ALBERT

Description: Extension of the continental lithosphere leads to the formation of rift basins and ultimately may create passive continental margins. The mechanisms that operate during the early-stage of crustal extension are still intensely debated. We present the results from coincident multichannel seismic and wide-angle seismic profiles that transect across the northern Tyrrhenian Sea basin. The profiles cross the Corsica Basin (France) to the Latium Margin (Italy) where the early-rift stage of the basin is well preserved. We found two domains, each with a distinct tectonic style, heat-flow and crustal thickness. One domain is the Corsica Basin in the west that formed before the main rift-phase of the northern Tyrrhenian Sea opening (~8-4 Ma). The second domain is rifted continental crust characterized by tilted blocks and half-graben structures in the central region and at the Latium Margin. These two domains are separated by a deep (~10 km) sedimentary complex of the eastern portion of the Corsica Basin. Travel-time tomography of wide-angle seismic data reveals the crustal architecture and a sub-horizontal 15-17 ± 1 km deep Moho discontinuity under the basin. To estimate the amount of horizontal extension we have identified the pre-, syn-, and post-tectonic sedimentary units and calculated the relative displacement of faults. We found that major faults initiated at angles of 45-50° and that the rifted domain is horizontally stretched by a factor of β=1.3 (~8-10 mm/a). The crust has been thinned from ~24 to 17 km indicating a similar amount of extension (~30%). The transect represents one of the best imaged early-rifts and implies that the formation of crustal-scale detachments, or long-lived low-angle normal faults, is not a general feature that controls the rift initiation of continental crust. Other young rift basins, like the Gulf of Corinth, the Suez-Rift or Lake Baikal, display features resembling the northern Tyrrhenian Basin, suggesting that half graben formations and distributed homogeneous crustal thinning are a common feature during rift initiation.