MODELLING THE KINEMATIC EVOLUTION OF VALLEY-SCALE FOLDING IN SURGE-TYPE GLACIERS

Glacier surges, episodic and dramatic increases in glacier flow velocity, produce iconic valley-scale fold trains which encode a history of polyphase deformation derived from these cyclic changes in glacier dynamic behaviour. The folding is passive, resulting from disturbances to ice foliation during surging flow that is characterized by extremely efficient basal sliding, and subsequently altered during quiescent flow during which polycrystalline ice creep primarily drives glacier motion. We conduct the first investigation of the kinematic evolution of these kilometre-scale folds using a full-Stokes numerical ice-flow model. We model the folds through multiple surge cycles within a set of synthetic glacier confluence configurations, and identify how differences in glacier flow regimes imprint themselves on three-dimensional fold geometry. We present an archetype of kinematic evolution that describes the transition from cylindrical vertically plunging gentle folds emplaced during the surge phase, to complex depth-varying folds following multiple cycles of surging and quiescent flow. The initial fold geometry is controlled by longitudinal and lateral shear stress regimes during surging, while fold evolution is governed primarily by lateral shearing after emplacement. We examine the role of valley geometry, glacier dynamics, and climate-driven mass balance in controlling the variation in fold geometry across simulations. Finally, we illustrate the potential of our approach to reconstruct more complex fold geometries as observed in nature, by applying it to a large surge-type glacier found in the St. Elias Mountains along the Yukon-Alaska border.