De Carboneras breukzone: Spanje's eigen San Andreasbreuk

Vlak voordat de zon verdwijnt achter de toppen van de Sierra Cabrera is de Rambla de la Granatilla op z’n mooist. Rode, gele en paarse kleuren geven de vallei een warme gloed. Eén stukje van de Rambla trekt vooral de aandacht; een zone met een breedte van zo’n honderd meter waarin kleurige rechtopstaande banden elkaar afwisselen op meterschaal. Een spel van rood, paars, geel en grijs, en alle kleuren daar tussenin. En het mooiste is: dit is niet alleen een prachtig stukje landschap, het is ook geologisch een zeer interessante plek. Dit is waar de Carboneras breukzone op z’n best is ontsloten

Intracrystalline deformation of calcite

It is well established from observations on natural calcite tectonites that intracrystalline plastic mechanisms are important during the deformation of calcite rocks in nature. In this thesis, new data are presented on fundamental aspects of deformation behaviour of calcite under conditions where 'dislocation creep' mechanisms dominate. The data provide a better understanding of the rheological behaviour of calcite rocks, and provide a basis for meaningful texture (crystallographic preferred orientation) modelling for calcite polycrystals. In chapter 1, previous work on intracrystalline plastic mechanisms in calcite is summarized. Aspects of deformation behaviour hitherto insufficiently understood are highlighted, thus defining the scope for the present study. Chapter 2 describes uniaxial compression experiments performed on optical quality calcite single crystals at temperatures and constant strain rates in the range 400 to 800°C and 3x10-4 to 3x10-8 sec-1 respectively (mostly under controlled CO2 pressure). The tests were carried out with the compression direction parallel to [4041], Le. parallel to the intersection of two cleavage rhombs. At temperatures below -600°C, the crystals deformed largely bye-twinning. At higher temperatures, deformation occurred by slip on a single r system plus a single f system in the so-called positive sense, as identified by slip line analysis. The effective slip direction within the active f-plane was of type rather than the type reported previously. In the slip dominated regime, the samples exhibited steady state flow behaviour. The flow stresses were found to be relatively insensitive to strain rate and can be empirically described by a power law creep equation with a stress exponent ranging from -13 at 550-600 °C to -9.5 at 700-800 °C. Irregular dislocation networks, observed in TEM, are prominent features of the dislocation substructure. These networks may have developed from dislocation interactions involving double cross slip. A related network recovery mechanism is implied by the steady state flow behaviour of the single crystals. In chapter 3, results are presented for uniaxial compression experiments on calcite single crystals in a second orientation, namely with the compression direction at 30° to the c-axis and 23° to the pole on a cleavage rhomb (Le. subparallel to [2243]). The tests were performed at temperatures in the range 300-800 °C, mostly at a constant strain rate of 3x10-5 sec-1. The stress-strain curves exhibited multistage hardening behaviour, steady state only being approached at higher temperatures and/or high strains (>10%). The active glide systems were found to be two r systems and one single f system, all in the so-called negative sense. In addition, at T;:::600 °C, definitive evidence was found for slip on the basal plane. The observed work hardening behaviour is attributed to the absence of a network-related recovery mechanism. Chapter 4 compares the strength characteristics of the r, f and c glide systems in calcite. Based on yield data obtained from multi-stage stress-strain curves, it is shown that no significant difference exists in strength between positive and negative glide on the r and 1 systems. Considering present results as well as previous data, it is also concluded that two regimes of slip system activity exist: 1) a low temperature regime involving e-twinning, r glide and 1 glide, and 2) a higher temperature regime characterized by r, 1 and c slip. A major texture transition is to be expected passing between these regimes In chapter 5, the stress (0") vs. dislocation density (p) relation for calcite single crystals is experimentally determined. The relationship obtained is found to be in good agreement with the well-known theoretical relation, 0" DC pO.5, based on theory of dislocation interaction. Data on calcite polycrystals, however, deviate from this. Using a concept of non-homogeneous deformation related to grain size, a simple model is put forward to account for this. In chapter 6, the flow data obtained for single crystals compressed in the [4041] orientation (chapter 2) are fitted to various microphysical models of dislocation creep. By considering the fitting results and microstructural observations, and comparing these with existing data on other materials, it is proposed that the steady state deformation of single crystals is best explained by a dislocation cross slip controlled creep mechanism. Mechanical behaviour and microstructures characteristic of calcite polycrystals, deformed at roughly identical conditions, show various similarities with the single crystals, and their creep behaviour may well be rate-controlled by the same mechanism. Finally, chapter 7 reports observations on deformed limestones from a small scale shear zone in SW Wales, UK, where maximum PT conditions were 130 MPa, and 200°C respectively. Flattened, strained calcite grains, high dislocation densities and a weak but distinct crystallographic preferred orientation indicate that deformation occurred predominantly by intracrystalline mechanisms. Using conventional paleopiezometers (dislocation density, recrystallized grain size, twinning frequency) and failure criteria, a paleostress within the range 70-410 MPa was inferred for the shear zone. Values computed by extrapolating various experimentally determined flow equations for calcite materials show a far wider range of stresses. Notably, power laws are unsuccesful in reliably predicting paleostresses in deformed calcite rocks at low metamorphic grade. In contrast, the cross slip controlled creep equation established for single crystals in the [4041] orientation may offer a method for estimating minimum flow stresses in limestones deformed at low temperature

