microstructures and c axis fabrics indicative of quartz dissolution in sheared quartzites and phyllonites abstract quartzites sheared in low metamorphic grade shear zones that extend along the contact with migmatitic gneissic complexes in the quadrilatere ferifero region southeastern brazil show quartz microstructures indicating deformation by dissolution dissolution occurred adjacent to the foliation planes preferentially in quartz porphyroclasts oriented with their 0001 planes parallel to the foliation dissolution worked as an efficient mechanism to remove porphyroclast edges smoothing foliations and enabling strain accommodation both dissolution and crystal plasticity were competitive crystallographically controlled processes operating in different domains of the evolving microstructure dissolution occurred along the foliation planes while isovolume crystal plasticity and accompanying recrystallization operated essentially in the quartzose domains between folia phyllonites formed in the most strained domains of the shear zones fine grained quartz grains in phyllonites show uncommon c axis fabrics c axis oriented at low angles to the stretching lineation which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavourably orientated to undergo dissolution domainal c axis fabrics reflect a deformation partitioning path with increasing participation of solution transfer strengthening c axis maxima at low angles with the stretching lineation at the expense of crystal plastic processes during progressive phyllonitization

calibration of rock fall modeling parameters abstract rock falls can be a hazard for many facilities in mountainous terrain particularly in areas with high rainfall and freezing temperatures and where seismic events occur design of protection measures against rock falls such as fences barriers and sheds requires information on impact energy to determine the strength of the structure and on trajectories to determine its location and size this paper presents documentation on rock falls at five locations in north america and japan involving impacts on rock talus colluvium asphalt and concrete for which values of the normal and tangential coefficients of restitution have been calculated the field results show that the normal coefficient of restitution is related to the impact angle with coefficients of about 0 2 for normal impacts and values as great as 2 5 for shallow impacts these values for the normal coefficient of restitution are essentially independent of the slope material the tangential coefficient of restitution ranges between 0 3 and 0 8 and is related to the friction coefficient at the impact point and is independent of the velocity and normal force it is also shown that the calculated field values for the coefficients of restitution are consistent with the principles of impact mechanics

theory and calibration of the pierre 2 stochastic rock fall dynamics simulation program the use of computer models to determine rock fall hazards is increasingly common with increasingly complex models being developed in most practical applications slopes potentially affected by ro

channel and floodplain response to recent abrupt climate change the tyne basin northern england this paper examines the timing nature and magnitude of river response in upland piedmont and lowland reaches of the tyne basin northern england to high frequency 20 30 year changes in climate and flood regime since 1700 ad over this period fluvial activity has been characterized by alternating phases of river bed incision and stability coinciding with non random decadal scale fluctuations in flood frequency and hydroclimate that appear to be linked to changes in large scale upper atmospheric circulation patterns episodes of widespread channel bed incision 1760 1799 1875 1894 1955 1969 result from a higher frequency of large floods 20 year return period and cool wet climate under meridional circulation regimes phases of more moderate floods 5 20 year return period corresponding to zonal circulation types 1820 1874 1920 1954 are characterized by enhanced lateral reworking and sediment transfer in upper reaches of the catchment and channel narrowing and infilling downstream rates of fluvial activity are reduced in intermediate periods 1800 1819 1895 1919 with no dominant circulation regime associated with lower flood frequency and magnitude the results of this study provide a valuable guide for forecasting probable drainage basin and channel response to future climate change

relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation field observations of mafic dykes intruded into felsic rocks from the lewisian complex nw scotland suggest that the mafic rocks are weaker than the felsic ones contrary to experimental results in order to resolve this conflict samples were studied to constrain the deformation mechanisms active under amphibolite facies conditions crystallographic preferred orientation cpo in plagioclase is used to infer deformation by dislocation creep on the 112 11 0 11 2 110 001 11 0 and 001 11 0 slip systems with increasing strain this cpo became weaker due to grain boundary sliding that accompanied diffusion creep in rocks where grain size reduction of plagioclase occurred by chemically dominated recrystallisation there is no cpo suggesting deformation was wholly accommodated by diffusion creep and grain boundary sliding in metamorphosed dykes plagioclase grains have orientations that are not consistent with dislocation creep deformation amphibole has a cpo consistent with either dislocation creep on 100 or deformation by diffusion creep with anisotropic dissolution and precipitation rates it is inferred that metamorphism of the dykes lead to the production of fine grained amphibole and plagioclase both of which deform by diffusion creep but that anisotropic dissolution and precipitation in the amphibole produced a cpo the mafic dykes are weaker than the felsic gneisses because grain size reduction is more extreme in the dykes even though both may be deforming primarily by grain size sensitive mechanisms this work highlights the variation in processes active in polyphase rocks and how these can lead to variations in cpo of the same mineral among different rock types

grain boundary sliding and development of grain boundary openings in experimentally deformed octachloropropane abstract in situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure and simple sheared octachloropropane at 75 85 of its absolute melting temperature and at strain rates of 10 5 10 6 s 1 are discussed discontinuities in the strain rotation and or translation components of deformation across the grain boundary induce grain boundary sliding the influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding grain boundary diffusion also causes grain boundary migration types ii and iii that is different from conventional grain boundary migration type i in being non conservative and in the details of boundary movement with respect to material points within grains grain boundary openings filled with fluid octachloropropane vapor are strongly associated with grain boundary sliding openings develop preferentially along grain boundaries at low angles to the shortening direction once openings grow they are closed by thrusting of sliding grains and by diffusion in faster strain rate experiments and entirely by diffusion in slower strain rate experiments an approximately steady openings ratio of 0 5 3 of the sample volume persists without the development of any large scale fracture all grain boundary openings disappear during static readjustment of the microstructure after deformation

rockfall analysis along transportation corridors in high hill slopes the paper discusses the problems pertinent to rockfall activities in highly jointed rock cut slopes excavated without detailed geotechnical and geological investigations along the right bank of river sutlej luhri himachal pradesh india the cut slopes are near vertical to sub vertical and overhangs are ubiquitous at several locations initially probable zones of slope failure were identified from field inspection and kinematic analysis the study then uses 2d rockfall analysis for estimating loss of energy due to the impact of falling blocks on the slope face and subsequently on the road several parameters were also assessed from the study viz bounce height travel distance kinetic energy and translational velocity the result from rockfall analysis shows that the energy and velocity achieved by the falling blocks are considerably high capable enough to cause severe damage in relation to energy and velocity bounce height achieved is not very large because of removal of trees from the slopes face and no measures to capture falling blocks the problems have been aggravated and therefore most of the rock covers very large distance to reach up to the valley crossing the roadways endangering travelers and infrastructures based on the results preventive measures have been suggested to prevent future rockfall hazards

