a new indicator of movement direction during orogenesis measurement technique and application to the alps the orientation of axes of sigmoidal staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism a simple technique is presented in detail to find this axis in 3d by radially sectioning a horizontal slab cut from an oriented sample when viewed from one direction the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis this technique is independent of the model adopted for the formation of sigmoidal or spiral shaped inclusion trails and can be used to evaluate the mechanism by which they form it can also be used to evaluate theories of folding and orogenesis measurements of spiral and sigmoid axes in garnet porphyroblasts from the european alps show that they reflect the movement of the african plate relative to europe better than linear indicators preserved within the same rocks

spherulites in calcrete laminar crusts biogenic caco 3 precipitation as a major contributor to crust formation abstract spherulites calcitic fibro radial spherulitic polycrystals are a major component of calcite layers in quaternary calcrete laminar crusts to explain the formation of spherulites petrographic and chemical studies were performed on pleistocene calcrete laminar crusts holocene biological crusts and laboratory in vitro cultures of cyanobacterial strains spherulites were found to be formed of acicular and radiating crystals either smooth and regular edged or twisted and irregular edged spherulites were composed of low mg calcite the concentration of mg in the spherulites increasing from nucleus to periphery the shape and chemistry of the in vitro spherulites and laminar crust spherulites were identical the structure elementary crystal shape and size of spherul tes can be used to differentiate them from microcodium with which they are commonly confused in the literature spherulite genesis can be linked to cyanobacterial activity involving photosynthetic uptake of hco 3 leading to caco3 precipitation the bicarbonate causing release of equivalent amounts of oh in the mucilaginous sheath the main steps in cyanobacteria biomineralization producing spherulites are 1 uptake of co2 and or hco 3 from the medium as the inorganic carbon source for photosynthesis 2 release of oh in the sheath 3 carbonate ion formation resulting from the reaction of oh and hco 3 followed by 4 caco3 precipitation this process involves two steps in desert environments during dry spells the cyanobacterial mat calcifies leading to spherulite production and the fo mation of thin clear microsparitic layers during wet periods a thin sedimentary deposit forms mixing micrite detrital particles and inherited or new tiny spherulites with the mucilaginous mat of cyanobacteria which traps all of this material an organo micritic layer forms during the following dry spell the process is repeated and the laminar crust is formed in conclusion the presence of spherulites in the laminar crust is indicative of subaerial exposure the crust being a terrestrial stromatolite

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

texture of microbial sediments revealed by cryo scanning electron microscopy abstract textures of modern lacustrine stromatolites on kiritimati line islands central pacific ocean and of buried layers in the stromatolitic carbonate sediments from french polynesian atoll lakes kopara have been studied using cryo scanning electron microscopy sem equipped with a freeze drying sample preparation device this study confirms that microscopic three dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria and from other extracellular polymer secretions are common components of microbial sediments of which they may form the framework in addition to this role in sediment cohesion and formation of microstructure the organic framework appears to be involved in carbonate precipitation with n the stromatolites through chemical nucleating steric and hydrodynamical controls the role of the dead organic constituents of the stromatolites is not only confirmed in micrometer size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments and to the formation of peloids that evolve into spherulites bacterial including nannobacterial carbonate bodies and carbonate impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied all these carbonate precipitation processes may cooperate in lithification of a given sediment

visual aspects of the flanking line in severe thunderstorms abstract photographs are presented that illustrate the various forms in which flanking lines exist in nature flanking lines may appear not only as the commonly observed sloping line of cumulus congestus but also as erect towers with a vertical face and as a line of altocumulus castellanus above and parallel to a band of stratocumulus lenticularis it is suggested that the slope of the tops of the flanking line towers with respect to the ground is related to a quantity that is similar in form to the bulk richardson number and that the orientation of the flanking line is a function of the mean wind in the lowest 6 km and storm motion these hypotheses need to be verified when high resolution rapid scan visible and infrared satellite imagery become available on a daily basis

porphyroblast inclusion trail geometries in the grand canyon evidence for non rotation and rotation abstract porphyroblasts in paleoproterozoic supracrustal rocks of the grand canyon grew during a progressive deformation that involved shortening and heterogeneous development of ne striking subvertical foliation s2 that crenulates and transposes an earlier foliation s1 inclusion trails in syn s2 porphyroblast cores show a remarkably consistent nw strike across the regional transect suggesting a lack of appreciable porphyroblast rotation relative to the trace of the ne striking s2 foliation these apparently non rotated porphyroblast cores thus preserve a regional nw striking s1 orientation porphyroblast rims and younger porphyroblasts locally overgrow ne striking s2 indicating that porphyroblast growth and continued non rotation spanned s2 apparent non rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening in contrast single thin sections show domains of these non rotated garnets with sigmoidal inclusion trails adjacent to domains where the characteristic sigmoids are rotated by about 45 this documents local rotation of garnets relative to s2 during a heterogeneous late or post s2 reactivation of the foliation thus these types of studies should continue to look for both rotational and non rotational behaviors at all scales and continue to specify the frame of reference

recycling of foliations during folding bedding on the limbs of early formed regional folds generally lies oblique to the major directions of bulk shortening in the crust that is the horizontal and vertical during subsequent deformations matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering consequently the schistosity parallel to bedding s0 s1 in multiply deformed rocks contains the relics of many deformation events and the 2 or 3 oblique foliations record only the very youngest history this lengthy early history is preserved as inclusion trails within porphyroblasts recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to s0 s1 for some orientations and combinations of shear senses both limbs of a pre existing fold can be reactivated from the commencement of a new deformation event this can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development for other combinations one limb of a pre existing fold will behave in this manner whereas a new foliation does develop fully on the other limb however subsequent phases of deformation switch which limb shears versus develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with s0 s1

