On the Regional Flexures Inferred from Gravity Datain Tsushima Island
1976 Volume 26 Issue 137 Pages 191-206
Details
1976 Volume 26 Issue 137 Pages 191-206
Tsushima Island is covered largely by Tertiary Taishu Group consisting mainly of thick alternation of major amount of shale and thin sandstone beds, which shows abundant NE-SW trending folds throughout the island. The group is intruded by many igneous dikes and stocks having the same trend. To the west offshore of the island large N-S trending faults are recognized (after Tomita et al., 1974, 1975).
Based upon field data, the authors examined a distributional characteristics of wavelength, amplitude and plunge of the folds, and found the following facts; (1) the island can be divided into two tectonic provinces of a short-period folding province in the northwest and a long-period folding province in the southeast which are bounded by a NE-SW trending tectonic line. (2) the short-period folding province can be further divided into two tectonic units of a medium-period folding sub-province in the west and a long-period folding sub-province in the east which are bounded by a N-S trending tectonic line. (3) the first order trend surface of the folds in these three tectonic units have different dip and strike each other. Because any remarkable fault is never found along the boundaries among each unit, the tectonic lines must represent the axis of two different flexures of the Taishu Group.
Based upon these results, the authors reprocessed and reinterpreted the gravity data, and came to the conclusion that the NE-SW trending axis of the flexure reflects a step-like structure of high-density basement, which is inferred to consist of gneissose rocks or weak-metamorphosed Palaeozoic System, and the N-S trending axis of the other flexure corresponds to the axial zone of the basement flexure, a inclination of which is nearly horizontal in eastern side and about 20 degrees toward west in the western tectonic units.
The complicated fold system of the Taishu Group is an reflection of structural movement resulted from the older NE-SW folds and younger N-S flexures of the Taishu Group and the basement.
