Identification of ancient impact structures: Low-angle faults and related geological features of crater basements
Identification of ancient impact structures: Low-angle faults and related geological features of crater basements
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古代撞击构造的识别：低角度断层及陨石坑基底的相关地质特征
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
D. Stöffler
中科院分区：
文献类型：
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作者：
T. Kenkmann;B. Ivanov;D. Stöffler
古老的撞击坑通常被后来的构造深深地侵蚀、变质和/或变形。在这种条件下，用微观结构或矿物学标准来识别这种冲击结构是非常困难的。提出了火山口基底断层模式可用于侵蚀构造的诊断。根据在法国Rochechouart陨石坑的野外观测和数值计算，我们认为存在向陨石坑中心缓慢倾斜的低角度断层。在最终直径为20-30公里的陨石坑中，低角度正断层的横向范围可能约为5公里，主要发生在距离陨石坑中心约3-8公里的地方。我们的数值模型表明，它们形成于原始陨石坑表面以下0.5至4公里的深度。当瞬态陨坑腔的陡峭边缘开始塌陷时，在陨坑改造过程中发生低角度正断层。重力驱动滑动发生在黏聚力低、摩擦系数减小的声流化岩石中。结果表明，流化程度随冲击中心的深度和距离而变化。沿缓倾剪切带的断裂可能是撞击引起的火山口底流变分层和中央峰隆起引起的剪切带被动旋转的结果。快速和无约束的单滑动事件局部导致摩擦熔化和伪石的形成。野外观测结果有力地支持了陨石坑蚀变过程中块体振荡的观点。
Ancient impact craters are commonly deeply eroded, metamorphosed and/or deformed by later tectonics. The identification of such impact structures using microstructural or mineralogical criteria are very difficult to apply under such conditions. It is proposed that fault patterns in the crater basement can be used diagnostically in eroded structures. On the basis of field observations in the Rochechouart crater, France, and numerical computations, we suggest the existence of low-angle faults dipping gently towards the crater centre. In craters with final diameters of 20–30 km, low-angle normal faults might have a lateral extent of approximately 5 km and predominantly occur at a distance of approximately 3–8 km away from the crater centre. Our numerical model suggests that they formed at a depth of 0.5 to 4 km below the original crater surface. Low-angle normal faulting occurs during crater modification when the steep rim of the transient crater cavity starts to collapse. Gravity-driven sliding occurs in acoustically-fluidized rocks with low cohesion and reduced friction coefficients. It is suggested that the degree of fluidization changes with depth and distance from the impact centre. Faulting along gently dipping shear zones may be the result of an impact-induced rheological stratification of the crater floor and a passive rotation of the shear zones due to the uplift of the central peak. Rapid and unconstrained single slip events locally lead to frictional melting and the formation of pseudotachylites. Field observations are presented that strongly support the idea of block oscillation during crater modification.