growth of a zoned plagioclase porphyroblast in a mylonite zoning patterns in a plagioclase porphyroblast in a blastomylonite derived from lewisian gneiss near loch eriboll northwest scotland suggest that the crystal grew during deformation discontinuities in the concentrations of cao k 2 o and feo mark the boundaries of overgrowths and define a core overgrowth structure the simultaneous changes in concentration of cao and k 2 o are used to construct a qualitative chemical potential diagram that reflects changes in a ca a 2 h and a k a h in the coexisting fluid during growth increases in these activity ratios are believed to have caused supersaturation of the grain boundary fluids with respect to plagioclase the repeated periods of growth indicate repeated overstepping of equilibrium which could have been caused by ion exchange on surfaces newly exposed during deformation these data fit a model that relates cyclic variations of strain rate in fault zones with cyclic periods of crystal growth which in turn could have a major influence on fault rock rheology

local deformation in compacting flows development of pressure shadows abstract small scale structures produced by deformation and compaction are common in igneous metamorphic and sedimentary rocks and are often used to define foliation and lineation directions analytic solutions for models of flow round a rigid stationary sphere are obtained using the equations that describe viscous compaction the finite deformation associated with these flows shows many of the features observed in deformed rocks some of which resemble those generated when the ratio of the bulk viscosity to the shear viscosity is small under these conditions the particle paths of the matrix are almost unaffected by the presence of the sphere it is encouraging that the continuum compaction equations can describe the observed behaviour even though the length scale on which the deformation occurs is only mm to cm it is unlikely that the observations can be used to distinguish between d arcy flow of the low viscosity phase and mass transport by diffusion

strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

development of structures under the influence of heterogeneous flow field around rigid inclusions insights from theoretical and numerical models rocks that are mechanically heterogeneous due to the presence of stiff or rigid inclusions floating in a ductile matrix commonly show a variety of micro to macro scale structures developing under the influence of heterogeneous flow field in the neighbourhood of the inclusions it is of fundamental importance to apprehend the nature of strain heterogeneity around inclusions to understand progressive development of structures associated with rigid inclusions such as strain shadow foliation drag porphyroclast mantle porphyroblast inclusion trails intragranular fractures etc the development of these diverse types of structures can be analyzed with the help of a suitable hydrodynamic theory in this paper we review different continuum models that have been proposed to characterize the heterogeneous flow field around rigid inclusions focusing on recent developments recent studies reveal that jeffery s proc r soc lond a 120 1922 161 theory dealing with the motion of ellipsoidal rigid bodies in an infinitely extended viscous medium is more general in nature and applicable for modeling the heterogeneous flow around both equant and inequant shapes of inclusions and ideal or non ideal shear deformation of the matrix the application of this theory therefore has advantages over other models based on lamb s lamb h 1932 hydrodynamics cambridge university press cambridge theory dealing with spherical inclusions the review finally illustrates numerical simulations based on hydrodynamic theories highlighting the controls of physical and kinematic factors on the progressive development of the structures mentioned above

mathematical model for folding of layering near rigid objects in shear deformation abstract the asymmetric arrangement of deformed layering around rigid objects in shear zones is among the most reliable criteria for inferring shear sense in rock a mathematical model which simulates shear deformation of a ductile medium containing a rigid inclusion gives insight into the genesis of this type of shear sense indicator the model which is based on the velocity field solution for viscous flow around a sphere demonstrates that many common strtuctures can arise in a mechanically passive matrix as the model simulates increasing shear strain structures in the matrix pass through distinct evolutionary stages similar results have been obtained in experimental models suggesting that it may be possible to read shear strain magnitude from the geometry of deformed layering near clasts and porphyroclasts in sheared rocks

variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore namibia results from sediment archives the coastal upwelling system offshore namibia is ideally suited to address a focal question of the integrated marine biogeochemistry and ecosystem research programme what are the mechanisms that drive long term changes in ecosystems considerable interannual variability in climatic forcing is indicated by long time series of meteorological and remote sensing observation these accompany considerable interannual to interdecadal changes in the upwelling intensity over the last 100 years as well as a centennial trend on longer time scales the only archives available are sediment records spanning the late holocene to decipher the sediment record we mapped surface sediment patterns of proxies for physical sea surface temperature sst from alkenone unsaturation indexes and nutrient 15n on bulk sedimentary n variables their present day surface sediment patterns outline the coastal upwelling cells and filaments and associated high productivity area analysed in an array of dated sediment cores the spatial patterns of sst suggest long term 100 years variability in the location and intensity of individual upwelling cells the patterns of 15n outline an area of intense denitrification near the coast and advection of water with low oxygen concentrations in the undercurrent from the north 15n exhibits considerable downcore variability in particular over the last 50 years the variability appears to be governed by differences in extent of denitrification and thus of the shelf oxygen balance which appears to have deteriorated in the last 50 years together the data suggest that sst and denitrification conditions have remained in the narrow bounds outlined by today s patterns in surface sediments but that spatially small variability in upwelling intensity and make up of upwelling feed waters induced considerable changes in the lower trophic levels of the coastal upwelling ecosystem over the last 6 000 years attempts to correlate proxy records from sediments with observational time series and regional climate reconstructions were not successful possibly because annual to interannual environmental signals are erased in the process of sediment formation

