Upper crustal deformation
The concept that the lithosphere is in a general critical, or near-critical, mechanical state has previously assumed spatial intersection of fractures at the critical point. This paper provides an initial basis for an alternative mechanism in which elastic interactions between aligned, open, spatially separated (micro-)cracks in rock can, by themselves, lead to a continuous phase change at a critical threshold of crack density lower than that required for crack coalescence. The existence of a critical density of aligned cracks is first demonstrated on a regular 2D hexagonal grid when subjected to a central stress perturbation; at this critical density the elastic tensile stress interactions have a long-range effect. From the results of those calculations an approximation of the tensile stress field around a crack in 3D is provided. The percolation behaviour of such 3D bodies is discussed and a critical crack density in 3D for elastic interaction of 0.035 is deduced. This falls within the range of crack densities (0.015–0.045) interpreted from observations of shear-wave splitting in many different rocks. The possibility that critical crack interactions can be mostly elastic provides one explanation of the long-range nature of correlations in flowrate fluctuations in oilfields without large-scale seismicity.
Figures & Tables
Ernie Rutter has made, and continues to make, a significant impact in the field of rock deformation. He has studied brittle and plastic deformation processes that occur within both the oceanic and continental crust, as well as other key properties such as the permeability and seismic velocities of these rocks. His approach has been one that integrates field observations, laboratory experiments and theoretical analyses. This volume celebrates Ernie’s key contribution to rock deformation and structural geology by bringing together a collection of papers that represent this broad approach. The papers within the volume address key issues that remain within these fields. These range from fundamental studies of brittle and plastic behaviour along with the resultant structures and microstructures from both the field and laboratory, to applied problems where a better understanding of the deformation and properties of the crust is still needed.