prolonged acadian orogenesis revelations from foliation intersection axis fia controlled monazite dating of foliations in porphyroblasts and matrix numerous phases of garnet growth are revealed by detailed studies of foliation inflection intersection axes preserved in porphyroblasts fias in acadian metamorphic rocks in the chester dome region of vermont u s a a regionally consistent succession of four different fia tends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe these monazite inclusions which lie within the various foliations that define the fias provide absolute ages for multiple periods of deformation and episodic phases of garnet growth the monazite inclusions reveal a progression in foliation ages from 431 2 to 349 3 million years within porphyroblasts and from 366 3 to 327 5 million years in pervasive matrix foliations three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts through the medians to the rims and then into the matrix ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the fias the four fia sets began forming prior to 424 3 405 6 386 6 and 366 4 million years ago respectively thus multiple stages of garnet growth occurred throughout acadian deformation and metamorphism in vermont and orogenesis was far more prolonged than previously thought invariably garnet growth occurred early in the accompanying deformation event dating of monazite inclusions without careful separation of phases of garnet growth by fia studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history analysis of monazite grains in the matrix alone will likely only present ages for the youngest events these ages can be amalgamated from grain to grain to yield apparently precise ages but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis or the overall continuity of the deformation and metamorphism that accompanies plate motion fia and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed this in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories

marine carbonate cements biofilms biomineralization and skeletogeneis some bivalves do it all recent years have seen a growing interest in the direct and indirect roles of organisms and more explicitly of organic matter in the crystallization of minerals in sediments two bivalve genera form extensive biofilms that are apparently responsible for the growth of a variety of isolated crystals together with a marine cement granicorium indutum and samarangia quadrangularis are infaunal species that cover their shells with a cemented coating of sand sculpted to mimic the surface ornament typical of many bivalves granicorium has an exceptionally mobile mantle margin that produces large volumes of mucus harboring a diverse microbial community the sand grains surrounding both species are initially bound by a biofilm that provides structural integrity but also acts like others as a template for the crystallization of a variety of carbonate polymorphs these include varied prismatic crystals rice grain and wheatsheaf forms and more importantly large volumes of acicular crystals indistinguishable from typical marine cements the distribution of these crystals is related to positions accessible to the mantle of the animal and in samarangia appears to result from the active emplacement of grains and mucus several times a year summing the evidence it seems that these species are responsible not only for the biomineralization involved in the formation of their shells but also for crystallization mediated through biofilms and for the generation of cements that are morphologically indistinguishable from typical marine cements the boundaries between these three strategies may be closer than we think

polymetamorphic origin of muscovite cordierite staurolite biotite assemblages implications for the metapelitic petrogenetic grid and for p t paths metapelites containing muscovite cordierite staurolite and biotite ms crd st bt are relatively rare but have been reported from a number of low pressure andalusite sillimanite regional metamorphic terranes paradoxically they do not occur in contact aureoles formed at the same low pressures raising the question as to whether they represent a stable association a stable ms crd st bt assemblage implies a stable ms bt qtz crd st al 2 sio 5 chl h 2 o invariant point ip1 the latter which has precluded construction of a petrogenetic grid for metapelites that reconciles natural phase relations at high and low pressure petrogenetic grids calculated from internally consistent thermodynamic data bases do not provide a reliable means to evaluate the problem because the grid topology is sensitive to small changes in the thermodynamic data topological analysis of invariant point ip1 places strict limits on possible phase equilibria and mineral compositions for metamorphic field gradients at higher and lower pressure than the invariant point these constraints are then compared with natural data from contact aureoles and reported ms crd st bt occurrences we find that there are numerous topologi cal textural and compositional incongruities in reported natural assemblages that lead us to argue that ms crd st bt is either not a stable association or is restricted to such low pressures and fe rich compositions that it is rarely if ever developed in natural rocks instead we argue that reported ms crd st bt assemblages are products of polymetamorphism and from their textures are useful indicators of p t paths and tectonothermal processes at low pressure a number of well known ms crd st bt occurrences are discussed within this framework including south central maine the pyrenees and especially sw nova scotia

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

the role of magnesium in the crystallization of calcite and aragonite in a porous medium abstract morphological development of calcite crystals is related to supersaturation conditions during growth crystallization of calcium carbonate calcite and aragonite as well as mg calcite was studied under controlled supersaturation conditions by the counter diffusion of ca2 and co32 ions through a porous transport medium a column of silica gel under our experimental conditions where ion transport is constrained to be diffusion controlled nucleation and growth take place under conditions of high supersaturation the actual threshold value of the supersaturation depending on the supersaturation gradient in the pure caco3 system calcite grows at lower supersaturation than aragonite the calcite develops relatively simple rhombohedra whil the aragonite grows as spherulites presence of mg2 in the interstitial fluid inhibits nucleation increasing the threshold supersaturation at which crystallization begins the resulting mg calcite crystals show a range of morphologies depending on the mg content and the supersaturation at the point of crystallization at high values of supersaturation up to 15 mol mgco3 is incorporated into the calcite and the crystals form spheres at lower supersaturations mg content decreases and morphologies change progressively through a well defined and reproducible sequence from spheres to dumbbell like forms to wheat sheaf like bundles and eventually single crystals with steep rhombohedral faces the crystals are compositionally zoned showing both sector and oscillatory zoning the compositional evol tion is related to the supersaturation and interface roughness during crystal growth

an analysis of cold season supercell storms using the synthetic dual doppler technique abstractcold season tornadic outbreaks occur with regularity in the southeastern united states however detailed analyses of parent supercell storms in the cold season environment often low cape high shear are scarce this is often because storms do not always move close enough to radars for a comprehensive single doppler analysis and significant topography or trees in the southeast make it difficult for mobile radars to operate thus limiting dual doppler coverage however during the super tuesday tornado outbreak of 5 6 february 2008 two tornadic supercell storms passed within 30 40 km of the weather surveillance radar 1988 doppler wsr 88d sites in memphis and nashville tennessee knqa and kohx respectively the relative steadiness of these storms during passage along with the large motion vector from the southwest at 20 25 m s 1 allowed the application of a synthetic dual doppler sdd analysis as such a detailed analysis of these storms was completed including examinations of low lev

