basement of the western sierras pampeanas 87 sr 86 sr 1105 values of 0 70543 and nd of 4 2 indicate a rather immature source for its origin with similar initial isotopic ratios to those found in orthogneisses from elsewhere in the western sierras pampeanas on the other hand sm nd model ages t dm for the etg range from 1 20 to 1 39 ga geochemical and isotopic signatures of the etg could be explained by low pressure partial melting from a basaltic source under high geothermal gradient conditions this thermal anomaly could be associated with the 1 1 ga global period of enhanced mantle plume activity developing widespread global magmatism

the development of discrete shear zones in a granite stress strain and changes in deformation mechanisms abstract the development of millimetre scale continuous shear zones in the sidi ali bou nab algeria granite and their progressive transformation into discrete shear zones are presented increases in strain stress and strain rate magnitudes as well as changes of deformation mechanisms are suggested by an evolution of the microstructure across these narrow shear zones temperatures are assumed to be constant from boundaries to core the most striking aspects of this evolution are 1 1 a decrease in the angle between the foliation and the shear zone 2 2 a decrease in the recrystallized grain size of quartz together with the development of a crystallographic orientation characterized by a strong concentration of c axes around the y axis of the finite strain ellipsoid 3 3 the development of very fine grained feldspathic aggregates or layers probably through dynamic recrystallization of original large crystals and 4 4 the replacement of original biotite by muscovite and a new generation of biotite in the continuous shear zones the deformation is controlled by the behaviour of quartz this mineral undergoes extensive dynamic recrystallization which probably favours a higher strain rate and a change in the active slip system from a dominant basal one in the outermost part of the structures to a dominant prismatic one in its core paleopiezometry and strain computations show a shear strain up to 10 and an inferred steady state differential stress to 100 130 mpa respectively the high stress level is suspect because the estimates are derived from recrystallized grain size which may be sensitive to strain and variations of chemical environment discrete shear zones develop in relation with dynamic recrystallization of feldspars in the core of continuous shear zones dynamic recrystallization of feldspars produces very fine grained aggregates grain size of 6 7 m which coalesce and form a continuous layer at the central part of the shear zones this fabric may result from 1 1 a change in the driving flow mechanism from dislocation creep of quartz to dislocation creep or and grain boundary sliding of feldspar neoblasts and 2 2 a large strain softening that would favour strain concentration in the fine grained matrix and the development of sharp shear zone boundaries

age constraints on the tectonic evolution and provenance of the pie de palo complex cuyania composite terrane and the famatinian orogeny in the sierra de pie de palo san juan argentina abstract new u pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the pie de palo complex represent a mesoproterozoic fragment of suprasubduction zone oceanic crust a gabbroic pegmatite interpreted to have formed during arc rifting or subsequent back arc spreading yielded a u pb age of 1204 5 3 4 7 ma highly tectonized ultramafic mafic cumulates occurring at the structural base of the pie de palo complex and previously interpreted to represent remnants of a primitive arc phase prior to rifting and back arc spreading could not be dated but should be older than 1204 ma if these inferences are correct tabular sill like bodies of leucogabbro diorite and calc alkaline tonalite granodiorite sills yielded ages of 1174 43 and 1169 8 7 ma respectively they may represent a younger more evolved arc phase established after arc rifting or a younger tectonically unrelated mesoproterozoic arc shrimp analysis of metamorphic zircon rims with low th u ratios in vvl 110 gave a 206pb 238u age of 455 10 ma similar to lower intercept dates determined by discordia lines combined these data indicate that the bulk of the amphibolite facies metamorphism present in the pie de palo complex was generated during the famatinian orogeny analysis of six single detrital zircon grains in a metasedimentary quartzofeldspathic garnet mica schist tectonically interleaved with the igneous rocks of the pie de palo complex and tentatively correlated with the difunta correa metasedimentary sequence of other workers yielded three age populations 1150 1160 ma 1050 1080 ma and 665 ma indicating that these sedimentary rocks were deposited during the late neoproterozoic or early paleozoic in addition they confirm structural evidence that intercalation of rocks of the pie de palo complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications these ages are also consistent with a laurentian provenance and earlier interpretations that these rocks once represented a sedimentary cover to the pie de palo complex the zircon population of 1050 1080 ma could be derived from grenville age felsic plutons identified elsewhere in the pie de palo complex by other workers however no evidence has been found in our samples for a grenville age orogenic event invoked previously to explain accretion of the oceanic pie de palo complex to laurentia prior to the late neoproterozoic early cambrian rifting and drift of cuyania

