Superplastic flow in finegrained limestone

Creep of Solnhofen limestone at temperatures between 600° and 900° C was found to fall into three different flow regimes: regime 1 with an exponential stress-dependence of strain rate, regime 2 with power-law creep and n ~ 4.7 and finally a superplastic regime 3 with n ~ 1.7. Within the superplastic regime the creep behaviour is strongly grain-size dependent, the strain rate increasing markedly with decrease in grain size at a given stress. Microstructural observations indicate that in regimes 1 and 2 intracrystalline plasticity is dominant whereas the superplastic regime is characterized by grain-boundary sliding. The crystallographic preferred orientation within the superplastic regime is weaker and of different geometry when compared with that in flow regimes 1 and 2. In a discussion on the deformation mechanisms it is suggested that flow regimes 1 and 2 are regimes of dis location creep in which the rate controlling step is diffusion assisted; for the superplastic regime existing models of grain-boundary sliding are compared with the observations Finally, the tectonophysical importance of superplasticity is discussed and by extra polating the observed creep behaviour to geological strain rates it is found that super plasticity in rocks is to be expected under a wide range of conditions, particularly at smal grain sizes.     

Deformation mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic flow in the lower continental crust

Microstructures and crystallographic preferred orientations in a fine-grained banded quartz-feldspar mylonite were studied by optical microscopy, SEM, and TEM. Mylonite formation occurred in retrograde amphibolite facies metamorphism. Interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity. Strain incompatibilities during grain sliding in the superplastic quartz-feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar, the rate of which probably controls the rate of superplastic deformation.

There is evidence for equal flow stress levels in the superplastic and crystal-plastic domains. In this case mechanism partitioning results in strain-rate partitioning. Fast deformation in the superplastic bands therefore dominates flow, and deformation is probably best modelled by a superplastic law.

If this deformational behaviour is typical, shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress, or at low differential stresses in case of fixed strain rates.     

Limestone mechanical deformation behavior and failure mechanisms: a review

In this paper, several mechanical deformation curves of limestone are reviewed, and the effects of temperature, confining pressure, and fluid are discussed. Generally, Mohr–Coulomb is used for limestone brittle fracture. The characteristic of low temperature cataclastic flow and the conditions and constitutive equations of intracrystal plastic deformation such as dislocation creep, diffusion creep, and superplastic flow are discussed in detail. Specifically, from the macroscopic and microscopic view, inelastic compression deformation (shear-enhanced compaction) of large porosity limestone is elaborated. Compared with other mechanics models and strength equations, the dual porosity (macroporosity and microporosity) model is superior and more consistent with experimental data. Previous research has suffered from a shortage of high temperature and high pressure limestone research; we propose several suggestions to avoid this problem in the future: (1) fluid-rock interaction research; (2) mutual transition between natural conditions and laboratory research; (3) the uniform strength criterion for shear-enhanced compaction deformation; (4) test equipment; and (5) superplastic flow mechanism research.     

Influence of host lithofacies on fault rock variation in carbonate fault zones: A case study from the Island of Malta

Relatively few studies have examined fault rock microstructures in carbonates. Understanding fault core production helps predict the hydraulic behaviour of faults and the potential for reservoir compartmentalisation. Normal faults on Malta, ranging from <1 m to 90 m displacement, cut two carbonate lithofacies, micrite-dominated and grain-dominated carbonates, allowing the investigation of fault rock evolution with increasing displacement in differing lithofacies. Lithological heterogeneity leads to a variety of deformation mechanisms. Nine different fault rock types have been identified, with a range of deformation microstructures along an individual slip surface. The deformation style, and hence type of fault rock produced, is a function of host rock texture, specifically grain size and sorting, porosity and uniaxial compressive strength. Homogeneously fine-grained micrtie-dominated carbonates are characterised by dispersed deformation with large fracture networks that develop into breccias. Alternatively, this lithofacies is commonly recrystallised. In contrast, in the coarse-grained, heterogeneous grain-dominated carbonates the development of faulting is characterised by localised deformation, creating protocataclasite and cataclasite fault rocks. Cementation also occurs within some grain-dominated carbonates close to and on slip surfaces. Fault rock variation is a function of displacement as well as juxtaposed lithofacies. An increase in fault rock variability is observed at higher displacements, potentially creating a more transmissible fault, which opposes what may be expected in siliciclastic and crystalline faults. Significant heterogeneity in the fault rock types formed is likely to create variable permeability along fault-strike, potentially allowing across-fault fluid flow. However, areas with homogeneous fault rocks may generate barriers to fluid flow.     

