Vredefort shatter cones revisited
Vredefort shatter cones revisited
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重新审视 Vredefort 破碎锥
DOI：
10.1029/1998jb900068
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发表时间：
1999
影响因子：
--
通讯作者：
W. Reimold
中科院分区：
文献类型：
--
作者：
L. Nicolaysen;W. Reimold
破碎锥体在世界范围内被描述为许多圆形和多边形结构，其起源可归因于大型地外抛射物的影响或灾难性的内生过程。尽管它们与神秘的灾难性过程有关，但破碎锥的性质及其形成所涉及的物理问题尚未得到全面研究。本文介绍了对南非Vredefort穹顶三个区域的破碎圆锥体进行的详细的野外和实验室研究结果。Vredefort碎裂锥与一种广泛存在的断裂现象直接相关，这种现象被称为“多条纹状关节集(MSJS)”。MSJ是平面到曲线平面的裂缝，发生的距离为1到几毫米。关节集具有分形性。当在现场使用新的测量方案时，包括研究所有关节表面和暴露在这些表面上的所有台阶和条纹，就可以获得关于MSJS的成因和意义以及它们与条纹锥形骨折的关系的新信息。通过在一套平行的薄切片中记录了许多断裂的精确几何形状，详细地检查了具有MSJS的岩石标本的内部组成。破碎锥体表面的台阶和条纹具有位移断裂(微断层)的特征，沿其观察到熔融的证据。破碎锥体和MSJS表面通常覆盖着玻璃状薄膜；我们评估这些断裂现象是否与假速生(摩擦)熔体的形成有关。我们的野外和岩石学观察可以解释为与Vredefort构造形成较晚的破碎锥/MSJs的生成一致。这一情景与普遍持有的观点形成鲜明对比，即破碎锥体是在冲击事件的早期“压缩”阶段形成的，冲击事件影响了水平地层。
Shatter cones have been described from a number of circular and polygonal structures worldwide, the origin of which has been alternatively ascribed to the impacts of large extraterrestrial projectiles or to catastrophic endogenic processes. Despite their association with enigmatic, catastrophic processes, the nature of shatter cones and the physics involved in their formation have not been comprehensively researched. Results of detailed field and laboratory studies of shatter cones from three areas in the collar of the Vredefort Dome in South Africa are presented. Vredefort shatter cones are directly related to a widely displayed fracture phenomenon, termed “multiply striated joint sets (MSJS)”. MSJs are planar to curviplanar fractures occuring at spacings of < 1 to several millimeters. The joint sets have a fractal character. When a new measurement protocol is used in the field, involving study of all joint surfaces and all steps and striae exposed on these surfaces, new information is gained on the genesis and significance of the MSJS and on their relationship to striated conical fractures. The internal constitution of a rock specimen with MSJS was examined in detail, by documenting the precise geometry of many fractures in a suite of parallel thin sections transecting the specimen. The steps and striae on shatter cone surfaces have the characteristics of displacement fractures (microfaults), along which evidence of melting is observed. Shatter cone and MSJS surfaces are often covered with glassy films; we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic (friction) melt. Our field and petrographic observations can be interpreted as consistent with the generation of shatter cones/MSJS relatively late in the formation of the Vredefort structure. This scenario contrasts sharply with the widely held view that shatter cones are formed during the early “compression” phase of a shock event that affected horizontal strata.