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Postcollisional exhumation history of the Tanzawa Tonalite Complex, inferred from (U-Th)/He thermochronology and fission track analysis

(U-Th)/He熱年代学とフィッション・トラック分析から見た丹沢トーナル岩複合岩体の後衝突削剥史

山田 国見; 田上 高広*

Yamada, Kunimi; Tagami, Takahiro*

新たな熱年代学的手法により島弧島弧衝突による丹沢トーナル岩複合岩体の削剥史を調査した。(U-Th)/He年代はアパタイトで2.0$$pm$$0.2Ma,ジルコンで3.3$$pm$$0.2Ma,ジルコンのフィッショントラック年代は4.5-6.9Maで、地温勾配を40$$^{circ}$$C/kmとすると削剥史は$$sim$$7-3.3Ma: 0.5-1.5mm/yr; 3.3-2.0Ma: $$sim$$2mm/yr; 2.0-0Ma: $$sim$$0.8mm/yrであった。7Maごろ以降、丹沢山地周辺のテクトニクス条件の変化にもかかわらず、削剥速度は大きな変化を受けていないことが明らかになった。

We performed new thermochronometry for the Tanzawa Tonalite Complex to investigate the exhumation history caused by arc-arc collision. Typical (U-Th)/He ages were 2.0 $$pm$$ 0.2 Ma on apatite and 3.3 $$pm$$ 0.2 Ma on zircon and 4.5 to 6.9 Ma on zircon using the fission-track method. The exhumation rate has been calculated to be 0.5 to 1.5 mm/yr from $$sim$$ 7 to 3.3 Ma, $$sim$$ 2 mm/yr from 3.3 to 2.0 Ma and $$sim$$ 0.8 mm/yr from 2.0 to 0 Ma with an assumed geothermal gradient of 40$$^{circ}$$C/km. The exhumation rate has not changed significantly despite the simultaneous tectonic changes around the Tanzawa Mountains after $$sim$$ 7 Ma.

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