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An active fold system revealed by interbedded tephra layers is visible on the ablation surface of Johnsons Glacier (Livingston Island, South Shetland Islands, Antarctica). Johnsons Glacier is a cirque-shaped glacier of 5 km2 area located in a temperate ice cap. Converging flow-lines as a consequence of the reducing channel section extend from ice divides and terminate in a calving ice-cliff. Recent tephra layers from the volcano on Deception Island constitute excellent markers of the internal structure of the glacier and, when dated, provide valuable information about deformation kinematics.

Detailed field mapping of the clearly visible tephra markers revealed several folded layers that define a set of folds with sub-horizontal and fan-distributed axes being sub-parallel to the converging flow-lines. Folds become tighter towards the centre of the channel, where a cylindrical anticline is clearly exposed. Related deformational structures, comprising minor folds, foliations, thrust faults and crevasses, are observed in the ablation area. Strain-rates reflect extensional flow and higher deformation values where maximum confluence occurs.

We examine how the occurrence of this structure results from the transverse shortening in response to the reducing channel section and the consequent confluence of ice masses, increase in differential flow-rates between the centre of the glacier and the margins, and the development of passive folding processes.

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