cyanobacterial precipitation of gypsum calcite and magnesite from natural alkaline lake water results from transmission electron microscopy provide direct evidence for cyanobacterial biomineralization of gypsum and calcite in aquatic environments laboratory simulations using filter sterilized natural lake water inoculated with synechococcus sp isolated from fayette ville green lake new york revealed epicellular biomineralization of gypsum calcite and magnesite experimental electron microscopical and sedimentological evidence indicates that synechococcus is responsible for a major proportion of the marl sediment and carbonate bioherms in green lake the elucidated role of synechococcus in biomineralization and its ubiquitous distribution in nature have widespread implications for cyanobacterial mineralization in marine and freshwater environments since late archean time

the dodge city tornadoes on 24 may 2016 damage survey photogrammetric analysis combined with mobile polarimetric radar data abstractpolarimetric measurements recorded by a mobile x band radar are combined with photographs of the dodge city kansas tornado to quantitatively document the evolving debris cloud an inner a

isograds and mineral assemblages in the eastern axial zone montagne noire france implications for temperature gradients and p t history detailed petrography across a metapelitic sequence in the eastern axial zone of the montagne noire france is the basis for a sequence of isograds marking the first appearance of biotite cordierite staurolite andalusite and sillimanite the juxtaposition of low grade biotite free rocks against medium grade rocks at the gently dipping biotite cordierite isograd is attributed to tectonic telescoping of the metamorphic sequence study of mineral assemblages with respect to an afm reaction sequence indicates the staurolite isograd is related to changes in rock composition and complex assemblages in the sillimanite zone may be the result of unstable persistence of minerals formed when metamorphic grade was lower these assemblages are interpreted to contain a record of part of the p t history during which pressure decreased as temperature increased p t profiles show that temperature gradients of 200 300 c km suggested by previous workers are not required to explain the isograd pattern gradients of 37

a shortened intraplate rift system in the proterozoic mount isa terrane nw queensland australia the leichhardt river fault trough of the mount isa terrane developed a complex extensional architecture between approximately 1800 and 1600 ma forming the underlying template upon which compressional structures were superimposed during the 1590 to 1500 ma isan orogeny basin fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity bounded sequences initial e w extension was associated with a massive magmatic event half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites thereafter a period of n s extension resulted in southward tapering north tilted half graben in which synrift basaltic and siliciclastic strata accumulated n s extension was followed by regional postrift subsidence and the deposition of a laterally continuous quartzite carbonate package a multiphase period of e w to nw se extension ensued during which time two unconformity bounded sequences accumulated the stratal architectures of these sequences are strongly asymmetric in cross section exhibiting a pronounced rotational thickening toward the east consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width finally a period of n s extension resulted in the development of e w trending f1 drag synclines in the highest level cover rocks the association of angular unconformities and block bounding faults e w trending synclines and e w striking faults and the unique internal fold geometries of fault blocks suggest that many fault bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening

deformation history of the otago schists new zealand from progressively developed porphyroblast matrix microstructures uplift collapse orogenesis and its implications abstract foliation overprinting relationships preserved in porphyroblasts and the surrounding matrix in the otago schists indicate development of up to six penetrative foliations during the late paleozoic mesozoic rangitata orogeny s1 s3 and s5 formed with steep dips and s2 s4 and s6 formed with gentle dips s1 and s2 are preserved only as inclusion trails in porphyroblasts because of matrix foliation reworking during progressive deformation s3 s6 are preserved in the matrix as well as in porphyroblasts and are generally correlatable from one to the other inclusion trail asymmetries define the location of the orogenic uplift core during the rangitata orogeny when the porphyroblasts grew the location of this core is consistent with macroscale geological and structural observations alternating steep and gently dipping foliations are inferred to indicate alternating horizontal compression and extension predictions made by an orogenic model compatible with continually alternating horizontal compression and extension compare favorably with geometries observed in the schists reconstruction of the gross geometry of the otago schists during the rangitata orogeny based on the three dimensional geometry of porphyroblast inclusion trails is similar to reconstructions based on regional geological considerations and plate tectonic calculations

experimental formation of spiky calcite through organically mediated dissolution abstract cultures of known species of fungi placed on crystals of iceland spar calcite resulted in extensive dissolution of the calcite this organically mediated dissolution produced large patches of spiky calcite within a period of 253 days the dissolution of the calcite occurred via surface reaction controlled kinetic processes that were mediated by the fungi this occurred despite the lack of vast quantities of fluids undersaturated with respect to calcite locally at least 10 m of calcite was removed from the original crystal surface

kinetic control of morphology composition and mineralogy of abiotic sedimentary carbonates abstract conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid my ca ratios and crystal morphology crystal composition and carbonate mineralogy resulting in the formation of meteoric equant low magnesium calcite and marine acicular high magnesium calcite and aragonite problematically calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals and aragonite may coexist with calcite in either environment commonly however a systematic relation exists between crystal morphology composition mineralogy and rates of reactant supply to growing crystal surfaces for example equant rather than acicular crystals of calcite form in modern deep and or cold marine meteoric phreatic and deep burial settings where the degree of carbonate saturation and or rates of fluid flow are low in areas of higher saturation and or fluid flow such as in warm shallow marine and meteoric vadose environments acicular calcite may predominate this relation is also seen in systems in which aragonite and calcite form in intimate association aragonite precipitation is favored when rates of reactant supply are high calcite forms when rates are low such relations suggest that crystal morphology composition and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants principally carbonate ions at growth sites precipitating phases are the ones which can best accommodate such excess reactants ambient mg ca ratios only indirectly control the nature of inorganically precipitated carbonate phases

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

a framework of overprinting orogens based on interpretation of the mount isa deep seismic transect a deep seismic reflection transect and coincident refraction study reveal the crustal structure developed during mesoproterozoic orogenesis preserved in the mount isa inlier of northwest queensland australia geologic interpretation of the reflection data indicates a two stage evolution for east west shortening the first stage involved west directed folding and thrusting above a decollement in the eastern part of the terrane this decollement system was the forehand of an orogenic system rooted east of the present day inlier the second stage is characterized by a network of basement cutting reverse and strike slip faults that dissected the decollement this overprinting system was linked to crustal thickening within the mount isa terrane the refraction profile indicates the crust beneath the inlier is currently approximately 55 km thick rocks with transitional velocities 7 km s in the lower 15 km are interpreted as mafic underplate formed during the pre orogenic rift history of the inlier the refraction profile also reveals two west dipping high velocity layers within the crust evaluating the origin of these layers in light of the kinematics of crustal thickening suggests they are portions of the underplated material thrust to midcrustal levels during the second stage of shortening the overprinting orogenic scenario has implications for the geodynamic evolution of the inlier that should have a direct bearing on mineralizing systems localized by reverse faults developed during the second stage of shortening this basement cutting stage established a linked fault network able to tap deeper crustal circulation during the intrusion of late to postorogenic granitoids

