The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite |
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| Authors: | MICHIBAYASHI KATSUYOSHI; MAINPRICE DAVID |
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| Institution: | 1 INSTITUTE OF GEOSCIENCES, SHIZUOKA UNIVERSITY, SHIZUOKA 422-8529, JAPAN
2 LABORATOIRE DE TECTONOPHYSIQUE, CNRS UMR 5568, UNIVERSITÉ MONTPELLIER II, MONTPELLIER 34095, FRANCE |
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| Abstract: | Structural and fabric analysis of the well-exposed Hilti mantlesection, Oman ophiolite, suggests that shear zone development,which may have resulted from oceanic plate fragmentation, wasinfluenced by pre-existing mantle fabric present at the paleo-ridge.Detailed structural mapping in the mantle section revealed agently undulating structure with an east–west flow direction.A NW–SE strike-slip shear zone cuts across this horizontalstructure. The crystal preferred orientation (CPO) of olivinewithin the foliation is dominated by (010) axial patterns ratherthan more commonly observed (010)100] patterns, suggestingthat the horizontal flow close to the Moho involved non-coaxialflow. Olivine CPO within the shear zone formed at low temperatureis characterized by (001)100] patterns and a sinistral senseof shear. The olivine CPO becomes weaker with progressive mylonitizationand accompanying grain size reduction, and ultimately developsinto an ultra-mylonite with a random CPO pattern. The olivine010]-axis is consistently sub-vertical, even where the horizontalfoliation has been rotated to a sub-vertical orientation withinthe shear zone. These observations suggest that the primarymechanical anisotropy (mantle fabric) has been readily transformedinto a secondary structure (shear zone) with minimum modification.This occurred as a result of a change of the olivine slip systemsduring oceanic detachment and related tectonics during cooling.We propose that primary olivine CPO fabrics may play a significantrole in the subsequent structural development of the mantle.Thus, the structural behavior of oceanic mantle lithosphereduring subduction and obduction may be strongly influenced byinitial mechanical anisotropy developed at an oceanic spreadingcenter. KEY WORDS: mantle lithosphere; anisotropy; shear zone; olivine CPO; Oman ophiolite |
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| Keywords: | : mantle lithosphere anisotropy shear zone olivine CPO Oman ophiolite |
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