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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Mechanism of caldera collapse and resurgence: Observations from the northern part of the Kumano Acidic Rocks, Kii peninsula, southwest Japan
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Mechanism of caldera collapse and resurgence: Observations from the northern part of the Kumano Acidic Rocks, Kii peninsula, southwest Japan

机译：破火山口塌陷和死灰复燃的机理：日本西南纪伊半岛熊野酸性岩北部的观测

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The northern part of the Kumano Acidic Rocks on the Kii peninsula, southwest Japan, formed during Miocene felsic igneous activity that involved caldera-forming eruptions and subsequent intrusion of a massive sheetlike body of granite porphyry into intracaldera deposits. Detailed mapping has revealed that the northern part of the Kumano Acidic Rocks has two separate collapse structures that show the structural margin of a caldera. One is an arcuate fault zone characterized by several normal faults with gentle dips. Pyroclastic deposits characterized by the dominance of crystal-rich welded ash vary in thickness across the fault zone. The ash beds contain numerous boulders on the inner side of the collapse structure, representing caldera-collapse breccias sourced from the fault scarp during the caldera-forming eruption. The other is a 1-3 km wide ring dike of granite porphyry that encloses the prevolcanic sedimentary rocks and the pyroclastic deposits, and is interpreted as a magma conduit along a subsurface circular fracture. The caldera collapse area enclosed by these two structures is elliptical and about 20 × 30 km in size. Our observations suggest a coherent, piston-type collapse for the northern part of the caldera, while asymmetric subsidence along the fault zone resulted in trapdoor or piecemeal subsidence in the southern part. The ring dike is connected to the sheetlike body of granite porphyry. We suggest that this sheetlike intrusion is a laccolith and may have caused resurgent doming of the caldera floor.
机译：日本西南部纪伊半岛上的熊野酸性岩石的北部，是在中新世长新世火成活动期间形成的，其活动涉及火山口形成喷发，随后大量的片状斑岩斑岩体侵入火山口内沉积物中。详细的地图显示，熊野酸性岩石的北部有两个单独的坍塌结构，显示了破火山口的结构边缘。一个是弧形断层带，其特征是几个正常的断层带缓和的倾斜。以富晶体焊接灰分为主的火山碎屑沉积物在整个断裂带的厚度各不相同。灰烬床在坍塌结构的内侧包含许多巨石，代表火山灰崩塌角砾岩，火山灰崩塌角砾岩是在火山灰形成爆发期间源自断层陡崖的。另一个是一个1-3公里宽的花岗岩斑岩环堤，它包围着火山前的沉积岩和火山碎屑沉积物，并被解释为沿地下圆形裂缝的岩浆导管。这两个结构所包围的破火山口塌陷区为椭圆形，大小约为20×30 km。我们的观察结果表明，破火山口北部存在连贯的，活塞式塌陷，而沿断层带的不对称沉降导致南部出现陷井或零碎沉降。环形堤坝与花岗岩斑岩的片状体相连。我们建议这种片状侵入是漆器，可能导致破火山口地板重新隆起。

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