- © The Mineralogical Society Of America
Crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales. Advances in our understanding have come from a combination of field observations, laboratory experiments and theoretical models. What necessarily links all these investigations is a focus on process. Crystal plasticity (dislocation creep) is one of several grain-scale deformation processes or mechanisms; each process is dominant over a certain range of conditions in the crust, has a particular form of the flow law, and produces a characteristic set of microstructures. Characterizing the deformation mechanisms for different minerals, over a range of conditions, allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces. The information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks, and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings. Iterations among field, theoretical, and experimental approaches are critical for making progress in our understanding.
The deformation process of crystal plasticity is the main focus of this volume. Crystal plastic deformation of the crust is in many ways more complex than that of the mantle, because most crustal rocks are polyphase aggregates, and the strengths of common crustal silicates (such as quartz, feldspar, and mica) are very different at a given set of conditions. Yield strength envelopes for the crust, as shown schematically in Figure 1⇓, are commonly plotted using the dislocation creep flow law for quartz, which inherently assumes that quartz is the controlling phase and undergoes steady state flow. But is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite, let alone a crustal ‘plum pudding’ of regions and layers of different lithologies? The crust …