Anisotropy of magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite, Italy
Anisotropy of magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite, Italy
复制标题
意大利坎帕尼亚熔岩沉积过程的磁化率各向异性研究
DOI：
10.1007/s00445-002-0241-2
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发表时间：
2002
影响因子：
3.5
通讯作者：
R. V. Fisher
中科院分区：
文献类型：
--
作者：
M. Ort;G. Orsi;L. Pappalardo;R. V. Fisher
摘要：晚更新世粗面状坎帕尼亚火山灰岩位于意大利那不勒斯附近的坎帕尼亚平原的大部分地区之下，并出现在平原周围山区的山谷中，距离其源头Campi Flegrei破火山口约80 km。在Campi Flegrei 15公里范围内的站点，磁化率（AMS）主方向的各向异性表明，在没有显著地形的情况下，沉积来自大致径向移动的流动。AMS研究的更远的熔结凝灰岩揭示了下坡和/或downvalley流动方向之前的沉积，即使这些方向是在一个大角度的一般径向运输方向从通风口。在罗卡蒙菲纳火山的侧翼，水流直接向下，仿佛熔结凝灰岩的源头就是火山的顶峰。在大多数地方，熔结凝灰岩由单一的块状存款组成。然而，在亚平宁山脉的一些地方，山区多个排水系统的汇合导致了多个局部流动单元，这些流动单元不能在山谷之间相互关联。对马萨附近平坦的蒂特尔诺河谷中的熔结凝灰岩的详细研究表明，AMS组构并不是由于沉积或压实过程中的后期蠕变造成的。从马萨地区的垂直和横向截面来看，定义明确但不平行的AMS结构最好解释为沿山谷和陡峭山谷两侧流下的重力流合并形成单一加积存款。在Mondragone的碎屑成分和AMS轴表明，火山碎屑流遇到了Monte Massico Baglif，并被部分阻挡，因此沉积过程中的流动是朝向Campi Flegrei。从坎帕尼亚平原边缘沿着的站点获得的类似AMS数据表明，亚平宁山脉第一个山脊的回流距离其底部至少5公里。坎帕尼亚期火山灰岩是由密度分层的火山碎屑流沉积而成。沉积体系由水流较低、密度较大的部分组成，并受地形控制。AMS轴的分组被解释为指示在层流条件下发生的沉积。
Abstract.The late Pleistocene trachytic Campanian Ignimbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic susceptibility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encountered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions.