Mantle peridotites from the Alpine-Apennine ophiolites record different tectonomagmatic events depending on their palaeogeographic setting in the Jurassic Ligurian Tethys. Peridotites from Ocean-Continent Transition (OCT) zones provide information on the tectonic and metamorphic processes related to mantle exhumation during lithospheric extension. Peridotites from More Internal Oceanic (MIO) settings provide information on processes in the extending lithospheric mantle related to percolation of asthenospheric melts.
Field, structural and petrologic-geochemical data show that: (i) the subcontinental lithospheric mantle consisted of fertile spinel lherzolites equilibrated along a continental geotherm at T ∼ 1000 C; (ii) extension caused significant thinning of the lithosphere and concomitant adiabatic upwelling and decompressional melting of the asthenosphere; (iii) asthenospheric MORB-type melts migrated by diffuse porous flow through the mantle lithosphere; (iv) reactive interaction (pyroxene dissolution and olivine precipitation) between the continental mantle and the percolating melts caused significant thermal and compositional modifications of the lithospheric mantle, characterized by increasing temperatures (T > 1250 C); (v) further percolating melts underwent interstitial crystallization at T ∼ 1250 C, and caused significant impregnation and refertilization by basaltic components in the form of microgabbroic aggregates.
The abundance of reactive/impregnated rock types among the ophiolitic peridotites indicates that significant parts of the extending lithospheric mantle were asthenospherized as a consequence of lithosphere extension and thinning, asthenosphere partial melting and melt percolation. Accordingly, parts of the lithospheric mantle underwent chemical and thermal erosion by the percolating melts and attained asthenospheric rheological characteristics. This process resulted in a significant (up to one order of magnitude) decrease in Total Lithospheric Strength (TLS), down to ∼ 10E12 N/m, achieved in a relatively short time (< 5 Ma). The softening of the lithospheric mantle was equally effective in the case of bulk impregnation (extension by pure shear) as in the case of impregnation along trans-lithospheric shear zones (extension by simple shear). The former, however, requires the impregnation of large volumes of lithospheric mantle, and consequently extension along melt-softened shear zones crossing the mantle lithosphere is a more favourable process, as also evidenced by the structural features of many peridotite massifs.
The relatively rapid TLS decrease associated with asthenospherization of the lithospheric mantle is potentially sufficient to cause whole lithosphere failure under appropriate far-field tectonic forces, and therefore it is a factor to be considered in the transition from continental extension to sea-floor spreading, and in the formation of a-volcanic or poorly volcanic passive continental margins.
