Deep subduction and exhumation of continental crust in the Alps

The Adula Nappe in the Central Alps and the Pohorje Nappe in the Eastern Alps are among the highest-pressure metamorphic complexes in the Alps. In both cases, Variscan continental crust containing post-Variscan intrusions was subducted, during the Cenomanian-Turonian in the case of Pohorje and during the Eocene in the case of Adula.

The Pohorje Nappe is exceptional in that ultrahigh pressures of 3.0 to 4.0 GPa are recorded by different rocks contrasting in rheology: competent lenses of kyanite eclogite and garnet peridotite as well as the surrounding incompetent matrix of diamond-bearing paragneiss. If pressure had been strongly non-lithostatic, rheologically different rock types would be expected to record different pressures. This is not the case, which rather suggests near-lithostatic pressure and, consequently, subduction to >100 km depth. Lu-Hf ages for UHP metamorphism in eclogite and garnet peridotite are similar (c. 96–92 Ma). Paragneiss yielded Permian to Triassic zircon cores and Cretaceous (c. 92 Ma) rims grown during UHP metamorphism. Hence, the rocks were subducted and exhumed together as a coherent, although strongly deformed unit.

The Adula Nappe originated from the southern passive continental margin of Europe. It was buried in and exhumed from a south-dipping subduction zone after Europe-Adria continent collision. Previous interpretations as a tectonic mélange were based on the mixture of gneiss with eclogite and garnet peridotite lenses. However, the eclogites also record an older (Variscan) metamorphism and thus do not represent Mesozoic oceanic crust but pre-Alpine continental basement, just like the gneisses. The Alpine subduction culminated around 37 Ma. Alpine metamorphic pressures show a strong gradient from c. 1.2 GPa at the front of the nappe in the North to >3 GPa in garnet peridotite and eclogite in the southernmost part (e.g. Alpe Arami), over a north-south distance of only c. 40 km. In contrast to Pohorje, indications of UHP metamorphism have not yet been found in the gneissic matrix surrounding eclogite and peridotite. During exhumation, the nappe was intensely sheared and folded but stayed coherent and did not mix with the surrounding units.  The exhumation of the Adula from deep in the subduction zone is recorded by mylonitic shearing in the gneissic matrix. Structures, strain, and textures indicate strongly three-dimensional, non-plane-strain flow. Differential loading, not buoyancy, is proposed to have caused the exhumation.

The main results from these two case studies are: (1) Subduction of continental crust to mantle depth is real and not a misinterpretation of non-lithostatic pressure; (2) not all subducted units are mélanges but some stay coherent during subduction and exhumation.