contrasting textures in metamorphic and anatectic migmatites an example from the scottish caledonides a method for the quantitative analysis of the spatial relations of minerals is described dispersed distributions are formed by annealing and destroyed in post tectonic migmatization aggregate distributions characterize solid state differentiation whereas leucosomes formed in systems of high fluid rock ratio in the examples studied anatectic melts show random distributions quantitative textural analysis can be used to indicate whether migmatization was post tectonic or earlier though caution is necessary if post migmatite cooling is slow or if there is some minor deformation more importantly it can be used to discriminate melt present from melt absent leucosomes this is exemplified by a suite of metamorphic and anatectic migmatites from the scottish caledonides the textural evolution of anatexites with increasing melt percentage is traced initial feldspar porphyroblastesis occurs by ostwald ripening via grain boundary melts subsequently ophthalmites develop with fabrics and chemistry inherited from the palaeosome at greater than 30 melt these inherited fabrics are wholly destroyed deformation prompts segregation into melanosome and leucosome resultant leucosomes contain no inherited crystals the scale of anatectic systems is fixed at the point at which segregation begins ophthalmites provide evidence for melt and crystal transfer beyond original palaeosome boundaries in contrast metamorphic migmatites are necessarily small scale systems because of diffusive constraints and melanosomes are invariably produced

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

interpretation of porphyroblast inclusion trails limitations imposed by growth kinetics and strain rates porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock however there remains considerable controversy over whether porphyroblasts rotate during bulk non coaxial deformation with reference to an area of the scandinavian caledonides and utilizing existing data from theoretical and experimental modelling this study demonstrates that both straight and s shaped inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to newtonian simple shear at crustal strain rates of 10 14 s 1 and porphyroblast growth times of 0 1 1 0 ma it is shown that a maximum of 5d 9d angular rotation would occur during growth at faster strain rates of 10 12 s 1 e g those in a shear zone porphyroblast angular rotations of 90d are shown to occur in 0 1 0 25 ma i e times comparable with or faster than porphyroblastesis in view of this s shaped inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain whereas straight inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates

do smoothly curving spiral shaped inclusion trails signify porphyroblast rotation smoothly curving spiral shaped inclusion trails may form in porphyroblasts during the overprinting of near orthogonal foliations in schists that are relatively quartz rich and in general graphite poor the smooth curvature results from the combined effects of 1 the protection from progressive shearing provided by projecting crystal faces and 2 greater competency and generally wider zones of progressive shearing in these rock types than in ones that are less quartz rich and more graphtic therefore lack of truncations or abrupt changes in compositional zoning profiles and the presence of smoothly curving spiral shaped inclusion trails do not necessarily signify that the porphyroblast containing them rotated as it grew this has considerable significance for shear sense determinations in deformed and metamorphosed rocks

fluid induced seismicity guided by a continental fault injection experiment of 2004 2005 at the german deep drilling site ktb 1 recent hydraulic experiments at the ktb site have shown that seismicity induced by long term fluid injection directly into a continental crustal fault remains guided by this fault the seismicity is triggered by pressure perturbations as low as 0 01 1 bars at the hypocenters a combination of sequential one year fluid extraction 2002 2003 and one year fluid injection 2004 2005 experiments has shown that only positive pore pressure perturbation i e injections was able to induce the seismicity moreover the onset of seismicity roughly coincides with the time of compensation of the extracted fluid volume by the following injection this confirms that the pressure diffusion is a dominant mechanism of seismicity triggering by fluid injections the probed fault shows a significant anisotropy and non linearity of its hydraulic behaviour its hydraulic diffusivity is up to one order of magnitude larger than that of surrounding rocks

rate and mechanism in prograde metamorphism for a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature disequilibrium depends on the slowest of three sequential steps a surface detachment of reactant minerals b transport of material to the site of mineral growth and c nucleation and growth of the product mineral we have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism the dissolution and growth steps are linear functions of the driving force g of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself

a new spin on non rotating porphyroblasts implications of cleavage refraction and reference frames abstract it has been claimed that rigid porphyroblasts which grow before or during folding and concurrent cleavage development do not rotate with respect to the geographical reference frame grf even if the straining is non coaxial bell 1985 bell and johnson 1990 the explanation offered is based on strain partitioning it is argued that the initial orientations of early fabrics included as internal foliations s i in the porphyroblasts have been preserved after polyphase deformation and even after successive orogenies according to the strain partitioning model the porphyroblasts are fixed in domains of coaxial straining microlithons and are isolated from the non coaxial straining associated with the enveloping septa s e this hypothesis and also its discussions both pro and contra suffer from insufficient attention to reference frames we therefore attempt to demonstrate a the need for rigorous treatment of reference frames in geological interpretations b that in a folding situation grains that do not rotate with respect to their immediate matrix generally rotate with respect to the grf c that lack of porphyroblast rotation with respect to the grf demands a rare folding mechanism slip fold model and d that the non rotation hypothesis is in conflict with heterogeneous deformation cleavage refraction finally we question the validity of the evidence in a study by fyson 1980 cited in support of non rotation with respect to the grf during folding fyson reported orientations of s i that are constant after folding over a large area this scenario is a product of selective data acquisition in summary our investigation shows that the lack of porphyroblast rotation with respect to a grf during folding while possible is not universal the development of microstructures e g curved s i is only related to the local deformation path the characterisation of which does not rely on the grf

high rate injection is associated with the increase in u s mid continent seismicity an unprecedented increase in earthquakes in the u s mid continent began in 2009 many of these earthquakes have been documented as induced by wastewater injection we examine the relationship between wastewater injection and u s mid continent seismicity using a newly assembled injection well database for the central and eastern united states we find that the entire increase in earthquake rate is associated with fluid injection wells high rate injection wells 300 000 barrels per month are much more likely to be associated with earthquakes than lower rate wells at the scale of our study a well s cumulative injected volume monthly wellhead pressure depth and proximity to crystalline basement do not strongly correlate with earthquake association managing injection rates may be a useful tool to minimize the likelihood of induced earthquakes

