活褶曲研究史と問題点 (第四紀テクトニクス特集)

概要

論文の詳細を見る
History of active fold study in Japan is critically reviewed and current problems in this branch are discussed with introduction of a case study of active fold along the River Shinano. The term active fold is here used for a fold (structure), the last deformation of which occurred in a very recent period (less than 105 years B. P.).Active fold, as reported first by Otuka (1941, 1942a) and Ikebe (1942), was discovered by deformed river terraces and vertical displacement of bench marks, both of them being concordant to underlying Neogene fold structures. Significance of this discovery lies in that the tectogenic stress which yielded the fold structure is in action at present and that we can observe the time mode of deformation of the structure.During the period 1943-1959, besides accumulation of examples found in the inner belt of Northeast Japan, a direct approach was tried by Miyamura (1943, 1949, 1956) for obtaining data of vertical displacement by bench marks newly set up for this particular purpose (Fig. 1). In the course of his study, the Futatsui Earthquake (1955, M. 6.2) happened to occur near the newly set levelling route. This event associated with possible pre-seismic deformation and coseismic progress of active fold, much stimulated general interest in the relationship between active fold and seismic activity.Since 1960, still more examples have been added, partly due to impact of UMP and earthquake prediction project. Based upon the accumulated results, active folding has been discussed with related crustal movement such as crustal unduration and active faulting. The following conclusions seem to be almost established by the recent work.1. Active fold has been formed with the maximum time rate of folding (tilting) of the order of 10-6 per year during the past 104∼5 years. This rate is abnormally high compared with deformation in other continental parts of the globe.2. Time rate of deformation is generally inversely correlated with wave length. The above maximum rate is attained by the fold with wave length of several kilometers or less. This high-rate, short-wave fold is characteristic of areas with thick semiconsolidated sediments such as the inner belt of Northeast Japan. This fact suggests that folding of this sort is genetically related to the presence of thick soft material.3. Due to the high rate, displacement observed within ten years or less, is able to be correlated to tectogenic deformation.An active fold along the lower course of the River Shinano is described referring mainly to Ota (1969). The distribution of river terraces is shown in Fig. 3 along with a syncline and an anticline of terrace deformation. The axes of this active fold almost duplicate those of the fold in the underlying Plio-Pleistocene deposits. Exact correlation of terraces is a difficult but important problem in an area of high rate of deformation like this. Fig. 4 is a cross section of folded terraces. Fig. 5 presents the distribution of bench marks near Sekihara shown in Fig. 2 and its relation to fold axis of terrace plains. Fig. 6 shows a part of the bench mark net set up for the study of active fold, vertical displacement during the past nine years, and the distribution of the Geodimeter baseline. This area apparently experienced steady progress of folding with a rate of 10-6 during the past 70 years, without appreciable earthquakes.From the foregoing review and a case study, future problems as below were summarized and discussed:1. Further accumulation of reliable examples.2. Quantitative study of original form of terrace topography.3. Age determination of topographic plain.4. Nature and distribution of active fold and its dependence on the physical property of underlying rocks which is the historic result of geology of the area.5. Mechanism of active fold, its dependence on depth and measurement of horizontal strain.
日本第四紀学会の論文

活褶曲研究史と問題点 (第四紀テクトニクス特集)

スポンサーリンク

概要

著者

関連論文

スポンサーリンク

活褶曲 研究史と問題点 (第四紀テクトニクス特集)

スポンサーリンク

概要

著者

関連論文

スポンサーリンク

活褶曲研究史と問題点 (第四紀テクトニクス特集)