Abstract: A New Adventure in Alpine Tectonics: Transorogenic Connections of Foreland Thrusts Across the Miocene Alps

Hans Laubscher

In the middle to late Miocene Alps, typical thin-skinned foreland fold and thrust belts developed on both sides of the orogene. Where they ramp down into basement, stacks of duplexes developed into conspicuous external massifs. The central zone of the orogene between the external massifs was little deformed in that phase, behaving more or less as a rigid lid. Projection of the basal thrusts into the center of the Alps results in a junction in about 20 km depth, within the brittle-ductile transition, establishing a lower boundary for the lid. This boundary appears to be a detachment zone as the warm masses below it were presumably deformed disharmoniously by ductile creep similar to other metamorphic terrains.

Balanced sections from both thrust belts reveal approximately linear increases of shortening along strike, which suggest a dextral rotation of the lid by a few degrees around its center. When trying to follow this suggestion to its logical conclusions, lid segments bounded by transalpine shear zones of the corresponding age interval are found. These have in places been analyzed in great detail by independent workers starting not in the foreland but in the interior of the Alps. For their tying to foreland tectonics the most recent results are the most interesting. They may be seen in the context of a kinematic model of simultaneous complementary dextral and sinistral shear associated with north-south compression and east-west extension.

Rotation of the shallow lid is consonant with a dextral component of deep lithospheric plate boundary deformation that imparted a dextral twist on the overlying lid. Therefore, both shallow- and deep-seated simultaneous dextral motion in the Alps separated by a surface of disharmony ought to be envisaged. The deep-seated motion follows a curved boundary concave to the south, suggesting an anticlockwise rotation of the southern (Adriatic) lithosphere. Such a rotation, in turn, appears necessary to produce the young thrust belts of the southwestern Alps in Provence. It would appear, therefore, that the model of simultaneous deep-seated and induced shallow rotations succeeds in explaining a series of puzzles in Alpine tectonics.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers