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Glacial ripping: geomorphological evidence from Sweden for a new process of glacial erosion
Geografiska Annaler: Series A, Physical Geography ( IF 1.5 ) Pub Date : 2020-06-05 , DOI: 10.1080/04353676.2020.1774244
Adrian M. Hall 1 , Maarten Krabbendam 2 , Mikis van Boeckel 1 , Bradley W. Goodfellow 3 , Clas Hättestrand 1 , Jakob Heyman 4 , Romesh N. Palamakumbura 2 , Arjen P. Stroeven 1, 5 , Jens-Ove Näslund 6
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ABSTRACT In low relief Precambrian gneiss terrain in eastern Sweden, abraded bedrock surfaces were ripped apart by the Fennoscandian Ice Sheet. The resultant boulder spreads are covers of large, angular boulders, many with glacial transport distances of 1–100 m. Boulder spreads occur alongside partly disintegrated roches moutonnées and associated fracture caves, and are associated with disrupted bedrock, which shows extensive fracture dilation in the near surface. These features are distributed in ice-flow parallel belts up to 10 km wide and extend over distances of >500 km. Our hypothesis is that the assemblage results from (1) hydraulic jacking and bedrock disruption, (2) subglacial ripping and (3) displacement, transport and final deposition of boulders. Soft sediment fills indicate jacking and dilation of pre-existing bedrock fractures by groundwater overpressure below the ice sheet. Overpressure reduces frictional resistance along fractures. Where ice traction overcomes this resistance, the rock mass strength is exceeded, resulting in disintegration of rock surfaces and ripping apart into separate blocks. Further movement and deposition create boulder spreads and moraines. Short boulder transport distances and high angularity indicate that glacial ripping operated late in the last deglaciation. The depths of rock mobilized in boulder spreads are estimated as 1–4 m. This compares with 0.6–1.6 m depths of erosion during the last glaciation derived from cosmogenic nuclide inventories of samples from bedrock surfaces without evidence of disruption. Glacially disrupted and ripped bedrock is also made ready for removal by future ice sheets. Hence glacial ripping is a highly effective process of glacial erosion.

中文翻译:

冰川撕裂:来自瑞典的冰川侵蚀新过程的地貌证据

摘要 在瑞典东部的低地势前寒武纪片麻岩地形中,磨损的基岩表面被芬诺斯坎第冰盖撕裂。由此产生的巨石铺展是大的有角巨石的覆盖物,其中许多具有 1-100 m 的冰川运输距离。巨石扩展发生在部分解体的木桃和相关的裂缝洞穴旁边,并与破碎的基岩相关,在近地表显示出广泛的裂缝扩张。这些特征分布在宽达 10 公里的冰流平行带中,延伸距离超过 500 公里。我们的假设是,该组合是由(1)液压顶升和基岩破坏,(2)冰下撕裂和(3)巨石的位移、运输和最终沉积造成的。软沉积物填充表明冰盖下方地下水超压对预先存在的基岩裂缝的顶升和扩张。过压降低了沿裂缝的摩擦阻力。当冰的牵引力克服了这种阻力时,就会超过岩体强度,导致岩石表面解体并撕裂成单独的块体。进一步的运动和沉积会产生巨石蔓延和冰碛。短的巨石运输距离和高棱角表明冰川撕裂在最后一次冰消期后期进行。巨石扩展中移动的岩石深度估计为 1-4 m。相比之下,末次冰期的侵蚀深度为 0.6-1.6 m,这是由基岩表面样品的宇宙成因核素清单得出的,没有破坏的证据。冰川破坏和撕裂的基岩也准备好被未来的冰盖清除。因此,冰川撕裂是一种高效的冰川侵蚀过程。
更新日期:2020-06-05
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