Formation mechanisms for ice‐stream lateral shear margin moraines
Formation mechanisms for ice‐stream lateral shear margin moraines
复制标题
冰流侧向剪切边缘冰碛的形成机制
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
C. Stokes
中科院分区：
文献类型：
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作者：
R. Hindmarsh;C. Stokes
古冰流床的横向剪切边缘偶尔会出现大型曲线形冰碛，但很少有工作涉及它们是如何形成的。在本文中，我们回顾了“横向剪切边缘冰碛”的特点，并提出了两个定量模型，它们的形成：（i）不同的侵蚀速率相关的横向变化的水流速度和（ii）横向入侵从流间脊后，较薄的流撤退，因此剪切边缘冰碛只是一个终端冰碛的流间脊。其他可能的形成机制定性解决。我们的两个定量理论在原则上都是可行的，但入侵模型不太可信，而且与观测结果不一致。差异侵蚀模型可以预测几十米厚、几公里宽的冰碛，这些冰碛可以在一千年内形成。冰碛的宽度取决于两项之间的竞争，一项是在冰流中变化的，因为它取决于冰的速度，总是会导致侵蚀，另一项是与局部积累率有关的，如果是正的，就会导致侵蚀，如果是负的，就会导致沉积。这些术语联合收割机在消融区域的冰流剪切边缘产生沉积的可能性。冰碛的宽度有随河流宽度增加的趋势。允许剪切边缘冰碛形成的条件相当有限：太多的消融和沉积发生在到处，而不仅仅是在冰流的侧缘附近;太少的消融和到处都是侵蚀。这与侧剪边缘冰碛的罕见观测结果一致。版权所有© 2008约翰威利父子有限公司。
The lateral shear margins of palaeo‐ice stream beds are occasionally marked by large curvilinear moraines, but little work has addressed how they might form. In this paper we review the characteristics of ‘lateral shear margin moraines’ and present two quantitative models for their formation: (i) differential erosion rates related to lateral variations in the stream velocity and (ii) lateral incursion from inter‐stream ridges after the thinner stream retreats, where the shear margin moraine is consequently just a terminal moraine of the inter‐stream ridge. Other possible mechanisms of formation are addressed qualitatively. Both of our quantitative theories are, in principle, feasible, but the incursion model is less plausible and inconsistent with observations. The differential erosion model can predict moraines tens of metres thick and several kilometres wide, which can be formed in a thousand years. The width of the moraine depends on the competition between a term that varies across the ice stream because it is dependent on ice velocity and that always causes erosion, and a term related to the local accumulation rate, which causes erosion if is positive and deposition if negative. These terms combine to create the possibility of deposition at the shear margins of ice streams in ablation areas. The width of the moraine has a tendency to increase with the width of the stream. The conditions that permit shear margin moraines to form are quite constrained: too much ablation and deposition occurs everywhere rather than just near the lateral margins of the ice streams; too little ablation and there is erosion everywhere. This is consistent with the infrequent observations of lateral shear margin moraines. Copyright © 2008 John Wiley & Sons, Ltd.