quartz fluid inclusion abundance and off fault damage in a deeply exhumed strike slip seismogenic fault abstract off fault damage zones comprise highly fractured rocks surrounding the dynamic slip surface of faults these damage zones modify fault zone rheology and rupture dynamics by changing the bulk elastic properties and modulating fluid flow damage zones in the brittle upper crust reaching widths 100 m are commonly characterized by measuring fracture density but identifying the extent of off fault damage in deeply exhumed faults is challenging due to post and inter seismic viscous plastic deformation we measured fluid inclusion abundance in quartz deformed at 400 500 c from an ancient seismogenic strike slip fault shear zone to evaluate whether it can be a proxy for damage zone width in contrast to upper crustal fault zones displaying a high low trend of healed microfracture density the shear zone has a low high low trend of fluid inclusion abundance from the core toward the host rock we find that pattern explicable through removal of fluid inclusions by recrystallization after co seismic deformation allowing us to use fluid inclusion abundance to measure damage zone extent which is 80 m the low high abundance region from the shear zone core our findings indicate that extensive co seismic damage zones may extend from earth s surface to the base of the seismogenic zone well within the frictional to viscous transition

fluid assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone semail ophiolite this study reports on feedback mechanisms between fluid migration ductile deformation and strain localization processes in an incipiently forming mantle wedge the basal banded unit of the semail ophiolite these peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction during this stage they were affected by coeval ductile deformation forming proto mylonites at 900 800 c to ultramylonites at 700 c and interaction with subduction fluids from the petrological and microstructural study of these hydrated peridotites and their protolith preserved lenses of porphyroclastic tectonites we show that peridotite interaction with hydrous fluids triggered dissolution precipitation processes dissolution of coarser grains and precipitation of new and smaller ones mainly pyroxenes spinel and amphibole resulted in a drastic grain size reduction phase mixing and a switch from olivine dislocation to grain size sensitive creep in proto mylonites data also evidence a feedback process of fluid focusing in actively deforming shear zones this rapid switch in olivine deformation mechanism driven by subduction fluid peridotite interaction triggered an intense weakening of the peridotites at 900 800 c and a transition from a mechanically decoupled to coupled plate interface as witnessed by the detachment and underplating of a high temperature metamorphic sole to the base of the ophiolite it also explains the intense strain localization 1 km thick shear zone along this ductile portion of the plate interface considering that similar mechanisms take place in mature subduction zones they may explain plate interface coupling at subarc depths in worldwide subduction zones

the strain dependent spatial evolution of garnet in a high pressure ductile shear zone from the western gneiss region norway a synchrotron x ray microtomography study reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono metamorphic processes however they are usually analysed in two dimensions omitting important information regarding the third spatial dimension we applied synchrotron based x ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite garnet eclogite in four dimensions where the 4th dimension is represented by the degree of strain in the investigated samples which cover a strain gradient into a shear zone from the western gneiss region norway we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain the microtomographic data allowed quantification of garnet volume shape and spatial arrangement evolution with increasing strain the microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe empa and electron backscatter diffraction ebsd analysis to correlate mineral composition and orientation data with the x ray absorption signal of the same mineral grains with increasing deformation the garnet volume almost triples in the low strain domain garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample we also observed that garnet coronas in the gabbros never completely encapsulate olivine grains in the most highly deformed eclogites the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics we interpret the aligned garnet aggregates to direct synkinematic fluid flow and consequently influence the transport of dissolved chemical components ebsd analyses reveal that garnet shows a near random crystal preferred orientation that testifies no evidence for crystal plasticity there is however evidence for minor fracturing neo nucleation and overgrowth microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies becoming more almandine rich we interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming

late holocene floodplain incision and alluvial fan formation in the central grampian highlands scotland chronology environment and implications a section cut across an alluvial fan and the underlying floodplain terrace in the central grampian highlands provides an unusually complete record of late holocene events at ca 2 7 2 4 cal kyr bp floodplain aggradation was replaced by net floodplain incision pollen evidence and charcoal counts provide no evidence for contemporaneous anthropogenic landscape change and the timing of the transition suggests that it reflects an increase in high magnitude erosive flood events following overall climatic deterioration the overlying fan was deposited by torrential hyperconcentrated flows during three brief storm generated depositional events at ca 2 2 2 1 1 9 1 8 and 0 9 0 7 cal kyr bp separated and succeeded by prolonged periods of stability and peat accumulation during these three events a cumulative total of ca 6750 m3 of sediment was deposited probably in no more than a few hours over a timescale of two millennia these findings imply that proposed links between human activity and the development of alluvial fans or debris cones require reassessment and that different elements of the holocene alluvial landscape have responded in different ways to the same climatic inputs aggregation of dating evidence relating to aggradation or incision of alluvial landforms at different scales therefore may produce misleading results copyright 1999 john wiley sons ltd

an sem study of porosity and grain boundary microstructure in quartz mylonites simplon fault zone central alps a backscattered and secondary electron sem study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde simplon fault zone established three characteristic components 1 fine isolated pores 1 m diameter are scattered across two grain interfaces preferentially concentrated on surfaces in extension pores are uncommon on three grain junctions and there is no evidence for fluid interconnectivity along three and four grain junctions the fine porosity may develop by accumulation of original mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding dynamic cavitation implies that the ductile mylonitic deformation is at least locally dilatant and therefore pressure sensitive 2 large vug like pores up to mm scale extend along multi grain boundaries observed in all samples they are most common in the higher initial temperature coarse grained samples with a microstructure dominated by grain boundary migration recrystallization grains bordering this connected porosity develop perfect crystal faces undecorated by fine pores or pits the irregular lobate optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces the coarse porosity is probably due to accumulation during dynamic recrystallization of co2 rich fluid with a high wetting angle against quartz 3 in one sample interconnected sinuous ridges 0 2 m high are observed to follow three and four grain junctions and disjoint into more isolated worms and spheroidal globules on two grain interfaces these are transitional to more branching vein like or convoluted brain like forms the brain like and globular forms have been observed with varying frequency through the range of samples with the globules attaining sizes of up to 60 m vein structures have also been observed on intragranular fractures these topologies do not match across adjoining surfaces and must have developed into free space the ridge vein brain spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established they could represent unstable meniscus necking of a thin grain boundary phase of low viscosity developed due to quasi adiabatic shear and or local stress induced dilatancy during microcracking

quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep abstract in the gran paradiso metagranodiorite western alps small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture polycrystalline quartz aggregates in the mylonite deform by dislocation creep developing a crystallographic preferred orientation cpo and a monoclinic surface orientation distribution function odf the polymineralic matrix of the mylonite and the ultramylonite deform by diffusion creep in the ultramylonite the quartz cpo is randomized and the surface odf becomes orthorhombic the transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates concomitant with the precipitation of k feldspar biotite between quartz grains in quartz reduction from the dynamically recrystallized grain size in the aggregates 110 m to the size of the dispersed grains in the ultramylonite 25 m occurs through the following processes k feldspar precipitates at opening sites along grain boundaries strain incompatibility pinning the grain size in quartz aggregates coalescence of k feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates solution precipitation reduces the size of the dispersed grains to less than subgrain size 45 m

effects of frictional viscous oscillations and fluid flow events on the structural evolution and re os pyrite chalcopyrite systematics of cu rich carbonate veins in northern norway abstract mesothermal chalcopyrite pyrite magnetite bearing calcite dominated vein deposits in the repparfjord tectonic window northern norway have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation the veins cut across paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured coarse grained margins and the fine grained mylonitic cores they formed under overall viscous conditions although cyclically increased fluid pressures caused transient embrittlement as mineral precipitation sealed the fractures deformation was accommodated again viscously leading to mylonitization of the vein s core local brecciation of the calc mylonite demonstrates the cyclicity of this process re os chalcopyrite pyrite and k ar fault gouge dates define an almost continuous age range from 2540 ma to 460 ma regression of three re os analyses yields a 2069 14 ma age 187os 188os 0 18 0 04 interpreted as the age of vein emplacement sulfide precipitation and initial frictional viscous deformation k ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 ma hence indicating structural reactivation connected with silurian caledonian orogenic compression the spread in re os model ages reflects this reactivation wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing dynamic recrystallization and isotopic disturbance of the sulfides the study provides evidence for fluid flow during viscous deformation and demonstrates that strain and flow of oxidizing fluids can have a significant yet localized control on the integrity of the re os systematic in pyrite and chalcopyrite

a cavitation seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi brittle field vivero fault nw spain abstract bands of multi phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions indeed they are one of the most typical manifestations of strain localization in high strained polymineralic rocks in the lithosphere especially in the semi brittle field yet many questions remain on their origin and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove particularly for the inter grain mixing of phases here we present a case study of ultramylonite bands developed in a coarse grained granite deformed at conditions near the base of the seismogenic zone i e showing coseismic deformation microstructures overprinted by thermally activated ones we found that in the mylonite to ultramylonite transition two deformation mechanisms coexist promoting inter grain phase mixing through kfs precipitation 1 dislocation accommodated grain boundary sliding in quartz and 2 diffusion accommodated grain boundary sliding in feldspar the model proposed to explain grain mixing is based on a cavitation seal mechanism which is strongly dependent on grain boundary sliding for the opening of transient micro cavities and on the diffusivity of potassium feldspar for their progressive sealing

creep cavitation bands control porosity and fluid flow in lower crustal shear zones shear zones channelize fluid flow in earth s crust however little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone this study investigates the deformation mechanisms fluid rock interaction and development of porosity in a monzonite ultramylonite from lofoten northern norway the rock was deformed and transformed into an ultramylonite under lower crustal conditions temperature 700 730 c pressure 0 65 0 8 gpa the ultramylonite consists of feldspathic layers and domains of amphibole quartz calcite which result from hydration reactions of magmatic clinopyroxene the average grain size in both domains is 25 mm microstructural observations and electron backscatter diffraction analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains festoons of isolated quartz grains define c type bands in feldspathic layers these quartz grains do not show a crystallographic preferred orientation the alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites as evidenced from synchrotron x ray microtomography such c type bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding mass balance calculation indicates a 2 volume increase during the protolith ultramylonite transformation which is consistentmore with synkinematic formation of creep cavities producing dilatancy thus this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain size sensitive creep thereby stabilizing strain localization in the polymineralic ultramylonites less

statistical evaluation of rockfall energy ranges for different geological settings of new south wales australia abstract structures used in rockfall protection are designed on the basis of the expected impact energy this quantity is usually assessed using commercial lumped mass models that stochastically predict possible block trajectories on a given slope in new south wales australia it is estimated that rockfall hazard involves values of energy which are much lower than those in europe canada or the usa however this view has not been supported by any systematic study across the whole state such a study is presented in this paper it applies to the five geological situations that are the most prone to rockfall in eastern australia previous experimental findings relating to block size distribution or restitution coefficients reviewed herein have been used to perform this statistical analysis a newly formulated lumped mass program which incorporates a relationship between the normal restitution coefficient and the impact angle allows the adoption of normal restitution coefficients in excess of unity for impact angles lower than 30 the results confirm that in three out of the five geological situations the 95th percentile of impact energy is lower than 200 kj and less than 2000 kj for the other two situations

change in the frequency of extreme events as the indicator of climatic change in the holocene in fluvial systems abstract the transformation of natural systems of climatic origin in the holocene may be realised by a change in the type of extreme events or by a change in their frequency or by both the rapid warming at the younger dryas preboreal transition exemplifies a parallel drop in frequency and a change in the type of events several wetter phases of the holocene are characterised by rises in frequency of various types of extreme rainfalls the fragmentary records of events in the fluvial and slope systems coincide with smoothed variations in stable ecosystems and shifts of ecotonal zones such as tree line and lake levels

toward objective rockfall trajectory simulation using a stochastic impact model the accuracy of rockfall trajectory simulations depends to a large extent on the calculation of the rebound of falling boulders on different parts of a slope where rockfalls could occur the models commonly used for rebound calculation are based on restitution coefficients which can only be calibrated subjectively in the field to come up with a robust and objective procedure for rebound calculation a stochastic impact model associated with an objective field data collection method was developed and tested in this study the aims of this work were to assess the adequacy of this approach and to evaluate the minimum amount of field data required to obtain simulation results with a satisfactory level of predictability to achieve these objectives the rebound calculation procedure developed was integrated into a three dimensional rockfall simulation model and the simulated results were compared with those obtained from field rockfall experiments for rocky slopes the simulations satisfactorily predict the experimental results this approach is advantageous because it combines precise modelling of the mechanisms involved in the rebound and of their related variability with an objective field data collection procedure which basically only requires collecting the mean size of soil rocks the approach proposed in this study therefore constitutes an excellent basis for the objective probabilistic assessment of rockfall hazard

flood alluviation and entrenchment holocene valley floor development and transformation in the british uplands the morphology sedimentary properties and sequence of recent coarse grained flood deposits and earlier holocene alluvial fills were investigated in thinhope burn a small 12 km2 catchment in the northern pennine uplands northern england twenty one large flood events are recorded by distinctive cobble boulder bars sheets and splays and boulder berms and lobes lichenometric analysis showed that all but one of these floods dated from the mid eighteenth century the timing of large floods between 1766 and 1960 corresponds with major hydroclimatic trends evident in northern britain and northwest europe over this period discharge estimates suggest that flood magnitudes have decreased since the mid eighteenth century channel and flood plain metamorphosis in late roman times and in the eighteenth century following major valley floor entrenchment locally as much as 8 m would appear to have been caused by increased runoff and flood magnitude this was linked to a shift to a wetter climate with flow augmented by iron age and roman woodland clearance and drainage of the catchment in more recent times results from this study suggest that current models of longer term holocene and pleistocene valley floor development in the british uplands may need to be re evaluated

seismic anisotropy and its impact on imaging the shallow alpine fault an experimental and modeling perspective the transpressional alpine fault in new zealand has created a thick shear zone with associated highly anisotropic rocks low seismic velocity zones lvz and high seismic reflectivity are recorded in the alpine fault zone but no study has explored the underlying physical rock parameters of the shallow crust that control these observations protomylonites are the volumetrically dominant lithology of the fault zone here we combine experimental measurements of p wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged numerical models that account for the porosity free real samples fabric elastic tensors from ebsd are calculated by mtex and a finite element model fem while microfractures are modeled with differential effective media dem theory at effective pressures representative of the alpine fault brittle zone experimental wave speeds are lower than those predicted by mtex fem a possible dem model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures 70 mpa such micro porosity in the form of foliation and mica basal plane parallel microfractures and grain boundaries is validated with synchrotron x ray microtomography and tem images finally by modeling anisotropy of seismic reflection coefficients with angle of incidence we demonstrate that the high reflectivity and lvz observed at the alpine fault can only be explained if this microporosity is accounted for throughout the brittle fault zone even at depths of 7 10 km

deformation mechanisms and reaction of hornblende examples from the bergell tonalite central alps abstract hornblendes in the bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization during the crystallization of the magma deformation commences by magmatic flow in this stage the hornblendes behave as rigid particles in a viscous matrix melt the rotation and alignment of hornblende as elongate rigid particles have produced a strong preferred orientation of both particle long axes and crystallographic directions with progressive solidification of the melt there is a gradual transition from magmatic flow to solid state deformation in the bergell tonalite the crystallographic and shape preferred orientation that have developed during solid state deformation are identical to those of magmatic flow this identical fabric development during solid state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods i e hornblendes have always acted as rigid particles in a viscous matrix tem observations show no evidence for intracrystalline plasticity in hornblende grains the modal abundance of hornblende decreases progressively with increasing solid state deformation to yield more biotite epidote and quartz small hornblende matrix grains form by cataclastic processes the observed changes in grain size shape and abundance of hornblende occur mainly by fracturing dissolution and reaction so that the solid state deformation of the tonalite is a complex process to which fracturing dissolution of hornblende and metamorphic reactions all contribute the relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction it is inferred that generally hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650 750 c in the presence of an aqueous fluid crystal plasticity could become dominant at higher temperatures and or lower aqueous fluid activities

the ultimate fate of a synmagmatic shear zone interplay between rupturing and ductile flow in a cooling granite pluton abstract the neoarchean cundimurra pluton yilgarn craton western australia was emplaced incrementally along the transpressional cundimurra shear zone during syndeformational cooling discrete networks of cataclasites and ultramylonites developed in the narrowest segment of the shear zone showing the same kinematics as the earlier synmagmatic structures lithological boundaries between aplite pegmatite veins and host granitic gneiss show more intense pre cataclasite fabrics than homogeneous material and these boundaries later became the preferred sites of shear rupture and cataclasite nucleation transient ductile instabilities established along lithological boundaries culminated in shear rupture at relatively high temperature 500 600 c here tensile fractures at high angles from the fault plane formed asymmetrically on one side of the fault indicating development during seismic rupture establishing the oldest documented earthquake on earth tourmaline veins were emplaced during brittle shearing but fluid pressure probably played a minor role in brittle failure as cataclasites are in places tourmaline free subsequent ductile deformation localized in the rheologically weak tourmaline rich aggregates forming ultramylonites that deformed by grain size sensitive creep the shape and width of the pluton shear zone and the regime of strain partitioning induced by melt present deformation and established during pluton emplacement played a key role in controlling the local distribution of brittle and then ductile subsolidus structures

episodic tremor and slip explained by fluid enhanced microfracturing and sealing episodic tremor and slow slip events at the deep extension of plate boundary faults illuminate seismic to aseismic processes around the brittle ductile transition these events occur in volumes characterized by overpressurized fluids and by near failure shear stress conditions we present a new modeling approach based on a ductile grain size sensitive rheology with microfracturing and sealing which provides a mechanical and field based explanation of such phenomena we also model pore fluid pressure variation as a function of changes in porosity permeability and strain rate dependent fluid pumping the fluid enhanced dynamic evolution of microstructures defines cycles of ductile strain localization and implies increase in pore fluid pressure we propose that slow slip events are ductile processes related to transient strain localization while nonvolcanic tremor corresponds to fracturing of the whole rock at the peak of pore fluid pressure our model shows that the availability of fluids and the efficiency of fluid pumping control the occurrence and the p t conditions of episodic tremor and slip

transformation weakening diffusion creep in eclogites as a result of interaction of mineral reactions and deformation abstract the deformation of eclogites and the driving forces for their fabric development are an important topic potentially allowing to determine deformation rates and stresses in subduction zones where the greatest number of large earthquakes occurs here fabric studies of grain size and shape texture and chemical composition from two locations of variscan and alpine eclogites are presented all samples show a well developed crystallographic preferred orientation cpo of omphacite with a strong maximum of 001 in the lineation direction and a weaker maximum of poles to 010 normal to foliation garnet shows no systematic cpo anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde in terms of pressure change p t path the individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations chemical microstructural and cpo data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures the dominant deformation is inferred to be diffusion creep where dissolution of material took place in reacting mafic phases plagioclase pyroxene and precipitation took place in the form of new eclogite facies minerals omphacite garnet zoisite this type of diffusion creep deformation represents a transformation process involving both deformation and metamorphic reactions it is emphasized that the weakening is directly connected to the transformation and therefore transient the weakening facilitates diffusion creep deformation of otherwise strong minerals pyroxene garnet zoisite at far lower stresses than dislocation creep the results imply low stresses during the deformation of eclogite blocks in subduction zones these results can be applied to other rock types too

