Aspects of Collision Tectonics and Intraplate Deformation

Mike P. Coward

Alpine collisional tectonics occurred episodically over the past 100 m.y., closing various small Tethyan basins and causing ripples of basin contraction and tectonic inversion across western Europe. Both at the Tethyan margin and in the smaller basins, deformation styles were controlled by existing fault geometries, in particular, (1) the position, dip, and detachment levels of the important bounding normal faults, (2) the locations of northwest-southeast trending lateral ramps/tear faults, which compartmentalize and "tram-line" the deformation, and (3) the distribution and thickness of Mesozoic postrift sediments.

Collision began in the middle Cretaceous, with the closure of Ligurian and Valais basins and the associated reactivation of northwest-southeast strike-slip faults and small basins as far away as the Atlantic margin. This movement was associated with the earliest orogenic flysch deposits, the subduction of Tethyan ophiolites, and local A-type subduction and high-pressure metamorphism close to the Tethyan continental margins. Major crustal shortening occurred in southern Europe (Spain and southern France) in the Late Cretaceous-Paleogene associated with closure of Pyrenean basins, but in the Alps, the major shortening continued throughout the Neogene. Section restorations based on regional studies, linked to commercial and deep seismic data, indicate well over 100 km of crustal shorteni g in the western and central Alps, with subduction of lower European crust and lithospheric mantle beneath the southern Alps and the Po plain.

Thrust directions cannot be related directly to major plate motions but are modified by (1) indentation tectonics, where small subplates and segments of the European crust were squeezed out sideways to the main plate compression, (2) rotations, in particular a major counterclockwise rotation of the Apulian plate, as well as rotations of thrust sheets where faults were pinned laterally, (3) gravitational spreading of the thickened crust, and (4) Tertiary rift propagation from the opening west Mediterranean basins.

The results are a complex, highly arcuate fold-and-thrust belt where propagation and displacement rates and directions, as determined from foreland basin analysis, vary dramatically in time and space.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.