EXPLORING THE COUPLING OF DEFORMATION AND METAMORPHISM

Structural geologists and petrologists have long recognized the intimate relations between deformation and metamorphism. For example, the recognition that some porphyroblasts grew while deformation was occurring in the surrounding rock goes back nearly a century, when geologists were making hand drawings of spiral-shaped inclusion trails and other syndeformational microstructures. With the power of modern analytical equipment, workers are now able to investigate details of the coupling between deformation and metamorphism. For example, we have known for decades that the development of crenulation cleavage involves a redistribution of volume, but recent workers have used x-ray compositional mapping techniques to suggest balanced reactions involved in the mass transfer, and to identify specific microstructural sites where dissolved material is redeposited. Other analytical techniques like electron backscatter diffraction and in-situ age dating are also contributing enormously to our understanding.

With the explosion of computer technology, the coupling of deformation, reaction and diffusion can now be investigated with microstructure modeling platforms like ELLE (http://www.microstructure.info/index.php). This code allows both mechanical and microstructural evolution to be evaluated to arbitrarily high strains by sequentially simulating specific grain-scale metamorphic and deformation processes such as dynamic recrystallization, grain nucleation, lattice and grain-boundary diffusion, and lattice rotation. Modeling allows the investigator to explore the feedback relations among processes and evolving fabric, and how fabric evolution affects rheological and other rock properties.

Experimental investigations of rocks and polycrystalline analog materials provide an essential tool in our exploration of coupled deformation, reaction and diffusion. Results from experiments give valuable insights on the progressive evolution of dynamic systems, quantitative tests for mathematical descriptions of coupling, and calibration for numerical codes that seek to simulate processes and their feedbacks. Technological advances in this area must keep pace with analytical and numerical advances.