LATE QUATERNARY DENUDATION RATE OF EAST PART OF KOREA : IN STEADY-STATE OR TRANSIENCE?

Generally, a landscape swings between steady and transient states as a response to change in magnitude of tectonic disturbance and the latter is likely to have some characteristic landscapes such as knickzones to accommodate increased stream power caused by tectonic uplift. Korean peninsula has been considered as tectonically inactive region since the pulse of uplift at mid-Miocene. Many scientists investigated recent tectonic movements using numerical dating methods such as radiocarbon, Optimally Stimulus Luminescence (OSL), cosmogenic nuclides to quantify the uplift rate and understand the landscape evolution state of the Korean peninsula over the late Pleistocene. However, to determine the geomorphic state of Korean peninsula, the uplift rate should be compared with denudation rate having equal geomorphic time scale (10³~10⁶ years). Here, we provide the catchment wide denudation rate (CWDR) using in-situ cosmogenic nuclide beryllium-10 (¹⁰Be) from the riverine sediment samples taken from 13 catchments in east part of Korea. The uplift rates derived from previous studies on marine and fluvial terraces over ~200 ka are 150-350 mm·ka^-1 within the study area. Our CWDR data indicates that the mean denudation rate is ~78 mm·ka^-1 which is consistent with the long-term, geologic exhumation rate (~80 mm·ka^-1) since mid-Miocene, but is ~ 2-3 times lower than the uplift rates derived from marine and fluvial terraces. In addition, we analyze the local relief and slope, and construct longitudinal profiles, and then compare them with CWDRs. The relationship between CWDRs and morphologic indices indicates most of the study area is near to the geomorphic steady state. However some major knick zones are found along longitudinal profiles. The knickzones may have been produced from base level change caused by tectonic events, lithologic difference, and sea level change. Our longitudinal analysis indicates that the tectonic events produced some knickzones, implying that tectonic signal probably caused by the late Pleistocene tectonic uplifts occurs in the channel, but has not propagated all the way up to whole catchment. Our results suggest that eastern part of the Korean peninsula has not approached the geomorphic steady state fully, but been the transient state since late Pleistocene.