Conclusive Remarks
Present knowledge on structure and petrology of the Alpine-Apennine ophiolitic peridotites from the Jurassic Ligurian Tethys basin shows that the evolution of the lithosphere-asthenosphere system during the rifting stages of the basin was driven by interdependent and mutually enhancing tectonic and magmatic processes. Far-field tectonic forces, already active during Triassic times, caused the progressive extension and thinning of the continental lithosphere and the adiabatic upwelling and decompression melting of the asthenosphere starting from early Jurassic times. The subcontinental lithospheric mantle underwent significant extension by means of a network of extensional shear zones that caused its progressive exhumation and final sea-floor exposure at the marginal OCT settings of the basin.
MORB melts from the asthenosphere percolated by porous flow through the mantle lithosphere along the axial zone of the extending system. Melt-peridotite interaction caused significant modification of the lithospheric mantle and the migrating melts were entrapped in the lithosphere by interstitial crystallization. Melt reactive percolation and entrapment in the lithospheric mantle during Jurassic times was responsible for the extreme heterogeneity of the lithospheric mantle which was exhumed and exposed during more advanced Late Jurassic oceanization stages at more distal intra-oceanic settings of the basin.
The entrapment of asthenospheric melts in the shallow lithospheric mantle during the rifting stages prevented melt extrusion during the continental break-up, which resulted in the non-volcanic nature of the passive rifted margins. Lithosphere melt impregnation and asthenosphere-lithosphere interaction induced significant reduction of the total strength of the sub-continental mantle along the axial zone of the extending system that, most probably, enhanced the transition from ultra-slow diffuse lithosphere extension to focused oceanic spreading.
The known ophiolite sequences from the distal intra-oceanic settings are still characterized by sub-continental lithospheric upper mantle, strongly modified by the melt-peridotite interaction they experienced during pre-oceanic rifting. In fact, the depleted spinel peridotites of the distal settings are reactive peridotites formed by melt-peridotite interaction and do not represent refractory residua after oceanic-type asthenosphere partial melting.
Acccordingly, the Ligure-Piemontese basin most probably did not reach a mature oceanic stage characterized by the sea-floor exposure of cogenetic refractory residua after Jurassic oceanic melting and the related melts.