Paleomagnetic Study of the Upper Cretaceous Izumi Stike-Slip Basin Along the Median Tectonic Line in Southwest Japan

  1. John W. Hillhouse
  1. Kazuto Kodama

Published Online: 18 MAR 2013

DOI: 10.1029/GM050p0239

Deep Structure and Past Kinematics of Accreted Terranes

Deep Structure and Past Kinematics of Accreted Terranes

How to Cite

Kodama, K. (1989) Paleomagnetic Study of the Upper Cretaceous Izumi Stike-Slip Basin Along the Median Tectonic Line in Southwest Japan, in Deep Structure and Past Kinematics of Accreted Terranes (ed J. W. Hillhouse), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM050p0239

Author Information

  1. Department of Geology, Faculty of Science, Kochi University, Kochi 780, Japan

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1989

ISBN Information

Print ISBN: 9780875904542

Online ISBN: 9781118666609

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Keywords:

  • Geology, Structural—Congresses;
  • Geodynamics—Congresses;
  • Earth—Crust—Congresses

Summary

Along the Median Tectonic Line in southwest Japan, there is a narrow and elongate sedimentary basin of the Upper Cretaceous age, called the Izumi basin, which extends almost 300 km from northwestern Shikoku to western Kii Peninsula. The strata are folded into a syncline that dips approximately 30° to the east. Eastward migration of the Izumi depocenter resulted in a cumulative thickness over 50 km of Late Cretaceous sediments. The structural and depositional patterns are quite similar to other examples of strike-slip basins and suggest that the Izumi basin was formed by a left-lateral strike-slip motion along the Median Tectonic Line. Paleomagnetic studies were carried out in the succession of strata from the lowest horizons in northwestern Shikoku to the upper horizons in Awaji Island 200 km to the east. A total of 730 samples were taken from layers of fine-grained sandstone and mudstone at 94 stratigraphic levels, covering almost half of the succession. Alternating-field and thermal demagnetizations show that most of the specimens have a single and stable magnetization component carried by magnetite. One reversed polarity zone was identified at the lowest portion of the section, and two short reversed zones were noted in the upper portion. Mean directions of the normal and reversed polarities are I = 50°, D = 76° and I = −47° , D = 268°. The mean pole position is 25°N, 203°E. The declinations concentrated to the northeast-southwest is owing to Neogene clockwise rotation of southwest Japan through 50°–60°. The inclinations are compatible with the present latitude of the basin. The pattern of magnetic polarity zones matches a geomagnetic polarity change during the early Campanian to earliest Maastrichtian in accord with previous biostratigraphic studies. Sedimentation rates based on the polarity zonation are 0.7 to 1 cm per year. The extremely high sedimentation rate, as well as other structural characteristics, suggests that the formation of the basin was controlled by extension and a left-lateral motion along the Median Tectonic Line. Despite the unique and complicated structure of the basin, the thick sedimentary pile provides a continuous record of geomagnetic polarity changes. The Izumi basin is an important element in the collision-accretion tectonics of the Japanese arc since the Late Cretaceous.