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization crystal size distributions csds measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates growth times and the degree of overstepping t of reactions during metamorphism csds are described through use of a population density function n dn dl where n is the cumulative number of crystals per unit volume and l is a linear crystal size plots of ln n vs l for olivine pyroxene and magnetite in high temperature 1000 c basalt hornfelses from the isle of skye define linear arrays indicating continuous nucleation and growth of crystals during metamorphism using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at t 10 c and nucleation rates between 10 4 and 10 1 cm3 s garnet and magnetite in regionally metamorphosed pelitic schists from south central maine have csds which are bell shaped we interpret this form to be the result of two processes 1 initial continuous nucleation and growth of crystals and 2 later loss of small crystals due to annealing the large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth the extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known finally the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs contact metamorphosed rocks because contact metamorphism involves high temperatures for short durations resulting csds are linear and unaffected by annealing similar to those produced by crystallization from a melt because regional metamorphism involves prolonged cooling from high temperatures primary linear csds are later modified by annealing to bell shapes

rotated staurolite porphyroblasts in the littleton schist at bolton connecticut usa staurolite porphyroblasts 1 5 8cm in length and 0 3 2cm in width in the littleton schist at bolton connecticut contain curved quartz inclusion trails which document synkinematic rotations of at least 135 the orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation serial sections of four differently orientated crystals and u stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical symmetry axis characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are in all cases parallel to the long axis of the porphyroblast the cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c axis section through the initial core of a crystal the amount of rotation about the c axis decreases linearly along the length of the crystal away from the nucleation site the sense and amount of rotation recorded by a porphyroblast is related to its orientation a tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c axes the total rotation reflected by the inclusion trails increases as a function of the angle between the c axes of the staurolite crystals and the slip direction initially random staurolite porphyroblasts rotated during growth as a consequence of laminar shear in the surrounding viscous matrix this interpretation is quantitatively consistent with the staurolite preferred orientation its coincidence with the apparent slip direction the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast and the twisted conical shape of the family of surfaces defined by the inclusion trails

a rigid circular inclusion in an anisotropic host subject to simple shear abstract the solution for velocity and rate of deformation in an incompressible homogeneously anisotropic viscous host embedding a cylindrical rigid inclusion subjected to a far field shearing parallel to the anisotropy direction is presented the rotation rate of the inclusion is equal to half the ambient shear rate independently of the anisotropy factor the flow in the matrix localizes into conjugate shear bands sub parallel to the anisotropy directions as the anisotropy factor increases the presented anisotropic analytical solution which neglects the bending stiffness approximates the flow in a layered medium in the limit of infinitely thin layers an initially planar layering is deflected adjacent to the inclusion and fold trains propagate into the host with progressing deformation numerical simulations show that the structural development leads to a decrease in the inclusion rotation rate and eventually to inclusion stagnation at high strains the fold shapes become more angular and the fold trains reach further out into the host as the anisotropy factor is increased increasing the layering thickness up to only 9 layers intercepting the inclusion has no significant effect on how the layer inclusion rotation rate evolves with strain a coarse layering in the host leads to a strongly reduced outreach of the fold trains and the absence of tight folds the analytical solution which is derived for a planar anisotropy in the host can be employed to approximate the structural development for a weakly anisotropic host or small deformation structural development of an anisotropic medium plays a major role in determining the system behavior for large deformation

the rotation rate of cylindrical objects during simple shear the rotational behaviour of circular objects is modelled using a two dimensional finite element simulation we present results for linear and non linear viscous rheologies and explore parameter space for the rheology contrast between object and matrix and for object size relative to the shear zone width l for high our results confirm analytical descriptions for l 0 in that the rotation rate of the object is half of the bulk shear strain rate however we show that for 0 1 l 0 9 the rotation rate as a function of l can have a minimum a maximum or it can decrease or increase depending on the stress exponent and in fact for some rheologies the rotation rate may decrease to 0 3 of the shear strain rate at intermediate l 0 5 20 area of porphyroblast in thin section these counter intuitive results have important consequences for the interpretation of the rotational behaviour of minerals in deforming rocks for example in rocks where garnet constitutes a volumetrically significant proportion of the bulk rock the effective shear zone boundaries may be given by neighbouring crystals corresponding to intermediate l as described here thus the interpretation of the rotation rate of crystals in such rocks can only be performed if the distance to the effective system boundaries is known

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

maximum magnitude estimations of induced earthquakes at paradox valley colorado from cumulative injection volume and geometry of seismicity clusters the paradox valley unit pvu a salinity control project in southwest colorado disposes of brine in a single deep injection well since the initiation of injection at the pvu in 1991 earthquakes have been repeatedly induced pvu closely monitors all seismicity in the paradox valley region with a dense surface seismic network a key factor for understanding the seismic hazard from pvu injection is the maximum magnitude earthquake that can be induced the estimate of maximum magnitude of induced earthquakes is difficult to constrain as unlike naturally occurring earthquakes the maximum magnitude of induced earthquakes changes over time and is affected by injection parameters we investigate temporal variations in maximum magnitudes of induced earthquakes at the pvu using two methods first we consider the relationship between the total cumulative injected volume and the history of observed largest earthquakes at the pvu second we explore the relationship between maximum magnitude and the geometry of individual seismicity clusters under the assumptions that i elevated pore pressures must be distributed over an entire fault surface to initiate rupture and ii the location of induced events delineates volumes of sufficiently high pore pressure to induce rupture we calculate the largest allowable vertical penny shaped faults and investigate the potential earthquake magnitudes represented by their rupture results from both the injection volume and geometrical methods suggest that the pvu has the potential to induce events up to roughly mw 5 in the region directly surrounding the well however the largest observed earthquake to date has been about a magnitude unit smaller than this predicted maximum in the seismicity cluster surrounding the injection well the maximum potential earthquake size estimated by these methods and the observed maximum magnitudes have remained steady since the mid 2000s these observations suggest that either these methods overpredict maximum magnitude for this area or that long time delays are required for sufficient pore pressure diffusion to occur to cause rupture along an entire fault segment we note that earthquake clusters can initiate and grow rapidly over the course of 1 or 2 yr thus making it difficult to predict maximum earthquake magnitudes far into the future the abrupt onset of seismicity with injection indicates that pore pressure increases near the well have been sufficient to trigger earthquakes under pre existing tectonic stresses however we do not observe remote triggering from large teleseismic earthquakes which suggests that the stress perturbations generated from those events are too small to trigger rupture even with the increased pore pressures

