The deformation, resulting from a surge in 1985, of Glaciar Horcones Inferior is analyzed using structural geological models. During the surge, previously continuous debris cover was deformed by the formation of regularly separated and rotated ice blocks, suggesting a system of linked rotational extensional faults. Block tilting was measured from photographs taken shortly after the surge, showing rotation of the debris-covered surface. Fault inclination was assumed to be coincident with the debris-free side of the block. Glacier advance during the surge was obtained by comparing pre-surge aerial photographs with the position of maximum advance after the surge. Glacier thinning was estimated from the debris surface average lowering (relief generated at lateral scarps coincident with shear zones) and ice thickness measurements after surge termination. Three independent sets of information, geometry of the deformation (i.e. depth of detachment, fault traces, fault spacing, block rotation), glacier thinning and net advance, limit possible interpretations. Surface geometry suggests a domino-style or a linked planar rotational extensional fault system. In the observed configuration, however, these models can only explain a 12–13% extension. Glacier thinning suggests 30% local extension, and total glacier advance implies 16% minimum extension, which does not account for some frontal compression, as observed. A linked curved rotational extensional fault model fits the data well, implying a significant degree of internal deformation within each block. This model satisfactorily explains the observed deformation produced by the surge. It may also explain some modes of fast glacier flow, since the observed style of block tilting is present in other glaciers with high relief.
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