Non-linear feedback loops in the rheology of cooling-crystallizing felsic magma and heating-melting felsic rock
Jean-Pierre Burg, Jean-Louis Vigneresse, 2002. "Non-linear feedback loops in the rheology of cooling-crystallizing felsic magma and heating-melting felsic rock", Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives, S. de Meer, M. R. Drury, J. H. P. de Bresser, G. M. Pennock
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At least six major parameters control the rheology of partially molten systems: melt content, rate of melt production, reaction to strain of the solid component, reaction to strain of the molten component, temperature and chemical composition of the source rocks. We examine their interactions to understand the rheology of partly molten rocks and partly crystallized magmas. In particular, this paper focuses on the rheology in the transitional domains between two pairs of thresholds that bracket a transitional regime between solid state and fluid behaviour during melting and crystallization, respectively. We review related information and point out non-linear effects that develop during heating of melting rocks and cooling of crystallizing magmas. Owing to the non-linear interactions, positive or negative feedback loops accelerate or damp the system. Melt in migmatite experiences shear-softening which, along with strain partitioning, facilitates melt segregation. Conversely, the increasing number of rigid crystals during cooling increases the suspension viscosity (shear hardening), which soon inhibits magma movement. These effects reinforce the asymmetry between solid-to-melt and melt-to-solid transitions. They severely contradict the concept of one rheological critical melt percentage valid for both melting and crystallization transitions. Fabric acquisition competes with nucleation and crystal growth, thus leading to hysteresis of the stress-strain rate curves. Implications for field observations include horizontal magma segregation, magma extraction and successive magma intrusions.
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The motion and deformation of rocks are processes of fundamental importance in shaping the Earth, from outer crustal layers to the deep mantle. Reconstructions of the evolution of the Earth therefore require detailed knowledge of the geometry of deformation structures and their relative timing, of the motions leading to deformation structures and of the mechanisms governing these motions. This volume contains a collection of 22 papers on field, experimental and theoretical studies that add to our knowledge of these processes. They are a mixture of review papers oh selected topics in the field of structural geology and tectonics and papers on current issues and new techniques and are grouped into four themes:
The effect of fluids on deformation
The interpretation of microstructures and textures
Deformation mechanisms and rheology of crust and upper mantle minerals
Crust and lithosphere tectonics
The volume will appeal to researchers in the fields of structural geology and tectonophysics, both in academia and industry.