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses

influences of temperature and mg ca ratio on caco3 precipitates from seawater the mg ca ratio at which the calcium carbonate mineral that has pseudohomogeneously precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature results indicate a dramatic change in the critical mg ca ratio over a relatively small temperature range for example in normal seawater with a mg ca of 5 1 the critical temperature is 6 3 c at a temperature of 25 c however mg ca must be less than 1 4 0 1 for calcite to form these data indicate that paleoclimatic conditions at the time of carbonate deposition must be considered when attempting to make inferences about variation in other factors such as paleo p co 2 p pressure and paleo mg ca ratios that may control carbonate mineral formation by abiotic processes however the inverse relation between inferred global paleoclimatic conditions and the dominant type of caco 3 polymorph being abiotically deposited i e warm conditions and low mg calcite versus cool conditions and aragonite suggest that substantial changes must have taken place in ocean chemistry over phanerozoic time

microstructural evidence for n s shortening in the mount isa inlier nw queensland australia the preservation of early w e trending foliations in porphyroblasts revealed by independent 3d measurement techniques 3d microstructural analyses of porphyroblast inclusion trails using both the asymmetry switch method for determining foliation intersection axes preserved in porphyroblasts fias and the recently developed fitpitch method reveal w e and n s trending fia sets in the white blow formation of the mount isa inlier each method reveals two subsets of fias centered on each of these major trends these were distinguished based on the relative timing trend and orientation of inclusion trail patterns thirty six samples were analyzed using both techniques and produced very similar results pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin sections and trends in horizontal sections yield distinct near orthogonal modal orientations from all the analyzed samples this indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes w e and n s trending fias have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages garnet and staurolite growth during bulk north south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain this period of bulk shortening and associated metamorphism formed during a period of orogenesis called o1 w e shortening formed n s striking foliations that preserve a period of orogenesis o2 and another succession of metamorphism involving more phases of porphyroblast growth preserving n s trending fias overprinting of successive fia trends wsw ene wnw ese nnw sse and ssw nne suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from n s to w e overall with a major tectonic break or decompression between o1 and o2 the porphyroblast inclusion trail patterns preserving w e trending fias provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations

metasomatic albitites and related biotite rich schists from a low pressure polymetamorphic terrane snake creek anticline mount isa inlier north eastern australia microstructures and p t d paths rocks of the snake creek anticline are mainly pelitic schists psammitic schists and quartzites that were metamorphosed during multiple high t low p events extending from d1 to d5 with the metamorphic peak occurring late to post d3 albitites are widespread but are concentrated in five areas they are typically fine to medium grained and consist of albite with or without combinations of quartz biotite staurolite cordierite garnet andalusite sillimanite kyanite gedrite and tourmaline from the presence or absence of albite inclusions in porphyroblasts the albitites are interpreted as forming early in the d3 event as a result of infiltration of external fluids psammitic schists and quartzites were preferentially altered but pelitic schists were also albitized in localities where the alteration was more extreme with the replacement of muscovite total and the replacement of quartz and biotite variable structural controls on albitization include fracturing and syn d3 shear zones in fold hinges biotite schists with abundant porphyroblasts combinations of staurolite garnet andalusite and cordierite occur adjacent to albitites and it is argued that they formed by the addition of fe and mg sourced from the albitites in several albitite rich areas cordierite grew early in d3 and was partly or entirely replaced during or after d3 by combinations of biotite andalusite tourmaline staurolite and sillimanite a postulated p t d path involved an increase in pressure with or without a decrease in temperature subsequent to early d3 albitization followed by an increase in temperature up to the metamorphic peak late d3 to early d4 the metamorphism was contemporary in part with the emplacement of the williams batholith c 1550 1500 ma which probably supplied the na rich fluids

the progressive development of a macroscopic upright fold pair during five near orthogonal foliation producing events complex microstructures versus a simple macrostructure abstract a macroscopic anticline in the adelaide fold belt with a profile similar to first deformation folds with an axial plane slaty cleavage elsewhere in the orogen folds bedding schistosity and a differentiated crenulation cleavage all these structures are readily visible in outcrop and a young crenulation cleavage has locally developed parallel to the axial plane of the fold yet foliations preserved as inclusion trails in andalusite garnet and staurolite porphyroblasts reveal a history of five foliations that formed successively in steep shallow steep shallow and finally steep orientations without producing a refold geometry in bedding at the macroscopic scale the predominantly dextral looking north asymmetry of curvature of overprinting foliations on both limbs of this fold tightly constrains any model for its development the anticline formed as one hinge of an open fold couplet with a sub vertical axial plane slaty cleavage during the first deformation it was progressively tightened during each subsequent event without producing a refold geometry in bedding steep axial plane foliations were overprinted by shallow differentiated crenulated cleavages that transected the fold couplet these shallow foliations were then folded during the next deformation producing another steeply dipping foliation as the fold couplet tightened thus simple macroscopic folds in orogens with a multideformational history may appear to have formed as late structures because they fold differentiated crenulation cleavages and a bedding parallel schistosity but actually formed very early during orogenesis and were then tightened by a succession of near orthogonal foliation producing deformations such deformation histories provide a simple explanation for the lack of large scale refold geometries in many fold belts with a complex history of deformation small scale structures are not always a direct reflection of larger scale geometries in fold belts

