Strain. Stress. Strain. Stress. Recoverable ~ no microstructures. Non-recoverable ~ microstructures formed. Lecture Practical

Size: px

Start display at page:

Download "Strain. Stress. Strain. Stress. Recoverable ~ no microstructures. Non-recoverable ~ microstructures formed. Lecture Practical"

Frederica Atkinson
5 years ago
Views:

1 LECTURE 6 DEFORMATION MECHANISMS AND MICROSTRUCTURES LECTURE PLAN 1) INTRODUCTION 2) DIFFUSIVE MASS TRANSFER 3) CRYSTAL PLASTICITY 4) FRICTIONAL SLIDING, FRACTURE PROCESSES AND CATACLASIS 1) INTRODUCTION In this Lecture, the mechanisms which allow rocks to change shape during deformation are described. There are 2 fundamental categories of deformation:- Recoverable ~ no microstructures Strain Stress Peak stress and strain Ascending stress/strain Descending stress/strain Failure or yield stress, beyond which does not occur Non-recoverable ~ microstructures formed a) Recoverable- elastic deformation and thermal expansion. Atomic bonds are elastic and are not broken during recoverable deformation. When the source of stress or heat are removed, the body resumes its original pre-deformational shape. b) Permanent- If the stress exceeds a critical value termed the yield stress, then permanent deformation will occur. In this lecture, we will deal only with permanent deformation, involving diffusive mass transfer, crystal plasticity and fracture processes. Strain Stress Peak and final stress and strain Ascending stress/strain Failure or yield stress, beyond which the material is permanently deformed through the development of microstructures Lecture Practical Course Homepage Contact Staff

The activation of specific deformation mechanisms is reliant on the prevailing temperature, stress magnitudes, fluid pressures, strain rate, chemical conditions and deformation history of the

Brittle strengths increase with increasing pressure because frictional strength increases and normal stresses fight against dilatancy. Gouges generally form.

2 The activation of specific deformation mechanisms is reliant on the prevailing temperature, stress magnitudes, fluid pressures, strain rate, chemical conditions and deformation history of the material (see below). Brittle deformation (generally low temperature) is defined as strongly-pressure dependent deformation involving an increase in volume (dilatancy). Brittle strengths increase with increasing pressure because frictional strength increases and normal stresses fight against dilatancy. Gouges generally form. Plastic deformation (generally high temperature) is defined as strongly temperature and time dependent deformation which is a constant-volume mechanisms. Plastic strengths are insensitive to pressure, but usually decrease exponentially with temperature. Mylonites with internally deformed grains/crystals generally form. BRITTLE CATACLASTIC FLOW (high confining pressure) High confining pressures do not allow grains to slide past eachother. Cataclastic flow occurs within zones of fault gouge composed of a multitude of small fragments of the original grains, formed by grain size reduction during frictional grain boundary sliding. (1) (2) BRITTLE CATACLASTIC FLOW (low confining pressure) (1) Starting material composed (2) of spherical particles. Low confining pressures allow grains to slide past eachother. Brittle deformation where frictional grain boundary sliding has re-arranged the grains.further slip between grains causes rotations. Note senses of rotation and shear between grains. PLASTIC FLOW Original Deformed (3) Volume changes (dilatancy) occur as the Plastic deformation through internal shape (4) Sliding and grain-rotation alternate grains move past eachother. The volume changes mean the deformation is sensitive changes in the grains/crystals. Volume to the confining pressure changes are unimportant so the deformation is pressure insensitive Either (low temperature) Or (high temperature) Ductile deformation with no visble breaks or discontinuities in the deformation (at the scale of viewing) So: - the terms plastic and brittle describe separate deformation mechanisms. - the term ductile describe the geometry of the deformation where no breaks or discontinuities can be seen in the deformation at a particular scale of viewing (e.g. macroscopic flow). Close-up Gneiss Mylonite Therefore ductile deformation by microscopic fracturing is termed cataclastic flow, whilst ductile deformation by crystal plasticity is plastic flow. Close-up view of a low-temperature thrust zone cutting Cretaceous chalk with thin chert (flint) beds. Material within the thrust zone has been fractured and grain-size reduced during frictional sliding involving fracture. The resulting fault rock is a gouge. Note the smaller-scale R and P shears within the fault zone. The black spots in the gouge are grain-size reduced chert. The deformation is brittle. Close-up view of a high temperature mylonitic thrust zone emplacing amphibolite facies gneisses of the Pre-Cambrian Lewisian complex onto Cambrian quartzites (not show), Ben Arnaboll, N.W. Scotland. The pink, orthoclase-rich pegmatites and gneisses have been stretched into thin layers within the myloniti c foliation. The deformation appears ductile and plastic.

