On the use of anisotropy of magnetic remanence to estimate the rotational
motion of faulted blocks within the Mukawa Serpentinite Body in Hokkaido, Japan
Gautam Pitambar 1) and Katoh Takayuki 2)
1) Department of Earth and Planetary Sciences, Graduate School of Science,
Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan. E-mail: gautam@cosmos.sci.hokudai.ac.jp
2) Earth Science Co. Ltd., 2-1, N39 W3, Kita-ku, Sapporo 001-0039, Japan.
Abstract
A large landslide commonly known as "O-kuzure" occurs at the eastern slope
of Mt. Bouzu (791m) between 470-250m altitudinal range. Geologically, the
area lies within the Mukawa Serpentinite Body which constitutes the southern
part of the Kamuikotan Belt in Hokkaido. The major rock constituting the
area is serpentinite derived originally from harzburgite which has been
intruded by dunite dykes. The landslide slope dips 40-42 degree towards the
east. The landslide comprises of the main (southern) block and northern
block being separated by a major fault trending E-W. Geological observations
suggest that there is wide variation in the structural elements of the rocks
and joint system across the fault. In particular, judging from the major
joint system, the main block has undergone 30-40 degree anticlockwise
rotation and 10-20 degree westward tilt relative to the northern block.
In order to estimate the rotation quantitatively, block samples were
collected at 4 sites across the fault and subjected to paleomagnetic
measurement and anisotropy of the magnetic remanence as well. Analysis of
the magnetic remanence revealed very high remanence intensities dominated by
a secondary viscous direction of secondary origin carried out by magnetite.
Anisotropy of magnetic remanence revealed clearly defined magnetic
anisotropy axes. Well-defined sub-vertical magnetic foliation in all sites
suggests that there is systematic rotation across the fault. In the
immediate vicinity of the fault, the rotation that occurred after the
formation of the major joint system amounts to 50 degree. This amount of
rotation in the horizontal plane decreases with increase in distance from
the fault.
raeg98@tansa.kumst.kyoto-u.ac.jp
Last modified: Mon Oct 19 15:40:55 1998