comparison of microstructures in superplastically deformed synthetic materials and natural mylonites mineral aggregation via grain boundary sliding we conducted compressional tensile and torsional creep experiments on fine grained forsterite plus ca bearing pyroxene aggregates a distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments the stress strain rate relationship grain size dependent flow strength and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding gbs as a result of gbs grain switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression we identify similar aggregated microstructures in previously reported micrographs of polymineralic granite origin ultramylonites mineral phase mixing through gbs which helps to retain fine grain size in rocks due to grain boundary pinning has been speculated to occur during formation of mylonites however our results contradict this hypothesis because mineral aggregation through gbs promotes demixing rather than mixing of the mineral phases gbs processes alone will not promote a transformation of well developed monomineralic bands to polymineralic bands during mylonitization

diffusion pathways of fe 2 and fe 3 during the formation of ferrian chromite a µxanes study the alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity fo2 and fe2 fe3 exchange during fluid rock interaction chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid rock interaction frequently overprints the record of earlier stages of alteration to shed light on that process we have studied the distribution of fe2 and fe3 in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron based microscopic chemical imaging and spatially resolved x ray absorption near edge structure spectroscopy our results show that early stages of alteration do not involve changes in cr3 and fe2 contents or in fe speciation but only depletion in al3 and mg2 resulting in the formation of porous chromite with ongoing alteration fe3 migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries sheared type chromitites record the maximum rates of fluid rock interaction because in these chromitite types the accommodation of deformation on porous chromite allowed higher rates of diffusion of fe3 and fe2 a magnetite component with fe3 fetotal 0 66 into the newly formed neoblasts in porous chromite type texture all the original chromite grains fully transformed to porous chromite the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains in contrast in partly altered type texture chromite grains with original cores surrounded by porous chromite such processes are only restricted to the porous rims giving rise to zoned chromian spinel ferrian chromite grains

in situ rockfall testing in new south wales australia abstract despite the significance of rockfall hazards in australia this phenomenon is still poorly characterised in many regional environments in particular the relationship between slope rock properties and rockfall motion parameters needs better definition in the context of rockfall prediction it is important to quantify the normal and tangential restitution coefficients referred to as k n and k t and the equivalent rolling coefficient which are site specific several series of rockfall tests have been conducted in three different geological environments in new south wales the results of the tests show a large variability of the motion parameters which is due to the natural variability of the blocks and to the randomness of the impact positions also values of k n consistently and systematically higher than the benchmark values from the literature have been inferred despite there being no clear correlation between the restitution coefficients and the rotational energy rotational phenomena are believed to be at the origin of such results

use of multi proxy flood records to improve estimates of flood risk lower river tay scotland abstract proxy flood records from sediment stacks in floodplain palaeochannels provide an opportunity to extend short instrumental records and thus improve current estimates of flood risk the bloody inches a meander cutoff on the lower river tay scotland has been infilling with flood deposits since c 1761 agricultural flood embankments locally breach with flows 850 m3 s 1 introducing silts into the palaeochannel and extensively fail with flows 1200 m3 s 1 which deposit sand repeated cores at the site up to 1 4 m in depth consistently reveal sand rich flood units in the upper section of the core 137cs dating enables these units to be correlated with clusters of floods in the post 1950 discharge record sand units in the lower part of the profile are correlated with major floods from 1814 onwards using a 210pb based chronology these dates are independently corroborated by flood marks inscribed on smeaton s bridge in perth 15 km downstream estimates of historic high flows back to 1814 can also be recovered from these flood stage levels at smeaton s bridge when combined with the 48 year long flow record these historic floods yield an augmented pot series extending back to 1847 using a relatively high threshold of 1361 m3 s 1 and the generalised pareto distribution to model this pot series estimates of the 50 100 and 200 year floods are 1875 2250 and 2050 m3 s 1 respectively these results provide independent and robust confirmation of the flood risk at perth using the standard statistical procedures advocated in the flood estimation handbook if successfully replicated at other sites radiometric dating of sediment stacks in palaeochannels provides a new technique to further advance palaeohydrology and the recovery of historic floods

weaker than weakest on the strength of shear zones thin laterally continuous ultramylonites within kilometer scale ductile shear zones may control mid lower crustal strength where deformation is localised interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach yet fine grained polyphase mixtures are commonly found in the cores of high strain zones we study a continental strike slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite to greenschist facies conditions a brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites respectively whereas the ultramylonites comprise a fine grained mixture of the host rock minerals the localisation of strain in ultramylonites demonstrates how fine grained polyphase mixtures can be weaker than and supersede interconnected phyllosilicate networks with increasing shear strain this contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach

the role of deformation reaction interactions to localize strain in polymineralic rocks insights from experimentally deformed plagioclase pyroxene assemblages abstract in order to study the mutual effect of deformation and mineral reactions we have conducted shear experiments on fine grained plagioclase pyroxene assemblages in a griggs type solid medium deformation apparatus experiments were performed at a constant shear strain rate of 10 5 s 1 a confining pressure of 1 gpa and temperatures of 800 850 and 900 c while the peak stress of plagioclase orthopyroxene assemblages reaches values between those of the end member phases the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase plagioclase this weakening correlates with the coeval development of high strain shear zones where new phases are preferentially produced including new pyroxene plagioclase and amphibole the reaction products mostly occur as intimately mixed phases within fine grained and interconnected shear bands together with different compositions with respect to the starting material this indicates that deformation significantly enhances the kinetics of mineral reactions which in turn strongly weaken the deforming sample here attributed to a switch to grain size sensitive diffusion creep through phase nucleation and grain size reduction such an interplay between deformation and mineral reactions may have strong implications for the initiation development and durability of shear zones in the lower crust

rock strength and texture evolution during deformation in the earth s ductile lithosphere a two phase thermodynamics model building upon the two phase and grain damage theory we propose a new formulation allowing to track the evolution of phase mixing segregation during ductile deformation of a two phased aggregate our model is based on a set of variables characterizing a rock texture the mean grain sizes and the phase proportion during ductile deformation activation of different micro mechanical processes impacts the aggregate texture dislocation and diffusion creep are the two main deformation processes considered we only account for the effect of zener pinning in slowing down grain growth and allow for active grain size reduction mechanisms in the diffusion creep domain for this purpose an equation is proposed to track the phase mixing evolution during ductile deformation numerical application using anorthite rheology shows that any grain reduction mechanisms that could be active in the diffusion creep regime requires a very high partition fraction in order to reach the grain size size predicted by the feldspar piezometer application of this model to gabbroic composition relevant for the ductile crust demonstrates that the strong coupling between phases grain sizes and interface evolution results in steady state grain sizes far below the field boundary this effect is coeval with an important increase of mixing between the two phases in addition accounting for the phase mixing results in a drop of the global aggregate stress during deformation this model allows for further comparison of mylonitized textures evolution with natural shear zones at the local and regional scales

