Back-arc extension dated from natural time markers across the Western Tyrrhenian Basin

The rifting of the Eastern Sardinian margin is considered to have occurred during the Neogene, by back-arc extension related to the eastward migration of the Apennine subduction system, leading to the development of the Tyrrhenian Basin, Western Mediterranean Sea. The locus of the extension during the rifting is interpreted to have occurred first in the East-Sardinia Basin (proximal margin), and then in the Cornaglia Terrace (distal margin). However, the dynamics of the Western Tyrrhenian Basin during the rifting is still largely undated, and the kinematics of the development of the different domains of the Eastern Sardinian margin remains poorly understood. This is due to the sparsity of dated rock samples within the basin, mainly recovered locally during ODP Leg 103, and because the timing of activity of the structures of the basin has been addressed so far using regional data, whose resolution is insufficient to observe the timing of the extension at the scale of a fault plane.

In this study, we use a 2400 km-long high-resolution seismic-reflection dataset acquired along the Eastern Sardinian margin during the “METYSS” research cruises in 2009 and 2011, and specifically designed to observe the block-bounding faults and the syn-rift and post-rift units. We interpret the syn-tectonic markers of the crustal deformation across the Eastern Sardinian margin, and we use the seismic markers of the Messinian Salinity Crisis (MSC) that provide exceptional and accurate natural time markers, to estimate the age of faults development and understand their evolution within the Western Tyrrhenian Basin.

Our observations demonstrate that the rifting was polyphased across the Western Tyrrhenian Basin, and that syn-rift extension was active on the proximal margin long before the MSC, whereas the distal Cornaglia Terrace developed only a short time before the MSC. Surprisingly, our interpretations also evidence significant post-rift reactivation of some structures of the Eastern Sardinian margin. These reactivations, formerly considered to be very minor or absent in the Western Tyrrhenian Basin, started in the Pliocene and occurred up to very-recent times along local fault-planes, as shown by the deformation of the shallowest Pleistocene layers and the seafloor.

Our results permit to precise the timing of the syn-rift and post-rift evolution of the Eastern Sardinian margin. Given the rarity of natural time makers in offshore sedimentary basins to address the dynamics of basins development, we expect our results to provide useful comparisons to study the kinematics of extension at other back-arc basins worldwide.