HR: 0800h
AN: G51C-0094    [Abstracts]
TI: Clarification of the NS-trending faults by analysis of borehole data and gravity inversion using the Monte Carlo method in northwestern Shikoku, Japan.
AU: * Ichihara, H
EM: ichihara@sci.ehime-u.ac.jp
AF: Graduate scool of science and engineering, Ehime University, 2-5,Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Sakakibara, M
EM: sakakiba@sci.ehime-u.ac.jp
AF: Fculity of science, Ehime University, 2-5,Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Ohno, I
EM: ohno@sci.ehime-u.ac.jp
AF: Fculity of science, Ehime University, 2-5,Bunkyo-cho, Matsuyama, 790-8577 Japan
AB: In northwestern Shikoku, Japan, several EW-trending faults related to the median tectonic line (MTL) have been investigated. However, the existence of the NS-trending faults has not yet been demonstrated as being estimated from its lineaments. On the basis of gravity data and borehole data, we investigate the basement structure and Quaternary stratigraphy under the Horie lowland, the narrow and NNW-SSE-trending alluvial lowland at the north of the Matsuyama Plain, northwestern Shikoku. We developed the gravity inversion method using the Monte Carlo method in order to estimate these underground density structures. The borehole data indicates a discontinuous change of depth on the basement surface along the western margin of the lowland. The depth of the basement surface is approximately 200 m on the east side of the step and 0-20 m on the west side. The length of the step is horizontally distributed for more than 1 km. The deposits under the Horie lowland can be categorized into five formations based on the sedimentary faces. They are composed mainly of gravel, sand, silt and clay that are intercalated with thin tuff beds. Lower formations located eastward from the step are tilted to the west at an angle of 98/1000 at a maximum, which decrease in ascending order. The Bouguer anomaly distribution was demonstrated by the gravity data of approximately 2500 points. Based on this distribution, a regional anomaly is assumed by the upward continuation method (continuation height = 2 km). This distribution shows a negative anomaly belt (minimum -3.3 mGal) in the same region of the Horie lowland. A high inclination of the anomaly value is distributed at the same location of the step of depth. A density structure was estimated by gravity inversion using the Monte Carlo method that employed known surface geology data and the density value of the basement rock and sediment as a constrained condition. The result shows the high inclined belt of the basement surface, which dip has an angle of 66 degree, distributed through the Horie Lowland. Further, its EW cross-section exhibits an asymmetric shape of the basement surface. These results are in good agreement with the basin structure on the basis of the borehole data. The existence of step of the basement surface and the anomaly tilt of the basal formation suggest fault movements by a NNW-SSE-trending fault, i.e., the Horie fault, along the western margin of the Horie lowland. It is inferred that the fault has been active during the Quaternary. The asymmetric shape of the basin and the tilted formations under the Horie lowland indicate that it is a half-graben due to an E-W extension stress. This graben can be traced for the distribution zone of the MTL active fault system on the basis of Bouguer anomaly.
DE: 5475 Tectonics (8149)
DE: 3260 Inverse theory
DE: 1219 Local gravity anomalies and crustal structure
DE: 1234 Regional and global gravity anomalies and Earth structure
DE: 0903 Computational methods, potential fields
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting