Tracking Adria indentation beneath the Alps by detrital zircon U-Pb geochronology: Implications for the Oligocene-Miocene dynamics of the Adriatic microplate

摘要

The Adriatic microplate is a key player in the Western Mediterranean tectonic puzzle, but its Oligocene-Miocene dynamics are not yet fully understood. In fact, even though the timing and magnitude of Adriatic slab rollback and backarc extension in the Apennines have long been established, the timing of progressive Adria indentation beneath the Central Alps and major strike-slip motion along the Insubric fault are still poorly constrained. Here we tackle these issues by utilizing detrital zircon U-Pb geochronology on Adriatic foredeep turbidites, i.e., by comparing the geochronologic fingerprints of the exhuming tectonic domes of the Central Alps (Ticino and Toce subdomes) with those of the Oligocene-Miocene turbidites chiefly derived from their erosion. We analyzed 11 sandstone samples ranging in age from 32 to 18 Ma. The ratio between Variscan and Caledonian zircon grains (which are dominant in the Toce and Ticino subdomes, respectively) sharply increases at ca. 24-23 Ma. This major provenance change marks the westward shift of the Adriatic indenter beneath the Central Alps and the associated right-lateral activity of the Insubric fault. The coexistence of strike-slip motion at the northern boundary of the Adriatic microplate at ca. 24-23 Ma and trench retreat during scissor-type backarc opening to the west requires a near-vertical rotation axis located at the northern tip of the Ligurian-Provencal basin. We propose that the rotation axis position was controlled by the interaction between the European and the Adriatic slabs, which may have collided at depth by the end of the Oligocene, triggering the westward shift of the Adriatic indenter beneath the Central Alps.