zircon a possible indicator of mass transfer in deformation zones a solution for the roffna gneiss controversy suretta nappe switzerland abstract this paper proposes a simple volume mass balancing technique as a means to assess the volume loss and tectonic flattening of mylonitic shear zones the method involves counting the number of zircon crystals in oriented polished thin sections analyzed subsequently in the electron microprobe using the bsei mode and a spectrometer setup for detection of the element zr here we present an application to the roffna gneiss eastern switzerland a porphyritic granite of permian origin which has been metamorphosed and transformed into a phengitic schist under low grade metamorphic conditions mass balances for this material have produced controversial interpretations in that a first analysis of chemical data of wall rock and mylonite suggested an isochoric mylonitic transition event while a dramatic volume loss of some 50 in the course this process was inferred after a re evaluation of the same data electron microscopy of zircon crystals that were retrieved from rocks which had undergone different degrees of deformation revealed only minor alterations and mechanical damage of single grains this stability of zircon justified the use of single crystals and in consequence of the element zr as passive markers in determining volume mass changes during deformation results obtained by this method allow us to confirm that the deformation of the roffna gneiss involved a volume loss of 29 6 accompanied by chemical alterations deformation induced mobility changes of various elements lead to small gains in mg and cr but significant losses si al na ca and sr during the metamorphic event all other elements including the ree underwent no substantial long range transport

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

measuring the sea ice floe size distribution sea ice is broken into floes whose diameters range from meters to a hundred kilometers this fragmentation affects the resistance of the ice cover to deformation and the melting at floe sidewalls in summer floes are broken by waves and swell near the ice edge and throughout the pack by isostatic imbalances thermal cracking winds and currents in winter they are welded together by freezing floe size can be measured by several properties p for instance area or mean caliper diameter two definitions of floe size distribution seem useful f p the fraction of area covered by floes no smaller than p and n p the number of floes per unit area no smaller than p f p can be measured by sets other than areas such as the fraction of a line or of a point set covered by floes no smaller than p if n behaves like for small where is mean caliper diameter must be greater than 2 so that the small floes occupy finite area if 2 1 the perimeter of small floes is infinite several summertime distributions have been measured on a log log graph their slopes local values of range from 1 7 to 2 5 one distribution follows a power law the others have steeper slopes for larger floes and more gradual slopes for smaller floes another sampling strategy is to measure the lengths of line segments on floes the distribution of these chord lengths is equivalent to the distribution of floe diameters the variance of an estimate of the fraction g of area covered by floes in any size range is g 1 g k 1 where k is the equivalent number of independent samples k can be found from the autocovariance of the indicator function for the chosen size range for line sampling of a narrow range of floe diameters k is the ratio of the sample length to the floe diameter

shear zone patterns and strain distribution at the scale of a penninic nappe the suretta nappe eastern swiss alps abstract shear zone patterns within the suretta nappe of the swiss alps are studied in order to understand finite strain and bulk tectonic displacements the suretta nappe is chosen for two main reasons the availability of late variscan granites deformed during alpine tectonics only and because of continuous outcrop from bottom to top of the nappe this alpine nappe recorded two major ductile deformation phases which developed under low grade metamorphic conditions the analysis of shear zone patterns in the roffna granite reveals that the first deformation is well developed in the entire nappe the second deformation however is more localized in the lower and upper parts the bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other penninic nappes in this area first a stacking of the nappe towards the nw followed by the top to the e shearing during post thickening extensional tectonics the shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale

brittle plastic deformation in mylonites an example from the meadow fork thrust western blue ridge province southern appalachians recent conceptual models for fault related rocks imply that mylonites with internal shear surfaces may exhibit frictional characteristics and that they potentially can form within the seismogenic zone this paper describes a lower greenschist facies quartz rich mylonite zone from the western blue ridge province in the southern appalachians that displays evidence for episodic deformation by both crystal plastic and brittle mechanisms strain was largely accommodated by quartz whereas feldspar behaved rigidly and passively plastically deformed monocrystalline quartz ribbons and the assemblage chlorite quartz muscovite are consistent with temperatures of deformation between 300 and 400 c and together with fluid inclusion measurements indicate that fluid pressures fluctuated between 90 and 300 mpa if one assumes that the maximum pressure represents lithostatic conditions this corresponds to a depth of about 12 km quartz c axis fabrics display both symmetrical and asymmetrical patterns with respect to the foliation and indicate that deformation occurred under coaxial and noncoaxial conditions healed microcracks and extensional quartz veins that are cut by quartz ribbons indicate that brittle and crystal plastic deformation occurred simultaneously or cyclically by analogy with experimental studies internal zones of localized shear in the mylonites may represent frictional behavior these results are discussed within the context of recently developed conceptual continental fault zone models