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

stratal extension in thrust footwalls makran accretionary prism implications for thrust tectonics small scale stratal extension is commonly associated with thrust faults in the makran accretionary prism of southwest pakistan low angle normal faults dipping in the direction of transport relative to the thrust are common in gouge zones and duplexes and may also cut the footwall and hanging wall where they cause stratal extension these faults are probably riedel shears we have studied a broad zone of more complex extensional deformation beneath one bedding parallel thrust the zone disrupts earlier compressional structures and affects a significant volume of the footwall normal faults initiating in a rearward sequence branch off the thrust and cut down into the footwall merging at a lower stratigraphic level part of the thrust displacement was thereby transfe red to that level and fragments of the footwall were detached and transferred southward beneath the hanging wall the latter remained undeformed and accommodation structures are confined to the footwall these effects may have been caused by variations in sliding resistance along the fault if a thrust transfers part or all of its displacement down section along normal faults in this way before eventually cutting back up toward the surface it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon this process can be referred to as footwall plucking it can explain rootless basement slices along thrusts for example and it has important implications for section restoration techniques and the interpretation of thrust belts

a theoretical model for aeolian impact ripples new insights into the grain bed impact process arising from both numerical and physical experiments involving single grain impacts lead to a more complete conceptual model of the aeolian saltation process that in turn allows a simple model of aeolian impact ripples to be developed the saltating population may be idealized as consisting of 1 long trajectory high impact energy constant impact angle successive saltations and 2 short trajectory low impact energy reptations it is argued that the spatial variations in mass flux due to the reptating population lead to the growth and translation of impact ripples using the sediment continuity equation an expression for the spatial variation in the ejection rate of reptating grains from a sinusoidally perturbed bed and a probability distribution for the reptation lengths a simple stability analysis demonstrates that the flat bed is unstable to small amplitude perturbations a fastest growing wavelength emerges that is roughly six times the mean reptation length and is only weakly dependent upon the detailed shape of the probability distribution of reptation lengths the results match well with the observed initial wavelengths in wind tunnel experiments

stibnite microstructures as imprints of fault movement the natural deformation of stibnite in relation to the kinematic history of a regional fault is resolved using microstructural analysis of massive stibnite along the biards fault in the massif central france stibnite underwent intra crystalline deformation associated with a strong linear and planar anisotropy lepidoblastic textures undulatory extinction of oriented grains growth of antimony between these grains and a colour change of stibnite from white to black result from dynamic recrystallisation a range of asymmetric microstructures indicate non coaxial deformation and a left lateral sense of shear related to the movement of the fault zone the computed displacement corresponds to the geological offsets measured along the fault we conclude that stibnite has microstructurally recorded the bulk kinematics of its host fault zone

s c cataclasite in granitic rock abstract foliated cataclasites in granitic rocks along the rokko awaji fault zone japan display s c fabrics similar to those of mylonites the s c cataclasites have s surfaces defined by shape preferred orientation of biotite fragments and aggregates of quartz and feldspar fragments and c surfaces defined by microshears all quartz and feldspar fragments show mainly brittle deformational microstructures the biotite fragments however have some fabric characteristics like those observed in s c mylonites such as biotite fish cleavage steps bending and folding these differences in microfabric between biotite quartz and feldspar suggest that there are marked differences in their deformational behavior under the same deformational conditions biotite deformed by a combination of brittle plastic shearing processes and quartz and feldspar were brittlely damaged one of the most significant microstructural differences between s c cataclasites and s c mylonites is the absence of dynamically recrystallized grains in the s c cataclasites the microstructural characteristics and the geological evidence suggest that the s c cataclasites in the rokko awaji fault zone formed at temperatures between 150 and 250 c corresponding to depths of 5 8 km at a continental geothermal gradient of 30 c km before being uplifted and exposed this study of cataclastically deformed rocks shows that the fault zone rheology of the upper 5 10 km of crust is greatly influenced by deformational processes in s c cataclasites