Intracrystalline deformation of calcite

It is well established from observations on natural calcite tectonites that intracrystalline plastic mechanisms are important during the deformation of calcite rocks in nature. In this thesis, new data are presented on fundamental aspects of deformation behaviour of calcite under conditions where 'dislocation creep' mechanisms dominate. The data provide a better understanding of the rheological behaviour of calcite rocks, and provide a basis for meaningful texture (crystallographic preferred orientation) modelling for calcite polycrystals. In chapter 1, previous work on intracrystalline plastic mechanisms in calcite is summarized. Aspects of deformation behaviour hitherto insufficiently understood are highlighted, thus defining the scope for the present study. Chapter 2 describes uniaxial compression experiments performed on optical quality calcite single crystals at temperatures and constant strain rates in the range 400 to 800°C and 3x10-4 to 3x10-8 sec-1 respectively (mostly under controlled CO2 pressure). The tests were carried out with the compression direction parallel to [4041], Le. parallel to the intersection of two cleavage rhombs. At temperatures below -600°C, the crystals deformed largely bye-twinning. At higher temperatures, deformation occurred by slip on a single r system plus a single f system in the so-called positive sense, as identified by slip line analysis. The effective slip direction within the active f-plane was of type rather than the type reported previously. In the slip dominated regime, the samples exhibited steady state flow behaviour. The flow stresses were found to be relatively insensitive to strain rate and can be empirically described by a power law creep equation with a stress exponent ranging from -13 at 550-600 °C to -9.5 at 700-800 °C. Irregular dislocation networks, observed in TEM, are prominent features of the dislocation substructure. These networks may have developed from dislocation interactions involving double cross slip. A related network recovery mechanism is implied by the steady state flow behaviour of the single crystals. In chapter 3, results are presented for uniaxial compression experiments on calcite single crystals in a second orientation, namely with the compression direction at 30° to the c-axis and 23° to the pole on a cleavage rhomb (Le. subparallel to [2243]). The tests were performed at temperatures in the range 300-800 °C, mostly at a constant strain rate of 3x10-5 sec-1. The stress-strain curves exhibited multistage hardening behaviour, steady state only being approached at higher temperatures and/or high strains (>10%). The active glide systems were found to be two r systems and one single f system, all in the so-called negative sense. In addition, at T;:::600 °C, definitive evidence was found for slip on the basal plane. The observed work hardening behaviour is attributed to the absence of a network-related recovery mechanism. Chapter 4 compares the strength characteristics of the r, f and c glide systems in calcite. Based on yield data obtained from multi-stage stress-strain curves, it is shown that no significant difference exists in strength between positive and negative glide on the r and 1 systems. Considering present results as well as previous data, it is also concluded that two regimes of slip system activity exist: 1) a low temperature regime involving e-twinning, r glide and 1 glide, and 2) a higher temperature regime characterized by r, 1 and c slip. A major texture transition is to be expected passing between these regimes In chapter 5, the stress (0") vs. dislocation density (p) relation for calcite single crystals is experimentally determined. The relationship obtained is found to be in good agreement with the well-known theoretical relation, 0" DC pO.5, based on theory of dislocation interaction. Data on calcite polycrystals, however, deviate from this. Using a concept of non-homogeneous deformation related to grain size, a simple model is put forward to account for this. In chapter 6, the flow data obtained for single crystals compressed in the [4041] orientation (chapter 2) are fitted to various microphysical models of dislocation creep. By considering the fitting results and microstructural observations, and comparing these with existing data on other materials, it is proposed that the steady state deformation of single crystals is best explained by a dislocation cross slip controlled creep mechanism. Mechanical behaviour and microstructures characteristic of calcite polycrystals, deformed at roughly identical conditions, show various similarities with the single crystals, and their creep behaviour may well be rate-controlled by the same mechanism. Finally, chapter 7 reports observations on deformed limestones from a small scale shear zone in SW Wales, UK, where maximum PT conditions were 130 MPa, and 200°C respectively. Flattened, strained calcite grains, high dislocation densities and a weak but distinct crystallographic preferred orientation indicate that deformation occurred predominantly by intracrystalline mechanisms. Using conventional paleopiezometers (dislocation density, recrystallized grain size, twinning frequency) and failure criteria, a paleostress within the range 70-410 MPa was inferred for the shear zone. Values computed by extrapolating various experimentally determined flow equations for calcite materials show a far wider range of stresses. Notably, power laws are unsuccesful in reliably predicting paleostresses in deformed calcite rocks at low metamorphic grade. In contrast, the cross slip controlled creep equation established for single crystals in the [4041] orientation may offer a method for estimating minimum flow stresses in limestones deformed at low temperature