proterozoic low pressure high temperature metamorphism and an anticlockwise p t t path for the hazeldene area mount isa inlier queensland australia in the hazeldene area situated in the mount isa inlier queensland the metamorphic grade changes from chlorite zone through biotite and cordierite zones to sillimanite k feldspar zone microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation s2 was replaced by biotite andalusite and sillimanite at the metamorphic peak late in s2 and in turn by kyanite chlorite adjacent to localized small post d2 shear zones although the anticlockwise p t t path is well defined the precise p t conditions are uncertain because of problems with experimental and thermodynamic data the best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600 c at 4 kbar the metamorphism has been dated at 1544 ma and was synchronous with a major crustal shortening event because proposed extensional events occurred more than 60 ma earlier their contribution to the peak metamorphic thermal perturbation would have been insignificant the syn metamorphic mica creek pegmatites the abundance of high heat producing elements in the nearby pre d2 sybella granite and advective heat by fluids which caused considerable metasomatism in the hazeldene area may have each contributed to the thermal budget however the metamorphic thermal gradient may be 80 c km 1 or higher strongly suggesting a local magmatic control as none are known in the area such syn metamorphic plutons would have to lie beneath the exposed high grade rocks

the synchronism of crustal thickening and high t low p metamorphism in the mount isa inlier australia 1 an example the central soldiers cap belt abstract in the soldiers cap group of the central eastern part of the early to middle proterozoic mount isa inlier tracing of mesoscopic structures has led to the recognition of an early nw closing fold nappe with an e w trending coaxially shortened frontal zone d1 folded around a n s trending upright syncline d2 an andalusite sillimanite prograde metamorphic event indicating a paleo geothermal gradient of approximately 50 c km occurred during early d2 times the coexistence of such moderately high paleo geothermal gradients and crustal thickening is common in the early to middle proterozoic mobile belts of northern australia crust mantle delamination has been suggested by etheridge et al 1987 to explain this coexistence here the thermal implications of this model and the additional thermal effects of waning crustal extension magmatic events and of the radioactive selfheating of the thickened crust are qualitatively discussed

geometry of a middle proterozoic extensional decollement in northeastern australia abstract an 80 km long belt of gneissic and mylonitic rocks is exposed in the core of a major anticlinorium in the middle proterozoic mt isa inlier of northeastern australia these rocks in the mary kathleen fold belt represent an early originally horizontal mid crustal shear zone which is interpreted as an extensional decollement between a lower ductile sheet and an upper more brittle plate the decollement was subhorizontal over a distance of at least 80 km and its depth was perhaps as shallow as 7 km shearing was accompanied by the syn tectonic intrusion of a bimodal complex of granitic and basic rocks over a period of 70 90 m y moderate to extreme ductile shearing was pervasive throughout the 1 1 5 km thickness of the lower plate that is now exposed massive dilation accompanied shearing and approximately 70 of the volume of the shear zone is occupied by igneous intrusive rocks which have been variably deformed to gneissic granite mylonite and amphibolite strain softening driven by this high thermal input is responsible for the pervasive ductile response at such shallow depths peak metamorphic conditions at the decollement were approximately 625 c to 675 c at 2 kbar intense ductile shearing in the upper plate is restricted to the lower few hundred metres above this zone brittle processes dominate although evidence is presented that the sheet as a whole has undergone a component of ductile shearing probably heterogeneously distributed large sheet like granite and dolerite bodies were introduced into dilation sites associated with synthetic faults in this plate but unlike their synchronous counterparts in the lower plate about 1 km below these are internally undeformed the middle proterozoic history of the area involves a cycle of extension and shortening with the shortening axis perpendicular to the earlier extension axis we speculate that the driving mechanism for both extension and shortening is mantle convection with switches in stress related to mantle upwelling

habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy abstract marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature which results in single crystals and aggregates of crystals crystal bundles the end member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms the most common of which is the dumbbell rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and consequently may serve to identify bacterially induced precipitates in the rock record viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted the medium viscosity controls the ion diffusion rate and thus the rate of precipitation and consequently the mineral precipitated in a liquid medium circulation and the ion diffusion rate are both high precipitation is rapid and aragonite forms in a gelatinous medium there is little circulation the ion diffusion rate is slow the rate of precipitation is slow and calcite is precipitated the overall form of the resultant precipitate rods or spheres was the same regardless of the mineral composition

inorganic calcite morphology roles of fluid chemistry and fluid flow abstract evaluation of calcite crystal habits and fabric and their relation to precipitating fluids in spelean environments indicates that habit variations are controlled primarily by degree of supersaturation at less than 6 times supersaturation crystal habits are limited to those with shallow to steep rhombohedral forms i e 1011 4041 with increasing supersaturation scalenohedrons extremely steep rhombohedrons pinacoids and hexagonal prisms are developed slight elongation due to differences in crystal dimensions of these forms may arise i e scalenohedrons versus shallow rhombohedrons at extreme supersaturation 12 times curvature of crystal faces or curved surfaces appear the development of fibrous versus equant fabrics is the result of changes or modification of reactant supply rates and the number of crystallites nuclei present under rapid flow conditions growth rates are accelerated and greater numbers of nuclei are present thus compromise boundaries parallel to c axis and perpendicular to flow and substrate are rapidly attained in the absence of flow fewer crystallites are present and larger well developed crystals or branching forms dictated by saturation conditions arise in the absence of flow at low supersaturation elongate composite crystals and hence fibrous fabrics may result from migration of the fluid crystal interface as a result of changing or oscillating water levels a flow control mechanism can be applied to numerous geological conditions in which transitions from fibrous to equant crystal morphologies are observed

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

freshwater phytoherm reefs the role of biofilms and their bearing on marine reef cementation abstract bioconstructions in carbonate rich freshwater environments generally lead to the development of thick autochthonous tufa deposits in rivers systems these large plant dominated freshwater reef frameworks phytoherms are capable of damming the water courses to produce chains of lakes the resulting phytoherms profoundly modify both local environment and sedimentation patterns in their vicinity and provide a wide range of niches for biota subaqueous fringe cements coat all exposed surfaces they bind the ephemeral plant frameworks providing a cement framework and give rigidity to the bioconstructions peloids are intimately associated with the development of these cements and frequently infil intra reef and framework cavities evidence is presented to suggest that both micritic fringe cements and peloids are the product of biological mediation associated with a procaryote microphyte biofilm and the presence of organic detritus the bladed low magnesium calcite fringes which develop subsequently on the micrite fringes and peloids are possibly slower formed inorganic precipitates the close morphological similarities between phytoherm fringe cements and peloids and their marine reef counterparts are considered to be more than coincidental it is argued that procaryote microphyte biofilms are also responsible for micritic precipitates in the shallow photic marine realm with bacterial films possibly carrying the process deep into unlit reef cavities and conduits and deep water lithoherms

