Heap, M.J.; (2009) Creep: time‐dependent brittle deformation in rocks. Doctoral thesis , UCL (University College London).

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Abstract

The characterization of time‐dependent brittle rock deformation is fundamental to understanding the long‐term evolution and dynamics of the Earth’s crust. The chemical influence of pore water promotes time‐dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short‐term failure strength. Reported here are results from a study of time‐dependent brittle creep in a suite of sandstones and a basalt under triaxial stress conditions. Results from conventional creep experiments show that creep strain rate is heavily dependent on the applied differential stress, a reduction of only 10% in differential stress results in a decrease in creep strain rate of more than two orders of magnitude. Conventional creep experiments have also demonstrated the possible existence of a critical damage threshold at the onset of the tertiary creep phase. The level of damage in the samples at the onset of acceleration to failure, regardless of the applied differential stress and the creep strain rate, is approximately similar. Furthermore, the relative proportions of the three phases classically used to describe creep are approximately the same, regardless of creep strain rate and rock type. Sample variability results however in significant scattering in the experimental data and numerous experiments are needed to clearly define a relation between the strain rate and the applied stress. Hence, it is demonstrated that stress‐stepping creep experiments provide a means to successfully overcome this problem. The influence of effective stress was investigated in stress‐stepping experiments with effective confining pressures of 10, 30 and 50 MPa (whilst maintaining a constant pore fluid pressure of 20 MPa). In addition to the expected purely mechanical influence of an elevated effective pressure results also demonstrate that stress corrosion appears to be inhibited at higher effective pressures. The influence of doubling the pore fluid pressure however, whilst maintaining a constant effective pressure, is shown to have no effect on the rate of stress corrosion. Stress‐stepping creep experiments have also demonstrated that the influence of an elevated temperature (from 20° to 75°C) has a profound effect on stress corrosion. For the same applied constant stress, creep strain rates are seen to increase by up to three orders of magnitude in sandstone. For basalt however, creep strain rates are only modestly affected. This is likely to be due to the high quartz content of sandstone that has been shown previously to be greatly influenced by temperature in double‐torsion experiments. All of the results are discussed in the context of microstructural analysis, acoustic emission hypocentre locations, mineralogy and fits to proposed macroscopic creep laws and a creep flow law. Elastic moduli are crucial parameters for defining relationships between stress and strain. Importantly, volcano monitoring techniques routinely running at Mt. Etna for example, such as seismic tomography and ground deformation modelling, rely on accurate knowledge of elastic moduli. Increasing‐amplitude stress‐cycling experiments have been performed on two extrusive basalts and two porous sandstones. Experiments have shown that stress‐cycling results in a reduction in sample stiffness, equating to a decrease in Young’s modulus and an increase in Poisson’s ratio. These changes are attributed to the growth of new cracks and the extension of pre‐existing cracks during each stress cycle and, hence, an increase in the total crack density. This interpretation is supported by the observation of the Kaiser ‘stress‐memory’ effect, where cracking‐associated microseismicity (acoustic emission or AE) on any stress cycle only occurs when the maximum stress in any previous cycle has been exceeded. It is also shown that in stress cycles in which the previous maximum stress is not exceeded, no AE output is generated and there are no further changes in elastic moduli. The results are interpreted in relation to measurements of tectonic‐volcano (VT) seismicity and deformation at Mt. Etna volcano.

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