A phenomenological numerical approach for investigating grain size evolution in ductiley deforming rocks

The sizes of recrystallised grains in exhumed ductile shear zones are often used to infer conditions of deformation (i.e. stress, strain rate and temperature). Here we present a simple numerical method of calculating the dynamic evolution of grain size during ductile deformation. Our phenomenological method is based on the fact that the dynamic competition between grain growth and recrystallisation will drive grains towards a steady-state size. At each time increment, grain growth and reduction contributions are calculated, with magnitudes which depend on the difference between the current grain size and a desired steady-state grain size. In our models we use a recrystallised grain size piezometer to calculate the steady-state grain size for a given stress. Our numerical routine is incorporated into the SULEC finite element package, allowing us to explore spatial and temporal changes in grain size.As a test, we compare model results to measured grain sizes in quartz layers thinned and recrystallised around rigid garnet porphyroclasts under simple shear dominated deformation in the Alpine Fault Zone of New Zealand. Numerical models are able to replicate observed grain size variations, with boundary conditions consistent with those constrained for the central Alpine Fault Zone.     

Low temperature recrystallisation of alluvial gold in paleoplacer deposits

Detrital gold particles in paleoplacer deposits develop recrystallised rims, with associated expulsion of Ag, leading to the formation of Ag-poor rims which have been recognised in most placer gold particles around the world. Recrystallisation is facilitated by accumulation of strain energy as the gold particles are deformed, particularly on particle margins, during transportation in a fluvial system. The recrystallisation process ensues after sedimentary deposition and can occur at low temperatures (<40 °C) over long geological time scales (millions of years). In the Otago placer goldfield of southern New Zealand, paleoplacers of varying ages contain gold with varying transport distances and these display differing degrees of rim formation. Narrow (1–10 µm) recrystallised rims with 0–3 wt% Ag formed on gold particles that had been transported <10 km from their source and preserved in Eocene sediments. Relict, coarse grained (∼100 µm) gold particle cores have 3–10 wt% Ag, which is representative of the source gold in nearby basement rocks. Gold in the Miocene paleoplacers was recycled from the Eocene deposits and transported >20 km from their source. The gold particles now have wider recrystallised rims (up to 100 µm), so that some particles have essentially no relict cores preserved. Gold in Cretaceous paleoplacers have wide (∼100 µm) recrystallised low-Ag rims, even in locally-derived particles, partly as a result of diagenetic effects not seen in the younger placers. Gold particles in all the paleoplacers have delicate gold overgrowths that are readily removed during recycling, but are replaced by groundwater dissolution and reprecipitation on a time scale of <1 Ma. The recrystallisation that leads to Ag-poor rim formation is primarily related to the amount of deformation imposed on particles during sedimentary transport, and is therefore broadly linked to transport distance, but is also partly controlled by the age of the paleoplacer on time scales of tens of millions of years. Gold particles that have been derived directly from basement sources can retain their original composition for long distances (tens to hundreds of kilometres) in a river system, with only minor recrystallised rim development. Gold particles that have been recycled through paleoplacer deposits can lose this link to source composition after relatively short transport distances because of extensive recrystallisation.     

Remarkable Cave,Tasmania

At Remarkable Cave, southern Tasmania, Triasssic sedimentary rocks are intruded by Jurassic dolerite to form a sheet-like body of unknown thickness to the east of and below Remarkable Cave. The sedimentary rocks near sea-level at Remarkable Cave show deformation and flow structures, and are composed of recrystallised predominantly quartz and feldspar with some darker alteration minerals and zeolite in the more massive parts of the recrystallised rock. The deformation was caused by the intrusion of dolerite. At the top of the sheet-like body to the east of Remarkable Cave there is rheomorphic veining into the dolerite following joint fractures. The dolerite is locally transgressive. In Remarkable Cave, dolerite must occur just below the floor of the cave to heat the sedimentary rocks to much higher temperatures than typical contact aureoles in the area, supporting a local feeder geometry, or compositional control.     

