Cosmogenic Be-10 and OSL dating of fluvial strath terraces along the Osip-cheon River, Korea: tectonic implications

Abstract

The Osip-cheon River flows along a linear valley that runs approximately parallel to the Osip-cheon Fault. The present study was undertaken to elucidate the tectono-geomorphic evolution of the Osip-cheon Basin during the late Quaternary by employing cosmogenic Be-10 surface exposure dating and optically stimulated luminescence (OSL) dating on a flight of fluvial strath terraces. Several morphometric indices (e.g., stream length-gradient index, asymmetry factor, transverse topographic symmetry factor, stream power incision) for relative tectonic activity are determined with the help of GIS analysis, suggesting minimal tilt of the blocks on either side of the Osip-cheon Fault, with displacement being mainly strike-slip. Thus, it is suggested that the Osipcheon River is a kind of subsequent stream that follows the weakened fault-line in the center of the basin. To determine the fluvial incision rate as an equivalent to the rate of local tectonic uplift, samples for cosmogenic 10Be dating and OSL dating were collected from the surfaces of strath terraces at two different areas along the Osip-cheon River. The terraces can be divided into three groups (T1: oldest to T3: youngest) depending on the height above the present river level. According to our data, the strath terraces formed at ca. 200 ka for T1, ca. 104 ka for T2, and ca. 14 ka for T3. The fluvial incision rate was determined by dividing the height of the terraces above the river by their respective ages. The fluvial incision rate has decreased over the last 200 ka, with a maximum rate of 0.28 mm/yr occurring between T1 and T2 (i.e., ca. 200 ka to 104 ka) followed by a rate of 0.21 mm/yr for the period from T2 to T3 (i.e., ca. 104 ka to ca. 14 ka), and a minimum rate of 0.17 mm/yr since the abandonment of T3 (i.e., over the last 14 ka). Tong-ri Canyon is deeply incised up to the major knickpoint of the Mi-in Falls. Here, the rock-cut strath terraces can be divided into two groups: the farther upstream part of the Tong-ri area (Site-1) was dated to 5.5 +/- 0.5 ka, equivalent to the incision rate of 0.26 mm/yr, and the downstream part of the Tong-ri area (Site-5) was dated to 18.5 +/- 1.8 ka, equivalent to the incision rate of 0.23 mm/yr. Given the active upstream migration of the knickpoint (Mi-in Falls) found in the upper reach of the river, the incision rate provided by the ages of the strath terraces in the Tong-ri Canyon should be higher than that for any other reach of the river. However, the incision rate of the reach below the mouth of Tong-ri Canyon is similar to the value of the reach above the major knickpoint, suggesting that knickpoint migration in the Tong-ri area was not active during the last 18.5 ka. Rather, it seems to have advanced to a certain level of incision in the past. Along the Osip-cheon River, the values of stream power incision and stream length-gradient show a contrasting distribution. The highest and most anomalous values of these indices are found along the upstream part of the Osip-cheon River. These indices values are likely to be associated with stream piracy. Given the geomorphic features (e.g., elbow of capture, wind gap, waterfall) found in the Tong-ri Canyon area, stream piracy might have been caused by Quaternary reactivation of the Osip-cheon Fault or by the ongoing but slowing, long-term regional tectonic uplift, with an axis along the Taebaek Mountain Range.