feldspar influenced rock rheologies progressive fracture subsequent cataclasis and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz mica phyllonite during early tertiary deformation in a 20 30 m wide shear zone in southeastern arizona the ductile deformation proceeded at temperatures and pressures approximating middle to upper crustal levels most of the deformation was taken up by the feldspar and its alteration products quartz deformed by both brittle fracture and crystal plastic mechanisms these results show that under certain temperature pressure strain rate conditions feldspars are weaker than quartz the observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature activated plastic deformation in quartz nor of the frictional behavior of rocks as inferred in the familiar strength vs depth curves it is more likely that the transition is complex and depends on a variety of parameters one such factor delineated in this study is the fracture and fracture facilitated syntectonic alteration of feldspar under hydrated conditions

stability of zircon in a low grade ultramylonite and its utility for chemical mass balancing the shear zone at mieville switzerland at mieville switzerland granitic basement rocks of the aiguilles rouges massif are cut by a subvertical shear zone the variscan shearing event produced a low grade ultramylonite zone which is up to 50 m wide and several kilometers long investigations of accessory zircons from the undeformed wall rock and from the most highly deformed ultramylonite show only minor alterations and mechanical damage of zircon crystals even in an extreme state of deformation this outstanding stability of zircon allows one to consider the element zr as immobile and therefore to use it as a passive marker for calculations of mass and volume changes during deformation processes application of this method confirms that the deformation at mieville proceeded under simple shear constant volume and isochemical conditions this is in agreement with the results of classical geochemical mass balancing in addition uncertainties inherent in the geochemical approach can be minimized by the new method

rare earth element mobility in the roffna gneiss switzerland the roffna gneiss a deformed hercynian granite porphyry within the penninic nappes of eastern switzerland underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies this resulted in the selective mobilization of major and trace elements over distances of 10 s to 100 s of meters and the resetting of the rb sr whole rock isotopic systems some 100 my ago the component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric the mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline albite and biotite and the formation of phengite and quartz the fluids introduced mg and h2o promoting the development of phengite and removed the na being released by the breakdown of albite the fluids were most probably derived from the surrounding triassic carbonates and quartzites these relatively high fo2 and carbonate rich fluids also introduced rare earth elements ree into the gneiss the gneiss was progressively enriched in eu up to 60 y up to 40 and yb up to 100 these enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite rich sample while the ree were mobile uranium and thorium were essentially immobile the formation of xenotime was suggested to explain the observed heavy ree enrichment when large differences in the ree contents were found for replicate analyses using hf and then lithium metaborate for dissolution these differences arose because xenotime like monazite can be difficult if not impossible to dissolve in hydrofluoric acid due to the possibility of incomplete sample dissolution we now recommend fusion with lithium metaborate for all ree lu hf or sm nd studies

evaluation of high resolution sea ice models on the basis of statistical and scaling properties of arctic sea ice drift and deformation sea ice drift and deformation from models are evaluated on the basis of statistical and scaling properties these properties are derived from two observation data sets the radarsat geophysical processor system rgps and buoy trajectories from the international arctic buoy program iabp two simulations obtained with the louvain la neuve ice model lim coupled to a high resolution ocean model and a simulation obtained with the los alamos sea ice model cice were analyzed model ice drift compares well with observations in terms of large scale velocity field and distributions of velocity fluctuations although a significant bias on the mean ice speed is noted on the other hand the statistical properties of ice deformation are not well simulated by the models 1 the distributions of strain rates are incorrect rgps distributions of strain rates are power law tailed i e exhibit wild randomness whereas models distributions remain in the gaussian attraction basin i e exhibit mild randomness 2 the models are unable to reproduce the spatial and temporal correlations of the deformation fields in the observations ice deformation follows spatial and temporal scaling laws that express the heterogeneity and the intermittency of deformation these relations do not appear in simulated ice deformation mean deformation in models is almost scale independent the statistical properties of ice deformation are a signature of the ice mechanical behavior the present work therefore suggests that the mechanical framework currently used by models is inappropriate a different modeling framework based on elastic interactions could improve the representation of the statistical and scaling properties of ice deformation