microbial sequestration of phosphorus in anoxic upwelling sediments organic phosphorus is removed from the ocean by its conversion to phosphorite laboratory incubations suggest that bacteria catalyse phosphorite formation and that the rate of conversion is greatest under anoxic conditions phosphorus is an essential nutrient for life in the ocean phosphorus burial regulates marine primary production1 2 phosphorus is removed from the ocean by sedimentation of organic matter and the subsequent conversion of organic phosphorus to phosphate minerals such as apatite and ultimately phosphorite deposits3 4 bacteria are thought to mediate these processes5 but the mechanism of sequestration has remained unclear here we present results from laboratory incubations in which we labelled organic rich sediments from the benguela upwelling system namibia with a 33p radiotracer and tracked the fate of the phosphorus we show that under both anoxic and oxic conditions large sulphide oxidizing bacteria accumulate 33p in their cells and catalyse the nearly instantaneous conversion of phosphate to apatite apatite formation was greatest under anoxic conditions nutrient analyses of namibian upwelling waters and sediments suggest that the rate of phosphate to apatite conversion beneath anoxic bottom waters exceeds the rate of phosphorus release during organic matter mineralization in the upper sediment layers we suggest that bacterial apatite formation is a significant phosphorus sink under anoxic bottom water conditions expanding oxygen minimum zones are projected in simulations of future climate change6 potentially increasing sequestration of marine phosphate and restricting marine productivity

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

an estimate of kinematic vorticity from rotated elongate porphyroblasts internal fabric in strain shadows around elongate rutile porphyroblasts indicates rotation of porphyroblasts relative to the incremental stretching direction during deformation within a regional scale shear zone in south central alaska for porphyroblasts of various aspect ratios and initial orientations this fabric is used to assess the sense of rotation relative to the direction of maximum incremental stretching the total rotation and the vorticity the total rotation of porphyroblasts relative to the external fabric based on these parameters varies as a function of porphyroblast orientation all 111 porphyroblasts regardless of orientation have rotated in a clockwise sense relative to the external fabric comparison of these results with previous models suggests that the deformation which affected the porphyroblasts took place in the field of continuous forward rotation with a maximum ratio of the rate of pure shearing to the rate of simple shearing sr of 0 21 or a kinematic vorticity number wk 0 92 the model curves showing rotation vs final orientation do not vary significantly over the range of sr and r that is relevant to this study the data have significant scatter yet there is remarkable agreement between the data and the model for a of 1 5 these results illustrate how strain shadows around porphyroblasts with large aspect ratios and variable initial orientations provide a more complete assessment of kinematics than inclusion trails within more equant porphyroblasts

numerical simulations of spiral shaped inclusion trails can 3d geometry distinguish between end member models of spiral formation numerical 3d simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that a 3d spiral geometry differs between the rotation and nonrotation end member models of spiral formation proposed in the literature and b 3d spiral geometry can be used as a criterion to distinguish between the two end member models in rocks four principal differences are identified between the two sets of simulations smoothness of spiral curvature spacing of foliation planes alignment of individual foliation planes either side of the sphere representing the porphyroblast and spiral asymmetry with respect to matrix shear sense of these differences only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end member models in the absence of a readily applied test to distinguish the end member models interpretation of spiral inclusion trails is problematic it is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation

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

induced seismicity associated with fluid injection into a deep well in youngstown ohio 1 over 109 small earthquakes mw 0 4 3 9 were detected during january 2011 to february 2012 in the youngstown ohio area where there were no known earthquakes in the past these shocks were close to a deep fluid injection well the 14 month seismicity included six felt earthquakes and culminated with a mw 3 9 shock on 31 december 2011 among the 109 shocks 12 events greater than mw 1 8 were detected by regional network and accurately relocated whereas 97 small earthquakes 0 4 mw 1 8 were detected by the waveform correlation detector accurately located earthquakes were along a subsurface fault trending ene wsw consistent with the focal mechanism of the main shock and occurred at depths 3 5 4 0 km in the precambrian basement we conclude that the recent earthquakes in youngstown ohio were induced by the fluid injection at a deep injection well due to increased pore pressure along the preexisting subsurface faults located close to the wellbore we found that the seismicity initiated at the eastern end of the subsurface fault close to the injection point and migrated toward the west away from the wellbore indicating that the expanding high fluid pressure front increased the pore pressure along its path and progressively triggered the earthquakes we observe that several periods of quiescence of seismicity follow the minima in injection volumes and pressure which may indicate that the earthquakes were directly caused by the pressure buildup and stopped when pressure dropped

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

contact metamorphism hydrothermal alteration of tertiary basalts from the isle of skye northwest scotland alkali olivine basalts from skye were simultaneously contact metamorphosed by tertiary gabbro and granite intrusions and altered by the hydrothermal convection system that the plutons induced four metamorphic zones were mapped around the plutons furthest from the intrusions in the primary olivine zone metabasalts are composed of combinations of igneous olivine augite plagioclase titaniferous magnetite ilmenite zeolites gyrolite sulfides and chlorite smectite intergrowths closer to the plutons in the smectite zone saponite and carbonate appear primary olivine and gyrolite disappear and zeolites decrease dramatically in abundance still closer to the plutons in the amphibole zone actinolite edenite chlorite sphene epidote andradite and quartz appear and saponite and chlorite smectite intergrowths disappear along parts of the contact between gabbro and basalt in the orthopyroxeneolivine zone orthopyroxene metamorphic olivine and biotite appear and amphibole chlorite sphene epidote andradite carbonate and quartz disappear whole rock chemical data indicate only minor change in the major element chemical composition of the metabasalts during progressive metamorphism hydrothermal alteration two pyroxene eothermometry and various mineral fluid equilibria suggest the range of peak temperatures attained in the metamorphic zones orthopyroxene olivine zone 900 1 030 c amphibole zone 400 900 c smectite and primary olivine zones 5 smow mineralogically more altered basalts from the smectite zone have whole rock 18o 2 to 5 still more mineralogically altered basalts from the amphibole zone with one exception have 18o 2 completely recrystallized hornfelses from the orthopyroxene olivine zone have 18o 0 the principal mechanism of isotope exchange between basalt and metamorphic hydrothermal fluid probably was heterogeneous mineralfluid reaction

the rotation of garnet porphyroblasts around a single fold lukmanier pass central alps abstract the orientation of the straight internal foliation s i within large 5 mm garnet porphyroblasts has been measured relative to the orientation of the external foliation s e around a single antiform of 0 5 m wavelength which folds the dominant regional foliation the internal foliation is not constant in orientation but varies consistently both with position around the fold and with the porphyroblast ellipticity the dip of s i hinge dip taken as zero is consistently less than the dip of s e it increases with increasing dip of s e and with increasing ellipticity of the porphyroblasts s i effectively defines a fold with an opening angle greater than that in the external foliation the opening angle of this fold in s i decreases with increasing porphyroblast ellipticity the observed variation in the orientation of s i can be explained qualitatively by a flattened flexural flow model for fold development as could be expected for folding of a pre existing strongly anisotropic foliation the measurements clearly demonstrate that rotation of porphyroblasts relative to geographical co ordinates did occur during the development of this fold and that a model based on the classical theories of rotation of stiff inclusions in a weaker viscous matrix is most appropriate

reference frame angular momentum and porphyroblast rotation abstract rotation of small rigid objects in a deforming ductile matrix can produce two different types of microstructure a shape fabric due to alignment of the principal axes of a population of elongate objects and the inclusion trail microstructure preserved in syntectonic porphyroblasts we use numerical modeling to show that inclusion trails of elongate porphyroblasts are expected to be extremely complex in contrast snowball garnets are readily interpretable but misuse of reference frame and kinematic misconceptions have obfuscated the discussion on the formation of porphyroblast inclusion trails in general and snowball garnet inclusion trails in particular we clarify this point models for snowball garnet formation that are based on the notion of garnets being irrotational with respect to the earth can be rejected on a geometrical and kinematic basis further the notion that rigid objects embedded in a deforming ductile matrix generally do not rotate is unsound it violates the fundamental physical law of balance of angular momentum

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

deep injection and closely monitored induced seismicity at paradox valley colorado the u s bureau of reclamation s paradox valley unit pvu extracts aquifer brine from nine shallow wells along the dolores river paradox valley in southwestern colorado and after treating high pressure injects the brine 4 3 4 8 km below the surface pvu injects at rates between 800 and 1300 l min since 1991 pvu has emplaced over 4 106 m3 of fluid and induced more than 4000 surface recorded seismic events the events are recorded on the local 15 station paradox valley seismic network the induced seismicity at paradox separates into two distinct source zones a principle zone 95 of the events asymmetrically surrounding the injection well to a maximum radial distance of 3 km and a secondary ellipsoidal zone 2 5 km long and centered 8 km northwest of the injection well the expansion of these zones has stabilized since mid 1999 about three years after the onset of continuous injection within the principal zone hypocenters align in distinct linear patterns showing at depth stratigraphy and the local wray mesa fracture and fault system the primary faults of the wray mesa system are aseismic striking subparallel to the inferred maximum principal stress direction with one or more faults probably acting as fluid conduits to the secondary seismic zone individual seismic events in both zones do not discernibly correlate with short term injection parameters however a 0 5 km2 region immediately northwest of the injection well responds to long term large scale changes in injection rate and the surpassing of a threshold injection pressure focal mechanisms of the induced events are consistent with simple double couple strike slip moments and subhorizontal extension to the northeast in addition the fault planes are consistent with principal stress directions determined from borehole breakouts more than 99 9 of the pvu seismicity is below human detection m 2 5 however approximately 15 events have been felt locally with the largest being a magnitude m 4 3 because of the m 4 3 and two earlier felt m 3 5 events and injection economics pvu changed injection strategies three times since 1996 these changes reduced seismicity from 1100 events year to as low as 60 events year

