Crustal thinning in the northern Tyrrhenian Rift: Insights from multichannel and wide-angle seismic data across the basin

1] Extension of the continental lithosphere leads to the formation of rift basins, or rifted continental margins if break-up occurs. Seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures, and different amount of extension and crustal thinning. Here, we compare two coincident wide-angle and multichannel seismic profiles across the northern Tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center. Tomographic inversion reveals that the crust has thinned homogeneously from ~24 km to ~17 km between the Corsica Margin and the Latium Margin implying a ?-factor of ~1.3-1.5. On the transect 80 km to the south, the crust thinned from ~24 km beneath Sardinia to a maximum of ~11 km in the eastern region near the Campania Margin (?-factor of ~2.2). The increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively towards the south-east. We interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting, as observed on MCS images. Locally, basalt flows are imaged intruding sediment in this zone and heat flow values locally exceed 100 mW/m2. Velocities within the entire crust range 4.0-6.7 km/s, which are typical for continental rocks and indicate that significant rift-related magmatic under-plating may not be present. The characteristics of the pre-, syn-, and post-tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting. In the western part of the southern transect, thick post-rift sediments were deposited in half-grabens that are bounded by large fault-blocks. Fault spacing and block size diminish to the east as crustal thinning increases. Recent tectonic activity is expressed by faults cutting the seafloor in the east, near the mainland of Italy. The two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure, to the asymmetric margins farther south. This structural evolution is consistent with W-E rift propagation and southward increasing extension rates.