Stress dependence of microstructures in experimentally deformed calcite

Optical measurements of microstructural features in experimentally deformed Carrara marble help define their dependence on stress. These features include dynamically recrystallized grain size (Dr), subgrain size (Sg), minimum bulge size (Lρ), and the maximum scale length for surface-energy driven grain-boundary migration (Lγ). Taken together with previously published data Dr defines a paleopiezometer over the range 15–291 MPa and temperature over the range 500–1000 °C, with a stress exponent of −1.09 (CI −1.27 to −0.95), showing no detectable dependence on temperature. Sg and Dr measured in the same samples are closely similar in size, suggesting that the new grains did not grow significantly after nucleation. Lρ and Lγ measured on each sample define a relationship to stress with an exponent of approximately −1.6, which helps define the boundary between a region of dominant strain-energy-driven grain-boundary migration at high stress, from a region of dominant surface-energy-driven grain-boundary migration at low stress

Stress dependence of microstructures in experimentally deformed calcite

Optical measurements of microstructural features in experimentally deformed Carrara marble help define their dependence on stress. These features include dynamically recrystallized grain size (Dr), subgrain size (Sg), minimum bulge size (Lρ), and the maximum scale length for surface-energy driven grain-boundary migration (Lγ). Taken together with previously published data Dr defines a paleopiezometer over the range 15–291 MPa and temperature over the range 500–1000 °C, with a stress exponent of −1.09 (CI −1.27 to −0.95), showing no detectable dependence on temperature. Sg and Dr measured in the same samples are closely similar in size, suggesting that the new grains did not grow significantly after nucleation. Lρ and Lγ measured on each sample define a relationship to stress with an exponent of approximately −1.6, which helps define the boundary between a region of dominant strain-energy-driven grain-boundary migration at high stress, from a region of dominant surface-energy-driven grain-boundary migration at low stress

The influence of water and supercritical CO2 on the failure behavior of chalk

Reduction of compressive strength by injection of water into chalk is a well-known mechanism responsible for increased compaction in chalk reservoirs. This raises the question of whether such effects might be enhanced in the context of long-term storage of CO2 or of CO2 injection for enhanced oil and gas recovery (EOR/EGR) purposes. Therefore, data regarding the effect of supercritical CO2 on the mechanical behavior of chalk are needed. The effect of supercritical CO2 on the short-term failure behavior of wet chalk was accordingly investigated by means of conventional triaxial deformation experiments, performed on Maastrichtian chalk cores under dry conditions, in the presence of saturated chalk solution and using CO2-saturated solution at temperatures simulating reservoir conditions (20-80°C) and effective confining pressures up to 7MPa. Increasing temperature from 20 to 80°C did not show any significant effects on the strength of the dry samples. Addition of aqueous solution to the samples led to drastic weakening of the chalk, the effect being more pronounced at high effective confining pressures (Peff>3MPa). Addition of 10MPa supercritical CO2 to wet samples did not produce any significant additional effect in comparison with the wet samples. All samples showed a yield strength envelope characterized by shear failure at low effective mean stresses giving way to a compaction cap at high mean stresses. The weakening effect of aqueous solution was explained in terms of a reduction in frictional resistance of the material, due to water-enhanced grain-contact cracking, and perhaps pressure solution, with a possible contribution by disjoining pressure effects caused by water adsorption. While CO2 does not seem to reduce short-term failure strength of wet chalk, processes such as intergranular pressure solution have to be considered for assessing mechanical stability of chalk in the context of long-term CO2 storage or EOR/EGR operations

Influence of deformation conditions on the development of heterogeneous recrystallization microstructures in experimentally deformed Carrara marble

Recrystallized grains are potentially useful as indicators of palaeostress in naturally deformed rocks, providing that well-calibrated relationships (palaeopiezometers) exist between the recrystallized grain size and stress. Rocks can exhibit microstructures that are heterogeneous, that is, containing recrystallized as well as deformed grains, and showing subgrains within grains that differ in size and character from the grain core to the mantle. Previous studies on palaeopiezometers only rarely took into account such heterogeneous microstructure. We used electron backscattered diffraction (EBSD) to accurately quantify the heterogeneous microstructures in experimentally deformed Carrara marble (flow stress 15-85 MPa, temperature 700-990 8C and natural strain 0.15-0.90). The sizes of bulges, recrystallized grains and deformed grains have been measured. We found that the overall character of the microstructures varies as a function of deformation conditions. In heterogeneous samples showing core-mantle microstructures, the sizes of the bulges and recrystallized grains are independent of strain and show an inverse dependency on stress. The recrystallized grains have been found to nucleate at grain boundary bulges. Our study illustrates that very different microstructures may develop in relation to the complexity of the recrystallization mechanisms. We therefore suggest that piezometers should be calibrated and applied for a single type of overall microstructure