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Journal of Petrology Advance Access published online on October 7, 2009
Journal of Petrology, doi:10.1093/petrology/egp057
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Microstructural and Rheological Evolution of a Mantle Shear Zone

Philip Skemer1,*, Jessica M. Warren2, Peter B. Kelemen3 and Greg hirth1

1Department of Geosciences, Brown University, Providence, RI 02912, USA
2Department of Terrestrial Magnetism, Carnegie Institution, Washington, Dc 20015, USA
3Lamont–Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

Received January 24, 2008; Revised typescript accepted August 4, 2009


  Abstract

We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2–3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domains—a relatively coarse-grained domain (~550 µm) that contains only olivine and a finer-grained domain (~250 µm) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker. The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution. The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.

KEY WORDS: olivine; orthopyroxene; mylonite; Josephine; LPO

*Corresponding author. Telephone: 401-863-1044. E-mail: philip_skemer{at}brown.edu


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