Role of mantle indentation in collisional deformation evidenced by deep geophysical imaging of Western Alps

Abstract

In collision belts, the first-order role of the mantle in localizing deformation has remained elusive, as the resolution of geophysical imaging remains too low to constrain crustal geometry. To address this issue, we geologically interpret a recent high-resolution shear-wave velocity model from ambient-noise tomography of Western Alps. We show that the lower crustal Alpine geometry is highly variable at depth, evolving from a preserved European crustal slab in the South to a smooth crustal root in the North. Moho morphology is controlled by numerous pre-existing major faults reactivated during the Alpine orogeny. Two mantle indenters located above the subducted European plate at different depths appear to control the locus of active deformation. The rigid nature of Adria mantle explains the localization of brittle deformation that is transferred towards the upper crust. The strain-field partitioning results in a combination of strike-slip with either shortening or extension controlled by the anticlockwise rotation of Adria.

Structural Evolution, Exhumation Rates, and Rheology of the European Crust During Alpine Collision: Constraints From the Rotondo Granite—Gotthard Nappe

“…It is interesting to note that such brittle-to-ductile evolution at peak metamorphic conditions has been reported from different crystalline units across the Alps spanning the whole range of "peak metamorphic conditions" recorded for the different case studies, from sub-greenschist to high-pressure amphibolite facies (see Ceccato et al, 2022; Figure 10a). Conversely, recent studies proposed an inherited origin for similar brittle structures occurring in the ECMs, suggesting their development during the Permo-Mesozoic rifting or previous Paleozoic collision (Ballèvre et al, 2018;Dall'Asta et al, 2022;Herwegh et al, 2020;Schwartz et al, 2024).…”

“…The Gotthard nappe includes a series of polymetamorphic Ordovician-Silurian crystalline units intruded by late-Variscan granitoids (Berger et al, 2017). Ballèvre et al, 2018, Schmid et al, 2004Schwartz et al, 2024). AA: Aar; AG: Argentera; AR: Aiguilles rouges; BD: Belledonne; DM: Dora Maira; GP: Gran Paradiso; GT: Gotthard; IVZ: Ivrea-Verbano Zone; LD: Lepontine Dome; MB: Mont Blanc; MR: Monte Rosa; PE: Pelvoux.…”

“…However, constraints on the factors controlling the "weakness" of the European crust are sparse, including whether the crust was weak since the beginning of burial and subduction, or if it was initially strong and then progressively weakened during collision. Both tectonic inheritance related to Mesozoic rifting (Bellahsen et al, 2014), as well as Variscan orogeny (Schwartz et al, 2024), and syncollisional Barrovian metamorphism (Rosenberg & Kissling, 2013) might have contributed to the weakening of the European continental crust in this part of the Alps.…”

“…Geological setting of the study area. (a) Tectonic sketch of the Central-Western Alps (redrawn fromBallèvre et al, 2018, Schmid et al, 2004Schwartz et al, 2024). AA: Aar; AG: Argentera; AR: Aiguilles rouges; BD: Belledonne; DM: Dora Maira; GP: Gran Paradiso; GT: Gotthard; IVZ: Ivrea-Verbano Zone; LD: Lepontine Dome; MB: Mont Blanc; MR: Monte Rosa; PE: Pelvoux.…”