Diversity and dynamics of pyroclastic eruptions at Aso caldera indicated by thermoremanent magnetization

通过古地磁研究，评价了分布在日本九州中部Aso火山口以北和以东的Aso火山碎屑流沉积的几个单元的侵位温度。岩样的天然剩余磁化的阻塞温度被假定为它们在环境磁场中安放的温度。结果表明，焊接件的基本断口大部分为单组分磁化，在560℃的热退磁条件下，基本不受阻挡。在居里温度以上冷却过程中，ASO-2A、2R、3B、4A和4B的焊接凝灰岩具有稳定的单组分磁化作用。对ASO-4B的古地磁测量显示，其磁化强度比其他凝灰岩强一个数量级。火山口北部具有挤压结构的Aso-1凝灰岩显示了两种成分，表明可能存在二次流动。相反，在火山口以东具有相似挤压结构的AsO-4只有一种成分，表明焊接凝灰岩的挤压部分位于阻塞温度以上。具有典型二次流动结构的ASO-2R显示了一种成分，表明凝灰岩侵位至少在500℃以上。另一个显著的特征是ASO-2A和2B具有较大的倾角，ASO-3B表示向东倾斜，这与以前报道的结果一致。

Paleomagnetic investigations were performed to evaluate emplacement temperatures of the several units from the Aso pyroclastic-flow deposits which are distributed north and east of Aso Caldera, central Kyushu, Japan. Blocking temperatures of natural remanent magnetizations of the rock specimens are assumed as temperatures at which they were settled in the ambient field. It is concluded that most of the essential fragments in welded parts carried one-component magnetizations which are almost unblocked in thermal demagnetization up to 560℃. Welded tuffs of the Aso-2A, 2R, 3B, 4A and 4B carried stable one-component magnetizations during cooling from above the Curie temperature. Palaeomagnetic measurements of the Aso-4B showed one order of magnitude stronger magnetization than those of the other tuffs. The Aso-1 tuff with squeeze-out structure in north of the caldera showed two components suggesting a possibility of secondary flowage. On the contrary, The Aso-4 with the similar squeeze-out structure in east of the caldera showed only one component suggesting that the squeeze-out part of the welded tuff was emplaced above blocking temperature. The Aso-2R with a typical secondary flowage texture indicated one component suggesting that the tuff was emplaced above at least 500℃.The other notable characteristics are that the Aso-2A and 2B showed larger angle on inclination and the Aso-3B indicates eastward declination as are consistent with the previously reported results.