Geomorphic disequilibrium in the Eastern Korean Peninsula: Possible evidence for reactivation of a rift-flank margin

• Catchment-wide denudation rates for Eastern Korea were measured.
• CWDRs are 2–3 times lower than terrace uplift rates.
• Denudation rates are consistent with long-term geologic exhumation rate.
• Major knickzones are found along the stem and tributary channels.
• Transient state occurred due to Quaternary reactivation of rift-flank margin.

Tectonically inactive since the middle Miocene, the Korean Peninsula is generally considered a typical passive continental margin with asymmetric relief with only low levels of seismic activity. Recent reports of uplift rates as high as 150–350 mm ka− 1 from geomorphic markers using numerical dating methods (e.g. radiocarbon, optically stimulated luminescence (OSL), and cosmogenic nuclides), however, cast doubt on the tectonic quiescence of Korea. Thus, we evaluate the geomorphic status of eastern Korea using denudation rates at geomorphic timescales of 103–106 years, which are equivalent to the rates previously reported at local points. To infer regional denudation rates, we measured catchment-wide denudation rates (CWDR) in 13 watersheds using in situ cosmogenic 10Be and 14C analyses of riverine sediment samples in eastern Korea. These CWDR data suggest that the mean denudation rate during the past 5–14 ka centers around ~ 74.7 ± 25.4 mm·ka− 1, which is consistent with the long-term, geologic exhumation rate we derive of ~ 74 ± 10.1 mm·ka− 1 since the middle Miocene. However, our CWDR data are ~ 2–3 times lower than rates of coastal uplift and river incision of 150–350 mm·ka− 1 derived from data on marine and fluvial terraces, respectively. In addition to isotopic analyses, we quantified topographic indices such as elevation, relief, and slope, and then compared these indices with CWDRs to examine morphological changes that might be related to tectonic disturbances since the beginning of late Pleistocene. The relationship between CWDRs and topographic indices indicates that most of the study area rests in a near steady-state condition. However, in contrast to the smooth concave longitudinal profile expected for a steady-state landscape, topographic analyses reveal the presence of major knickzones (confirmed by field reconnaissance). These knickzones may have been produced by changes in base level caused by tectonic events, lithologic variations or sea-level changes. Our 1-D modeling of knickpoint propagation suggests that tectonic disturbance is the factor most likely to produce knickzones in the study area, raising the possibility that erosional signals caused by late-Pleistocene tectonic disturbance affected channel gradients, but that the knickpoints have not yet propagated through the entire catchment area. Our results suggest that the eastern part of the Korean Peninsula is approaching, but has not completely reached, a geomorphic steady-state, and that transient conditions have been induced by reactivation of the rift-flank margin no later than the late Pleistocene, causing tilting of the central Korean Peninsula to the west at a rate of 7.0 × 10− 10 m/m per year over the past ~ 125 ka.