Late Miocene thermal history of the Hengchun Peninsula, Southern Taiwan: Apatite Fission-Track data from the Lilungshan Formation

This study aims to investigate the thermal history regarding the Late Miocene Formation of the Hengchun Peninsula with low-temperature thermal chronometry. The samples used in our study were from Lilungshan Formation, which included quartzite (conglomerates) and sandstones (matrix). Lilungshan Formation was an upper fan or feeder channel deposits in shelf environments. Measurements of paleocurrent indicate that these rocks were transported from the NW to the SE, which may represent its source area is a low-grade metamorphic orogenic belt (Yuli belt). In the Late Miocene, outcrops of Yuli belt were low-grade metamorphic rocks with low metamorphic temperatures. To do so, low-temperature thermal chronometry was applied to measure the time since the Lilungshan Formation cooling below the closure temperature. Apatite Fission-track thermochronology is used in this study, which is a radiometric dating method that refers to thermal histories of rocks within the closure temperature range of 110–135°C.

Our study indicates that the pooled age of apatite fission tracks of conglomerates is 3.3–5 Ma and the grain ages of sandstones are below 5 Ma. Those ages are lower than the formation age of Lilungshan Formation (NN11, > 5.6 Ma). In addition, the grain ages spectrum of sandstones is partial annealing, which implies that the conglomerate has suffered from the thermal event and rapidly brought to the earth’s surface within 4 Ma. This study also compares data of previous studies with regard to the fission tracks of conglomerates in Southern Taiwan and confirms the existence of thermal events. With the assumption that the thermal gradient of the accretionary prism is 40–45°C/km, we can suggest that Lilungshan Formation was located 3 km below the earth's surface in roughly 4 Ma.

 

Keywords: Hengchun Peninsula, Lilungshan Formation, Apatite Fission Track, thermal history, chronometry, Late Miocene