Back-arc opening and the mode of subduction

Trench-arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back-arc basins are associated with them. This suggests that subduction of an oceanic plate is not a sufficient condition for back-arc opening, though it may be necessary one. Mechanisms that cause the distinction between the two types have been investigated. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench-arc systems. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic. This difference seems to indicate that the stress state in the back-arc area differs between the two types: compression in the Chilean type and tension in the Marianas type. This difference in the stress state is also manifested in other tectonic features, such as topography, gravity, volcanic activity, and crustal movement. Two possible mechanisms for the difference between the two types are suggested: (1) The nature of the contact zone between upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction. The decoupling results in an oceanward retreat of the trench and back-arc opening. (2) The downgoing slab is anchored to the mantle, so that the position of a trench is also fixed with respect to the mantle. Since the motion in the mantle is slow compared to that of surface plates, it is the motion of the landward plate which controls the opening and nonopening of back-arcs

Fundamental Problems in the Future Studies of the Philippine Sea (Part VIII. General Discussion)

Edited by Tsunemasa Shik

21. An Interpretation of the Transient Geomagnetic Variations Accompanying the Volcanic Activities at Volcano Mihara, Oshima Island, Japan

The reversible change in the geomagnetic declination accompanying the major activities of Volcano Mihara, Oshima Island, has been interpreted as being due to heating and cooling, or demagnetization and remagnetization of a subterranean mass at a depth of a few km. In the present paper, an alternative way of interpreting the observation is proposed. In the new model, the heated region is assumed to be a cylinder (r&pcong;430m) extending from the depths to the surface crater. It is shown that the gradual heating of the cylinder from beneath would produce the apparently reversible change in the declination, without requiring any rapid cooling which has been the difficulty in the classical model. Even in the new model, the transfer of heat both in the heating and cooling of the cylinder would need some process such as convective transfer, which is much more effective than the ordinary conductive one.|伊豆大島三原山の火山活動に伴なつて,同島野増地磁気観測所では偏角に可逆的変化があることは,力武,横山らによつて確かめられている.この現象は地下数粁の深さに仮定された球状部分が,火山活動の消長に伴なつて加熱,冷却され,その磁性を失なつたり,再獲得したりするとして解釈されている.本論文では多少異なつた解釈法が提出される.新らしいモデルでは加熱される部分は半径約430mの円筒状の部分であつて,地下深所より火口に達するとされる.このような円筒が下部から徐々に加熱されるとすると,野増観測所における偏角は見掛け上,可逆的な変化を起すことが示される.この場合には,従来のモデルで必要とされたような急速な地下球体の冷却は要求されない.ただし,新らしいモデルにおいても,数年おきに火山活動が起り,これに伴って磁場変動もあるという観測事実を説明するためには,対流現象のような効果的な熱伝達機構を山体内に仮定する必要がある

22. Some Experiments on Thermal Shock Fracture of Rocks

Thermal shock resistance and thermal strain are examined experimentally on basaltic rocks: thermal strain was created in the sample, cylindrical in shape having a coaxially drilled inner hole, by suddenly pouring hot molten metal into its inner hole. It was observed that the thermal strain (tensile) measured on the outer surface of the sample increases with the increase in the temperature of the poured metal. When the thermal strain reached the critical strain, fracture was observed. The temperature of the poured metal when it causes a fracture was in the present case about 800℃ onwards.|玄武岩の熱衝撃抵抗および熱歪を実験的に調べた.熱歪は,円筒状内孔を有する玄武岩柱に,種々の温度の熔融金属を注入して生ぜしめられた.標本表面に発生する引つ張り歪みは,ストレーンゲージによつて測定されたが,注入金属の温度とともに増大することが観られた.また破壊を起す場合の金属温度はほぼ800℃以上であつた.800℃以上の温度差を有する高温岩漿が,地層岩石内に突然注入した場合には,熱衝撃破壊の発生が予想される

Magnetization of Four Pacific Seamounts near the Japanese Islands

The direction and magnitude of the magnetization of four seamounts in the Pacific near the Japanese Islands are computed, using the total magnetic force and topographic survey data reported previously (UYEDA et al., Bull. Earthq. Res. Inst., 42, 555-570, 1964). Computation is made following Vacquier's method in which uniform magnetization is assumed. The results indicate that these seamounts are magnetized normally in declination but much shallower or reversed in inclination compared with the present geomagnetic field. Depending on the relative intensity of induced magnetization and remanent magnetization, the estimated palaeomagnetic pole positions fall in the central to north Atlantic Ocean. Some Cretaceous index fossils were reported from one of the seamounts. Some speculations are made regarding the possible significance of the obtained pole positions on the movements of ocean floors.本州東北部および北海道沖の太平洋に存在する4個の海山について,その測深結果および,磁気測量(プロトン磁力計による全磁力)結果にもとづき,磁化方向,強度を推定した.推定にはVacquierの方法を用い,計算はスタンフォード大学のIBM 7090によつて行なわれた.海山はいづれも現在の地球磁場に比べて順方向に帯磁しているが,伏角はいづれも小さく,上向きの場合もあることが結論された.磁化は残留磁化が主成分と考えられるので,この結果にもとずき,古地磁気学的磁極を計算すると,大西洋北部に比較的よく集中する.これらの海山の生成年代は中生代と考えられるので,ここに得られた結果は,太平洋底の移動によるものかもしれない.但し,その可能性を確めるには,更に多くの海山について同様の研究を行うことが必要である