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

Sakugi大锅呈方形轮廓，被几乎垂直的断层所分隔。其中一些是剪切带较宽的正断层，另一些是剪切带较窄的逆断层。逆断层向外倾斜。石英斑岩脉和闪长岩在某些情况下侵入这些剪切带。坩埚充填物由安山岩凝灰岩、安山岩熔岩和英安岩凝灰岩组成。英灰质凝灰岩段夹在湖源凝灰岩砂岩和粉砂岩的细层状交替层间。也就是说，火山喷发期间存在一个火山口湖。采用三维有限元法对应力分布进行了分析。假设地壳是均匀压缩塑性体，直径为4公里的岩浆库将地壳从10公里深处抬升。研究结果如下：1。最小主拉应力3垂直于穹顶顶部附近。因此，该带以正断层为特征。它们主要由径向断裂或径向小裂谷带和同心正断层组成。初步形成径向断裂，随后形成同心正断层。这些裂缝形成了多角形大锅。2. 另一组同心正断层从岩浆库顶部向上发育，并与顶部附近的裂缝相连。沿樱木大锅边缘的正断层似乎是由初始穹隆破裂形成的。逆断层可能是岩浆房崩塌造成的。

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.