Bedrock Fracture Characteristics as a Possible Control on the Distribution of Surge-Type Glaciers
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| Authors: | Crompton, JW; Flowers, GE; Stead, D |
| Year: | 2018 |
| Journal: | J. Geophys. Res.-Earth Surf. 123: 853-873 Article Link (DOI) |
| Title: | Bedrock Fracture Characteristics as a Possible Control on the Distribution of Surge-Type Glaciers |
| Abstract: | Glacier surging has been studied extensively and is understood as a dynamic instability at the glacier bed. Yet an explanation for the heterogeneous distribution of surge-type glaciers at the scale of a mountain range remains elusive. Here we investigate bedrock discontinuity properties in the basins of 16 surge-type and nonsurge-type glaciers in the St. Elias Mountains of Yukon, Canada. Using scaled photographs of bedrock outcrops at the margins of each glacier, we digitize traces of the bedrock discontinuities and with automated purpose-built software, quantify discontinuity properties that are a function of length, orientation, and spacing of bedrock fractures. We obtain an unexpected result: outcrops in the basins of surge-type glaciers are less fractured than those in the basins of nonsurge-type glaciers. We hypothesize that the degree of bedrock fracture may control the extent and location of a clast-rich till transition zone at the glacier bed. This zone would provide flow resistance conducive to the development of an ice reservoir and thus to surging behavior. To reconcile our observations with the global distribution of surge-type glaciers, we speculate that surge-type glaciers may occur in geological settings characterized by an intermediate range of bedrock fracture. Plain Language Summary Surging is a perplexing form of glacier flow in which ice moves slowly for decades and then suddenly speeds up by a factor of 10 to 1,000. Surging gives us a window into some of the most dramatic forms of mass movement and sheds light on the general mechanics of glacier flow. Here we revisit an early hypothesis that glacier surging is related to fractures in the underlying bedrock. To test this hypothesis, we studied the bedrock characteristics around 16 glaciersnine of which are known to surgein the St. Elias Mountains of Yukon, Canada. We quantified the extent of bedrock fracture in each of the glacier basins from photographs, using software developed specifically for this project. Contrary to expectation, we find that surging glaciers in our study area inhabit basins with less fractured bedrock than the basins with nonsurging glaciers. To explain our results, we propose a hypothesis for surging that links bedrock fracture to the nature and distribution of rocky debris under the glacier, creating friction between the ice and bed. To reconcile our results with previous studies, we propose that surging glaciers worldwide may be found on bedrock that is not too fractured, but just fractured enough. |
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