Late Quaternary Stratigraphic Features in the Ilan Basin, an Active Tectonic Subsidence Basin in Taiwan

The Ilan Basin, located at the southwestern end of the Okinawa Trough, was mostly believed to be formed due to the expansion of the Okinawa Trough. However, recent marine surveys show that they may not be directly related. On the other hand, existing terrestrial surveys concentrated on the Oligocene to Miocene formations instead of the tectonic activities during the Last Glacial period to Holocene, and contradictions remain in the interpretation of the paleo-environment. This study analyzed 40 cores of Ilan Plain, reconstructed the paleo-sedimentary environment, and interpreted the seismic profiles. We found that the transgression of the Ilan Plain in the Last Glacial period was controlled by tectonic activities. The subsequent main transgression that happened in 17.5 ka and 15~14ka was driven by the rapid sea-level rise after the Last Glacial Maximum and the Melting-water Pulse 1A event. The tectonic subsidence of the Ilan Basin was centered on the deepest part of the basement. The combination of subsidence rate and sediment supply was generally stable before 4,000 years ago, but the subsidence rate has increased significantly since then, and the sediments supply has also been increased. The sediments not only filled the deepest area in the north of Lanyang River but also left the seismic facies of forwarding propagation on the Ilan shelf. In addition, there may be another sinking center in the south before 10 thousand years ago. This study continues to establish the complete sedimentary model of the Ilan Basin and to discuss the timing and causes of the main changes in the sedimentary environment. This study will improve our understanding of the tectonic subsidence model of the Ilan Basin and the sedimentary system in the basins with significant tectonic subsidence.