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.
The main goal of this project was to obtain experimental results to understand why plate tectonics occur on Earth but not on other planets such as Venus. Plate tectonics is a well-established paradigm (theory) in geological science htat explain a large number of observations in a systematic fashion. Geological and geophsyical observations show that the near-surface layer is relatively strong and does not deform much but in a limited narrow regions they do deform. This deformation causes earthquakes and much of hte volacnism and also is responsible for mountain building. This relatively strong layer is called lithospheric plate. The intermediate nature of mechanical behavior of "plate" is not easy to explain. The near surface layer of a planet is cold, and therefore the near surface layer is strong. However, if the lithosphereic plate is so strong, it would not deform and therefore plate will not be broken up and the surface layer would remain stagnant. This is the most natural mechanical behavior of geological material. And indeed all other terrestrial planets (Mars, Venus, Mercury, the Moon) show that their surface layers are not moving. Therefore a key question to be addressed is why the lithsopheric plate on Earth is not s strong to allow breaking up of plates and their "subduction" (going back to the deep interior. The key aspect of the mechanica behvaior of the lithospheric plate on Earth is that it undergoes localized deformation. The localized deformation occurs when a material becomes weak as it deforms. Understanding the process of this strain weakening is the central theme of this project. Based on our previous study, we suspected that a minor phase, orthopyroxene, might play a key role in promoting strain weakening. Therefore we have conducted large strain deformation experiments on the mixtures of olivine + orthopyroxene (model lithosphere material) and determined the conditions under which strain localization occurs. We found that strain weakening is triggered by the recrysrallization of orthopyroxene (formation of fine-grains of orhtopyroxene) that occurs only in a narriw temperature window corresponding to the mid-lithopshere temperature. These results provide a new insight into the origin of plate tectnonics on Earth.
