Evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation

Details

Serval ID
serval:BIB_DA90499CD5F9
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation
Journal
Journal of Geophysical Research - Solid Earth
Author(s)
Linckens J., Herwegh M., Muntener O., Mercolli I.
ISSN-L
0148-0227
Publication state
Published
Issued date
2011
Peer-reviewed
Oui
Volume
116
Pages
B06210
Language
english
Abstract
The influence of second phases (e.g., pyroxenes) on olivine grain size was studied by quantitative microfabric analyses of samples of the Hilti massif mantle shear zone (Semail ophiolite, Oman). The microstructures range from porphyroclastic tectonites to ultramylonites, from outside to the center of the shear zone. Starting at conditions of ridge-related flow, they formed under continuous cooling leading to progressive strain localization. The dependence of the average olivine grain size on the second-phase content can be split into a second-phase controlled and a dynamic recrystallization-controlled field. In the former, the olivine grain size is related to the ratio between the second-phase grain size and volume fraction (Zener parameter). In the latter, dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size. In both fields the average olivine and second-phase grain size decreases with decreasing temperature. Combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites (similar to 1100 degrees C) and mylonites (similar to 800 degrees C) formed under the predominance of dislocation creep. Since olivine-rich layers are intercalated with layer parallel, polymineralic bands in the mylonites, nearly equiviscous conditions can be assumed. In the ultramylonites, diffusion creep represents the major deformation mechanism in the polymineralic layers. It is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine-grained polymineralic domains at low temperatures (<similar to 700 degrees C), underlining the role of the second phases on strain localization in cooling mantle rocks.
Open Access
Yes
Create date
09/10/2011 20:09
Last modification date
20/08/2019 15:59
Usage data