Tectonic Structure of the Central Tyrrhenian Sea from wide-angle and near vertical seismics and gravity modelling

Abstract

We use coincident wide-angle (WAS), multichannel seismic reflection (MCS) images and gravity data acquired with the MEDOC cruise in 2010 to characterize the crustal domains and tectonic structure across the Tyrrhenian basin. We present a ~450 km-long, E-W-trending transect, which crosses the entire basin, from Sardinia (40N), across Sardina basin, the Cornaglia Terrace and the deep Magnaghi and Vavilov basins, to the Campanian margin (Italy). The joint interpretation of the WAS model and time-migrated MCS profile give information to understand the rifting phases leading to continental break-up and mantle exhumation. The WAS data , recorded on 26 OBH/S (Ocean bottom hydrophones/ seismometers) and 5 land stations, were modelled to obtain a P-wave velocity model of the basin and the geometry of the crust-mantle boundary by joint refraction and reflection travel-time tomography. The statistical uncertainty of the model parameters has been estimated following a Monte Carlo-like approach. Subsequently, a velocity-derived density model using existing relationships for different rocks was used to infer the composition of domains that fit gravity data being consistent with the velocity model. The model display abrupt lateral heterogeneity, showing four crustal domains based on velocity gradients. From West to East, the first domain consist of a ~23 ±2 km-thick continental crust beneath Sardinia and its shelf, with a mean velocity of 6.5 ±0.3 km/s. Eastwards, the crust thins from 22 ±2 km to 12 ±1 km in ~140 km below the Sardinia basin. This second domain is interpreted as a highly extended continental crust, containing numerous faults imaged in the coincident MCS profile. The third domain, in the central part of the profile, includes basins under the deepest water depth, and is interpreted as floored by exhumed mantle. In this domain, no crust-mantle reflections are identified, neither in the WAS data nor in the MCS images. Here, the velocity increases rapidly from 2.6 ±0,1 km/s at the sea-floor to ~7.8 ±0,15 km/s at ~5 km below. The vertical velocity gradient is twice larger than typical for oceanic Layer 2, and consistent with that observed in regions of mantle exhumation like the West Iberian Margin. In this third domain, we find three conspicuous velocity anomalies located under large volcanic seamount, formed by Upper Pliocene and Middle Pleistocene extension-related magmatism of the Magnaghi seamount, D'Ancona Ridge and Vavilov seamount, respectively. In the Eastern segment of the profile, beneath the Campanian margin, there are well-defined crust-mantle reflections in both WAS data and MCS profile, displaying a progressive thickening of continental crust towards mainland. The velocity gradient in this fourth domain is similar to that of the highly extended continental crust of the second domain, which approximately corresponds to its conjugate margin. Based on these seismic observations we conclude that, in this part of the Tyrrhenian basin, extension occurred slowly enough to exhume mantle rocks without producing significant synchronous magmatism that generated well-defined oceanic crust