Geodynamics of orogenic wedges has been for a number of years a major subject of research. The case of the Alpine mountain belt is a very well-established and documented example of internal dynamics. Studies have focused on the mechanics of the Alpine orogenic wedge and on the complex interactions between tectonics and surface processes and their consequences on both the dynamics of the orogenic wedge and the evolution of topography. Depending on their rates and localization in space and time, erosion and sedimentation modify the morphology and the internal structure of the wedge. Geological and geophysical studies do provide a global view of the Alpine orogenic wedge at a lithospheric scale, but this image is a ‘‘snapshot’’ in time (after Bonnet et al., 2007).

In the Alps, feedback mechanisms linking surface processes, tectonic processes, and structural heritage can be investigated at first hand. The Molasse foreland basin develops in the northern part of the orogen in response to the development of the mountain belt and receives the products of the erosion of the orogenic belt. The basin structure not only reflects the Tertiary history of the Alps, but also responds to the mechanics of the orogenic wedge. Indeed, the size of the basin, which is considered here as part of the wedge, influences the wedge mechanics such as for instance the sequence of thrusting. The exhumation of the External Crystalline basement Massifs, south of the Molasse Basin is largely driven by the subduction mechanism of the European plate under the Adriatic promontory. However, it appears that erosion of the overlying Penninic orogenic lid controls the localization, velocity, and magnitude of the exhumation. To the north of the Molasse Basin, the Jura foreland fold-and-thrust belt started to form during the Miocene, because of a major ‘‘jump’’ of the Alpine thrust front by about 100 km toward the north under the Molasse foreland basin. This foreland-directed propagation of the frontal thrust is due to a combined effect of the structural and lithological settings and the shape of the Molasse Basin; hence the nature and amount of sedimentation. Among other causes, the presence of a large thickness of Molasse deposits in the Alpine foreland allowed the activation as décollement level of the Triassic evaporite layers accumulated at the base of the European cover. The Alpine development of the orogenic wedge is further strongly influenced by a structural heritage of Mesozoic sedimentary half-graben type basins developed along major normal fault systems. In order to understand the complex interactions between erosion, sedimentation, structural heritage, and tectonics and to investigate the importance of sedimentation/erosion, we performed a series of analogue models.

Key stages of the analog model evolution. Here t0 is the initial stage; t10 is the climb of the lid onto the top of the basement ramp causing an intense internal deformation of the lid due to backthrusting; t13 is the first slice of recently deposited foreland sediments thanks to the activation of the basal de ́collement level in the basin; t16 is the inversion of the inherited basement normal faults and propagation to the surface as a second foreland sediment slice; t23 is underplating of the basement imbricates and development of a third foreland sediment slice; t29 is spontaneous underplating of the homogeneous part of the basement, initiation of a fold-and-thrust development in the foreland, and detachment of a nappe from the lid due to the uplift of basement units; and t38 is final stage picture showing the developed foreland fold-and-thrust belt, the remains of the detached nappe and the antiformal basement nappe stack.

On the basis of a section across the northwestern Alpine wedge and foreland basin, analog modeling is used to investigate the impact of surface processes on the orogenic evolution. The basis model takes into account both the structural and lithological heritages of the wedge. During shortening, erosion, and sedimentation are performed to maintain a critical wedge. Frontal accretion leads to the development of a foreland thrust belt; underplating leads to the formation of an antiformal nappe stack in the internal zones. Important volumes of analog materials are eroded out of the geological record, which in the case of the Alps suggests that the original lengths and volumes may be underestimated. The foreland basin evolves differently depending on the amount of erosion/sedimentation. Its evolution and internal structuring are governed by the wedge mechanics, thought to be the main controlling mechanism in the development of the Molasse basin in a feedback interaction with surface processes (after Bonnet et al., 2007).

Modeling reveals that two major types of mechanisms are active simultaneously: frontal accretion in the external parts leading to the development of a foreland thrust belt and underplating in the internal zones leading to the formation of an antiformal nappe stack. The basement imbricates in the antiformal stack are progressively steepened during its development. In the internal part of the Alpine belt, the originally shallow SE dipping axial surfaces and thrusts in the nappe stack also steepens to become sub-vertical to overturned dips. The reason for this geometry is the addition by accretion through tectonic underplating of frontal imbricates resulting in the rotation of the older trailing thrust sheets.
The sequential development in our model can also be favorably compared with the evolution of the stacking of the external basement massifs, the Molasse Basin development, and the initiation of the Jura fold-and-thrust belt. Stacking of prestructured basement units leads to important uplift creating topography which in turn causes higher erosion to maintain the overall wedge geometry. The developing geometries are linked to the preexisting structures. The propagation of the deformation to the unstructured basements causes a shift in the foreland tectonics, where a basal décollement beneath the autochthonous sediments causes the orogenic front to ‘‘jump’’ away from the orogen. This is similar to the initiation of the Jura, which can therefore be correlated with the development of basement nappes.

Learn more:

-> Malavieille, J., 2010. Impact of erosion, sedimentation and structural heritage on the structure and kinematics of orogenic wedges: analog models and case studies. Geological Society of America, account GSA Today, v. 20, no. 1,

-> Bonnet, C., J. Malavieille, and J. Mosar, 2007. Interactions between tectonics, erosion, and sedimentation during the recent evolution of the Alpine orogen: Analogue modeling insights, Tectonics, 26, TC6016, doi:10.1029/2006TC002048.

See also:

-> Henriquet, M., Dominguez, S., Barreca, G., Malavieille, J., Monaco, C., 2020. Structural and tectono-stratigraphic review of the Sicilian orogen and new insights from analogue modeling, Earth Science Review, 208, 103257,

-> Malavieille, J., Dominguez, S., Lu, C., Chen, C., & Konstantinovskaya, E., 2021. Deformation partitioning in mountain belts: Insights from analogue modelling experiments and the Taiwan collisional orogen. Geological Magazine, 158(1), 84-103. doi:10.1017/S0016756819000645