Deformation mechanisms, rheology and tectonics: microstructures, mechanics and anisotropy: introduction

This special publication of the Geological Society of London presents recent advances in the study of deformation mechanisms and rheology and their application to tectonics. We have subdivided the papers into two themed sections. The inference of deformation processes, conditions and rheology at depth in active tectonic settings is of fundamental importance to a quantitative geodynamic understanding of deformation in the Earth. The papers in the section on Lattice Preferred Orientations and Anisotropy are extremely important as they underpin our ability to make such geodynamic interpretations from global seismic data. These papers reflect the growing emphasis on the determination of elastic properties from microstructures, from which acoustic properties can be computed for comparison with in situ seismic measurements. The component of the microstructure that receives most attention is the lattice preferred orientation (LPO), otherwise known as the crystallographic preferred orientation (CPO) or the texture (the term used in material science and metallurgy). The papers include new LPO measurements (made almost exclusively by the relatively new technique of electron backscatter diffraction or EBSD), exploration of the significance of these data for seismic properties of both the crust and the mantle and modelling of LPO generation. An invited contribution from Mainprice and colleagues introduces a computational toolbox to help researchers calculate anisotropic physical properties from their LPO data. Rock microstructures evolve during deformation and rock physical properties, including both elastic properties and creep rheology, evolve with the microstructures as a function of strain and time. The section on Microstructures, Mechanisms and Rheology reflects the fundamental importance of understanding microstructural evolution to our ability to estimate deformation processes and conditions from recovered samples or geophysical data and to our modelling of tectonics. An invited contribution from Austin focuses on some of the key issues from the last few decades: how different mechanisms (grain size sensitive and grain size insensitive) compete and interact to control the evolution of grain size and LPO. Many of the other papers touch on these issues and make use of combinations of laboratory experiments, field studies and computational methods to explore the controls on microstructural evolution and to relate microstructural evolution to rheology and largescale tectonic processes. It is clear from this collection of papers that resolution of the controls on microstructural evolution in rocks remains at the cutting edge of Earth sciences.