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Deformation mechanisms and rheology of crust and upper mantle minerals

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Published:
January 01, 2002

Abstract

High pressure (HP) and ultrahigh (UHP) metamorphic rocks are exhumed from subduction zones at high rates on the order of plate velocity (cm/year). Their structural and microstructural record provides insight into conditions and physical state along the plate interface in subduction zones to depths of >100 km. Amazingly, many identified (U)HP metamorphic rocks appear not to be significantly deformed at (U)HP conditions, despite their history within a high strain rate mega-shearzone. Other (U)HP metamorphic rocks seem to be deformed exclusively by dissolution-precipitation creep. Indications of deformation by dislocation creep are lacking, apart from omphacite in some eclogites. Available flow laws for dislocation creep (extrapolated to low natural strain rates, which is equivalent to no deformation on the time scales of subduction and exhumation, i.e., 1 to 10 Ma) pose an upper bound to the magnitude of stress as a function of temperature along the trajectory followed by the rock. Although the record of exhumed (U)HP metamorphic rocks may only be representative of specific types or evolutionary stages of subduction zones, for such cases it implies: (1) strongly localized deformation; (2) predominance of dissolution-precipitation creep and fluid-assisted granular flow in the shear zones, suggesting Newtonian behaviour; (3) low magnitude of differential stress; which (4) is on the order of the stress drop inferred for earthquakes; and (5) negligible shear heating. These findings are easily reconciled with exhumation by forced flow in a low viscosity subduction channel prior to collision, implying effective decoupling between the plates.

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Geological Society, London, Special Publications

Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives

S. de Meer
S. de Meer
Utrecht University, The Netherlands
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M. R. Drury
M. R. Drury
Utrecht University, The Netherlands
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J. H. P. de Bresser
J. H. P. de Bresser
Utrecht University, The Netherlands
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G. M. Pennock
G. M. Pennock
Utrecht University, The Netherlands
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Geological Society of London
Volume
200
ISBN electronic:
9781862396081
Publication date:
January 01, 2002

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