Water pressures at the base of active glacial overdeepenings are known to fluctuate strongly on various time scales. Rapid peaks in basal water pressure can lead to fracturing of the glacier bed, a process that has been described at numerous sites around the world, mostly based on large hydrofracture systems. This article presents drill‐cores from the base of a >100‐m‐deep glacial overdeepening in the Lower Aare Valley in northern Switzerland that were investigated with high‐resolution imaging (including X‐ray computed tomography) as well as compositional and microstructural analyses. The drill‐cores recovered Jurassic limestones hosting palaeokarst voids infilled with blue clay. We identify this clay, based on its kaolinitic composition, as siderolithic Bolus Clay but in a rather atypical variety formed under reducing conditions. The surfaces of the palaeokarst walls show smoothly undulating as well as brecciated sections with form‐fit interlocking clasts, which are the result of an in situ brecciation process. We argue that these particular fractures are not related to (glacio‐)tectonics or frost action. Instead, we suggest an explanation by involving water‐pressure peaks that were transmitted to the void walls by the clayey karst filling, resulting in hydrofracturing. In addition to pervasive karstification and tectonic overprinting, this water pressure‐driven fracturing may have enhanced the deep incision of the overdeepening into the rheologically competent bedrock.

中文翻译：

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冰川覆盖的古喀斯特（瑞士北部下阿勒河谷）的断裂：冰期水压峰值的结果？

众所周知，活跃的冰川过度深化基础上的水压会在不同的时间尺度上剧烈波动。基础水压的快速峰值会导致冰川床破裂，这一过程已在世界许多地方进行了描述，主要是基于大型水力压裂系统。本文介绍了瑞士北部下阿勒河谷冰川深部> 100米深的岩心，这些岩心已通过高分辨率成像（包括X射线计算机断层扫描）以及成分和微观结构分析进行了研究。钻芯回收了侏罗纪石灰岩，其中容纳了充满蓝色粘土的古岩溶空隙。基于其高岭土组成，我们将这种粘土确定为铁质块状粘土，但在还原条件下形成的非典型粘土中。原位过水化过程。我们认为这些特殊的裂缝与（冰川）构造或霜冻作用无关。取而代之的是，我们提出了一种解释，其中涉及到由黏土岩溶充填物传递至空隙壁的水压峰值，从而导致水力压裂。除了普遍的岩溶作用和构造套印之外，这种水压驱动的压裂可能还增强了深变层在流变作用基岩中的深层切口。