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Particulate flow of rock and the formation of cleavage

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DOI: 10.1016/0040-1951(81)90243-2


Abstract Sliding on grain boundaries produces particulate flow during rock deformation. Intragranular deformation and particulate flow may be uncoupled (independent particulate flow), completely coupled (dependent particulate flow) or partially coupled (controlled particulate flow). In independent particulate flow grains are not deformed. In dependent particulate flow the grain sliding is limited and dependent on the incompatible deformation of grains. In controlled particulate flow, grain sliding is encouraged by other factors (e.g. pore-fluid pressure) but the rate of grain sliding is controlled by the deformation of grains. Cataclastic flow and superplasticity are two well known types of behaviour that involve intragranular deformation mechanisms and one or more types of particulate flow. The degree of coupling between intragranular deformation and particulate flow may change during the course of natural deformation. For example, pore-fluid pressure may decrease, causing a transition from controlled to dependent particulate flow. The early part of such a history would be dominated by particulate flow and intragranular deformation would predominate later. This explains bulk strains that exceed the strains of individual grains, and it explains cleavages (formed by the deformation of grains) which transect coeval folds and show no simple relationship with the bulk strain ellipsoid. It also indicates that strain data derived from grain shapes or intragranular features may not truly reflect bulk strain.

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