Volcanic and tectonic evolution of the Ulleung Basin, the East Sea

Abstract

In this thesis, I present results of my studies concerning Cenozoic tectonic and volcanic history of the Ulleung Basin region. The thesis comprises three chapters, which cover issues of multi-channel seismic reflection studies on (1) tecto-magmatic origin of the Ulleung Basin in the East Sea and (2) paleogeographic reconstruction based on wave-planation surfaces and a sedimentological and tephro-stratigraphic study on (3) the latest eruptive history of Ulleung Island, a volcanic island in the northern Ulleung Basin. Chapter 1 includes contents for the style of crustal extension in the Ulleung Basin. In this chapter, I provide evidences for variable asymmetric features of the basin, such as north-biased magmatism, unidirectional fault pattern, and apparent asymmetries in basement topography and Moho geometry. The evidences all together indicate that the Cenozoic back-arc opening in the Ulleung Basin was not achieved in a typical pure-shear-style crustal extension, but rather made in a mode of simple shear where most probably a north-dipping crustal detachment was a main control for variable asymmetries in basin configuration. Chapter 2 focuses on the subsidence history and paleogeographic evolution of the South Korea Plateau and the northern Ulleung Basin regions. In this section, I trace the paleo-sea level based on a concept of wave-planation which resulted in flattening and beveling of subaerially exposed basement and volcanic edifices before their deep submergence below sea level by tectonic subsidence. I provide formative models of all available types of wave planation surfaces, and estimate spatio-temporal variation of subsidence rate on varying types of substratum. Based on the information of subsidence rate, I lastly reconstruct the paleogeographic evolution of this region. Chapter 3 presents fieldwork-based results on pyroclastic deposits in the Ulleung Island, entitled Nari Tephra Formation, originating from the latest explosive eruptions in the Nari Caldera since 19ka B.P. In this chapter, I define 5 eruption episodes in the tephra formation, and unravel the style of pyroclastic transport-deposition process in conjunction with the eruption style of each episode. The chapter lastly provides reconstruction of regional-scale Ulleung tephro-stratigraphy with a review for the relationship between tephra-transport mechanism and geomorphic and meteorological effects.