Neogene lateral extension during convergence in the Central Alps: Evidence from interrelated faulting and backfolding around the Simplonpass (Switzerland)

Abstract

The Simplon Fault Zone occurs at the western end of the Lepontine metamorphic dome in the Central Alps and represents a major, low-angle (25° dip) normal fault associated with WSW-ENE-directed lateral extension. Movement began in the early Miocene and continued until at least 3 Ma, accumulating a total vertical throw of around 10 km, corresponding to 24 km displacement parallel to the fault. This displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic “detachment fault” (the Simplon Line). In the central region (Simplon village to Zwischbergen), some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone. Due to the higher exposed structural level, the importance of the cataclastic component increases towards the northwest. In the northern “steep belt” around the Simplonpass, retrograde greenschist-facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with SSE-vergent, large-scale backfolding, representing NNW-SSE shortening and crustal thickening. The combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen, allowing lateral escape of continental material. Normal faulting and the resultant lateral escape was not just superficial. It extended to depths of at least 15 km, and probably to depths exceeding 25 km. The overall geometry and kinematics of the Simplon Fault Zone, and in particular the interplay between folding and faulting, is very similar to that described from the metamorphic core complexes and detachment faults of the Cordillera of the southwestern USA. Within the Central Alps, extension was associated with rapid exhumation of the Lepontine metamorphic dome, as evidenced by very rapid cooling rates determined from mineral ages in the footwall. Similar to the Cenozoic tectonics of the Himalayas and Tibet, orogen-parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction. More limited extension of the same age and orientation is known from a large area of the European foreland. Greater extension in the mountain belt compared with the foreland was accommodated by major strike-slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern Alps of Haute Provence (France). Coeval folding and orogen-parallel extension, such as at the Simplonpass, were developed in a dextral transpressive regime due to oblique WNW-ESE convergence across the Alps during the Neogene.