Abstract
High-resolution 32–20 Ma-old stratigraphic records from the Molasse foreland basin situated north of the Alps, and Gonfolite Lombarda conglomerates deposited on the southern Alpine margin, document two consecutive sedimentary responses - an immediate and delayed response - to slab breakoff beneath the central Alps c. 32–30 Ma ago. The first signal, which occurred due to rebound and surface uplift in the Alps, was a regional and simultaneous switch from basin underfill to overfill at 30 Ma paired with shifts to coarse-grained depositional environments in the foreland basin. The second signal, however, arrived several million years after slab breakoff and was marked by larger contributions of crystalline clasts in the conglomerates, larger clast sizes, larger sediment fluxes and shifts to more proximal facies. We propose that this secondary pulse reflects a delayed whiplash-type erosional response to surface uplift, where erosion and sediment flux became amplified through positive feedbacks once larger erosional thresholds of crystalline bedrock were exceeded.
Highlights
Progradation of coarse-grained material in foreland basins has been related to tectonic uplift, which accentuates erosion through the generation of steeper slopes[1], or to shifts towards stormier climates, which enable the transport of larger clasts by more powerful floods[2,3]
From 25.5 Ma onwards, the conglomerates coarsen and thicken upwards towards a succession where amalgamated conglomerate beds with a massive fabric dominate the stratigraphic architecture. This change records a transition to a braided river system where overbank fines, represented by red-mottled mudstone interbeds, are rare[28]
The three Molasse sections described here are all characterized by a similar, yet diachronous, abrupt sedimentation change within an overall coarsening and thickening-upward trend. Such a shift in the stratal architecture could be interpreted as the stratigraphic response to local tectonic events in the immediate hinterland of each fan[7,9,20,28,33], driven by the northward shift of the Alpine orogen (e.g., Fig. 2d)[34]
Summary
Progradation of coarse-grained material in foreland basins has been related to tectonic uplift, which accentuates erosion through the generation of steeper slopes[1], or to shifts towards stormier climates, which enable the transport of larger clasts by more powerful floods[2,3] Most of these interpretations assume instantaneous process-responses, but recent physical models suggest that sediment supply signals linked with external perturbations can be buffered or even amplified[1,4,5], with a possible time lag[6]. We show that several millions of years later, the arrival of the second signal was marked by: distinct pulses in sediment discharge paired with the supply of material with larger grain sizes and larger relative abundance of crystalline clasts derived from the back of the Alps We propose that these sediment pulses reflect whiplash-type[27] erosional responses to slab breakoff, where erosion of Alpine streams became amplified once the larger erosional thresholds of the crystalline bedrock were exceeded. This implies that related responses in sedimentary basins to such events can be significantly protracted (several Ma) and possibly non-unique
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
