Two-dimensional numerical modeling of fracturing and shear band development in glacier fronts

摘要

In this contribution we present a two-dimensional numerical model of a deforming glacier front. The model is based on a hybrid lattice spring network approach where particles in the model can deform in a volume conservative visco-elastic manner but at the same time they can be compressed elastically and fracture by discrete failure. We restrict ourselves to a simple setting where the glacier sits on a fric-tionless slope that dips with 5-10°, the ice block is fixed on one side and has a free surface on the other. The glacier varies in viscosity and can flow at the base, whereas it is brittle at the top. Results show that the head of the glacier is unstable. Failure happens as a combination of extension fractures (crevasses) at the top surface of the glacier and shear fractures that are dipping toward the glacier head. Once the shear fractures intersect with the free side-wall of the glacier a triangular ice block is carving from the glacier head. During successive flow of the glacier the failure is stepping backwards into the glacier and large shear planes develop that connect the sliding ice at the base with crevasses at the top. Variations of overall viscosity of the glacier indicate that higher viscosities (and thus a more brittle glacier) lead to larger spacing of shear surfaces and thus to larger ice blocks that are carving from the head of the glacier. In addition the geometry of the deformation structures within the glacier does not vary significantly with the height of the ice indicating that larger glaciers carve larger blocks. A higher tilt of the ground surface, however, leads to tighter spacing of shear surfaces and a more pronounced crevasse development. This indicates that glacier heads that lie on steeper slopes will carve smaller blocks than glacier heads that lie on shallower slopes. Failure and carving of ice from the model glaciers is a combination of early developing closely spaced extension fractures (crevasses) and later developing wider spaced and more localized shear fractures or shear zones.