The Mechanics of Pseudotachylite Formation in Impact Events
The Mechanics of Pseudotachylite Formation in Impact Events
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DOI：
10.1007/3-540-27548-7_2
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发表时间：
2005
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影响因子：
--
通讯作者：
H. Melosh
中科院分区：
文献类型：
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作者：
H. Melosh
本文讨论了在大陨石撞击附近快速剪切岩石中形成假玄武玻璃岩的基本制约因素。许多地质学家普遍认为，假玄武玻璃岩是由岩石的摩擦熔融和/或与相邻岩石类型的差异冲击压缩相关的剪切作用形成的。一些关于摩擦熔化的物理研究表明，理论上，少量的运动（厘米或更少）就能产生非常薄的熔化岩石脉。更现实的模型表明，不规则的滑动表面上的晶粒尺寸的顺序可能仍然会产生初级熔体静脉高达几毫米厚。假玄武玻璃形成的主要奥秘并不在于摩擦会导致熔融，而在于它似乎形成了厚的块状物，宽达几米到几十米。然而，如此厚的岩体应该通过减少滑动岩体之间的摩擦来防止熔融。我建议这个难题的一个可能的解决方案是，通过在狭窄的剪切带上滑动产生的熔体被挤出到邻近的围岩中，从而保持滑动表面狭窄，同时在邻近的低压区中产生厚的熔体积聚，该低压区在剪切带的端部开口。为了使这种机制发挥作用，熔化的岩石必须具有足够的流动性，以便在陨石坑坍塌期间从剪切带中挤出。这对熔体的粘度和温度有很强的限制。这个模型可能会被测试的假玄武玻璃岩产状的几何形状的未来仔细调查。
This paper presents a discussion of the basic constraints controlling the formation of pseudotachylites in the rapidly sheared rocks in the vicinity of a large meteorite impact. The prevailing opinion among many geologists is that pseudotachylites are formed by friction melting of rocks and/or shearing associated with differential shock compression of adjacent rock types. Several physical studies of friction melting have shown that, in theory, small amounts of movement (centimeters or less) are capable of producing very thin veins of melted rock. More realistic models suggest that irregularities on the sliding surface of the order of the grain size may still create primary melt veins up to a few millimeters thick. The principal mystery of pseudotachylite formation is not that friction can cause melting, but that it seems to form thick masses of it, meters to tens of meters wide. However, such thick masses ought to preclude melting by reducing the friction between sliding rock masses. I propose that one possible solution to this conundrum is that the melt produced by sliding on narrow shear zones is extruded into the adjacent country rock, thus keeping the sliding surfaces narrow, while creating thick accumulations of melt in adjacent low pressure zones that open at the end of the shear zones. For this mechanism to operate, the melted rock must be fluid enough to extrude from the shear zone during the time available during crater collapse. This places strong constraints on the viscosity and temperature of the melt. This model may be tested by future careful investigation of the geometry of pseudotachylite occurrences.