Large-scale, thin-skinned thrusting in the southern Alps: Kinematic models
The tectonics of the southern Alps are dominated by thrusting on extensive ramp-flat systems and by folding, some of which is ramp-folding. Basement slivers are involved that, frontally, are not thicker than 5 km. The most unequivocal section is that of the Grigna Mountains east of Lake Como, for which kinematic modeling based on material balance considerations results in a shortening of ∼40 km. It is separated from the Grona section in the west and the Valtorta section in the east by probably pre-existing transverse zones that cut through the important decollement horizons and define blocks of different depths to basement. Local tectonics across these transverse zones change abruptly, while over-all shortening remains constant. In particular, both east and west of the Grigna section, basement ramp folds were wedged into the very incompetent Raibl beds, provoking back-thrusting of the thick Hauptdolomit and higher sediments with the substitution of the original cover of the frontal limb of the basement fold. This new kinematic interpretation is compatible with the information, facilitates the construction of balanced sections, and is required for lateral constancy of shortening.