DEM analysis of the onset of flow deformation of sands: linking monotonic and cyclic undrained behaviours

This study explores the link between the monotonic and cyclic undrained behaviour of sands using the discrete element method (DEM). It is shown that DEM can effectively capture the flow deformation of sands sheared under both monotonic and cyclic undrained loading conditions. When subjected to cyclic shearing, flow-type failure is observed for a loose sample, while cyclic mobility is observed for a dense sample. A strong correlation between the monotonic and cyclic loading behaviour that has been revealed experimentally is also confirmed in DEM simulations: (a) flow deformation occurs in the compressive loading direction when the cyclic stress path intersects the monotonic compression stress path prior to the monotonic extension stress path, and vice versa; (b) the onset of flow deformation in q–\(p^{\prime }\) space is located in the zone bounded by the critical state line and the instability line determined from monotonic simulations. Hill’s condition of instability is shown to be effective to describe the onset of flow failure. Micro-mechanical analyses reveal that flow deformation is initiated when the index of redundancy excluding floating particles drops to below 1.0 under both monotonic and cyclic loading conditions. Flow deformation induced by either monotonic or cyclic loading is characterized by an abrupt change of structural fabric which is highly anisotropic. The reason why the dense sample dilated during monotonic loading but showed cyclic mobility (temporary liquefaction) during cyclic loading is attributed to the repeating reversal of loading direction, which leads to the periodic change of microstructure.     

Deformation and recrystallisation of orthopyroxene from the giles complex, Central Australia

The microstructures and fabrics of naturally deformed orthopyroxenites from the Giles Complex, central Australia are described in some detail. Coarse grained enstatite is deformed and recrystallised where it is incorporated in planar gneissic (mylonite) zones which show a gradation in strain from their margins inwards. Deformation takes place by slip on (100) [001] to produce regular lattice bending and kinking, and recrystallisation takes place preferentially along grain boundaries and kink band boundaries (KBB's). The microstructures and preferred orientation of recrystallised grains along KBB's are interpreted in terms of possible nucleation mechanisms, and both bulge nucleation (Bailey and Hirsch, 1962) and subgrain coalescence (Hu, 1963) are likely contributors. Electron microprobe analyses have indicated a small compositional difference between new (recrystallised) and host (deformed) grains, which is related to the nucleation mechanism. The total preferred orientation patterns for host and new grains are discussed with special reference to previous measurements and interpretations.     

Investigation of water pressure transients beneath temperate glaciers using numerical groundwater flow experiments

Recent work has identified groundwater flow through basal till aquifers as a key control on melt‐season pressure transients beneath alpine glaciers, with potential implications for climate change studies, glacial geomorphology, tracer test interpretation, and the sediment load and chemistry of glacially derived waters. In this study, we investigate heuristically such subglacial Darcian flow processes using standard groundwater modelling techniques. Our primary result is that a one‐dimensional, transient, confined flow model with recharge, implemented numerically using time‐varying specified‐head and no‐flow boundaries, reproduces overall observed subglacial hydraulic behaviour very well and permits effective visualisation of aquifer responses under different conditions. Spatial variability in hydrogeological parameters is shown to have significant effects, but may be difficult to incorporate reliably into site‐specific models and to identify unambiguously in borehole pressure data. Time‐variance in areal recharge, if present, is apparently not observably expressed in the system, but time‐variance in transmissivity may be significant for some glaciers. Copyright © 2000 John Wiley & Sons, Ltd.     

