New Constraints on the Evolution of the Inner Northern Apennines by K-Ar Dating of Late Miocene-Early Pliocene Compression on the Island of Elba, Italy

The Northern Apennines (NA) orogenic wedge formed during Oligocene-Miocene convergence
and westward subduction of Adria beneath the European Plate. Extension ensued in the Mid-Late Miocene in
response to Adria roll-back, causing opening of the back-arc Northern Tyrrhenian Sea. Whether extension
continues uninterrupted since the Mid-Late Miocene or it was punctuated by short-lived compressional
events, remains, however, uncertain. We used the K-Ar method to date a set of brittle-ductile and brittle
deformation zones from the Island of Elba to contribute to this debate. We dated the low-angle Zuccale Fault
(ZF), the Capo Norsi-Monte Arco Thrust (CN-MAT), and the Calanchiole Shear Zone (CSZ). The CN-MAT and
CSZ are moderately west dipping, top-to-the-east thrusts in the immediate footwall of the ZF. The CSZ
slipped 6.14 ± 0.64 Ma (<0.1 μm fraction) and the CN-MAT 4.90 ± 0.27 Ma ago (<0.4 μm fraction). The ZF,
although cutting the two other faults, yielded an older age of 7.58 ± 0.11 Ma (<0.1 μm fraction). The ZF
gouge, however, contains an illitic detrital contaminant from the Paleozoic age flysch deformed in its
hanging wall and the age thus is a maximum faulting age. Removal of ~1% of a 300-Ma-old contaminant
brings the ZF faulting age to <4.90 Ma. Our results provide the first direct dating of brittle deformation in the
Apennines, constraining Late Miocene-Early Pliocene regional compression. They call for a refinement of
current NA geodynamic models in the framework of the Northern Tyrrhenian Sea extension.

- We provide the first isotopic geochronological constraints on brittle deformation in the NA by illite K-Ar dating of brittle fault rocks - A combined structural-geochronological approach constrains a Late Miocene-Early Pliocene regional compressive stress state