2) DIFFUSIVE MASS TRANSFER Formation of cements in pore spaces Diffusive mass transfer (DMT) induces deformation by the transfer of material away from zones of relatively high intergranular normal

eg Flattened quartz grains with beards of quartz & chlorite growing in the spaces between the grains. The removal of material can lead to volume losses and strain accommodation by chemical compaction.

3 2) DIFFUSIVE MASS TRANSFER Formation of cements in pore spaces Diffusive mass transfer (DMT) induces deformation by the transfer of material away from zones of relatively high intergranular normal stress to interfaces with low normal stresses. D C D = dissolution C = cementation Material dissolved from the stylolitic contact between grain may be reprecipitated as cements in pore spaces. eg Flattened quartz grains with beards of quartz & chlorite growing in the spaces between the grains. The removal of material can lead to volume losses and strain accommodation by chemical compaction. The driving force for DMT depends on the variation in chemical potential in the rock aggregate induced by stress variations within the aggregate, fluid pressure gradients or variations in the internal strain energy of grains. Bedding Cleavage Spaced pressure dissolution cleavage in Cretaceous pelagic limestones (Scaglia Rosata), Umbria Marche Thrust belt, central Italy. The cleavage has formed at angle to bedding due to c. NE-directed overthrusting. Material has been dissolved along the cleavage, and may have caused volume losses of a few tens of percent. The material is transported in solution and then re-precipitated elsewhere within the thrust belt. DMT is most likely to dominate the deformation in fine-grained material where the diffusion path length is low. Can be considered a 3 stage process:- Ooids with an anhydrite cement Peloids and small ooids with an anhydrite cement Mudstone with anhydrite crystals a) Source mechanisms- These occur along stylolites and dissolution seams. How the material enters a diffusion path. Include the processes which control the activation of diffusion, corrosion of existing material and reaction processes. Stylolite in Zechstein (Permian) carbonates within an oil/gas field in the Netherlands. Material has been dissolved along the cleavage, and may have caused volume losses of a few tens of percent. The material is transported in solution and then re-precipitated elsewhere within the region. Bitumen has accumulated along the stylolite, either as an insoluble residue or due to later fluid flow along the stylolite. Injected blue glue shows pores.

One of the source mechanisms is pressure dissolution. Areas of high internal stress in rocks such as point contacts between grains have high internal elastic strain.

Formation of pitted pebbles greatest stress and elastic strain dissolution Microstructures include stylolites, pitted pebbles and generally areas of dissolution.

4 One of the source mechanisms is pressure dissolution. Areas of high internal stress in rocks such as point contacts between grains have high internal elastic strain. The strain energy makes the stressed solid more soluble in the pore fluid than the un-strained material. Formation of pitted pebbles greatest stress and elastic strain dissolution Microstructures include stylolites, pitted pebbles and generally areas of dissolution. b) Migration or diffusion mechanisms- Includes:- Diffusion along:- i) the discontinuities within the crystal structure. ii) thin fluid film along grain boundaries iii) transport in a bulk fluid which is undergoing flow. Microstructures form such as stylolites, cleavage and pitted pebbles Microstructures may be difficult to pinpoint. c) Sink processes- where material is precipitated in the sites of crystal growth. Microstructures include cement overgrowths, pressure shadows and veins. PPL CL Two views of a calcite vein from the Vercors, thrust belt, French Alps. In plane-polarised light almost-clear calcite has been precipitated from pore-waters circulating through a fracture. Under cathodoluminescence (electic current passed across the thin-section in a vacuum and no light), the growth zones in the calcite produce different luminescence colours, revealing subtle variations in the pore water iron/manganese chemistry and the growth faces of the crystals. The crystals have grown into a fluid-filled cavity.

Low strain rates and low temperatures force deformation to occur via dissolution in the presence

5 Click to enlarge Click to enlarge overview overview Red stained stylolites that have allowed pressure dissolution of fossiliferous limestones (see on a polished table top made of marble ). Low strain rates and low temperatures force deformation to occur via dissolution in the presence of water within pore spaces. Insoluble residues (haematite in this case) accumulate along the stylolite.