granite emplacement during contemporary shortening and normal faulting structural and magnetic study of the veiga massif nw spain abstract the veiga massif belongs to the calc alkaline series of hercynian granitic rocks of the ibero armorican arc the veiga granodiorite intruded during the upper carboniferous into the core of the wnw ese n verging ollo de sapo antiform formed by precambrian and palaeozoic metasediments internal fabrics show that magma intrusion was contemporary with shortening measurements of feldspars orientations and anisotropy of magnetic susceptibility ams throughout the granite are consistent and indicate a foliation striking wnw ese parallel to folding with a constant dip of 75 85 n the zonation of bulk low field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders the difference in elevation between outcrops more than 600 m allows us to infer the three dimensional attitude of granite fabrics throughout the massif syn magmatic fabric folds are preserved in the inner part of the igneous body the highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion at the scale of the massif foliation is convergent toward the bottom of the intrusion along a line located at its northern border where the magma source is interpreted to be located in the western border of the massif the presence of c and s structures indicates that magma cooling was coeval with movement of the chandoiro fault a n s striking normal fault with a n290e hanging wall displacement direction these results indicate that emplacement of the veiga granite is coeval with nne ssw shortening and with an wnw ese extension direction parallel to the trend of the late folds

grain size segregation and stratigraphy in aeolian ripples modelled with a cellular automaton aeolian ripples are distinguished by coarse grained crests finegrained troughs and thin veneers of coarse grains on the upwind stoss slopes migration of these sorted bedforms during periods of net deposition results in a characteristic inversely graded stratigraphy that distinguishes aeolian sandstones from those of fluvial origin1 here we investigate the formation and evolution of aeolian ripples using a cellular automaton model of a sandbed which incorporates two grain sizes and is subject to the impacts of episodically hopping saltating grains using this model we identify the physical processes responsible for both the spatial sorting and the stratigraphic signature of the ripples high energy saltating grains are found to eject small grains preferentially from the impact site leading to a coarsening of the heavily bombarded stoss slopes coarse grains do not in general leap far enough to escape the shadow zones on the downwind lee slopes and therefore tend to accumulate and cycle around the ripple crests small grains on the other hand are ejected at higher velocities enabling them to hop further incorporating net deposition into the model results in a stratigraphy closely resembling thin planar laminae pinstriping found in aeolian sandstones the inverse grading of grain sizes is a direct consequence of their different hop lengths

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations

early paleozoic accretionary orogenies in nw argentina growth of west gondwana abstract two early paleozoic orogenic cycles in the gondwana margin of nw argentina were driven by subduction and interrupted by collision of continental ribbons of laurentian affinity subduction started at the passive margin of gondwana possibly in response to the end of amalgamation of the supercontinent the passive margin was underlain by a hyper extended continent ocean transition and low angle subduction gave rise to a wide and hot fore arc region during the pampean orogenic cycle the arrival of the first continent ribbon ended the pampean cycle steepened the subduction further heating the fore arc and restarting arc magmatism after a magmatic lull 250 300 km trenchwards initiating the famatinian orogenic cycle this cycle started with a 30 ma long period of extension and marine sedimentation followed by the arrival of the second continent ribbon and inversion the back arc initiating a 20 ma long shortening event culminating with the shutting down of the arc this event gave rise to a 300 km wide low topography hot orogeny thus the many peculiarities of the two paleozoic orogens of nw argentina result from a subduction history that efficiently transferred heat to a 300 400 km wide belt of turbidite dominated sediments that was first part of the pampean fore arc and then part of the famatinian retroarc developed at the extended continental margin of gondwana the two orogenies with continued high heat flux lasted 110 ma giving rise to two calc alkaline arcs separated by a 300 km belt of high t low p migmatites and peraluminous granites formed by anatexis of sediments deposited on the passive margin as well as those deposited in the wide pampean fore arc these turbidite dominated sequences were metamorphosed and melted to form a continental crystalline basement and accreted to the cratonic margin adding 500 km of crystalline rocks to the margin of gondwana at the same time that a similar process was happening in eastern australia along the same continental margin interestingly this wide accretionary orogen has now become the region where the andean system developed a wide orogeny above flat slab subduction

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization