Structural Evolution During Cyclic Glacier Surges: 2. Numerical Modeling.

The mesoscale structures of a glacier express the history of flow, temperature, and stress. Thus, in principle, numerical ice dynamics models have sufficient physics to examine the formation and transport of these structures. In this study we use a vertically integrated thermomechanical ice dynamics model to simulate the temporally evolving patterns of surficial moraine, stratification, foliation, and folding of glacier ice, and the density and orientation of traces of former crevasses. The modeled glaciers are simplified versions of Trapridge Glacier in northwest Canada that allow diagnostic modeling of influences on glacier structure and help to clarify the physics and numerics. In the model, surges occur every 50 years in response to a prescribed cyclic change in bed friction. Medial moraine patterns are simulated by tracking the englacial and supraglacial trajectory of debris injected at fixed points in the accumulation region. Stratification is assumed to be associated with isochronal surfaces, and vertical foliation is explained in terms of horizontal flattening of strain ellipsoids. Crevasses form when and where the intensity of tensile stress exceeds a prescribed threshold; crack damage is cumulative so that crevasse traces observed at sampling sites are a superposition of the damage accumulated en route. Folding is parameterized but not resolved. By evaluating the deformation gradient tensor along ice particle trajectories and applying the polar decomposition to this tensor, we isolate the cumulative effects of rotation and stretching by ice flow and calculate strain ellipsoids as well as other practical indicators of deformation. Plain Language Summary: A surge is a short‐lived event during which a glacier flows considerably faster than normal. The switch from slow to fast flow is related to ice instability, resulting from changing hydrology and sediment characteristics at the glacier bed. Building on half‐a‐century of field investigations on Trapridge Glacier in the St. Elias Mountains (Yukon), we use archived and contemporary field data to investigate and model numerically the structural evolution of the glacier through its most recent full, but unusually slow, surge cycle that ended in 2005. After documenting the initial layering in the glacier (stratification), both ductile structures (stratification, foliation, and folds) and brittle structures (crevasses, thrusts, and other fractures) were measured and their order of formation recorded. Using the data collected, the authors were able to simulate the development of these structures using newly derived models of the dynamics of glacier flow (Part 1). Modeling the structures observed in the field then underpinned the development of generic models for broadly similar glaciers (Part 2). The work offers new insight concerning the structural evolution of glaciers generally, notably how the various structures are formed and modified in response to changing stress and strain conditions. Key Points: Ice dynamics models have sufficient physics to explain many of the structural features of surging and nonsurging glaciersModel calculations of the deformation gradient indicate that convergent ice flow favors development of longitudinal foliation structuresThe propensity for folding can be parameterized using scalar invariants of a rank 3 tensor derived from the deformation gradient [ABSTRACT FROM AUTHOR]