重力異常からみた九州中部地域東部の第四紀テクトニクス Quaternary Tectonic Interpretation around the Eastern Part of Central Kyusyu, Japan, Based on Gravity Analysis
Gravity anomalies provide much information on subsurface structures, i.e., faults, depressions and dome structures. These structures are considered as an accumulation of the past crustal deformations. The subsurface structures, which are estimated by using gravity anomaly and other geophysical and geological information, can thus become a guidepost to discuss the tectonic model around low and/or high gravity anomalies.<BR>The eastern part of central Kyushu, Japan, including Beppu Bay is characterized by gravity low and has complicated structures, as an intersection of the Median Tectonic Line (MTL), the Oita-Kumamoto Tectonic Line (OKTL) and the Kurume-Hiji Line (KHL). Based on the subsurface structures estimated by gravity analysis, we tried to discuss the mechanical formation and the Quaternary tectonic history of this region. Results obtained show the possibility that this region experienced two crustal deformations; (1) the half-graben was formed along the OKTL before 1 Ma ago, and (2) tectonic basins in and around Beppu Bay were formed by right-lateral motions of the MTL and KHL, as a kind of pull-apart basins, in the last 1 Ma.