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

possible microbial effects on stable carbon isotopes in hot spring travertines recent and older early holocene to pleistocene hot spring travertine carbonates from central italy have two distinctive macrofabrics crystalline crusts and shrubs crystalline crusts are laminated slope deposits that formed abiotically following co 2 degassing from spring water the formation of shrub travertine irregular dendritic precipitates from pool environments is controversial and has been attributed to both abiotic and microbial processes oxygen isotope variation in our travertines can be explained by abiotic processes mainly co 2 degassing in contrast our carbon isotope data cannot be wholly explained by abiotic co 2 degassing invoked in earlier studies because photosynthesis is known to preferentially remove 12 c leaving ambient waters enriched in 13 c this fractionation should be recorded in delta 13 c values of microbially influenced travertine our shrub carbon isotope values are between 0 5 per thousand and 6 0 per thousand larger than values for associated abiotic precipitates and this difference is probably caused by microbial activity these isotope data support previous visual evidence that some shrub fabrics are microbially influenced

porphyroblast microstructures a review of current and future trends many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies this paper reviews ten current applications of these microstructures 1 porphyroblast growth timing criteria 2 tracking progressive foliation development relative to changing metamorphic conditions 3 timing of pluton emplacement relative to deformation and metamorphism 4 finite longitudinal strain determinations 5 kinematics and porphyroblast rotation 6 use of linear fabrics preserved in porphyroblasts 7 porphyroblasts and folding mechanisms 8 inclusion trail orientations and orogenic processes 9 inferring shear strain rates from porphyroblast growth rates and 10 in situ age determinations although there is still no concensus on the interpretation of some porphyroblast microstructures a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings

recent stromatolites in landlocked pools on aldabra western indian ocean abstract stromatolites occurring in three areas of landlocked pools on aldabra show distinctive morphologies petrography and biological characteristics related both to water chemistry and the physical characteristics of sites they are formed apparently by indirect precipitation of high magnesium calcite in association with growths of cyanobacteria and cladophora the three contrasted areas are dominated respectively by phormidium lyngbya and pleurocapsa by entophysalis lyngbya and pleurocapsa and by pleurocapsa dichothrix gypsophila and schizothrix reflecting an increasing concentration and increasing dissolved carbonate activity of pool waters centimeters to decimeters of carbonate encrustations are made up of irregular laminate columns porous layered coatings of planar or cerebroid form including oncolites and finely laminate branching columns laminae are typically alternations of dense and porous micrite with in one locality radiating fibrous sparry crystal growths although exclusively associated with cyanobacteria calcification typically begins several millimeters beneath living cells with few examples of carbonate coated unicells or filaments micrite consists of randomly arranged particles of varying size products of sparse homogeneous nucleation followed by both overgrowth and dissolution coarse cements of contrasted morphologies develop within internal cavities resembling the endostromatolites described by others chemical data indicate that waters in all three pools are supersaturated with respect to caco 3 calcite this is adequate but is not in itself sufficient to bring about precipitation of carbonate which is site specific cyanobacteria are not directly calcified but metabolic activities may promote precipitation through mechanisms such as extraction of co 2 or by the generation of specific organic molecules which promote nucleation or colonies may provide a closed passive environment in which other bacteria or inorganic processes bring about crystallization the precise path is not known but the reduction of possibilities carries implications both for the accretion of other stromatolites and the crystallization of carbonate in deposits such as tufas

preferred orientation of planar microstructures determined via statistical best fit of measured intersection lines the fitpitch computer program the fitpitch computer program allows characterisation of preferred orientation of planar microstructures in a rock from the orientations of their intersection lines on different sections measured pitch or strike angles of such intersection lines are fitted to the theoretical intersection lines of a single model plane or a combination of two or three model planes the degree of fit is quantified in terms of the deviation between natural and theoretical intersection lines normalised by uniform data a first application is described concerning porphyroblast inclusion trails whose orientation in 28 samples from the variscan iberian massif was measured analysis of these data illustrates how the method can be used to quantify preferred orientation of relict foliations contained in a group of porphyroblasts and how this complements existing techniques for determining foliation intersection axes fia in porphyroblasts a second described application deals with previously published orientation data for microfracture trace lines in quartz grains of a variscan granite in the rhine graben qualitatively established best fit planes derived originally from this data are compared with the numerically established planes using the fitpitch program and discussed in terms of the advantages and potential limitations of the method q 2002 published by elsevier science ltd

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

tectonic evolution of the montagne noire and a possible orogenic model for syncollisional exhumation of deep rocks variscan belt france structural analysis in the montagne noire including a study of inclusion trail patterns in andalusite garnet and staurolite porphyroblasts reveals a succession of five foliations in alternately steep and flat lying orientations this is inferred to reflect multiple gravitational collapse phases that intervened during continuous plate convergence initial crustal thickening by foliation development and folding d1 was responsible for eclogite facies metamorphism in the gneissic basement subsequent thrusting and recumbent folding d2 generated a subhorizontal crenulation cleavage during namuro westphalian times while previous steep structures became rotated petrological and microstructural evidence indicate that thrust nappes were emplaced by a gravitational spreading mechanism while undergoing internal thinning an important amount of basement exhumation took place synchronously these observations form the basis for a new orogenic model in which thermal reequilibration of a previously thickened orogen induces gravitational instability in the upper crust collapse of the upper crust causes it to spreadout horizontally over continuously converging relatively cool plates thereby creating low angle thrust nappes the upper crustal thinning facilitates simultaneous extrusion of lower crust in vertical root zones at the orogen center which on reaching the higher part of the orogen can be incorporated in the nappes and displaced horizontally following such a mechanism of nappe tectonics renewed horizontal shortening d3 produced the montagne noire gneissic dome by upright folding of nappe related foliations with orogen parallel stretching previous crustal thinning had led to an increased heat flux between basement and cover so that during d3 cover sequences were still rising in temerature while basement rocks had already commenced their retrograde path during d2 peak metamorphic conditions in the cover triggered a second gravitational collapse during namurian times d4 producing a 2 km wide subhorizontal shear zone in which orogen parallel stretching continued at the surface deformation was accommodated by decollements normal faulting and basin formation gneissic rocks below the shear zone were not affected by the collapse because a stronger rheology and horizontal shortening was probably continuous here retrograde cooling of the cover restored gravitational equilibrium and caused its renewed upright folding during late variscan times d5 permian postorogenic extension d6 oriented at a high angle to the previous orogen parallel stretching completed exhumation of the gneiss dome the dome structure of the montagne noire metamorphic core complex was acquired during compressional tectonics and later modified flattened by synorogenic and postorogenic extensional events a similiar polyphase origin appears not to be excluded for other metamorphic core complexes with poorly constrained preextension histories