the significance of geology for the morphology of potentially unstable rocks abstract from a consideration of the concepts of geological weathering and structure it can be expected that rockfall hazards should be characteristically different in different geological environments this paper tests this idea by looking at the geometric characteristics of rock fragments formed on natural slopes in four different geological environments in eastern australia where rockfall phenomena are often characterised by rolling of pre detached debris by measuring the three principle dimensions and making a systematic assessment of the shape characteristics of samples of rock debris in significant geological environments it is found that the distributions of size and shape for the surface debris are statistically different from the results it is shown that the size and shape of debris is directly controlled by the rock type its weathering characteristics and the structure of the parent rock mass the severity of rockfall hazards is shown to be relatively lower in areas of tertiary basalt as the size of rolling fragments is limited by closely spaced fracturing inherited from its formation and the tendency to deteriorate further as it weathers deeply and rapidly it is also lower in areas of palaeozoic volcanics since these tend to produce relatively angular fragments with higher proportions of fragments that are inherently more resistant to rolling by contrast thickly bedded sandstones form larger blocks with a larger proportion of shapes that are more prone to rolling the size distribution of fragments is shown to be well approximated by a log normal statistical distribution and using the data provided in this study it is possible to generate the size and shape data needed to undertake a stochastic assessment of rockfall trajectories in different geological environments

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

textural analysis and modelling of a flood deposit river severn u k samples of sediment collected from the severn floodplain between worcester and gloucester following the severe flooding in january and february 1990 were analysed for their grain size distribution the results show that most sand was deposited within 20 m of the channel bank but that fine sand may contribute to flood sediment across the width of the floodplain james 1985 numerical model of overbank sedimentation attempts to predict the transfer of sediment to the floodplain during flooding geometrical and hydraulic data relating to the severn flood are used as input for a computer program of james 1985 model the pattern of sediment concentrations predicted by the model was compared with that obtained from statistical analysis of the flood sediment the patterns were found to be similar so james 1985 model was considered to predict in a relative sense the distribution of flood sediment

conductivity response to intraplate deformation evidence for metamorphic devolatilization and crustal scale fluid focusing we present two dimensional electrical resistivity models of two 40km magnetotelluric mt profiles across the frome embayment to the east of the northern flinders ranges south australia the lower crust shows low resistivity of 10 m at around 30km depth the middle crust is dominated by resistive 1000 m basement rocks underlying the flinders ranges adjacent to the ranges conductive lower crust is connected to vertical zones of higher conductivity extending to just below the brittle ductile transition bdt at 10 km depth the conductive zones narrow in the brittle upper crust and dip at roughly 45 beneath the surface zones of enhanced conductivity coincide with higher strain due to topographic loading and sparse seismicity we propose that fluids are generated through neotectonic metamorphic devolatilization low resistivity zones display areas of fluid pathways along either pre existing faults or an effect of crustal compression leading to metamorphic fluid generation the lower crustal conductors are responding to long wavelength flexure induced strain while the upper crustal conductors are responding to short wavelength faulting in the brittle regime mt is a useful tool for imaging crustal strain in response to far field stresses in an intraplate setting and provides important constraints for geodynamic modeling and crustal rheology

replacement reactions and deformation by dissolution and precipitation processes in amphibolites the deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition fluid content pressure and temperature these mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation here we show that fracturing and fluid flow triggered coupled dissolution precipitation and dissolution precipitation creep processes which were responsible for the formation of a mylonitic microstructure in amphibolites this fabric is developed over a crustal thickness of 500 m in the lower seve nappe scandinavian caledonides amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence the albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the 001 cleavage planes and display lobate edges with embayments and peninsular features two albite oligoclase generations bright in cathodoluminescence resorbed and overgrew the porphyroclasts sealing the fractures electron backscattered diffraction shows that the two albite oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them these two albite oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation the amphibole crystals experienced a similar microstructural evolution as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c axis no crystal plasticity is observed in either mineral species plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies 600 c 0 75 0 97 gpa our results demonstrate that at middle to lower crustal levels the presence of h2o rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution precipitation and favours deformation by dissolution precipitation creep over dislocation creep in plagioclase and amphibole

experimental studies on fragmentation of rock falls on impact with rock surfaces rock fall is a common natural hazard causing significant damage to infrastructure and loss of life rock fragmentation is frequently observed during rock fall events and several authors have raised issues related to the impact of fragments on the protection structure however this phenomenon is not accounted for when designing the protection barriers the paper presents the results of 20 rockfall tests performed in a quarry in italy to provide new insight into the fragmentation phenomenon especially in the case of foliated materials the results have shown that the impacting angle plays a key role in the fragmentation of foliated rocks whereas the effect of the impacting energy tends to be of second order no threshold in impacting energy could be defined to explain what triggers the fragmentation it has been noticed that the proportion of impacting energy dissipated during fragmentation is relatively constant and depends on the choice of the normal restitution coefficient

the effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites abstract this study investigates the grain scale mechanisms that facilitated strain localization in the lower crust within the east athabasca mylonite triangle of northern saskatchewan canada there two distinct lithotectonic units the upper deck and northwestern domain are juxtaposed across a km scale shear zone that has been previously interpreted to record thrust and or normal sense displacement our study reports new quartz c axis opening angle ti in biotite and ti in quartz geothermometry and amphibole plagioclase geothermobarometry within syn kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies 725 c and 0 8 gpa while sharing similar conditions the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn kinematic hydrated mineral phases detailed microstructural and crystallographic preferred orientation cpo analyses reveals that specimens poor in hydrated minerals 10 pressure solution and re precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz cpo these results indicate a strong mineralogical and fluid control on deformation and therefore which strain softening process es may dominate in lower crustal shear zones