two subduction events in a polycyclic basement alpine and pre alpine high pressure metamorphism in the suretta nappe swiss eastern alps this study is essentially based on coupling macrostructures microstructures and metamorphic petrology in polymetamorphic mafic rocks from the swiss eastern alps suretta nappe penninic domain petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization deformation relationships and to establish a relative but precise sequence of tectono metamorphic events a first eclogite facies overprint and related exhumation occurred before emplacement of late palaeozoic intrusives during the alpine cycle the suretta nappe was part of the thinned european continental margin the tertiary burial due to subduction and collision is responsible for d1 ductile thrusting and blueschist facies metamorphism late deformation phases related to exhumation are responsible for the development of extensional structures under greenschist facies conditions quantitative metamorphic petrology based on gibbs free energy minimization domino by de capitani gives a constraint on the p t conditions during the polymetamorphic and polycyclic evolution the first high p metamorphic event related to pre alpine structures occurred at c 700 c and at least 2 0 gpa these conditions are compatible with pre alpine high p re equilibration already described in several alpine units the alpine high p metamorphism occurred under blueschist facies conditions at c 400 450 c and 1 0 gpa similar high p low t conditions have already been described in the mesozoic and permian rock types the two high p events are clearly related to two different geothermal regimes and geodynamic environments

strain and shape fabric variations associated with ductile shear zones abstract the foliated and compositionally banded granitic orthogneisses in the central core of the maggia nappe a lower pennine basement nappe of the central swiss alps are shown to be the sheared equivalent of late hercynian age granitic intrusions these ductile shear zones show mineral assemblages in amphibolite facies are alpine in age and form an anastomosing network enclosing remnant lozenge shaped pods of relatively undeformed rock the foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional through tear drop shapes to ribbon like aggregates these shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation induced planar surfaces

strain accumulation during basal accretion in continental collision a case study from the suretta nappe eastern swiss alps abstract stacked crystalline basement nappes are a typical feature in the core of many orogens the suretta nappe exposed in eastern switzerland consists of briancon derived crustal slices which were assembled in a south dipping subduction zone during the alpine orogenic cycle the nappe contains post variscan rocks of the rofna porphyry complex rpc and permo mesozoic cover sequences which are ideal to study the strain evolution during alpine nappe formation we present new structural cross sections finite strain analyses and a retrodeformation for the frontal part of the suretta nappe the overall geometry of the suretta nappe is the result of two main deformation phases 1 eocene top to the nnw directed thrusting and folding ferrera phase which is overprinted by 2 backfolding and backshearing niemet beverin phase we suspect that this backshearing was caused by the buoyant rise of light continental crust beneath the suretta nappe and by frictional resistance at the top contact of the suretta nappe which had been accreted to the upper plate in the course of nappe stacking in the lower and interior parts of the suretta nappe weakly to undeformed boudins are generally surrounded by l tectonites indicating wsw ene stretching foliated equivalents reveal a plane strain deformation state the upper part of the suretta nappe which was strongly affected by backshearing shows flattening strain strain data were used to reconstruct the palinspastic evolution of the suretta nappe at two distinct time slots prior to niemet beverin backfolding the suretta nappe is characterized by thrust faults and large scale detachment folds before the onset of ferrera phase nappe stacking jurassic normal faults and the rpc intrusion shape trigger the localization of subsequent deformation our structural study confirms a complex polyphase evolution for the suretta nappe which might be characteristic for crystalline basement nappes in continental collision zones elsewhere

marine oil source of the yingmaili oilfield in the tarim basin abstract although a large amount of petroleum has been discovered in the tarim basin the origin of the oils is still disputed yingmaili oilfield was selected as a case study to further examine hydrocarbon generation and accumulation in the basin detailed geochemical studies were conducted to unravel the formation mechanisms the oils in the field correlate well with middle upper ordovician o 2 3 rocks or similar rocks oil oil and oil source rock correlation by maturity parameters of saturate and aromatic hydrocarbons suggest that the yingmaili and adjacent oils are primarily comparable to source rocks deeper than 6000 m the results also showed that carbonates might be important potential source rocks as indicated by a relatively high abundance of aromatic sulfur compounds and relatively high c 29 c 30 hopane and c 35 c 34 hopane ratios of the oils which are typical features of carbonate source rocks we suggest the oil from wells yg2 and yg2 1c are more suitable to be end members from the o 2 3 source rocks or similar rocks than the oil from well ym2 as previously suggested we also suggest that the previous model of hydrocarbon migration along the ordovician unconformity should be revised

