Structure and changing dynamics of a polythermal valley glacier on a centennial timescale: Midre Lovénbreen, Svalbard

Most polythermal glaciers in Svalbard, other than those of surge type, have receded steadily since the early 20th century. Midre Lovénbreen, a slow-moving, 4-km-long valley glacier terminating on land, is a typical example, and its internal structures reflect changing dynamics over this period. The three-dimensional structural style of this glacier and the sequential development of structures have been determined from surface mapping, ground-penetrating radar, and numerical flow modeling. In order of formation the structures observed today at the glacier surface are (1) primary stratification that has become folded about flow-parallel axes; (2) axial plane longitudinal foliation associated with this folding; (3) several sets of intersecting crevasse traces; (4) arcuate upglacier-dipping fractures developed as part of a thrust complex near the snout; and (5) longitudinal splaying fractures in the snout area. The long-term evolution and dynamic significance of these structures can be ascertained from historical ground and aerial photographs. Modeling indicates that stratification and foliation continue to evolve today as a result of internal deformation, especially in zones of converging flow, where simple shear is most pronounced, but within the tongue are carried passively toward the snout. Crevasse traces appear to be no longer actively forming but are interpreted as relict structures when the ice was more dynamic and mostly wet based. The interpretation of arcuate fractures near the snout as thrusts is supported by the matching orientations of modeled strain ellipses, which illustrate the importance of longitudinal compression.