use of fallout 137cs in investigations of overbank sediment deposition on river floodplains abstract river floodplains have been recognised as important sinks for storing suspended sediment and associated contaminants mobilised from the upstream catchment however information on contemporary rates of overbank sedimentation is difficult to obtain using conventional methods measurements of the 137cs content of floodplain sediments provide an alternative approach to obtaining estimates of medium term rates ca 40 years of overbank sediment deposition the 137cs approach requires only a single site visit and minimum sample preparation furthermore it is capable of providing information on spatial patterns of sediment deposition on floodplains which is needed to improve our understanding of the processes involved in overbank flow and sediment deposition this paper reviews the basis for using 137cs measurements in floodplain sedimentation studies and presents improved procedures for interpreting 137cs profiles in floodplain sediments and for obtaining estimates of sedimentation rates from single measurements of the total 137cs inventories of bulk sediment cores the results obtained from a case study undertaken on the floodplain of the river stour dorset uk are presented

a combined radiometric and mineral magnetic approach to recent geochronology in lakes affected by catchment disturbance and sediment redistribution abstract as a result of empirical testing of 210 pb dating models in a wide variety of lakes we conclude that there are many situations where despite major changes in sedimentation rates as a result of lake or catchment disturbance the crs constant flux model provides a sound basis for establishing recent chronology factors which may complicate or vitiate the assumptions of the model include short water residence time bioturbation for which we propose a modified dating model discontinuous or disturbed sedimentation sediment resuspension and shifts in sediment source in the case of the last three processes we illustrate ways in which the problems can be identified and evaluated using mineral magnetic measurements we propose an evaluative approach to 210 pb dating projects with emphasis on independent dating evidence on tests of internal consistency and on the use of data from several correlatable cores

modelling flood hydraulics and overbank deposition on river floodplains this paper outlines a numerical model for the prediction of floodplain inundation sequences overbank deposition rates and deposit grain size distributions the model has two main components first a simplified hydraulic scheme which predicts floodwater flow depths and velocities and second a sediment transport element which employs a mass balance relation describing suspended sediment dispersion by convective and diffusive processes and sediment deposition as a function of particle settling rates these relationships are solved numerically on a finite difference grid that accurately replicates the complex topographic features typical of natural river floodplains the model is applied to a 600 m reach of the river culm devon u k using data derived from a range of field and laboratory techniques continuous records of river stage and suspended sediment concentration provide the model s upstream boundary input requirements these are supplemented by measurements of the in situ settling characteristics of the suspended sediment load the model s sediment transport component is calibrated with the aid of a dataset of measured overbank deposition amounts derived from flood events over a 16 month period the model is shown to predict complicated floodwater inundation sequences and patterns of suspended sediment dispersion and deposition which are largely a product of the complex topography of the floodplain these results compare favourably with observations of overbank processes and are an improvement over those of previous models which have employed relatively simple representations of floodplain geometry 1997 by john wiley sons ltd

how ductile are ductile shear zones three premises of ductile deformation in the middle and lower crust are widely accepted 1 rocks flow with power law viscous rheology 2 localization in ductile shear zones involves strain softening and 3 fluid flows into and is channelized within ductile shear zones ductile viscous shear zones should therefore initially develop along planes of maximum shear stress strain soften and rotate as material planes into the field of progressive extension however the mean stress or pressure within a weak elongate viscous band being stretched is higher than the surrounding matrix and this is difficult to reconcile with premise 3 in contrast mohr coulomb brittle faults always have lower pressure within the fault zone flow of fluid and melt into high temperature shear zones therefore implies that ductile shear zones are not perfectly viscous but have a pressure dependent viscoplastic rheology the continued pressure dependence may reflect significant microcracking on the grain scale even when localized deformation does not produce larger scale discrete fractures

the sensitivity of scottish rivers and upland valley floors to recent environmental change abstract rivers in humid zones are delicately adjusted within narrow limits to evacuate the water and sediment supplied from upstream without significant changes in channel type and the morphology of the valley floor prevailing climate and land use are the major controls determining the nature of that adjustment significant environmental change either climate and or land use has the potential to change radically the channel type and destabilise the adjacent valley floor channel metamorphosis in which a river shifts from an aggrading to a degrading regime or from a single thread to multiple thread channel often provides the clearest evidence that environmental change has occurred this paper examines the stability of river channels and their associated valley floors in the uplands of scotland since 1750 changes in global scale atmospheric circulation patterns during this period have resulted in regionally distinct flood rich and flood poor periods with potential geomorphic impacts changes in the planform of nine active rivers in the piedmont zone of upland scotland are examined from series of rectified maps and aerial photographs at scales close to 1 10 000 at none of the sites other than one affected by an artificial increase in base level is there evidence of abrupt changes in river behaviour or channel metamorphosis however rates of lateral channel shift and extents of bare gravels do vary and these are clearly driven by changes in the incidence of floods since the mid 19th century the scale of valley floor reworking is less dramatic than that recently reported for comparable sites in northern england differences in geomorphic history land use and degree of slope channel coupling are identified as the likely causes of marked differences in the behaviour of active rivers in scotland and northern england over the last 250 years

tracking metamorphic dehydration reactions in real time with transmission small and wide angle synchrotron x ray scattering the case of gypsum dehydration we present a review of a unique non destructive method for the real time monitoring of phase transformations and nano pore evolution in dehydrating rocks transmission small and wide angle synchrotron x ray scattering saxs waxs it is shown how saxs waxs can be applied to investigating rock samples dehydrated in a purpose built loading cell that allows the coeval application of high temperature axial confinement and fluid pressure or flow to the specimen because synchrotron sources deliver extremely bright monochromatic x rays across a wide energy spectrum they enable the in situ examination of confined rock samples with thicknesses of 1 mm at a time resolution of order seconds hence fast kinetics with reaction completion times of about hundreds of seconds can be tracked with beam sizes of order tens to hundreds of micrometres it is possible to monitor multiple interrogation points in a sample with a lateral extent of a few centimetres thus resolving potential lateral spatial effects during dehydration and enlarging sample statistics significantly therefore the saxs waxs method offers the opportunity to acquire data on a striking range of length scales for rock samples with thicknesses of 10 3 m and widths of 10 2 m a lateral interrogation point spacing of 10 5 m can be achieved within each irradiated interrogation point volume information concerning pores with sizes between 10 9 and 10 7 m and the crystal lattice on the scale of 10 10 m is acquired in real time this article presents a summary of the physical principles underpinning transmission x ray scattering with the aim of providing a guide for the design and interpretation of time resolved saxs waxs experiments it is elucidated 1 when and how saxs data can be used to analyse total porosity internal surface area and pore size distributions in rocks on length scales from 1 to 300 nm 2 how waxs can be employed to track lattice transformations in situ and 3 which limitations and complicating factors should be considered during experimental design data analysis and interpretation to illustrate the key capabilities of the saxs waxs method we present a series of dehydration experiments on a well studied natural gypsum rock volterra alabaster our results demonstrate that saxs waxs is excellently suited for the in situ tracking of dehydration kinetics and the associated evolution of nano pores the phase transformation from gypsum to bassanite is correlated directly with nano void growth on length scales between 1 and 11 nm for the first time a comparison of the saxs waxs kinetic results with literature data emphasises the need for future dehydration experiments on rock specimens because of the impact of rock fabric and the generally heterogeneous and transient nature of dehydration reactions in nature it is anticipated that the saxs waxs method combined with in situ loading cells will constitute an invaluable tool in the ongoing quest for understanding dehydration and other mineral replacement reactions in rocks quantitatively

