Holocene Sedimentation Pattern in the Backarc-Opening Ilan Plain, Taiwan: Implications for Regional Tectonic Subsidence and Basin Shape

Abstract:

The triangular Ilan Plain in northern Taiwan has well-preserved sedimentary records which provide opportunities for understanding the subsidence and sedimentary processes at the southernmost tip of the backarc-opening Okinawa Trough. To better examine the deposition and tectonic history of the Ilan Plain, we analyzed data from 13 boreholes and used ¹⁴C dates to reconstruct basin sedimentary layers during the Holocene time. The borehole depths and their correspondent ¹⁴C ages are used to reconstruct the overall age models in the Ilan Plain. The sedimentation rates from the borehole locations vary significantly from 0.5 to 2.0 cm/yr. Age models were fitted using quadratic equations instead of linear equations. The linear age models, although commonly used by previous studies, may not be desirable because most age distributions show decreasing sedimentation rates, particularly after 6 ka BP. Six boreholes show very good fit using quadratic equations in the age models and five boreholes, mostly located along the coastal areas, show relatively linear relations. Two other boreholes do not have enough ¹⁴C dates and the reconstructed age models are less reliable in the two locations. Contour maps of the apparent sedimentation rates every thousand years are derived from the interpolated apparent sedimentation rates through the quadratic age models. Based on our 3D reconstruction of age models, the pattern of sedimentary layers in the Ilan Plain can be explained by the seaward-dipping basin shape and the propagation of sediment fronts during the Holocene time. The analyzed sedimentation pattern does not prefer noticeably localized faults or large estimates of tectonic subsidence rates in the backarc-opening environment.