Deposition of rheomorphic ignimbrite D (Mogán Formation), Gran Canaria, Canary Islands, Spain
Deposition of rheomorphic ignimbrite D (Mogán Formation), Gran Canaria, Canary Islands, Spain
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DOI：
10.1007/s004450050246
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发表时间：
1999-03
影响因子：
3.5
通讯作者：
G. Kobberger;H. Schmincke
中科院分区：
文献类型：
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作者：
G. Kobberger;H. Schmincke
流变型火成岩D （Gran Canaria上Mogán组，13.4 Ma）是一种多流单冷却单元，在构造和变形火山碎屑的有限应变上存在不同的4个主要构造带。它们的结构特征表明了流变过程中不同的变形机制。这些带是：(a)垂直于片理的纯单轴平坦的基生带（玻璃体）；(b)上覆剪切带，其特征是不对称结构和明显较高的有限应变，具有类似于拉伸扁圆体的椭球几何形状；(c)一个中心区域，其有限应变几何形状与下面的剪切区域相似，但没有旋转应变分量的证据；(d)一个轻微变形到未变形的顶部带，其中亚球形火山碎屑的方向几乎是随机的，表明保存了原始的同沉积碎屑形状。D中的流变流是热火山碎屑流沉积前后再活化的结果，基于：(a)有限应变和组构分析得出的整体垂直构造分带；(b)剪切感与地形的关系；(c)流动底部计算的垂直冷却过程（玻璃体的形成）与应变几何的相关垂直变化之间的相互关系；(d)复杂岩化史；(e)变形流的后续机制。由于火山碎屑物质在坡度一般为6-8°的垂直堆积增加，荷载压力导致流变流。我们认为D的每一层新沉积的火山碎屑流物质在开始流变变形之前首先经过一个以压实（纯压扁）为主的焊接过程。
Rheomorphic ignimbrite D (13.4 Ma, Upper Mogán Formation on Gran Canaria), a multiple flow–single cooling unit, is divided into four major structural zones that differ in fabric and finite strain of deformed pyroclasts. Their structural characteristics indicate contrasting deformation mechanisms during rheomorphic flow. The zones are: (a) a basal zone (vitrophyre) with pure uniaxial flattening perpendicular to the foliation; (b) an overlying shear zone characterized by asymmetric fabrics and a significantly higher finite strain, with an ellipsoid geometry similar to stretched oblate bodies; (c) a central zone with a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component; and (d) a slightly deformed to non-deformed top zone where the almost random orientation of subspherical pyroclasts suggests preservation of original, syn-depositional clast shapes. Rheomorphic flow in D is the result of syn- to post-depositional remobilization of a hot pyroclastic flow as shown by kinematic modeling based on: (a) the overall vertical structural zonation suggested by finite strain and fabric analysis; (b) the relation of shear sense to topography; (c) the interrelationship of the calculated vertical cooling progression at the base of the flow (formation of vitrophyre) and the related vertical changes in strain geometry; (d) the complex lithification history; and (e) the consequent mechanisms of deformational flow. Rheomorphic flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally 6–8°. We suggest that every level of newly deposited pyroclastic flow material of D first passed through a welding process that was dominated by compaction (pure flattening) before rheomorphic deformation started.