Analysis of gravity measurements to understand the structure and evolution of the Ulleung Basin (East Sea/Sea of Japan) and numerical modeling on the genesis of adakites in subduction zones

Abstract

This thesis presents the results of my studies concerning the tectonic evolution of the Ulleung Basin, East Sea using geological and geophysical data and the genesis of adakites and boninites by partial melting of oceanic crust in subduction zones. My thesis comprises two chapters and first is about the effect of time-varying subduction parameters on the Cenezoic adakites and boninites and next is the analysis of gravity measurements in the Ulleung Basin and its implications for the Moho depth variations. Chapter 1 addresses the effect of time-evolving slab age and convergence rate of the incoming oceanic plate on the genesis of adakites and boninites in the subduction zones. I made a series of two-dimensional numerical models using the time-dependent subduction parameters and compared with the geochemical evidence in each subduction zones. The results show that my model calculations successfully explained the adakites and boninites in some subduction zones. However, in the other regions such as Mariana and northeastern Japan subduction zones, the other tectonic settings of back-arc spreading, inflow of hot mantle and ridge subduction are needed to explain the partial melting of oceanic crust in subduction zones. Chapter 2 includes the analysis of gravity measurements in the Ulleung Basin and its implications for the crustal structure. I examined the gravity anomaly of the Ulleung Basin using more extensive data sets focusing on the crustal thickness and also calculated the effect of temperature on the gravity anomaly using a simple thermal model. My analysis shows that the Moho discontinuity is varied from 16 to 22 km, but within the central part of the basin, the variation is only about 10-20 %. Such finding appears to be consistent with previous studies using ocean bottom seismometers. Based on the gravity analysis, I provide the information of crustal structure and infer the important tectonic processes shaped the Ulleung Basin during post-rift period.