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Paleozoic to Oligocene metasedimentary rocks present in the Alpi Apuane region of the Northern Apennines, Italy, have been sequentially deformed during a Tertiary progressive deformation. In an attempt to date the individual deformation episodes, over 50 conventional K-Ar and 1140 Ar/39Ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known. Mineralogy, X-ray diffractometry, and thin-section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3–4 kbars and 300–400°C during all deformational phases. Pre-existing micas were variably crenulated during each subsequent deformational phase. Both K-Ar and 40Ar/39Ar analyses were carried out on 0.6-2μm, 2–6 μm and 6–20 μm size separates of the phengitic white mica. Although the K-Ar apparent ages range from 11 to 27 Ma and are consistent with available stratigraphic constraints, the 40Ar/39Ar age spectra display variable internal discordancy. These isotopic data indicate that: (1) both the K-Ar and 40Ar/39Ar total-gas ages decrease as the degree of crenulation increases; (2) the K-Ar and 40Ar/39Ar total-gas ages decrease as grain size decreases; (3) for each sample, characteristics of the 40Ar/39Ar age spectra depend upon grain size, with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and (4) phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures. The isotopic results are interpreted to indicate that the major deformation phase (D1) occurred at approximately 27 Ma with subsequent pulses ending by c. 10 Ma. These results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10−15 and 10−14 s−1. A 27 Ma age indicates Mid-Oligocene initiation of plate tectonic activity in the Western Mediterranean and concomitant deformation in the Northern Apennines.