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

flow mixing object matrix coherence mantle growth and the development of porphyroclast tails abstract the kinematic stability fields of common porphyroclast types have been defined using a computer program that simulates low reynolds number newtonian shearing flow in the vicinity of a rigid spherical object the principal variables governing the geometry of porphyroclast tails at a given finite shear strain are 1 porphyroclast matrix coherence and 2 mantle recrystallization history both of which affect the extent to which mantle material becomes mixed with the matrix as deformation progresses

how useful are millipede and other similar porphyroblast microstructures for determining synmetamorphic deformation histories oppositely concave microfolds ocms in and adjacent to porphyroblasts can be classified into five nongenetic types type 1 ocms are found in sections through porphyroblasts with spiral shaped inclusion trails cut parallel to the spiral axes and commonly show closed foliation loops type 2 ocms commonly referred to as millipede microstructure are highly symmetrical the foliation folded into ocms being approximately perpendicular to the overprinting foliation type 3 ocms are similar to type 2 but are asymmetrical the foliation folded into ocms being variably oblique to the overprinting foliation type 4 ocms are highly asymmctrical only one foliation is present and this foliation is parallel to the local shear plane type 5 ocms result from porphyroblast growth over a microfold interference pattern types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths respectively during porphyroblast growth however theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing given particular initial foliation orientation and timing of porphyroblast growth type 3 ocms can also form during these intermcdiatc deformation paths and are commonly found in the same rocks as type 2 ocms type 4 ocms may indicate highly noncoaxial deformation during porphyroblast growth but may be difficult to distinguish from type 3 ocms thus types 1 3 and possibly 4 reflect the finite strain state giving no information about the rotational component of the deformation s responsible for their formation furthermore there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation s during the growth of porphyroblasts containing ocms type 2 ocms that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening but there is a lack of supporting physical or computer modelling it is possible that microstructures in the matrix around ocms formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another however determining degrees of noncoaxiality from rock fabrics is a major long standing problem in structural geology

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

glacial interglacial variations in marine phosphorus cycling implications for ocean productivity 1 using a box model of organic carbon c and phosphorus p cycling in the global ocean we assess the effects of changes in the continental supply of reactive p oceanic mixing and sea level on the marine p cycle on glacial interglacial timescales our model results suggest that mixing is a dominant forcing during early glaciation causing retention of p in the deep ocean thereby lowering primary production and associated organic c and p burial sea level fall is the dominant forcing during late glaciation when reduced coastal trapping of reactive p enhances its transfer to the open ocean restoring primary production and p burial during postglacial periods changes in circulation and weathering dominate open ocean processes and oceanic primary production peaks as primary production is reduced upon glaciation changes in the marine p cycle are unlikely to drive enhanced primary production and co2 drawdown during glaciation

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

geological structure of the paradox valley region colorado and relationship to seismicity induced by deep well injection the bureau of reclamation reclamation operates a deep injection well at paradox valley in western colorado seismicity has been recorded since 1985 with more than 5900 likely induced earthquakes detected since injection operations began in 1991 making this project a unique and valuable case study in the long term development of injection induced seismicity likely induced earthquakes occur in a complex pattern with clusters separated by aseismic gaps of 2 km or more and have been observed up to 16 km from the injection well to evaluate how the subsurface geologic structure may control the occurrence of induced seismicity we compare the earthquake locations to a set of geologic models that reclamation commissioned prior to drilling the injection well our analysis indicates that many aspects of the observed seismicity pattern can be attributed to the complex geologic structure the earthquake depths and locations are generally consistent with the structural trends of the primary injection target formation within the uncertainty of the models and some of the mapped basement faults appear to serve as boundaries in the earthquake locations suggesting that they are barriers to flow additionally the majority of events follow the predicted fluid flow pattern which trends northwest southeast and wraps around paradox valley some features of the seismicity pattern such as the early onset of seismicity to the northwest of the injection well and the distinct clustering are not explained by the current geologic models and may be related to unmapped geologic features or local variations in the stress field

an estimate of the efficiency of the iron and manganese driven dissolved inorganic phosphorus trap at an oxic euxinic water column redoxcline 1 geochemical records suggest the ocean has undergone periods of at least partial deeper ocean anoxia or euxinia two counteracting feedback loops involving redox control of the dynamics of the phytoplankton nutrient dissolved inorganic phosphorus dip might coexist helping to stabilize the redox state of the atmosphere and oceans this concept implies that during deeper ocean anoxia the dip transfer from the deep anoxic into the oxic surface ocean is uninhibited by processes taking place at the redoxcline this implicit assumption requires testing because iron fe and manganese mn dynamics at oxic anoxic water column redoxclines have the potential to form a dip trap inhibiting dip transport from anoxic deep into oxic surface waters using a time series data set of fe mn dip and dissolved oxygen distributions in the eastern gotland basin of the baltic sea we provide estimates of the efficiency of this fe and mn driven dip trap this efficiency was estimated by calculating the ratios of 1 the downward flux of dip adsorbed onto and or coprecipitated into the settling authigenic fe and mn rich particles just above the redoxcline and 2 the upward turbulent diffusive dip flux across the redoxcline depending on the assumed particle densities we find average 1 sd trapping efficiencies of 0 38 0 29 and 0 63 0 45 the efficiencies are significant in that they seem to impact cyanobacterial dynamics in the central baltic sea we discuss possible implications of the trapping mechanism for and propose two hypotheses relating to the potential importance of fe controlled dip trapping at redoxclines during ocean anoxic events

phosphorus cycling in marine sediments advances and challenges abstract phosphorus p is a critical macronutrient for all living cells as it provides the phosphate ester backbone of nucleic acids plays a crucial role in the transmission of chemical energy by the atp molecule and is also a structural constituent in many cell components while p cycling in the marine water column has been the subject of many studies and reviews many aspects of the sedimentary p cycling remain poorly understood this is mainly due to low p concentrations in sediments and analytical difficulties associated with isolating distinct p forms from sediment samples the aim of this review is to highlight the advances made in our understanding of p cycling in marine sediments as a result of improved instrumentation and novel approaches some of these techniques range from sediment sequential extractions sedex to spectroscopic techniques 31p nmr and innovative isotope tracer experiments stable oxygen ratios in phosphate radioactive p isotopes while our understanding of p cycling has considerably improved numerous aspects of sedimentary p cycling including the fate of organic p in sediments as a function of redox conditions organic matter content and burial depth as well as turnover rates of different sedimentary p compounds remain poorly constrained the techniques presented in this review along with yet to be utilized innovative approaches used in soil and lacustrine sediment p research will play an important role in addressing these important questions and in furthering our understanding of this critical component of the p global biogeochemical cycle

