Late Oligocene to early Miocene lateral extrusion at the eastern border of the Lepontine dome of the central Alps (Bergell and Insubric areas, eastern central Alps)

Abstract

How the internal domain of the central Alps deformed during late stages of the continental collision remains poorly understood. To fill this gap in knowledge, we present new data constraining the late Alpine brittle and brittle-ductile tectonics in Oligocene intrusions of the Bergell Alps (eastern central Alps). Syncollisional late normal faulting is widespread at all investigated scales, but part of the deformation observed is also associated with oblique-slip and transcurrent displacements. The faults are explained by stress permutations in space and time implying the three principal stress axes and correspond to different homoaxial states of stress of one single transtensive tectonic event. Motion on these faults accommodated coeval orogen-parallel extension and orogen-perpendicular contraction, during oblique indentation by the Southern alpine crust. Extensional and transcurrent structures likely formed by lateral extrusion of the Bergell Alps via distributed eastward extrusion of imbricated wedge-shaped crustal blocks, during the late Oligocene and lower Miocene. A first-order wedge-shaped crustal block in the Bergell Alps is represented by three coeval segments of the Periadriatic Fault System (namely the Tonale, Engadine, and Forcola faults). The eastern central segment of the Alpine belt appears to have deformed during the late Oligocene and lower Miocene in a similar way to its extremities where lateral escape tectonics occurred at least until the upper Miocene, according to free boundary availabilities. Copyright 2008 by the American Geophysical Union.