Reconstructing basal ice flow patterns of the Last Glacial Maximum Rhine glacier (northern Alpine foreland) based on streamlined subglacial landforms

Based on high-resolution (sub)glacial geomorphological mapping, we present a first digital inventory of streamlined bedforms within the footprint of a Last Glacial Maximum (LGM) Alpine piedmont glacier. A total of 2460 drumlins were mapped across the Rhine glacier foreland. Glacial lineations and one field of subglacial ribs (ribbed/Rogen moraines) - the first record of this type of subglacial landform on the Alpine foreland-were identified. Two flowsets, associated with (i) the Rhine glacier's LGM maximum advance (Schaffhausen stadial) and (ii) a late LGM readvance (Stein am Rhein stadial), are differentiated. The vast majority of streamlined bedforms occur in fields aligned in a 16- to 30-km-wide swath upstream of the Stein am Rhein frontal moraines. Orientation and elongation of drumlins and glacial lineations set the basis for the reconstruction of paleo-ice flow. Basal flow paths of the LGM maximum advance are visually interpreted and restricted to the zone proximal to the former ice front. The flow field reconstructed for the late LGM glacier readvance (Stein am Rhein stadial) extends tens of kilometres upstream and is modelled implementing a recently published kriging routine. The derived basal flow patterns paired with information on ice surface levels from lateral and frontal moraines and combined with relative ice velocity differences inferred from spatial changes in bedform elongation reveal detailed insights on ice flow geometries, particularly during the glacier readvance. Reconstructed flowlines highlight basal flow under shallow ice that is strongly controlled by local topography evidenced by diverging around basal bumps and converging in (narrow) valley sections and troughs, where basal flow velocities, steered by topography, are high. Gained paleo-ice basal flow patterns offer new insights on landscape evolution of the northern Alpine foreland and provide evidence-based flow data to validate future physical modelling results. More than 2500 bedforms (drumlins, glacial lineations and subglacial ribs) were mapped in the footprint of the Last Glacial Maximum Rhine glacier piedmont lobe (6000 km2). Elongation and direction of mapped subglacially streamlined bedforms were used to reconstruct the paleoglacier's flow pattern during a late LGM readvance. Basal trajectories that diverge around bedrock highs and converge in narrow valley sections, as well as identified fast flow areas, that largely correspond to valley troughs, underline the strong topographical control on ice flow.image