Comment on "Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy", by G. Vignaroli, C. Faccenna, L. Jolivet, C. Piromallo, F. Rossetti

We discuss the model for the reversal of the subduction polarity in the Alps-Apennine system, proposed by Vignaroli et al. [Vignaroli, G., Faccenna, C., Jolivet, L, Piromallo, C.. Rossetti, F., 2008. Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy. Tectonophysics 450, 34-50]. This model integrates recently published tomographic data with a re-interpretation of the surface geology of the Voltri Massif to suggest a Tertiary extensional regime of nappe excision. The structural reconstruction proposed by Vignaroli et al. [Vignaroli, G., Faccenna, C., Jolivet, L, Piromallo, C., Rossetti, F., 2008. Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy. Tectorrophysics 450,34-50] individuates in the Voltri Massif two major tectonic complexes, an eclogite-facies unit (the Lower Tectonic Complex, LTC) underneath a low-grade tectonic element (the Upper Tectonic Complex. UTC), which attained significantly lower P-T conditions. The two complexes are separated by a major extensional contact. Most of the arguments exposed by Vignaroli et al. [Vignaroli, G., Faccenna, C., Jolivet, L, Piromallo, C., Rossetti, F., 2008. Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy. Tectorrophysics 450, 34-50] to substantiate such a nappe pile reconstruction and the proposed extensional regime do not properly consider the field data, the datasets available in the literature and the published geological maps. Also, the available P-T data and metamorphic histories of rocks pertaining to the LTC and UTC do not provide evidence for a significant pressure jump between the two complexes. In addition the expected thermal effects achieved during metamorphism in a regime of crustal thinning and astenospheric uprise do not occur in the study area. We conclude that the evidence supporting the geodynamic model proposed by Vignaroli et al. [Vignaroli, G., Faccenna, C., Jolivet, L, Piromallo, C.. Rossetti, F., 2008. Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy. Tectonophysics 450, 34-50] is poorly convincing and not supported by the field and petrologic observations: both the re-interpretation of the surface geology and the resulting crustal reconstruction have several major flaws. (C) 2008 Elsevier B.V. All rights reserved.