Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 110; no. 41; pp. 16355 - 16360
• Main Authors: Farla, Robert J. M., Karato, Shun-ichiro, Cai, Zhengyu
• Format: Journal Article
• Language: English
• Published: Washington, DC National Academy of Sciences 08.10.2013; NATIONAL ACADEMY OF SCIENCES; National Acad Sciences
• Subjects: crystallization; Deformation; Earth sciences; Earth, ocean, space; Evolution, Planetary; Exact sciences and technology; Geological Phenomena; Grain size; Internal geophysics; Kinetics; Lithosphere; Low temperature; Microstructure; Minerals - chemistry; Models, Theoretical; Olivine; Physical Sciences; Plastic deformation; Plate tectonics; Recrystallization; Rheology; Shear stress; Shear zones; Solid-earth geophysics, tectonophysics, gravimetry; Strain rate; tectonics; Temperature; strain; nesosilicates; orthopyroxene; chain silicates; rheology; lithosphere; electron probe; olivine; deformation mechanisms; electron probe microanalysis; shear stress; two-phase models; Zener pinning; pyroxene; silicates; deformation; two-phase; laboratory studies
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1218335110

For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine–orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window. The reduction in strength is associated with the formation of small orthopyroxene and olivine grains. Our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones: fine-grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated. A model is developed to apply these results to geological conditions. Such a model, combined with our experimental observations, suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range, leading to long-term weakening associated with strain localization in the lithosphere.