Control of increased sedimentation on orogenic fold-and-thrust belt structure – insights into the evolution of the Western Alps

Abstract

Abstract. We use two-dimensional thermomechanical models to investigate the potential role of rapid filling of foreland basins in the development of orogenic foreland fold-and-thrust belts. We focus on the extensively studied example of the Western European Alps, where a sudden increase in foreland sedimentation rate during the mid-Oligocene is well documented. Our model results indicate that such an increase in sedimentation rate will temporarily disrupt the formation of an otherwise regular, outward-propagating basement thrust-sheet sequence. The frontal basement thrust active at the time of a sudden increase in sedimentation rate remains active for a longer time and accommodates more shortening than the previous thrusts. As the propagation of deformation into the foreland fold-and-thrust belt is strongly connected to basement deformation, this transient phase appears as a period of slow migration of the distal edge of foreland deformation. The predicted pattern of foreland-basin and basement thrust-front propagation is strikingly similar to that observed in the North Alpine Foreland Basin and provides an explanation for the coeval mid-Oligocene filling of the Swiss Molasse Basin, due to increased sediment input from the Alpine orogen, and a marked decrease in thrust-front propagation rate. We also compare our results to predictions from critical-taper theory, and we conclude that they are broadly consistent even though critical-taper theory cannot be used to predict the timing and location of the formation of new basement thrusts when sedimentation is included. The evolution scenario explored here is common in orogenic foreland basins; hence, our results have broad implications for orogenic belts other than the Western Alps.

Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations

“…The Helvetic nappe system is commonly subdivided into Infrahelvetic, Helvetic and Ultrahelvetic units (Fig. 1c) (e.g., Masson et al, 1980;Escher et al, 1993;Pfiffner et al, 2011). The nappes consist mainly of Jurassic to Paleogene sediments that were deposited on the Mesozoic European passive margin before the Alpine orogeny ( Fig.…”

“…1a). This passive margin inherited half-grabens and horsts from the Mesozoic, pre-Alpine extensional phase (e.g., Masson et al, 1980;Escher et al, 1993). The stratigraphy of the nappes is generally characterized by shale-rich units, totaling several kilometers in thickness, and two major units of massive platform carbonates, the so-called Quinten (Malm) and Urgonian (Lower Cretaceous) limestones, with a thickness of several hundred meters (e.g., Masson et al, 1980;Pfiffner, 1993;Pfiffner et al, 2011).…”

“…Tectonic nappes were discovered more than a hundred years ago and are considered typical tectonic features of orogenic belts (e.g., Price and McClay, 1981), particularly in the Alps (e.g., Lugeon, 1902;Termier, 1906;Argand, 1916;Tollmann, 1973;Trümpy, 1980;Escher et al, 1993;Pfiffner, 2014). Several definitions of a nappe have been proposed (see discussion in Price and McClay, 1981), for example, after Termier (1922): "A nappe is a rock packet not in its place, resting on a substratum that is not its original one".…”

“…Bertrand (1884) argued also convincingly for an overthrust nappe (he used "masse de recouvrement" and "lambeaux de recouvrement" instead of nappe) in the Glarus region, so that finally also Heim (1906) accepted the overthrust interpretation instead of the earlier preferred double-fold interpretation ("Überschiebungsfalte" instead of "Doppelfalte"). Although the important controversies and observations supporting tectonic nappes are related to the Glarus region, which is part of the Helvetic nappe system, the true birth date of the nappe concept in the Alps, according to Trümpy (1991), is the publication by Schardt (1893) who worked in the Prealps, belonging to the Penninic domain (e.g., Escher et al, 1993). Schardt (1893) realized that Jurassic breccias have been thrust over Tertiary flysch and that large regions of the Prealps have been actually emplaced as a major overthrust.…”

Towards a nappe theory: Thermo-mechanical simulations of nappe detachment, transport and stacking in the Helvetic Nappe System, Switzerland

“…Lithospheric scale numerical models can self-consistently model the generation of orogenic wedges and major crustal shear zones, including effects of isostasy, flexure and back-folding (e.g. Erdős et al, 2014;Jaquet et al, 2018;Jourdon et al, 2019;Erdős et al, 2019). With higher numerical resolution such lithosphere models may eventually be able to resolve the upper crustal deformation with a resolution as applied in our model.…”