[S-CG46] Hard-Rock Drilling Science: From Continental to Deep Sea Drilling, and Oman Project

[SCG46-03] Comparing the rheological properties of ductile shear zones within the megamullions of mid-ocean ridges and back-arc basins

*Kohei Nimura¹, Katsuyoshi Michibayashi^1,2, Yumiko Harigane³, Yasuhiko Ohara^2,4 (1.Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, 2.JAMSTEC, 3.Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 4.Hydrographic and Oceanographic Department of Japan)

Keywords:megamullion, ductile shear zone, gabbroic rock, flow law, mid-ocean ridge, back-arc basin

Among more than 200 megamullions identified on mid-ocean ridges (MORs) and in back-arc basins, only four have been examined for their rheological properties such as deformation mechanism, deformation temperature, shear strain rate and thickness of the shear zone). Furthermore, since comparative studies of their rheological properties have been limited to the megamullions of mid-ocean ridges, the similarities and differences of ductile shear zones within the megamullions of MORs and back-arc basins are poorly understood. To address these issues, we investigated the rheological properties of ductile shear zones within the Mado Megamullion of the Shikoku Basin in the Philippine Sea by analyzing deformed gabbroic rocks. Microstructural observations and crystallographic preferred orientation analyses suggest that the dominant deformation mechanism of both the plagioclase and amphibole in the deformed gabbroic rocks was dislocation creep. The amphibole–plagioclase geothermometer indicates that the gabbroic rocks underwent crystal plastic deformation in a temperature range of 870–690 °C. By applying the wet-plagioclase flow law, we estimated the shear strain rate of the ductile shear zone within the Mado Megamullion to be higher than 10^–10 s^–1 and its thickness to be less than 7 m. We compared these deformation conditions with those of typical megamullions around the world, and we found that the ranges of crystal plastic deformation temperatures were almost identical in the gabbroic rocks of megamullions on mid-ocean ridges and in back-arc basins, but that there were clear differences in the shear strain rates and thicknesses of the shear zones. Here, we show that megamullions can be classified roughly into two types: wet types with a thin shear zone and high shear strain rate (e.g., Mado and Godzilla) and dry types with a thick shear zone and low shear strain rate (e.g., Kane and Atlantis Bank). Our model suggests that the thicknesses and shear strain rates of ductile shear zones within the megamullions of back-arc basins are thinner and higher, respectively, than those of ductile shear zones within the megamullions of mid-ocean ridges.