systematics of internal zircon morphology in major variscan granitoid types the internal morphologies of zircon crystals from different types of granitoids alkaline calcalkaline and anatectic are revealed by cathodoluminescence imaging and are described in terms of growth rates of the crystal faces relative to each other zircons in the alkaline granitoids are characterized by high and constant growth rates of 010 relative to the pyramidal forms and by symmetric grwoth of 011 zircons in the calcalkaline and anatectic granitoids are characterized by fluctuating or gradually decreasing relative growth rates of 010 by asymmetric and highly variable growth of 011 and by a tendency of 110 to become grwoth inhibited corrosion events are interspersed during zircon growth in the calcalkaline magmas in the calcalkaline and anatectic magmas a discontinuity breaks the morphological evolution at late stages of crystallization the discontinuity coincides with a sharp drop in cathodoluminescence the growth behaviour of each crystal form is analysed and compared with predictions made by the periodic bond chain pbc theory it is argued that the relative growth rate of 010 depends on supersaturation that the growth rates of 011 faces are changed in response to different ratios of adsorbing cations na k al and that 110 faces become growth inhibited by the adsorption of h2o or trace elements enriched in the residual liquid morphological and chemical discontinuities at late stages of crystallization are reasonably explained by the formation of larger growth units from smaller ionic entities in the residual liquid important factors controlling the zircon morphology in different types of granitoids are high cooling rates alkaline magmas magma mixing calcalkaline magmas enrichment of h2o and trace elements in residual liquids calcalkaline and anatectic magmas and the major element chemistry of the magma possibly the ratio of na and k to al agpaicity

evidence for limited ree leaching from the roffna gneiss switzerland a discussion of the paper by vocke et al 1987 cmp95 145 154

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

effective rheology across the fragmentation transition for sea ice and ice shelves funding was provided by the nerc grant ne p011365 1 calving laws for ice sheet models calismo data files for the plots are found at https doi org 10 5285 76d7d3ca 7b83 4bb0 aae5 a8e92c7da5b0

a damage mechanics assessment of the larsen b ice shelf prior to collapse toward a physically based calving law calving is a primary process of mass ablation for glaciers and ice sheets though it still eludes a general physical law here we propose a calving framework based on continuum damage mechanics coupled with the equations of viscous deformation of glacier ice we introduce a scalar damage variable that quantifies the loss of load bearing surface area due to fractures and that feeds back with ice viscosity to represent fracture induced softening the calving law is a standard failure criterion for viscous damaging materials and represents a macroscopic brittle instability quantified by a critical or threshold damage we constrain this threshold using the ice sheet system model issm by inverting for damage on the larsen b ice shelf prior to its 2002 collapse by analyzing the damage distribution in areas that subsequently calved we conclude that calving occurs after fractures have reduced the load bearing capacity of the ice by 60 10 2012 american geophysical union all rights reserved

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

volume loss fluid flow and coaxial versus noncoaxial deformation in retrograde amphibolite facies shear zones northern malawi east central africa two retrograde amphibolite facies shear zones were studied to explore the relationship between retrograde mineral reactions volume strain fluid flow mylonitization and coaxial versus noncoaxial deformation the two shear zones are the contractional mafwewu hills shear zone and the transcurrently displacing mkamasa river shear zone of northern malawi in general shear zone formation is characterized by the breakdown of feldspar and biotite and the formation of sillimanite quartz and water silica alkali and alkali earth elements were mobile mass balance calculations based on major and trace element geochemistry indicate as much as 50 60 volume loss in mylonite fluid to rock ratios estimated from the calculated depletions of silica are as much as 200 400 indicating that the initiation and activity of the shear zones were accompanied by large amounts of fluids that infiltrated the shear zones these fluids caused an almost complete loss of alkalies within the shear zone leading to the development of almost dry and sheet silicate free mylonite it is proposed that the associated destabilization of micas destroyed the pathways for fluid circulation and ultimately caused the cessation of shearing in the mafwewu hills shear zone shear zone related stretching lineations are oriented parallel to tectonic transport in the mkamasa river shear zone the stretching lineations curve from a subhorizontal attitude in the wall rock into a subvertical attitude in mylonite characterized by large volume loss shear bands in the mkamasa river shear zone indicate a sinistral sense of shear a synoptic analysis of mesoscale s and z asymmetric folds indicates that the orientation of the stretching lineations in mylonite characterized by large volume loss is subperpendicular to the regional tectonic transport direction aspect ratios of feldspar and quartz in xz sections x y z principal axes of finite strain ellipsoid show oblate grain shapes and show a positive correlation with volume strain mean kinematic vorticity numbers are 0 26 0 44 in the wall rock and 0 57 0 79 in mylonite in mylonite small kinematic vorticity numbers correlate with large volume loss the transport perpendicular orientation of the stretching lineations in strongly volume deficient mylonite is thought to be the result of a pronounced component of shortening normal to the shear zone the resulting higher degree of coaxial deformation controlled the final orientation of the stretching lineations

physical and chemical response of zircons to deformation an investigation of u pb isotopic systematics in zircons from mylonitized henderson gneiss sinha and glover 1978 revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic pb at 460 m y to further investigate the relationship between pb loss u gain and grain size reduction associated with increasing strain in the shear zone we have characterized the chemistry and morphology of zircons in the mylonitic rocks using both electron microprobe analysis and scanning electron microscopy