フィリピンルソン島中央コルディエラ山地の削剥史の地質; 熱年代学的予備研究

A Preliminary geo- and thermochronological study on the exhumation history of the Central Cordillera, Luzon, Philippines

中嶋 徹   ; 末岡 茂   ; 長田 充弘   ; Kohn, B.*; Ramos, N.*; 堤 浩之*; 田上 高広*

Nakajima, Toru; Sueoka, Shigeru; Nagata, Mitsuhiro; Kohn, B.*; Ramos, N.*; Tsutsumi, Hiroyuki*; Tagami, Takahiro*

島弧における山地隆起のメカニズムとしてドーム状隆起と断層ポップアップモデルの2つの端成分モデルが提唱されている(Sueoka et al., 2015; Fukuda et al., 2021)。本研究では、地質-熱年代学的手法をフィリピンルソン島中央コルディエラ山地に適用し、これらのモデルを検証するとともに山地隆起メカニズムを議論する。

In this study, we apply geo- and thermochronological methods to the Central Cordillera, Luzon, Philippines, to discuss the uplift mechanisms. We collected five diorite samples from the east and west blocks of the Abra River Fault, a strike-slip fault along the western margin of the Central Cordillera. The zircon U-Pb Concordia ages range from 32.5 to 6.1 Ma, indicating that the intermediate magma intermittently intruded the middle and upper crust during the Cenozoic volcanism. The zircon fission-track (ZFT) pooled ages range from 36.7 to 6.9 Ma, which are generally comparable with the U-Pb ages. These results suggest that these diorites were rapidly cooled below 200-300 degree, the partial anneal zone of the ZFT, immediately after the intrusions. On the other hand, the apatite (U-Th-Sm)/He (AHe) weighted mean ages ranging from 9.3 to 1.8 Ma indicate a decrease in cooling rates around 80-200 degree. This observation indicates that the ZFT ages reflect the initial cooling of the intermediate magma, whereas the AHe ages reflect the cooling process associated with the regional exhumation after the initial cooling. The AHe ages do not show a younging pattern toward the axial part of the mountain range and do not support the doming-uplift of the Central Cordillera (Sueoka et al., 2015). Inverse analysis using a thermokinematic model (Braun et al., 2012) proposes a pop-up uplift mechanism with almost no exhumation during the Neogene and rapid exhumation (ca. 3.0 mm/yr) during the Quaternary. A significant difference in exhumation rates was also not observed between the eastern and western blocks of the Abra River Fault, suggesting that faulting had little contribution to the regional exhumation. Our initial geo- and thermochronological data infer that exhumation of the Central Cordilleras during the Quaternary resulted from the pop-up uplift of the entire mountain range.