injection induced seismicity poroelastic and earthquake nucleation effects the standard model of injection induced seismicity considers changes in coulomb strength due solely to changes in pore pressure we consider two additional effects full poroelastic coupling of stress and pore pressure and time dependent earthquake nucleation we model stress and pore pressure due to specified injection rate in a homogeneous poroelastic medium stress and pore pressure are used to compute seismicity rate through the dieterich 1994 model for constant injection rate the time to reach a critical seismicity rate scales with t r2 cfc where r is distance from the injector c is hydraulic diffusivity and fc is a factor that depends on mechanical properties and weakly on r the seismicity rate decays following a peak consistent with some observations during injection poroelastic coupling may increase or decrease the seismicity rate depending on the orientation of the faults relative to the injector if injection induced stresses inhibit slip abrupt shut in can lead to locally sharp increases in seismicity rate tapering the flux mitigates this effect the maximum magnitude event has been observed to occur postinjection we suggest the seismicity rate at a given magnitude depends on the nucleation rate the size distribution of fault segments and if the background shear stress is low the time varying volume of perturbed crust this leads to a rollover in frequency magnitude distribution for larger events with a corner that increases with time larger events are absent at short times but approach the background frequency with time larger events occurring post shut in are thus not unexpected

porphyroblast inclusion trail orientation data eppure non son girate extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non coaxial ductile deformation of the matrix subsequent to their growth this has been demonstrated for all tectonic environments so far investigated the work also has provided new insights and data on metamorphic structural and tectonic processes including 1 the intimate control of deformation partitioning on metamorphic reactions 2 solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones and relative plate motion and 3 a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens

phosphorus accumulation in marine sediments and the oceanic phosphorus cycle ideas about key factors in the oceanic mass balance of dissolved reactive phosphate have changed substantially i present an integrated overview of these here with an emphasis on evaluating the burial sinks for p and defining areas needing further research the major source of reactive p to the ocean is river input reactive p is delivered to the oceanic sediment water interface primarily in particulate organic matter p scavenged by hydrothermal iron rich oxyhydroxide particles with uptake in proportion to deep water phosphate concentrations represents a substantially smaller flux to the sediment water interface diagenetic transformations are important influences on the form of reactive p burial in marine sediments p burial occurs with organic carbon burial and as p associated with iron rich oxyhydroxide particles and coatings formation of authigenic p rich phases presumably apatite at the expense of organic p and oxide associated p is significant in open ocean marine sediments the authigenic p sink may represent a substantially larger portion of the sedimentary burial than indicated by previous estimates focused on p burial in organic rich continental margin sediments

a comparative geochemical study of pelitic schists and metamorphosed carbonate rocks from south central maine usa whole rock major element chemical analyses of progressively metamorphosed impure carbonate rocks and pelitic schists collected from the same metamorphic terrain reveal similarities and differences in the chemical response of these rock types to the metamorphic event relative to a constant aluminum reference frame both schist and carbonate exhibit no detectable change in their contents of fe mg ti si and ca with change in metamorphic grade carbonate rocks become progressively depleted in k and na with increasing grade of metamorphism while schists exhibit no statistically significant change in their contents of k and na both rock types become depleted in volatiles principally co2 and h2o with increasing grade

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

toward a unified conceptual framework for shear sense indicators abstract the growing lexicon for shear sense indicators has created a need for a unified conceptual system of classification many common shear structures can be described by linear combinations of two kinematic variables which quantify the relative importance of objects vs foliations in controlling the shear zone velocity field

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

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

rapid 20th century increase in coastal upwelling off northwest africa near shore waters along the northwest african margin are characterized by coastal upwelling and represent one of the world9s major upwelling regions sea surface temperature sst records from moroccan sediment cores extending back 2500 years reveal anomalous and unprecedented cooling during the 20th century which is consistent with increased upwelling upwelling driven ssts also vary out of phase with millennial scale changes in northern hemisphere temperature anomalies nhtas and show relatively warm conditions during the little ice age and relatively cool conditions during the medieval warm period together these results suggest that coastal upwelling varies with nhtas and that upwelling off northwest africa may continue to intensify as global warming and atmospheric co 2 levels increase

rotation of porphyroblasts in non coaxial deformation insights from computer simulations porphyroblast inclusion trails have the potential to provide critical information about tectonometamorphic events recently however traditional interpretations of inclusion trails have been called into question by the suggestions that porphyroblasts do not rotate during non coaxial deformation and that apparent spiral inclusion trails can be generated in coaxial deformation we present a new computer model that simulates inclusion trail development model results suggest 1 that the extent of porphyroblast rotation is controlled by conditions at the porphyroblast matrix boundary 2 that curved inclusion trails may develop in unrotated porphyroblasts 3 that classic snowball inclusion trails are most simply explained by rotational growth histories and 4 that some of the observations used to support the view that porphyroblasts do not rotate e g weakly sigmoidal inclusion trails apparent truncations of inclusion trails can be accounted for by variations in the growth rate of rotating porphyroblasts

