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，中间几乎是垂直的断层。它们有的是剪切带较宽的正断层，有的是剪切带较窄的逆断层。反向断层向外倾斜。在某些情况下，石英斑岩脉和闪长岩脉侵入这些剪切带。火锅填充物由安山凝灰岩、安山岩熔岩和英安凝灰岩组成。英安质凝灰岩段夹有凝灰质砂岩和湖源粉砂岩的细小层状交替层。即火山喷发时存在一个破火山口湖，用三维有限元方法分析了其应力分布。假定地壳为均质压缩塑性体，直径4公里的岩浆室将地壳从10公里处抬升。结果表明：1.最小主拉应力3垂直于穹顶附近。因此，该带以正断层为特征。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。初步形成放射状断裂，随后形成同心正断层。这些裂缝产生了多边形大锅。2.另一组同心正断层从岩浆房顶板向上发育，与峰顶附近的断裂相连，沿Sakugi锅边缘的正断层似乎是由初始穹隆的破裂形成的。反向断层可能是岩浆室坍塌造成的。

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.