地壳不同构造层次岩石变形机制及其构造岩类型

构造岩记录地壳构造变形演化重要信息,其成因、分类与命名一直没有统一认识。本文对构造岩变形机制、控制因素和构造岩分类进行系统总结。认为构造岩形成受物质成分、变形机制、应变速率、流体、温度、压力等因素控制,是物质成分与物理化学条件、变形机制等众多变量的函数。变形机制包括破裂作用、碎裂流动、晶质塑性、物质扩散、重结晶作用和超塑性流动,不同变形机制出现在不同地壳构造层次中,形成不同的显微组构。依据成因机制、物质组成和组构等标志对构造岩分类与命名进行重新修订,将构造岩划分为碎裂岩系列和变质构造岩系列,前者发育在地壳浅构造层次上,以破裂作用和碎裂流动变形机制为主;后者发育在中深部构造层次上,以晶质塑性、重结晶作用、物质扩散作用和超塑性流动作用为主。碎裂岩系列划分碎裂岩、角砾岩、微角砾岩、超碎裂岩、断层泥和假玄武玻璃;变质构造岩系列划分为构造片岩、糜棱岩和构造片麻岩。依据岩石流变性质、变形机制和构造岩分布,地壳构造层次划分为:脆性域,变形机制以碎裂作用和碎裂流动为主,发育碎裂岩系列;脆-韧性转换域,以晶质塑性、物质扩散和重结晶作用为主,并伴随有碎裂作用,形成糜棱岩、千糜岩和构造片岩;低温韧性域,以晶质塑性、物质扩散和重结晶作为主,发育糜棱岩与构造片岩;高温韧性域,以超塑性蠕变和重结晶作用为主,形成构造片麻岩。     

新疆西准噶尔地区阿尔卑斯型超基性岩的成因及其演化

本地区阿尔卑斯型超基性岩在温度约800—1200℃、压力约0.9—1.68GPa、差异应力0.2—0.35GPa和比较干的条件下发生了塑性变形作用,形成一套以残碎斑结构为主并有糜棱结构的超基性岩质糜棱岩。在残碎斑状矿物和造矿的铬尖晶石中的原生全结晶化硅酸盐熔体包裹体和熔流体包裹体的发现和研究,证明该类岩石是在温度约1200—1300℃、压力约1.1-1.38 GPa,大致相当40-50 km深处,庄物理化学条件相对恒定的环境中,经硅酸盐熔浆液态分异形成的岩浆岩。在重结晶次生橄榄石和辉石中未发现任何包裹体,说明超基性岩塑性变形作用是在岩浆成岩之后。     

Magnetic susceptibility used in mapping of amphibolite facies recrystallisation in basic dykes

The dense tholeiitic Koster dyke swarm, west Sweden, has undergone progressive deformation under amphibolite facies metamorphic conditions. The varying influence of this event on the dyke rock forms the basis for a regional division of the swarm into three sectors: the sectors of igneous dolerites, partly recrystallised metadolerites, and lineated amphibolites, respectively. Systematic measurements of the magnetic susceptibility in the dykes have demonstrated a correlation between the regional pattern of recrystallisation and the pattern of mean susceptibility given as mean susceptibility per 0.5 km². The transformation of dolerites to metadolerites and finally to lineated amphibolites is accompanied by a marked decrease in the susceptibility, which is related essentially to the degree of alteration of titanomagnetite to sphene + Fe-ions incorporated in the silicates.     

Pyrrhotite Textures and Their Genetic Implications in the Hongtoushan Massive Sulphide Deposit,Liaoning Province,China

The Archaean massive sulphide Cu-Zn deposit at Hongtoushan has been metamorphosed to upper amphibolite facies.An investigation on ore textures shows that pyrrhotite in the deposit was originally produced by submarine exhalation,and subsequently metamorphosed,annealed and recrystallised.Deformation textures formed by progressive metamorphism of pyrrhotite were erased almost completely under peak conditions,and the presently-existing deformation textures and ore mylonites are largely resultant from diaphthoresis.Deformation and annealing of pyrrhotite could have been enhanced by fluds.Retrogressive fluids are characterized by high oxygen fugacity.     

中国东部主要成矿时代磷块岩的组成

本文研究了我国震旦、寒武纪磷块岩的矿石岩石学,主要矿石矿物为碳氟磷灰石,重要伴生矿物为伊利石,白云石和自生石英。磷块岩的化学组成有三个显著特点,一是品位随矿石类型的不同而异;二是随成矿域不同而异;三是随成矿时代不同而异。成矿背景,成矿环境、磷块岩相以及矿石组成等因素的差异是导致上述矿石组成变化的主要原因。     