intense biological phosphate uptake onto particles in subeuphotic continental margin waters elucidating the processes that affect particulate phosphorus p export from the euphotic zone and burial in sediments is important for models of global phosphorus nitrogen and carbon cycling we investigated dissolved inorganic pi incorporation into particles 0 2 m in the sub euphotic zone and benthic boundary layer bbl of high productivity mauritanian and namibian shelf waters using 33po43 tracer experiments combined with a sequential chemical extraction analysis pi uptake 5 4 to19 9 nmol p l 1d 1 by particulate matter was biologically mediated 50 into the organic fraction and similar to estimated rates of heterotrophic growth thus a substantial fraction of pi must be recycled through a particle associated microbial pool rapid adsorption of 33p in the anoxic waters of namibia indicated the additional existence of a large pool of surface exchangeable p particle associated pi recycling and adsorption may influence the export flux and ultimate fate of particle bound p in continental shelf waters

an x ray computed tomography study of inclusion trail orientations in multiple porphyroblasts from a single sample inclusion trails in 58 garnet porphyroblasts in a single sample from the cram hill formation in southeast vermont were imaged using high resolution x ray computed tomography texturally the porphyroblasts have a large core with a sub vertical foliation that has a well defined geometry with a mean orientation of 113 72e for a 95 confidence cone semi angle of 5 3 this steep foliation curves into a sub horizontal foliation the foliation intersection inflection axes fias defined by hinge lines for this curvature are tightly clustered with a mean plunge of 11 towards 201 with a 95 confidence cone semi angle of 4 1 eigenvalue analysis indicates that the clustered distribution of the foliation and fia data are unlikely to be the result of a random event there is evidence in the specimen for multiple foliation forming events subsequent to garnet nucleation and the preservation of these clustered distributions in their wake strongly suggests that the porphyroblasts have not rotated with respect to a geographic reference frame fias represent a measurable structural element that can provide information on tectonic events at the time the porphyroblasts grew a comparison of fia data collected using high resolution x ray computed tomography with data collected using the asymmetry method demonstrates that the asymmetry method is a valid technique for defining the mean fia orientations in a sample

phosphorus imbalance in the global ocean the phosphorus budget of the pre human modern ocean is constrained applying the most recent estimates of the natural riverine eolian and ice rafted input fluxes the phosphorus burial in marine sediments and the hydrothermal removal of dissolved phosphate from the deep ocean this review of current flux estimates indicates that the phosphorus budget of the ocean is unbalanced since the accumulation of phosphorus in marine sediments and altered oceanic crust exceeds the continental input of particulate and dissolved phosphorus the phosphorus mass balance is further tested considering the dissolved phosphate distribution in the deep water column the marine export production of particulate organic matter rain rates of phosphorus to the seafloor benthic dissolved phosphate fluxes and the organic carbon to phosphorus ratios in marine particles these independent data confirm that the phosphate and phosphorus budgets were not at steadystate in the pre human global ocean the ocean is losing dissolved phosphate at a rate of 11 6 x 1010 mol yr 1 corresponding to a decline in the phosphate inventory of 4 5 kyr 1 benthic data show that phosphate is preferentially retained in pelagic deep sea sediments where extended oxygen exposure times favor the degradation of particulate organic matter and the up take of phosphate in manganese and iron oxides and hydroxides enhanced c p regeneration ratios observed in the deep water column 400 m water depth probably reflect the preferential burial of phosphorus in pelagic sediments excess phosphate is released from continental margin sediments deposited in low oxygen environments the dissolved oxygen threshold value for the enhanced release of dissolved phosphate is 20 m benthic phosphate fluxes increase drastically when oxygen concentrations fall below this value

phosphate oxygen isotopes insights into sedimentary phosphorus cycling from the benguela upwelling system abstract marine sediments are the main sink in the oceanic phosphorus p cycle the activity of benthic microorganisms is decisive for regeneration reflux or burial of inorganic phosphate pi which has a strong impact on marine productivity recent formation of phosphorites on the continental shelf and a succession of different sedimentary environments make the benguela upwelling system a prime region for studying the role of microbes in p biogeochemistry the oxygen isotope signature of pore water phosphate 18op carries characteristic information of microbial p cycling intracellular turnover of phosphorylated biomolecules results in isotopic equilibrium with ambient water while enzymatic regeneration of pi from organic matter produces distinct offsets from equilibrium the balance of these two processes is the major control for 18op our study assesses the importance of microbial p cycling relative to regeneration of pi from organic matter from a transect across the namibian continental shelf and slope by combining pore water chemistry sulfate sulfide ferrous iron pi steady state turnover rate modeling and oxygen isotope geochemistry of pi we found 18op values in a range from 12 8 to 26 6 both in equilibrium as well as pronounced disequilibrium with water our data show a trend towards regeneration signatures disequilibrium under low mineralization activity and low pi concentrations and microbial turnover signatures equilibrium under high mineralization activity and high pi concentrations these findings are opposite to observations from water column studies where regeneration signatures were found to coincide with high mineralization activity and high pi concentrations it appears that preferential pi regeneration in marine sediments does not necessarily coincide with a disequilibrium 18op signature we propose that microbial pi uptake strategies which are controlled by pi availability are decisive for the alteration of the isotope signature this hypothesis is supported by the observation of efficient microbial pi turnover equilibrium signatures in the phosphogenic sediments of the benguela upwelling system

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

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

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

evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the californian continental margin peruvian continental slope north carolina continental slope and from the santa monica basin california are reported comparison of these sites indicates that o2 depleted bottomwaters enhance p regeneration from sediments diminishing overall phosphorus burial efficiency based on these observations a positive feedback linking ocean anoxia enhanced benthic phosphorus regeneration and marine productivity is proposed on shorter timescales these results also suggest that o2 depletion in coastal regions caused by eutrophication may enhance p regeneration from sediments thereby providing additional p necessary for increased biological productivity