Missing evidence of landscape transience: Examination of the knickzones in a steep marginal escarpment in the Korean Peninsula

High-elevation steep escarpments in the passive continental margins are known to have resulted from long-lasting erosion following rifting-related marginal uplift. However, recent studies have revealed that rifting-related processes alone do not explain the high-elevation marginal escarpment. Particularly, post-rift rejuvenation could contribute to the growth of the relief of the long-lived escarpment. A steep escarpment rising above 1,700 m occurred along the eastern margin of the Korean Peninsula. It is known to be a compartment of the passive continental margin, which has been tectonically stable since the extension of a back-arc sea (i.e., the East Sea). Nevertheless, coastal terraces indicative of enhanced tectonic uplift are observed along the eastern coastline. Thus, the steep escarpment has been tectonically reactivated since the Late Quaternary. Knickzone is a geomorphic marker that can decipher the role of tectonics in landscape evolution. This study examined and classified the knickzones of the catchments draining the marginal escarpment, and simulated the response of river profiles to an enhanced tectonic forcing to geomorphologically assess the post-rift tectonic perturbation and understand its effects on the evolution of the marginal escarpment. Firstly, the knickzone analysis identified 52 knickzones, of which 15 were slope-break knickzones, usually interpreted as being transient. However, all slope-break knickzones were not transient but were stationary since they were associated with spatial heterogeneity in erosional resistance. Secondly, river profile evolution modeling illustrated that an abrupt slope break of a transient knickzone developed by an enhanced uplift rate is diffused rapidly during its migration under the condition of a steep marginal escarpment with high channel concavity. Additionally, the tilted uplift pattern induced a newly adjusted river profile of a lower channel concavity, consequently decreasing the steepness of the marginal escarpment. In conclusion, the marginal escarpment in the study area would have been being adjusted since the post-rift tectonic enhancement without the development of typical slope-break transient knickzones, reducing its steepness. These finding suggest that the slope-break knickzone should be used carefully in assessing landscape transience of the marginal escarpment.