An Experimental Study on the Strength of the Lithosphere: Large-strain shear deformation experiments of olivine + orthopyroxene aggregates

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Although plate tectonics is well documented on Earth, the reason why plate tectonics works on Earth but not on other terrestrial planets such as Venus, Mars is not well understood. In fact, plate tectonics is not an obvious mode of tectonics on Earth-like planets. On Earth-like planets, the near-surface layer is strong because of low temperature and the most 'natural' mode of tectonics is the so-called 'stagnant-lid' convection where the surface layer is rigid and does not deform (this is a case for Mars and most of other planets). In fact, if one uses a standard model of the strength of the lithosphere, the oceanic lithosphere on Earth will be too strong for plate tectonics to operate. Consequently, some weakening processes need to be invoked in order to understand why plate tectonics operates on Earth. After reviewing various new observations on the strength of Earth materials, we believe that the reduction in the strength in the deep lithosphere is a key to the operation of plate tectonics, and focus our attention to this issue.Based on our previous studies, we hypothesize that the presence of a secondary phase mineral orthopyroxene (the primary one being olivine) might have a key influence on the weakening. Consequently, we will conduct systematic experimental studies on deformation of a mixture of olivine and orthopyroxene for various ratios with a range of water contents. Rheological behavior (i.e., the stress-strain relationship) and the micro-structural evolution will be investigated. The results will then be interpreted by a model of weakening, and applied to Earth and other terrestrial planets. The results of this study will shed some lights on the reason for the operation of plate tectonics on Earth but not on other terrestrial planets.