6 3) CRYSTAL PLASTICITY Crystal plasticity involves the accumulation of strain by intracrystalline processes such as the movement of dislocations and twinning. Crystals commonly contain defects such as missing atoms or impurities which are orders of magnitude weaker than the crystal structure. Crystal plasticity involves the motion of these defects through the crystals in response to stress. Crystal plasticity through movement of dislocations straining grain dislocation adjacent grain Time 1 Time 2 strain in adjacent grain At low temperatures deformation occurs by dislocation glide where dislocation motion is confined to slip planes (low temperature plasticity). Strain Hardening Deformation may become become easier to accomplish if one of the atomic bonds is broken at a defect or dislocation, where there are less atomic bonds to break along a plane in order for it to slip. Strain Stress Ascending stress/strain Onset of strain hardening where strain accumulates with more stress required per unit strain. Leads to dislocation tangles and strain-hardening (characterised by an increasing resistance to straining during deformation). Twin gliding occurs in bands where the crystal structure is sheared into a mirror image of its neighbouring material.

7 At higher temperatures, thermally activated recovery processes such as dislocation climb (movement of the dislocation out of their slip planes) reduce the effect of the work hardening process. The deformation mechanism is termed dislocation creep. Microstructures which indicate the action of crystal plasticity include:- - Twins - preferred crystallographic orientations - undulose extinction indicating bent or twisted crystal structure - sub-grain structures within grains - new grains developed during dynamic recrystallisation of grain boundaries during grain-boundary migration or sub-grain rotation. - high dislocation densities within the crystals Strain Softening Strain Stress Ascending stress/strain Onset of strain softening, where strain accumulates with less stress required per unit strain. Strain-softening of shear zones may result as easy glide horizons become aligned.

Sample of a mylonite from the Moine Thrust Zone, N.W. Scotland. Note the intense foliation defined by crystals that have become aligned during crystal plastic deformation.

8 Sample of a mylonite from the Moine Thrust Zone, N.W. Scotland. Note the intense foliation defined by crystals that have become aligned during crystal plastic deformation. The green color is due to the presence of chlorite beteen the quartz crystals. The lighter bands have less chlorite because they were worm burrows before the deformation when the rock was a sedimentary silt/sandstone.

The ellipse shapes on the end of the sample are flattened worm burrows.

9 Views onto the top and end of the Moine mylonite sample. A stretching lineation is seen on the top, paralllel to the pen, defined by quartz crystals aligned by crystal plastic deformation. The ellipse shapes on the end of the sample are flattened worm burrows. Stretching direction Elliptical worm burrows flattened during the shearing Worm burrows Worm burrows Worm burrows Worm burrows The original sedimentary rock with worm burrows is progressively sheared at temperatures high enough to allows crystal plasticity. This results in a foliated rock with ellipses seen when viewed parallel to the direction of maximum elongation (known as the x direction).

$4) FRICTIONAL SLIDING, FRACTURE PROCESSES AND CATACLASIS a) Frictional grain-boundary sliding without fracture Deformation by frictional grain-boundary sliding involves the sliding of grains past$

10 4) FRICTIONAL SLIDING, FRACTURE PROCESSES AND CATACLASIS a) Frictional grain-boundary sliding without fracture Deformation by frictional grain-boundary sliding involves the sliding of grains past eachother. Individual grains are essentially undeformed and behave as rigid bodies. This deformation mechanism is termed independent particulate flow. Frictional grain boundary sliding without fracture Also known as independent particulate flow Sliding starts when the cohesion and friction between grains is overcome. Therefore, this is a pressure sensitive mode of deformation which is promoted by low confining pressures and high fluid pressures. The initiation of sliding is critically dependent on the amount and strength of cement bridges holding the grains together. Complex volume changes accompany this style of deformation as grains move apart and compact closer together to accomplish displacements. Deformation occurs through the following sequence of events:- Fault gouge on a normal fault near Delphi, central Greece. Although the grains have been formed by sliding involving fracture and grain size reduction, it is likely that at least some strain will have been accommodated by frictional grainboundary sliding without fracture (like ballbearings rolling past eachother). The microstructure produced by sliding without fracture will simply be grain re-packing. As you can see, it will be difficult to recognise! dilation + fluid influx disaggregation and displacement >collapse and grain alignment Microstructures may be difficult to recognise as grain sizes and shapes are not disturbed.

