Charlotte E. Cederbom

Climate-induced rebound and exhumation of the European Alps

Foreland basins record the regional isostatic compensation of mountain belts; during periods of crustal thickening, they subside, and when erosion unloads the mass of the mountains, the basins rebound and are eroded. In order to evaluate this mechanism for rebound, it is critical that the timing and magnitude of erosion are documented. We present data estimating the timing and magnitude of late orogenic or postorogenic erosion in the North Alpine Foreland Basin of Switzerland. Mineral cooling ages demonstrate that the basin underwent 1-3 km of erosion soon after 5 Ma. This erosion coincided with a decline in structural deformation in the Swiss Alps, and a doubling of sediment accu- mulation rates in surrounding depocenters. We propose that accelerated erosional un- roofing of the Swiss Alps triggered isostatic rebound and erosion of the foreland basin after 5 Ma. A projection of the isostatic rebound of the basin into the mountains suggests that at least 6.5 km of erosion should have occurred in the high topography of the Aar Massif. Accelerated erosion in the Swiss Alps at that time is explained by an increase in atmospheric moisture driven by an intensification of the Atlantic Gulf Stream at 4.6 Ma. Consequently, we propose that the changing erosional capacity of the climate triggered late orogenic to postorogenic mass reduction and isostatic rebound of the Swiss Alps and their neighboring foreland basin.

Linking the northern Alps with their foreland: The latest exhumation history resolved by low-temperature thermochronology

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U-Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6–5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis.