ORGANIC MOLECULAR PALEOHYPSOMETRY REVEALS EVIDENCE OF HIGH ELEVATION IN TAIWAN SINCE ~1.5MA

Taiwan is one of the world's youngest orogenic belts and serves as the planet’s endmember to study feedback among tectonics, erosion, and climate during an arc-continent collision. However, fundamental components of this model orogen are still widely debated due to a lack of paleotopographic records. The stable isotopes of terrestrial plant biomarkers provide information related to past hydrological interactions with topographic barriers, hence can be used to study paleotopographic change. We utilize the hydrogen isotope composition of leaf-wax biomarkers (δ²H_nC29) in Plio-Pleistocene sediments to reconstruct the source elevation of detrital organic matter exported from the Hsuehshan range of Northwest Taiwan. Our data show that prior to ~1.5 Ma the δ²H_nC29 is relatively constant and is characterized by a mean value of ~170‰. After 1.5 Ma, the δ²H_nC29 of fluvially transported organics decreases to <-180‰. Modern detrital sediments in Taiwan rivers indicate that organic δ²HnC29 is closely related to the source elevation of organic matter and approximates catchment mean elevation. The progressive decrease observed in detrital δ²H_nC29 is consistent with an increase in the average source elevation of organic matter exported from northwestern Taiwan. Based on modern data, this signature can be interpreted to show a rapid increase in catchment mean elevation from ~0.7 km to >1.7 km after 2 Ma. Integration of organic molecular isotope data with recent modeling of thermochronological data, which shows a steady increase in the rate of exhumation since about 6.0 Ma, suggests significant topographic growth of the Hsuehshan range occurred only after a threshold exhumation rate (~3km/Myr) is reached after 2 Ma. This timing of growth is consistent with records from the Southern Central Range and points to rapid topographic growth along the length of the Taiwan orogen after 2-1.5 Ma.