pressure temperature paths from p t pseudosections and zoned garnets potential limitations and examples from the zanskar himalaya nw india the extraction of p t histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny while continued re equilibration frequently obliterates early information garnet zonation and inclusion assemblages can often surmount this problem the task is more difficult in high variance assemblages or if inclusions are not preserved but one approach is to use pseudosections that are specific to the bulk composition of a given rock in the latter case the compositions and abundances of all the minerals are fixed at a given p t point such that if the effective bulk composition is known the garnet composition alone can be used to reconstruct the history here we explore this approach using examples from the zanskar himalaya nw india pseudosections have been calculated for four pelitic to semipelitic rocks from the zanskar himalaya and have been contoured for garnet composition the calculations adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite is found as inclusions within garnet moreover the pseudosections successfully model the garnet core compositions with all three independent compositional contours overlapping at a single pressure and temperature this occurs at 550 c and at pressures varying from 3 7 kbar for the four rocks studied we have been less successful however at modelling garnet compositions beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself however in this case a combination of the pseudosection and conventional thermobarometry using fe ti inclusions and matrix phases allows us to reconstruct the entire p t history the resulting p t paths record burial of 3 5 kbar without significant temperature increase followed by isobaric heating of 50 100 c this evolution is consistent with himalayan collision in the early tertiary but a combination of the p t data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the oligocene in addition the data demonstrate that no extra heat source is required to cause melting of the himalayan crust in the miocene while melting could have occurred both by dehydration during decompression or in the presence of a fluid the lack of garnet resorption does suggest decompression was rapid and followed quickly by cooling this scenario favours melting by decompression

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

beachrock formation via microbial dissolution and re precipitation of carbonate minerals cementation of beach sand in the intertidal zone produces beachrock such as that found on heron island heron reef great barrier reef australia although common to coastlines in many low latitude beach environments the cause of cementation is not fully understood in this investigation electron and x ray fluorescence microscopy were used to characterize previously undocumented features of beachrock two generations of beachrock were examined as a means of understanding the progression of cementation meniscus shaped attachments at point contacts appear to be the first cements to form in biofilms near the beachrock surface this is followed by isopachous fringe cements within the now enclosed beachrock composed of aragonite needles that are enriched in strontium and contain extracellular polymeric substances eps cement precipitation is driven by locally high concentrations of cations in solution undoubtedly generated via microbial dissolution of the detrital carbonate grains binding to negatively charged bacterial eps retains these cations within beachrock microenvironments the metabolism of cyanobacteria and associated heterotrophs induce the supersaturating conditions needed for cement precipitation deeper within beachrock mm to cm below the surface abundant microbialites are found on the edges of grains and contain trapped and bound detrital material these structures are laminated enriched in strontium in some layers and contain microfossils the results of this investigation clearly demonstrate a biological influence in the precipitation of aragonite cement involving internal recycling of cations through microbial dissolution and precipitation of carbonate minerals

polyphase deformation in the western mount isa inlier australia episodic or continuous deformation abstract the polyphase deformation history of the mount novit ranges western mount isa inlier australia involved the development of four generations of n s to nw plunging folds development of which alternated with movement on n s striking faults shear zones with west over east displacement the spatial relationships between successive structures and the distinct similarities in the kinematics style and asymmetry are all consistent with continuous deformation during e w to ne sw shortening additional support for this hypothesis is based on the development of the three major fold generations and the intervening faults and shear zones during low p high t metamorphism although the duration of this metamorphic event cannot be constrained recent studies have linked low p metamorphism to heat advection by migrating melts thermal modelling indicates that such metamorphic events will be short lived in comparison to those associated with thermal relaxation of over thickened crust low p metamorphism is unlikely to persist through two or more separate tectonic events therefore syn metamorphic polyphase deformation in the mount novit ranges apparently formed during a single continuous tectonic episode this structural history differs from both the regional scheme and that of previous workers in this area the main differences are i no evidence was found for early thrust faulting prior to n s folding ii there are four generations of n s to nw se folds as opposed to two and iii this study favors a single consistent tectonic regime in contrast to the interpretation of three discrete isolated tectonic events

hydromagnesite stromatolites and sediments in an alkaline lake salda golu turkey abstract salda lake in sw turkey is a highly alkaline ph 9 water body with waters enriched in magnesium microbial stromatolites along shorelines contain a microflora of diatoms and cyanobacteria with extensive associated biofilms together these are responsible for precipitation of hydromagnesite although distribution is not congruent with either cells or films deposition has continued over several thousand years and is independent of both lake volume and the general concentration of lake waters the lake is rimmed on three sides by serpentinites and on the fourth by dolomite the bulk of the water entering the lake is meteoric fed via extensive gravelly alluvial fan deltas with predominantly serpentinite pebbles whose total areas and catchments exceed that of the lake these waters are significantly cooler than surface waters of the lake and there is no evidence for derivation from hot springs the magnesium is thought to have been leached from the gravels extensive areas of recent sediments accumulating on shorelines have apparently formed by mechanical breakdown of microbialites similar deposits form conspicuous terraces in two areas and point to the long term effectiveness of these processes terrace deposits are modified by a variety of subaerial processes including growth of hydromagnesite cements no microbial structures are preserved in these and it is presumed that they are destroyed during reworking fine grained hydromagnesite sediment derived from shorelines and from terraces is suspended in the water column during storms and is probably redeposited as turbidites in the deeper parts of the lake sediments of this kind may be a useful analogue for some ancient and economically important stratiform magnesite deposits

