Tectono-stratigraphic modelling of the Sardinian margin of the Tyrrhenian Sea

Rifting of the east Sardinia passive margin was characterised by a southeastward migration of the site of extension through time. In the first stage of rifting (9-7 Ma), lithospheric thinning was mainly localised near Sardinia; crustal and subcrustal thinning factors averaged across the whole transect were 1.12 and 1.5, respectively. At 7 Ma, i.e. immediately before the onset of the Messinian salinity crisis, the basin floor was at depths of up to 1000 m in the west and less than few hundred metres in the east. During the second stage of rifting (between 7 and 5 Ma), the site of main extension was located in the central part of the future margin with average crustal and subcrustal thinning factors of 1.18 and 1.63, respectively. At 5 Ma, after the termination of the Messinian salinity crisis, the morphology of the basin floor had changed substantially with depths in general between 700 and 1000 m. Extension, during the last rifting stage before oceanic crust formation at 3.5 Ma, was mainly concentrated in the southeasternmost part of the profile; average crustal thinning factors increased dramatically to 1.95, while the subcrustal one remained almost unchanged (1.64). The analysis of temporal changes of thinning factors shows that the opening of the Tyrrhenian oceanic basin can be adequately explained with a depth-dependent pure-shear geometry with crustal and subcrustal thinning being accommodated on the same vertical column. Subcrustal thinning factors in the initial stages of rifting were higher than the crustal ones but the two become comparable approaching break-up. The regional compensation of changing loads coupled with a very deep depth of necking, is consistent with the observed subsidence trend and basin stratigraphy.