Rheology of Crustal Rocks at Ultrahigh Pressure

Abstract

An improved understanding of tectonic processes in the deep levels of subduction zones and collisional belts requires information on the mechanical behavior of continental crust during ultrahigh-pressure (UHP) metamorphism. Predictions are based on the results of experimental deformation of minerals stable at ultrahigh pressure and on the anticipated effect of pressure on deformation mechanisms. Flow laws for dislocation creep of coesite and aragonite indicate that, at ultrahigh pressure, the strength of continental material remains well below a few MPa at natural strain rates. We question the high strength that has been inferred for eclogites, based on preliminary experimental data on jadeite and on natural microstructures of omphacite. The (micro)structural record of natural UHP rocks indicates that strain localization into weak shear zones, albeit not yet identified, must be common. We propose that the presence of dense fluids or hydrous melts at grain and solid phase boundaries could accomplish deformation analogous to liquid phase sintering in ceramics. The low strength of continental material at ultrahigh pressure precludes notable shear heating, thus cool geotherms and P-T paths are implied. The low strength also places upper bounds on the stress drop of seismic events in presently subducted continental crust and limits the size of coherent subducted continental slices.