THE EXHUMATION HISTORY OF THE MIDDLE HSUEHSHAN RANGE, TAIWAN, AS REVEALED BY ZIRCON THERMOCHRONOLOGICAL MODELING

The Taiwan orogenic belt has high precipitation and high exhumation rates (3–6 km/Myr), making it a natural laboratory for understanding how erosion, tectonics, and climate interact. Understanding how rock exhumation rates have evolved since the late Miocene is key to unraveling climate–tectonic interactions. In this study, we present 8 new zircon fission track (ZFT) and (U-Th)/He (ZHe) cooling ages (including 2 unreset ages) from 5 samples from 1700 to over 3300 m elevation around the highly incised Dajia River in the core of the Hsuehshan Range (termed the mid-HR in this study) in central Taiwan. Two of the ZFT and ZHe ages (5.3 Ma and 2.6 Ma) are the oldest ages recorded in this region. These ages suggest that the cooling rate of the mid-HR is lower than that of the Backbone Range and Yushan. With these and previously published cooling ages, we modeled the exhumation and cooling history using a 2-D thermal diffusion equation with time-varying erosion. The ZFT and ZHe ages are best explained by a steadily increasing exhumation rate up to 3.4 + 0.5/−0.3 km/Myr from 7 to 1 Ma. The exhumation rate in the last million years is not as well constrained but appears to have increased at a slower rate. Our findings differ from those of previous studies that propose that the exhumation rate increased rapidly in the last 1–2 Myr. We propose that the steady increase in the exhumation rate in the mid-HR indicates increasing topographic relief, which is likely caused by progressive crustal thickening.