$Late fractures that formed during erosion and uplift Individual clast within the gouge Matrix between clasts is actua;y made of extremely fine grained clasts.$

11 A sample of carbonate fault gouge that has been stained to highlight the ferroan nature of the calcite within the sample. Sample from a thrust in the French Alps. Late fractures that formed during erosion and uplift Individual clast within the gouge Matrix between clasts is actua;y made of extremely fine grained clasts. Fault gouge with relatively coarse clast sizes Gouges such as these form during high strain rate slip events (earthquakes) through cataclasis. Cataclasis involves intense clastsize reduction due to the action of friction causing crushing of clasts. Fault gouge with extremely fine clast size Scratches on the surface of the specimen: ignore

$b) Frictional grain boundary sliding involving fracture Fracture processes involve the nucleation, propagation and displacement along new surfaces created during the deformation.$ $If the fracture occurs after distortion of a material, and the pieces no longer fit together, then this is termed ductile fracture.$

12 b) Frictional grain boundary sliding involving fracture Fracture processes involve the nucleation, propagation and displacement along new surfaces created during the deformation. If the pieces fit together after fracture then it is called brittle fracture. e.g. dropping a plate. If the fracture occurs after distortion of a material, and the pieces no longer fit together, then this is termed ductile fracture. Ductile fracture is accompanied by strain surrounding the fracture accommodated by another deformation mechanism (plasticity). e.g. bending a piece of metal until fatigues and fractures. The fragmentation of material, together with the rotation and associated grain-boundary sliding and dilation, constitute cataclasis which dominates faulting at shallow crustal levels and produces gouges and fault breccias. In the absence of water, frictional heating associated with rapid seismic slip may melt the fault rocks to produces glassy pseudotachylites. Thrust fault cutting steeply dipping beds of chalk and siliceous chert (the thin black layer). Frictional sliding involving fracture and grain-size reduction has broken both the chalk and the chert into fragments. The arrows indicate the contact between the gouge (where fragments are completely surrounded by a fine-grained rock flour) and brecciated chalk (where fractures surround clasts but the clasts are essentially in the place they originated). Note the irregular nature of the gouge-to-breccia contact. The movement sense is in and out of the page.

13 Fracture mechanisms are usually associated with fast crack propagation termed brittle failure. Propagation at lower velocities is termed sub-critical crack growth (e.g. ductile fracture). Mechanisms include:- - Elastic strain accumulation- Fractures follow weaknesses such as crystallographic cleavages and may be transgranular and exploit grain boundaries. - Crystal plastic processes- can cause fracture when dense dislocation tangles or intense twinning induce work hardening and failure. Also, voids can open at the grain boundaries between minerals whose ease of crystal plastic deformation is different during the same deformation. - Diffusion processes- The opening of voids during dissolution can lead to failure as the voids are linked by a propagating fracture. - Phase transformations- Volume changes associated with recrystallisation can open voids which can nucleate a fracture. - Fluid processes- High fluid pressures can cause fracturing by reducing the effective normal stress across a fault or fracture so that the frictional resistance to sliding is decreased. Also, corrosion of minerals at crack tips due to the presence of the fluid can induce sub-critical crack growth.

14 FURTHER READING AVAILABLE FROM THE ELECTRONIC LIBRARY John P. Craddock, Kimberly J. Nielson and David H. Malone, Calcite twinning strain constraints on the emplacement rate and kinematic pattern of the upper plate of the Heart Mountain Detachment, Journal of Structural Geology, 22, B. Bos, C. J. Peach and C. J. Spiers, 2000, Frictional-viscous flow of simulated fault gouge caused by the combined effects of phyllosilicates and pressure solution, Tectonophysics, 327, Peter Vrolijk and Ben A. van der Pluijm, Clay gouge, Journal of Structural Geology, 21, Trenton T. Cladouhos, Shape preferred orientations of survivor grains in fault gouge, Journal of Structural Geology, 21, J. F. HipperttF. D. Hongn, Deformation mechanisms in the mylonite/ultramylonite transition, Journal of Structural Geology, 20,

CREEP CREEP. Mechanical Metallurgy George E Dieter McGraw-Hill Book Company, London (1988)

CREEP CREEP. Mechanical Metallurgy George E Dieter McGraw-Hill Book Company, London (1988) CREEP CREEP Mechanical Metallurgy George E Dieter McGraw-Hill Book Company, London (1988) Review If failure is considered as change in desired performance*- which could involve changes in properties and/or