The Geometry of Crustal Shortening in the Western Alps

Abstract

The use of structural data to define the convergence history across orogenic belts has been handicapped by the absence of a coherent tectonic model within which it is possible to unravel the relative horizontal displacements, their lateral continuity and compatibility. Such a model can be provided by concepts of linked thrust analysis, this contribution applies this working hypothesis to the Eocene-Pliocene tectonic evolution of the western Alps. Deformation commenced with the obduction of oceanic material and a sheet of previously subducted continental crust onto the Franco- Swiss crust. Locally displacements were transferred onto the base of shelf sediments which were carried out onto the external zones as the Prealpine klippen. The obduction complex was then disrupted by back thrusts and then carried forwards by displacements in the frontal Pennine and external zones. Thrusting terminated in the Jura in latest Miocene times. Throughout the deformation the Franco-Swiss continent was transected by thrusts which followed several mid-crustal detachment levels. A series of balanced sections together with an appraisal of thrust sheet distributions suggest that displacements exceeded 400 km on a WNW-ESE axis in the western Alps at a rate of about 1 cm.yr-1. Substantial volumes of Penninic crust must have been subducted beneath the Po plain requiring wholesale transformation to eclogite facies assemblages to inhibit isostatic compensation. Using this model for western Alpine orogenesis the central Alps are interpreted as an oblique ramp zone within which oblique folds, thrust sheet rotations and complex strain patterns develop. Two dimensional section restoration will not be appropriate in this sector. However, displacements can be transferred onto the base of the Austroalpine sheets in the eastern Alps and, via transfer faults, onto the Apennine thrusts to the south.