Variable microstructure of peridotite samples from the southern Mariana Trench: Evidence of a complex tectonic evolution |
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| Authors: | Katsuyoshi Michibayashi Miki Tasaka Yasuhiko Ohara Teruaki Ishii Atsushi Okamoto Patricia Fryer |
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| Institution: | aInstitute of Geosciences, Shizuoka University, Shizuoka, Japan;bDepartment of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan;cOcean Research Laboratory, Hydrographic and Oceanographic Department of Japan, Tokyo, Japan;dOcean Research Institute, University of Tokyo, Tokyo, Japan;eGraduate School of Environmental Studies, Tohoku University, Sendai, Japan;fSOEST, University of Hawaii at Manoa, Honolulu, Hawaii, USA |
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| Abstract: | We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594–7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)100] and (010)001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region. |
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| Keywords: | Mantle wedge Crystal-preferred orientation Olivine Mariana Trench Microstructure Challenger Deep |
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