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

对分布在日本九州中部阿索火山口北部和东部的阿索火山碎屑流沉积进行了古地磁研究，评价了几个单元的侵位温度。假定岩石试样自然剩余磁化的阻挡温度为它们在环境场中沉降时的温度。结果表明，焊接件中的基本碎片大部分为单组分磁化，在560℃以下的热退磁过程中几乎不受阻挡。Aso-2A, 2R, 3B， 4A和4B的焊接凝块在居里温度以上冷却时具有稳定的单组分磁化。古地磁测量表明，Aso-4B凝灰岩的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型结构的Aso-1凝灰岩呈现出两种组分，提示有二次流动的可能。相反，在火山口东部具有类似挤压构造的Aso-4只显示出一个组分，表明焊接凝灰岩的挤压部分位于阻塞温度以上。具有典型二次流结构的Aso-2R显示一组分，表明凝灰岩放置温度至少在500℃以上。另一个显著的特征是，Aso-2A和2B的倾角较大，Aso-3B的倾角偏东，与前人报道的结果一致。

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.