On the validity of the flow rule postulate for geomaterials

This paper is concerned with a fundamental assumption in the theory of plasticity: the direction of plastic strain increments is independent of the loading (stress) increment direction. This assumption, also known as plastic flow rule postulate, works quite well for metal‐like materials. However, geomaterials such as sand present deformational mechanisms that are distinctive from those of metals when they are loaded. As such, we hereby examine the validity of this postulate for granular media accounting for their discrete nature. This is accomplished by analysing the mechanical behaviour of a cubic assembly of polydispersed spherical articles using a particle flow code. An extension to Gudehus' response envelope to three‐dimensional conditions is used to study the incremental character and influence of loading direction on the behaviour of these materials. It is found that plastic flow in granular media is governed by both current state variables and incremental loading direction and magnitude, especially under non‐axisymmetric stress conditions. The flow rule postulate of plasticity remains valid only in axisymmetric and biaxial conditions. We also verified that the plastic response might be significantly influenced by the stress path (or history) taken prior to loading. These findings raise the question of whether or not classic elastoplastic models based on the above postulate will have serious shortcomings, especially in true‐triaxial conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling of the physical behaviour of clay barriers close to water saturation

The physical properties of bentonite-based buffer materials for nuclear waste repositories have been investigated by a number of different laboratory tests. These tests have yielded a material model that is valid for conditions close to water saturation and is useful for describing: (a) the stress, strain and volume change behaviour; (b) the pore pressure and flow of water; and (c) the thermal and thermomechanical response.

The material model is based on the Drucker-Prager Plasticity model and a Porous Elastic Model. The effective stress concept and Darcy's law are applied and the swelling/consolidation and thermomechanical processes are coupled according to the separate mechanical properties of the pore water, the solids and the clay skeleton. The model can be used by the finite-element program ABAQUS.

The model has been tested in several laboratory and field verification tests. Comparison between measured and calculated behaviour shows that the general behaviour is described properly and several calculations of different scenarios have been made for the Swedish KBS 3 concept. However, certain processes, like the hysteresis effect at consolidation/swelling, the curved stress-strain relation at shearing, and the curved failure envelope, are not modelled in a perfectly accurate way and an improved material model is proposed here. It combines the behaviour of the Cam-clay model on the wet side with the more relevant plastic behaviour of a modified Drucker-Prager model with a curved failure envelope and the possibility to introduce strain softening after failure.

The paper presents some laboratory results that are the basis of the first model. It also shows the application of the model to finite-element calculations of some laboratory tests. Comparisons between the calculations and measured results expose some disadvantages of the model and a concept for an improved model is suggested.     

中、下地壳中长石的塑性变形机制及其证据——长石的显微构造研究综述

综述了有关长石的碎裂流动、位错蠕变、扩散蠕变、颗粒边界滑动及超塑性流动的特点及有关现象,并指出了长石塑性变形机制研究的重要意义。     

Rheological evidence for changes in the deformation mechanism of Solenhofen limestone towards low stresses

New rheological data on Solenhofen limestone at temperatures of 600–900°C show a transition into power-law creep with n-values around 4.5 followed by a second transition into low n-values (n 2). It is suggested that the mechanism associated with low n-values is probably the active one in many geological situations involving flow in fine-grained carbonates and corresponds to superplastic phenomena known to occur in fine-grained metals.     

Electron backscatter diffraction analysis to determine the mechanisms that operated during dynamic recrystallisation of quartz-rich rocks

Determination of the controlling nucleation and recrystallisation mechanisms from a samples microstructure are essential for understanding how the microstructure formed and evolved through time. The aim of our research was to apply a quantified analytical approach to the identification of the controlling nucleation, recrystallisation and microstructural modification mechanisms. We used electron backscatter diffraction to quantify the microstructures of naturally deformed quartz-rich rocks which were deformed at various temperature and pressure conditions. Our results show that ratios of the recrystallised grain size to the subgrain size with values less than 1 (0.5–0.7 in the data presented here) suggest bulge nucleation, whereas ratios of ∼1 suggest subgrain rotation nucleation. Other supporting evidence for subgrain rotation nucleation is an increase in misorientation from the centre of an original protolith ‘parent’ grain to the edge. All samples show evidence for modification of the microstructure due to grain boundary sliding including increased misorientation angles between grains and movement of recrystallised grains between parent grains. By systematically analysing sample microstructures it is possible to separate out evidence to determine the controlling nucleation and recrystallisation mechanisms, as well as being able to identify microstructure modification mechanisms. Using microstructural quantification via EBSD allows a systematic methodology to analyse samples from any location from an objective viewpoint.