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The transition from brittle to ductile deformation in the Sambagawa metamorphic belt, Japan
Author(s) -
TORIUMI M.
Publication year - 1990
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1990.tb00631.x
Subject(s) - geology , metamorphism , epidote , actinolite , metamorphic rock , metamorphic facies , greenschist , geochemistry , petrology , facies , paleontology , chlorite , quartz , structural basin
The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote‐pumpellyite‐actinolite zone and the epidote‐actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote‐actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote‐actinolite zone of the Sambagawa metamorphic belt. The sharp change of the ductile strain magnitude also takes place at the epidote‐actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300‐400°C. and is independent of pressure of metamorphism.