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Rifting in the northern Tyrrhenian Sea Basin: a multichannel and wide-angle seismic study

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Disciplines
  • Earth Science
  • Geography
  • Mathematics
  • Medicine

Abstract

Rifted continental margins or basins are the results of continental lithospheric extension and a fundamental process in geodynamics. Extension is expressed by normal faulting and block rotation in the brittle upper crust and ductile deformation in the lower crust. Conjugate rifted margins are often asymmetric and reveal different amounts of crustal stretching and thinning. Shallow dipping crustal-scale detachment faults are thought to be the controlling mechanism of this asymmetry, although the development of such features is debated. This PhD thesis presents the results of a seismic campaign from the northern Tyrrhenian Sea Basin. It is a continental back-arc basin exhibiting rift features preserved at the early stages of rifting. Two W-E orientated multichannel seismic profiles (MCS) cross the region between Corsica, Sardinia and mainland Italy. The data are processed up to prestack-depth and time migrated images in order to investigate the style of faulting and sedimentary structures. Coincident refraction- and wide-angle seismic data were acquired using OBH-stations. Joint refraction and reflection tomography reveal the crustal architecture and thickness. The MCS image of the northern profile shows that the geometry of upper Tortonian to Early-Pliocene sediments (8-4 Ma) reflects the active rift phase. Fault displacements are localized on major normal faults dipping mainly to the east (30°-40°) forming a succession of asymmetric half-grabens. Major blocks generate the majority of the entire horizontal extension which is estimated to be ~30%. Faults seem not to be linked at the brittle-ductile transition zone (~3-4 km). The tomographic model shows, that the crust has thinned from ~24 km to a thickness of ~17 km (ß=1.3). The thinning matches the estimated amount of horizontal extension (30%). Towards the southern transect, the width of the basin increases. Seismic data display an asymmetric margin configuration. This is expressed by crustal thinning from ~24 km in the west (Sardinia) to ~11 km in the east (ß-factor=2.2). Moreover, the Sardinia Margin comprises large fault-bounded blocks where the crust is relatively thick. Blocks of smaller size are observed further east, where the crust has thinned more. This zone (~ 100 km wide) is characterized by reduced p-wave velocities which correlates with intensive faulting as imaged on the MCS section. A reliable estimation of the amount of brittle extension is not possible due to the presence of complex small-scale faults. However, the thinning-factor of the lower crust matches the thinning-factor of the entire crust. This implies that the amount of brittle extension should be similar. In this zone, reflections within sedimentary sub-basins are related to basalt flows. A narrow zone with similar characteristics is also observed on the northern line. In the vicinity of the Sardinia Margin, post-rift sediments indicate that tectonism has ceased before Early-Pliocene time, whereas young faults cut the seafloor in the center and in the eastern region (near mainland Italy). The different ages of syn-tectonic sequences indicate an apparent W-E rift propagation. The comparison of both transects shows a NW-SE orientated crustal thinning resulting in mantle exhumation in the Vavilov Basin. Diminishing block sizes and concomitant crustal thinning reflect increasing strain rates in NW-SE direction. The data provide no evidence for potential long-lived detachment faults controlling the development of the asymmetric rift configuration. However, it can be explained by the regional plate-tectonic setting and subduction roll-back. Rifting propagated toward the east, following the retreatment of the subduction zone. The pole of rotation is located north of the basin. Thus, with increasing distance from the pole toward the south, stretching rates increase. This led to the differences of syn-sedimentary ages, block sizes and NW-SE crustal thinning.

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