[1] The timing and magnitude of channel bed erosion by three debris flows was measured in 2008 at the Illgraben catchment, Switzerland, using a scour sensor which consisted of a vertical array of erodible sensor elements. During the largest debris flow, sediment was entrained progressively and stepwise at the flow head within 20 s after front arrival, and onset of erosion started before maximum values for flow height and normal and shear stress, measured nearby, were reached. Erosion in one of the two smaller debris flows also occurred at the head of the flow, but the magnitude of erosion was at the detection limit for the sensor. For the other small debris flow, we were not able to determine the timing of erosion due to the presence of a sediment layer covering the sensors. Measurements of pressure fluctuations along the channel sidewall, which are produced by interparticle collisions within the flow, indicate that the entrainment of sediment is coincident with the largest mean and fluctuating pressures, suggesting that interparticle collisions may drive the erosion process at the front of debris flows. After erosion at the head of the debris flows, sediment was deposited on top of the erosion sensor columns, indicating that the bed was reworked to a larger depth than directly visible at the surface after the event.Observations from elsewhere along the channel support our measurements of the magnitude of net debris flow erosion and indicate that significant erosion can be expected on debris fans when the flow is confined to a channel. Debris flow entrainment and subsequent bulking of the flow influence the flow dynamics and therefore should be considered in debris flow models and hazard assessment.

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At the end of the Miocene, the European Alps ceased outward expansion, and tectonic uplift and exhumation shifted into the orogen interior. This shift is consistent with a change from orogenic construction to orogenic destruction, reflecting an increase in the ratio of erosional flux to accretionary flux. The coincidence of this change with an increase in sediment yield from the Alps suggests a climate-driven increase in erosional flux. The timing of deformation and sediment release from the southern Alps indicates that the tectonic change occurred synchronous with the last phase of the Messinian salinity crisis. We attribute the increase in erosional flux to a climatic shift to wetter conditions throughout Europe, likely augmented by the base-level fall that occurred during the Mediterranean dessication. This climate change is represented in the stratigraphic record by the Lago Mare deposits of the Mediterranean salinity crisis.

Magnetostratigraphic chronologies, together with lithostratigraphic, sedimentological, and petrological data enable detailed reconstruction of the Oligocene to Miocene history of the North Alpine foreland basin in relation to specific orogenic events and exhumation of the Alps. The Molasse of the study area was deposited by three major dispersal systems (Rigi, Höhronen, Napf). Distinguished by characteristic heavy mineral suites, conglomerate clast populations, and the presence of key clasts, these systems record three major phases of denudation of the Alpine edifice. The Rigi system eroded the Austroalpine and Penninic nappes of eastern Switzerland from 30 to 25.5 Ma as a result of backthrusting and uplift of these units along the Insubric Line. Subsequent uplift of the Aar massif some 40 km to the north appears to have controlled the duration of the Höhronen and Napf dispersal systems, spanning 24-22 Ma and 21.5-15 Ma, respectively. They record downcutting into the crystalline cores of the Penninic and Austroalpine nappes of eastern (Höhronen) and western (Napf) Switzerland. High-resolution reconstruction of the structural and geometrical evolution of the proximal Molasse reveals in-sequence and out-of-sequence thrusting events at the Alpine front and incorporation of the Molasse into the orogenic wedge by in-sequence thrusting and underplating. Furthermore, it reveals close relationships between periods of rapid denudation in the central Alps and phases of increased sediment accumulation rates at the proximal basin border. An initial increase in Molasse accumulation rates to >1 km/m.y. occurred between 30 and 25.5 Ma and coincides with the Insubric phase of back-thrusting along the eastern Insubric Line, where >10 km of vertical displacement is interpreted. During the same time span, the Alpine wedge propagated forward along the basal Alpine thrust, as indicated by the coarseningand thickening-upward megasequence and by occurrence of bajada fans derived from the Alpine border. The end of this tectonic event is marked by a basinwide unconformity, interpreted to have resulted from crustal rebound after initial loading. A subsequent increase in accumulation rates to >1 km/m.y. between 23 and 21.5 Ma coincides with initial uplift of the eastern Aar massif by at least 4 km. This phase of high accumulation rates is associated with incorporation of early Chattian conglomerates into the orogenic wedge. The third advance of the Alpine wedge between 21 and 15.5 Ma caused underplating of Molasse deposits, resulting in synsedimentary backthrusting of previously deposited Molasse sequences and in the development of a progressive unconformity. A rapid increase in accumulation rates from 0.35 to >1 km/m.y. between 15.5 and 15 Ma marks the final loading event in the wedge, which may be caused by further major displacement and loading of the Aar massif. This deformation is coeval with out-ofsequence thrusting of the Helvetic border chain and of the piggyback stack of North Penninic and Ultrahelvetic Flysch nappes along the bas...

The stratigraphies of foreland basins have been related to orogeny, where continent–continent collision causes the construction of topography and the downwarping of the foreland plate. These mechanisms have been inferred for the Molasse basin, stretching along the northern margin of the European Alps. Continuous flexural bending of the subducting European lithosphere as a consequence of topographic loads alone would imply that the Alpine topography would have increased at least between 30 Ma and ca. 5–10 Ma when the basin accumulated the erosional detritus. This, however, is neither consistent with observations nor with isostatic mass balancing models because paleoaltimetry estimates suggest that the topography has not increased since 20 Ma. Here we show that a rollback mechanism for the European plate is capable of explaining the construction of thick sedimentary successions in the Molasse foreland basin where the extra slab load has maintained the Alpine surface at low, but constant, elevations.

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