Control of glacial quarrying by bedrock joints
Control of glacial quarrying by bedrock joints
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DOI：
10.1016/j.geomorph.2012.02.012
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发表时间：
2012-06-01
期刊：
影响因子：
3.9
通讯作者：
Iverson, Neal R.
中科院分区：
文献类型：
--
作者：
Hooyer, Thomas S.;Cohen, Denis;Iverson, Neal R.
描述冰川对基岩开采的模型的一个主要假设是，床上的裂缝很小，而且是孤立的。假定这些裂缝在冰川荷载作用下缓慢生长，最终划定了采石场的区域。如果这一假设是正确的，那么采石场的方向将由与滑动有关的床上主应力的方向和由此产生的冰床从岩石壁架和岩口分离的方向来控制，而不是由冰川加载之前的节理的方向来控制。为了验证这一假设，我们比较了加拿大和瑞士9个最近冰川消退的冰川前田的采石面和冰前节理的方向，这些冰川前田具有不同的基岩岩性和床型。除1个前田外，其余前田采石面与主要节理组重合，采石面取向与滑动方向无系统关系，但明显不面向冰川。同一冰川前田的两个区域滑动方向相差64度，对采石面方向没有影响，两个区域开采的节理集相同。这些观察激发了对冰下基岩的一种不同于现有采石模型的新特征。床层被理想化为一组由冰期前节理分隔的岩块。采石所需的缓慢裂缝增长将被限制在连接节理面的小岩石体（桥梁）中。这种床层模型不允许将其视为通常假设的具有水力隔离裂缝的完整弹性固体，这将对采石力学的表征产生深远的影响。(C) 2012 Elsevier B.V.版权所有
A principal assumption of models that describe bedrock quarrying by glaciers is that cracks in the bed are small and isolated. These cracks are assumed to grow slowly in response to glacial loading to eventually delimit the areas of quarried surfaces. If this assumption is correct, then quarried-surface orientations will be controlled by orientations of principal stresses in the bed associated with sliding and resultant ice-bed separation downglacier from rock ledges and roches moutonnees, rather than by orientations of joints that predate glacial loading. To test this hypothesis, we compared orientations of quarried surfaces and preglacial joints in nine recently deglaciated forefields of glaciers in Canada and Switzerland with different bedrock lithologies and bedforms. In all but one forefield, quarried surfaces and major joint sets are coincident, with orientations of quarried surfaces bearing no systematic relationship to sliding direction other than their obvious tendency to not face upglacier. A difference in sliding direction of 64 degrees between two areas of one glacier forefield had no effect on the orientation of quarried surfaces, with the same joint set exploited in both areas. These observations motivate a new characterization of subglacial bedrock that differs from that of existing quarrying models. The bed is idealized as a group of rock blocks separated by preglacial joints. Slow crack growth necessary for quarrying would be confined to small bodies of rock (bridges) that connect the faces of joints. This model of the bed does not allow it to be treated as an intact elastic solid with hydraulically isolated cracks as is usually assumed, which would have a profound influence on characterization of quarrying mechanics. (C) 2012 Elsevier B.V. All rights reserved.