Structural Evolution During Cyclic Glacier Surges: 1. Structural Glaciology of Trapridge Glacier, Yukon, Canada

Abstract

Interpreting the relationships among the internal processes of glaciers and their mesoscale structural products has been a long‐standing challenge for glaciologists. Trapridge Glacier is a small polythermal surge‐type valley glacier that has been studied for 40 years. It offers an opportunity to investigate the structural evolution of a glacier through a series of surges and to apply novel modeling approaches to gain physical insight as to how different structures are formed. Following the glacier's most recent slow surge, the structural attributes were documented, with emphasis on their three‐dimensional geometry and sequential development: ice stratification (S0), longitudinal foliation (S1), and associated medial moraine, folding of stratification (F1), transverse foliation (S2), thrusts (S3), and recumbent folds (F3), fractures (surface crevassing and crevasse traces; S4). Efforts to represent these structures using models of glacier flow dynamics remain at an early stage but provide informative tests of model skill and of current understanding of the processes that control structure generation. Using field interpretations as a guide to the relevant processes of formation, structures on Trapridge Glacier are compared with computer‐simulated structures for the same glacier. Modeling achieved the greatest success in simulating moraine patterns, ice stratification, longitudinal foliation, and the downglacier decrease in the density of surface crevasse traces. The least successful effort was to simulate the orientation of crevasse traces.

Structural Evolution During Cyclic Glacier Surges: 2. Numerical Modeling

“…We begin by developing a digital elevation model (DEM) of bed topography that corresponds to a simplified version of Trapridge Glacier which we refer to as “Traplike Glacier.” Traplike Glacier flows from west to east and is symmetric about its centerline flow axis. This symmetry is produced by taking the DEM for Trapridge Glacier bed topography B ( x , y ) (derived from Flowers & Clarke, , and extended using a simplified version of Clarke et al, ) and identifying the central flow axis for Trapridge Glacier (refer to Figure 11 of Hambrey & Clarke, ), rotating the DEM to align with this axis, resampling the DEM in the rotated grid, and then cropping the new DEM to isolate the Trapridge Glacier catchment. We denote the gridded bed elevation for the new DEM as B i , j , where i =1,2,…, N x and j =1,2,…, N y and assume that N y is an odd number so that j =1 + ( N y − 1)/2 corresponds to the j index of the glacier centerline.…”

“…Large bedrock bumps complicate the flow of Trapridge Glacier (e.g., Hambrey & Clarke, , Figure 3). By avoiding the full complexity of Trapridge Glacier, we can systematically examine how subglacial topography influences ice flow and ice structure.…”

“…The right channel surge model examines the case where the southern half of the glacier experiences cyclic changes in the basal sliding friction while the northern half has constant sliding friction throughout the surge cycle (Figure ). Hambrey and Clarke () suggest that the south channel (i.e., to the south of the medial moraine) shows clearer evidence for surging than the north channel. The simplified model allows the consequences of such a channelization to be explored.…”

“…This contribution aims to provide a quantitative framework for interpreting a companion paper (Hambrey & Clarke, ) on the measured and modeled structure of Trapridge Glacier, Yukon, Canada (61°14 ′ N, 140°20 ′ W), a well‐studied glacier that from circa 1980–1999 experienced a slow surge (Frappé‐Sénéclauze & Clarke, ). Combining field observations and numerical modeling, Hambrey and Clarke () analyzed the postsurge structure of Trapridge Glacier including the medial moraine pattern, stratification and foliation, folding, and the density and orientation of crevasse traces. That paper was concerned with the observed structures and the skill of prognostic models to reproduce observations.…”

“…We follow the template of Table 1 in Hambrey and Clarke () and use standard structural geological notation to label glacier structures: S 0 , S 1 , and so forth, for sequential planar structures and F 1 , F 2 , and so forth, for successive fold phases. In the present paper we consider primary stratification (S 0 ), longitudinal foliation (S 1 ), transverse foliation (S 2 ), fractures (surface crevassing and crevasse traces; S 4 ), recumbent folds (F 3 ), and medial moraines.…”