Study on Structure of a Tertiary Cauldron in San-in District

Sakugi大锅显示出一个正方形的轮廓，被几乎垂直的断层所分隔。它们有的是具有宽剪切带的正断层，有的是具有窄剪切带的逆断层。逆断层向外倾斜。在某些情况下，石英斑岩脉和闪长岩体侵入这些剪切带。釜状充填物由安山岩凝灰岩、安山岩熔岩和英安质凝灰岩组成。英安质凝灰岩段夹有湖成因凝灰质砂岩和粉砂岩的薄层状互层。用三维有限元法分析了火山口的应力分布。假定地壳为挤压塑性体，一个直径为4km的岩浆房将地壳从10km深处抬升。最小主拉应力3在拱顶顶部附近是垂直的。因此，该地带的特征是正断层。它们主要由放射状断裂或放射状小裂谷带和同心正断层组成。放射状断裂首先形成，随后是同心正断层。这些裂缝形成了多边形坩埚。2.另一组同心正断层自岩浆房顶部向上发育，并与顶部附近的断裂相连接，沿着Sakugi火山口边缘的正断层似为初始穹隆破裂形成。逆断层可能是由岩浆房的坍塌产生的。

The Sakugi cauldron shows a square outline delimited with nearly vertical faults. Some of them are normal faults with wide shear zones and others are reverse faults with narrow shear zones. The reverse faults dip outward. Quartz porphyry dikes and diorite plutons intrude into these shear zones in some cases. Cauldron fills consist of andesitic tuff, andesite lava and dacitic tuff. The dacitic tuff member intercalates fine laminated alternating beds of tuffaceous sandstone and siltstone of lake origin. Namely, a caldera lake existed during eruptions.Stress distribution was analyzed by three-dimensional finite element method. It was assumed that the earth's crust is homogeneous-compressive plastic body, and that a magma chamber with a diameter of 4 km uplifts the crust from a depth of 10 km.The results are as follows: 1. Minimum principal tensile stress 3 is vertical near the crest of the dome. Accordingly, this zone is characterized by normal faults. They consist mainly of radial fractures or radial minor rift zones and concentric normal faults. The radial fractures are initially formed, which are followed by the concentric normal faults. These fractures produce the polygonal cauldron. 2. Another group of concentric normal faults develop from the roof of the magma chamber upward, and link with the fractures near the crest.The normal faults along the margin of the Sakugi cauldron seem to have been formed by fracturing of initial doming. The reverse faults may have been produced by collapse of the magma chamber.