use of fallout pb 210 measurements to investigate longer term rates and patterns of overbank sediment deposition on the floodplains of lowland rivers the potential for using fallout unsupported pb 210 210pb measurements to estimate rates of overbank sediment deposition on the floodplains of lowland rivers is explored a model which distinguishes the contribution from direct atmospheric fallout and the catchment derived input associated with the deposition of suspended sediment has been developed to interpret the fallout pb 210 inventories at floodplain sampling sites and to estimate average sediment accumulation rates over the past 100 years the approach has been successfully used to estimate rates of overbank sedimentation on the floodplains of the rivers culm and exe in devon u k a detailed investigation of the pattern of longer term sedimentation rates within a small reach of the floodplain of the river culm indicated a range of deposition rates between 0 07 and 0 59 g cm 2 a 1 which was in close agreement with estimates of current sedimentation rates obtained using sedimentation traps

amphibole fabric formation during diffusion creep and the rheology of shear zones we conducted experiments to examine the effect of amphibole forming hydration reactions on mineral fabric development and the strength of mafic rocks both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions 800 c 1 gpa amphibole that formed under hydrostatic conditions exhibits a random lattice preferred orientation lpo in contrast amphibole formed during deformation exhibits both a strong shape preferred orientation spo and lpo with a 001 maximum aligned subparallel to the shear direction plagioclase in both hydrostatic and deformed samples shows a very weak to random lpo at low effective strain rates 10 5 s 1 to 10 6 s 1 the stress exponent is 1 1 5 consistent with deformation accommodated by diffusion creep the correlation of the spo and lpo coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation the experimentally produced fabrics are strikingly similar to those observed in amphibolite grade natural shear zones supporting interpretations that such rocks deform by diffusion creep in addition the rheology of the fine grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite thus both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust

fluid present deformation aids chemical modification of chromite insights from chromites from golyamo kamenyane se bulgaria abstract chemical signatures of chromitites are commonly used to track the evolution of the earth s mantle however chemical modification during deformation may have important implications for the interpretation of chromites signatures here we describe the details of how deformation promotes chemical modification in chromite physicochemical characteristics of the chromites were quantified by measuring crystallographic orientation relationships using electron back scattered diffraction ebsd and electron microprobe analysis emp chromites show porphyroclastic textures with coarse grained porphyroclasts ca 0 2 5 mm and fine grained neoblasts 3 and cr and lower mg values than the cores of large grains we interpret f1 and f2 to represent chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization respectively crystallographic preferred orientation cpo and misorientation data on the well developed low angle subgrain boundaries in coarse grains and f2 grains indicate that deformation in chromite was accommodated mainly by dislocation creep with the dominant activation of the 111 slip system the retrograde p t exhumation path predicted by thermodynamic and chemical modeling suggests that these fine grained chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism 1 0 gpa and 500 700 c our results show that deformation in the dislocation creep regime in a chemically open system has induced chemical modification and homogenization within chromite aggregates as well as strain localization this close physicochemical link offers new avenues of interpreting the chemical signatures of chromites utilizing their microstructurally controlled variation or lack thereof

deformation at the frictional viscous transition evidence for cycles of fluid assisted embrittlement and ductile deformation in the granitoid crust abstract mid crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional to viscous transition fvt here we investigate an exhumed continental mid crustal basement section in order to envisage the relationship between ductile and brittle deformation at the fvt our detailed study from km to micro scale shows that under greenschist metamorphic conditions deformation is accommodated by a dense network of highly localized ductile shear zones in the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine grained polymineralic matrix consisting of quartz feldspar sheet silicates and epidote during viscous granular flow mass transfer processes under the presence of fluids promote a chemo mechanical mixing resulting in grain size reduction and reaction softening coeval with this ductile deformation fluid assisted embrittlement occurs as indicated by biotite coated fractures cataclasites and injection of non cohesive polymineralic gouge material into secondary fractures inside the host rock the embrittlement during predominant ductile deformation occurs in cycles i e prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation we interpret this fluid assisted cyclic embrittlement evidenced by injection of the fluidized material into off fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo seismic activity with exhumation and associated cooling localized deformation persists in the ultrafine grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure dependent frictional flow always showing cycles of slow interseismic flow and fast seismic injection events overall in the granitic crust of the aar massif brittle and ductile deformation coexist up to deformation temperatures of minimum 450 c indicating that the fvt has to be placed in a rather wide range from 8 km up to 18 20 km in the granitoid crust

new polygonal grains formed by dissolution redeposition in quartz mylonite abstract arrays of polygonal grains are common along original grain margins subgrain boundaries and intragranular fractures of deformed quartz porphyroclasts we investigated these internal polygonal grains within quartz porphyroclasts of a partially recrystallized quartz mylonite deformed in the presence of fluid in a greenschist facies shear zone from quadrilatero ferrifero southeastern brazil optical observation showed profuse fluid inclusions and muscovite flakes inside the deformed porphyroclasts observation by sem revealed the presence of crystal faces on the internal polygonal grains domainal c axis analysis showed that the internal polygonal grains have a preferred orientation similar to the host porphyroclasts c axes at low angles to the stretching lineation but different from the fabric of recrystallized matrix grains type 1 cross girdle based on these observations we suggest a mechanism of in situ solution reprecipitation where the new grains are nucleated in the intergranular fluid filled pores present along the internal discontinuities of the porphyroclasts the grains grow with their boundaries largely in contact with fluid allowing the development of crystal faces at least during the early growth stages after the new grain reaches the opposite pore wall further grain growth should occur through ordinary grain boundary migration mechanisms driven by the strain energy difference between the new grain and the host porphyroclast we conclude that in these rocks in situ solution reprecipitation operated in conjunction with crystal plastic processes accounting for the nucleation of new grains during deformation recrystallization in the presence of fluid under low grade metamorphic conditions