Abstract

Present knowledge on structure and petrology of the various masses of Alpine-Apennine ophiolitic peridotites provides evidence that Triassic-Jurassic rifting and spreading in the Ligurian-Piemontese domain was accomplished through successive stages of lithosphere evolution well recorded in the mantle peridotites.

The early stages of rifting, active during Triassic times, were dominated by overall extension and thinning of the continental lithosphere and exhumation of the sub-continental lithospheric mantle via km-scale extensional shear zones. This represents a tectonic-dominated stage.

Subsequent stages of rifting, starting from Early Jurassic times, were characterized by asthenosphere adiabatic upwelling and decompression melting triggered by lithosphere extension and thinning. These rifting stages were dominated by the interaction of tectonic and magmatic processes. MORB-type melts from the asthenosphere percolated through the overlying lithospheric mantle along the axial zone of the future oceanic basin and large volumes of the extending mantle lithosphere were modified by melt-rock interaction.

Melt percolation induced the thermo-mechanical erosion of the mantle lithosphere resulting in the weakening and softening of the lithosphere. It was a controlling factor in the transition from distributed continental deformation to localised oceanic spreading. MORB magmas intruded the sub-continental mantle along the axial zone of the future oceanic basin forming km-scale gabbroic bodies.

The oceanic stage (i.e. formation of the oceanic lithosphere) was characterized by complete failure of the continental crust during Late Jurassic times, and by the direct exposure of mantle peridotites on the sea-floor. Sub-continental peridotites were exposed at ocean-continent transition (OCT) zones and melt-modified sub-continental peridotites at more internal oceanic (MIO) settings.

MORB magmas reached the sea-floor, extruded above the mantle peridotites to form the oceanic lithosphere (i.e. the association of sub-continental peridotites and Jurassic MORB basalts) of the Ligurian Tethys basin.

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