Three-dimensional insight into Central-Alpine collision: Lower-plate or upper-plate indentation?

Accommodation of collisional shortening in the Central Alps varies dramatically along strike, and this change is inferred to result from along-strike changes of rheology. In the western Central Alps, 90% of shortening is accommodated in the thickened lower plate. In the eastern Central Alps, 90% of shortening is accommodated in the upper plate. In the central Central Alps, shortening is almost equally partitioned between the two plates. The lower crust of the Adriatic plate forms a wedge that reaches a maximum north-south extension of almost 70 km in the Engadine section, progressively decreasing westward and disappearing along the Simplon section. This difference indicates an along-strike increase of intra-plate decoupling, limiting shortening of the Adriatic plate to the middle and upper parts of the crust. Whereas the upper plate indents into the thickened accreted lower plate in the Simplon section, it is the lower plate that indents an intensely deforming upper plate in the Engadine section. In the west, the Ivrea mantle body increases the strength of the Adriatic upper plate, and Barrovian metamorphism weakens the lower plate. Therefore, along-strike transfer of shortening from one plate to the other appears to be a manifestation of along-strike changes in rheology deep in the crust.