Exposure ages deduced from cosmogenic 10Be and 26Al produced in situ: application to granite domes and tors in Korea

Abstract

The cosmogenic radionuclides 10Be and 26Al produced in situ were analyzed to determine the exposure age and to estimate the landform evolution of a granite dome and tor in Korea. Since the tor of Mt Sapae has no joints, and the 26Al/10Be ratio has a singular point near the steady state island, the sample is supposed to have undergone steady-state erosion over a long period. In the dome on Mt Bukhan, many sheeting joints having thickness 20–70 cm develop on the slope surface, to make step-like sheet structures by exfoliation. Since the 26Al/10Be ratio of samples from Mt Bukhan is plotted into the lower area apart from the steady erosion curve, we work with a two-stage exposure model. Results show that (1) the minimum surface exposure ages of the two landforms based on the 10Be model are about 8 and 71 kyr respectively, (2) the sample from Mt Sapae was taken to have undergone steady-state erosion at rate of 7.0×10−4 cm/yr, and (3) a two-stage exposure model on Mt Bukhan suggests that the samples were irradiated by cosmic radiation in the past several thousand years, after having being shielded at the depth of 300–400 cm.