EVIDENCE OF ACTIVE TECTONIC UPLIFT, SOUTHEASTERN KOREA

Recent field work, radiocarbon dating, and analysis of digital elevation data indicate that uplift related to tectonic movement has occurred along the Ulsan fault (UFT), SE Korea. The most recent sense of motion along the Ipsil fault (IFT; N05°E/68°-74°SE), a 4.5 km long segment of the UFT, is left-lateral reverse. The surveyed profile of a westward-flowing canyon that crosses the IFT reveals a prominent change in stream gradient, from 0.027 downstream to 0.077 upstream of the IFT. The steeper eastern gradient is consistent with high-angle reverse motion (top-up-to-the-west), and corresponds with several knick-points that merge upstream. A prominent terrace in the knick point zone is offset 6.5-m across the IFT. The bedrock strath is overlain by 10-m of fluvial sediments, from which other workers obtained ages of 39.1±2 ka (C14) and ~52±4 to 70±39 ka (OSL). If we assume a maximum age range of 37-109 ka for this terrace, which is 18 m above the modern channel bed upstream of the IFT, the bedrock incision rate is 0.3m/ky on the footwall and 0.5m/ky on the hanging wall of the fault. Additional stream-profile analyses of westward-flowing streams that might cross the IFT reveal prominent knick points as well, identifying the possible extension of the IFT. Approximately 35 km to the north of the ITF along the eastern coast, an uplifted sea cave near Masanri reveals evidence of possible Holocene coseismic uplift. This uplifted sea cave is located ~1-km from the NE-trending Ocheon fault. The bedrock floor of the cave, 1.4-m above sea level, is overlain by ~3.6-m of horizontal beach sediments. The sediments consist of pebbles and sands with occasional pockets of shells in a weakly fining-upward sequence. The sediments were deposited in a prograding marine beach environment that could have resulted either from rising sea level (eustatic sea level change) or an uplifting coast line (tectonic). C14 AMS dates from shell fragments and charcoal lenses yielded calibrated C14 dates of 1650-1820 AD near the base of the strata and 1420-1620 AD near the top of the strata. The sequence indicates either a sea-level high stand or tectonic uplift of as much as 5-m that occurred several hundred years ago. The latter is more likely, and might coincide with coseismic uplift during an historical earthquake that occurred in Gyeongju (1643 AD) or Gangwondo (1681 AD).