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

lack of porphyroblast rotation in noncoaxially deformed schists from petrel cove south australia and its implications abstract inclusion trails representing an s 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non coaxial deformations the pelitic schists of the kanmantoo group at petrel cove south australia contain two generations of porphyroblasts the first one consists of cordierite porphyroblasts that formed early in d2 and contain straight to slightly sigmoidal inclusion trails of s 1 the second generation consists of andalusite porphyroblasts that overgrew crenulated s 2 late during d3 several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection hence s 1 was planar and horizontal prior to d2 s 1 measurements on limbs and hinges of a mesoscale d2 foldpair show that folding had little effect on porphyroblast orientation as the s 1 orientation remained constant and subhorizontal around the fold hence porphyroblasts have not rotated during any of the non coaxial deformations accompanying and following their growth this is interpreted as a result of the partitioning of the deformation around them s 1 in the matrix has been totally destroyed by the formation of s 2 as a fully differentiated crenulation cleavage what has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning the presence of a subhorizontal s 1 foliation suggests horizontal movements e g thrusting or detachment faulting during the earliest phase of the adelaidian orogeny

rapidity of marine carbonate cementation implications for carbonate diagenesis and sequence stratigraphy perspective abstract lithification is a rapid process but how rapid is rapid a year after one of my visits to joulter s cay bahamas i found a sardine can from my previous visit the cementation of carbonate particles in and around the can was surprising 382 g of ooids and skeletal material had filled the can and lithified therein or had become cemented to the outside of the can the rapidity of marine carbonate cementation implies that during highstand of sea level shallow water platform carbonate sediments are subject to marine lithification which retards erosion this conclusion is at variance with that of other workers who rely on freshwater cementation during lowstands to generate cement which retards reworking clearly the historical debate over highstand versus lowstand of shallow water marine carbonates is not yet over

lack of porphyroblast rotation in the otago schists new zealand implications for crenulation cleavage development folding and deformation partitioning porphyroblasts of garnet and plagioclase in the otago schists have not rotated relative to geographic coordinates during non coaxial deformation that post dates their growth inclusion trails in most of the porphyroblasts are oriented near vertical and near horizontal and the strike of near vertical inclusion trails is consistent over 3000 km2 microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures this is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding several crenulation cleavage fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding in the light of accumulating evidence that porphyroblasts do not generally rotate the applicability of these models to deformed rocks is questionable whether or not porphyroblasts rotate depends on how deformation is partitioned lack of rotation requires that progressive shearing strain rotational deformation be partitioned around rigid heterogeneities such as porphyroblasts which occupy zones of progressive shortening or no strain non rotational deformation therefore processes operating at the porphyroblast matrix boundary are important considerations five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast a a thin layer of fluid at the porphyroblast boundary 2 grain boundary sliding 3 a locked porphyroblast matrix boundary 4 dissolution at the porphyroblast matrix boundary and 5 an ellipsoidal porphyroblast shadow unit

multiple deformations with successive subvertical and subhorizontal axial planes in the mount isa region their impact on geometric development and significance for mineralization and exploration the first orogeny to affect most rocks exposed in the mount isa province caused significant repetition of stratigraphy through north south shortening geological maps are dominated however by younger folds with subvertical north south striking axial planes these folds were produced by east west shortening during a second orogeny both orogenies produced several foliations however some foliations have negligible geometric consequences allowing the geometry of the region to be described generally in terms of just the three deformations d 1 d 2 and d 3 which produced macroscopic folds and the younger deformations which produced brittle faults one of the weak events that appears superficially to have had little or no geometric impact played a highly significant role in the generation of structurally controlled mineralization that role has been identified following advances in conceptual understanding of fold development and associated foliation the deformation which occurred between the d 2 and d 3 events produced geometric effects that were masked by overprinting of north south trending d 2 folds by similarly trending d 3 folds significantly these d 3 folds which control the localization or upgrading of much mineralization in the mount isa province did not form in rocks unaffected by this intermediate event because the near parallelism of s 2 and s 3 in such rocks resulted in intensification of s 2 rather than development of a new s 3 the intermediate deformation event called d sub 2 5 locally produced highly asymmetric deflections in bedding and s 2 with subhorizontal axial planes that were subsequently overprinted by d 3 folds and an s 3 cleavage no mesoscopic foliation was produced during d sub 2 5 and its rotational effects on bedding were commonly subsumed within younger d 3 structures thus d 3 disguises the presence of d sub 2 5 except in relatively rare locations where d 2 folds were isoclinal such as in the tommy creek block and on the northern end of the snake creek anticline southeast of cloncurry and northwest of mount isa in these three locations in the latter locations both limbs were affected resulting in large scale overturning of bedding on one limb prior to overprinting by d 3 the asymmetric folds developed during d sub 2 5 created sites where bedding could be pulled apart during d 3 resulting in implosion because of the high fluid pressure in the surrounding country rock with brecciation jostling fluid access and mineralization large scale zones of mineralization form where d sub 2 5 was localized at an earlier formed heterogeneity such as a thrust surface e g at mount isa mine or a granite country rock contact in these locations early formed angular bedding relationships were locally rotated into orientations which created sites that could implode during d 3 this role of d sub 2 5 in the generation of d 3 allows recognition of regional zones for exploration that previously have received little attention recognition of the role of d sub 2 5 reveals a cyclicity in successive subvertical and subhorizontal events not recognized by most workers in this terrain that may occur in other orogens around the world but has similarly gone unnoticed after the first orogeny finished a succession of three near vertical and two subhorizontal events occurred during the east west shortening orogeny at mount isa these were succeeded by younger kinks with east west to northeast southwest trending axial planes that appear to be the youngest ductile deformation to have affected the mount isa province