Strain partitioning in the axial NW Alps since the Oligocene

This work analyzes the postmetamorphic evolution of the northwestern Alps thanks to a multiscale field-geology study integrated by fission-track analysis. It illustrates how the fission-track method, constraining throws along major faults and age of mesoscale deformation, provides interpretation keys to detangle complex structural data sets. Results are discussed within the framework of the Alps-Apennines knot, and give new insights on strain partitioning within transpressional orogenic belts. The axial NW Alps acted since the late Oligocene as an orogen-scale shear zone accommodating, in the Aosta Valley, dextral strike-slip coeval with forward propagation of external thrusts. Within this transcurrent belt, right-lateral deformation was partitioned between the fast-exhuming Western Block and the already-unroofed Eastern Block. These blocks show contrasting structural characters. The former is cut by first-order longitudinal structures bounding major tectonostratigraphic units. The latter is fragmented by complex patterns of lower-order faults interpreted as subsidiary shears. The onset of mesoscale extension, widespread in the Eastern Block, is diachronous across the belt and migrated through time towards the European foreland. Transition from syn-collisional shortening to orogen-scale transcurrence observed in the NW Alps corresponds to a regional strain-field variation, documented in the mid-Oligocene all around the Alps-Apennines knot, which is possibly related to a change of the Adria trajectory relative to Europe. Since the late Oligocene, deformation associated with oblique plate-convergence was partitioned between strike-slip in the axial belt and thrusting in external areas. Most of the shortening was accommodated in the Helvetic chains (10e2 km). Deformation was limited elsewhere to an order of 10e1 km