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Constraining the timing of brittle deformation and faulting in the Toki granite, central Japan

土岐花崗岩における脆性破壊及び断層活動の時期の制約

山崎 誠子*; Zwingmann, H.*; 山田 国見*; 田上 高広*; 梅田 浩司

Yamasaki, Seiko*; Zwingmann, H.*; Yamada, Kunimi*; Tagami, Takahiro*; Umeda, Koji

断層帯の形成について時間的制約を与えることは、構造発達史を理解するうえで重要である。土岐花崗岩の断層ガウジに含まれる粘土鉱物のイライトを対象に、粒度ごとにK-Ar年代測定を実施した。その結果、より細粒の試料でより新しい年代値が得られ、細粒の試料には自生の結晶がより多く含まれていることが示唆された。0.1$$mu$$m未満の粒子の年代値は42.7$$sim$$46.5Maを示しており、母岩の冷却史におけるイライトの安定領域と脆性破壊の温度領域とに一致する結果が得られた。

Constraining of the timing of fault zone formation is of fundamental geotectonic importance to understand structural evolution and brittle fault processes. Here, we present authigenic illite K-Ar age data from brittle fault zones comprising two gouges within the Toki granite, central Japan. The gouge samples were collected from a shaft at the Mizunami Underground Research Laboratory, and were separated into five grain-size fractions. K-Ar ages of clay fractions decrease with grain size, suggesting enrichment in finer fraction of more-recently grown authigenic illites. The K-Ar ages of the fractions range from 53.6 to 42.7 Ma (Paleogene-Early to Middle Eocene). The $$<$$0.1 $$mu$$m fractions yield ages of 42.7 and 46.5 Ma. This age range is consistent with the stability field of illite and the main temperature field of brittle deformation within the cooling history of the Toki granite, supported by extensive thermochronological data of the host rock.

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パーセンタイル:33.22

分野:Geochemistry & Geophysics

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