ALBERT

Description: 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.

Description: The Tyrrhenian basin has been formed by extension of overriding continental lithosphere driven by roll back of the Ionian slab across the mantle. The basin is not actively extending but the tectonic structure provides information of the processes that controlled rifting and formation of conjugate margins. The basin opened from west to east, with rifting stopping after progressively larger stretching factors from north to south. The northern region stopped opening at extension factors about 1.8. Towards the south extension continued until full crustal separation that produced first intense magmatism that subsequently was followed by mantle exhumation. The final structure displays two conjugate margins with structures that evolved from symmetric to asymmetric as extension rates increase and a complex tectonic structure in between. The basin provides a natural laboratory to investigate a full rift system with variable amounts of extension. We present observations from a two-ship wide-angle (WAS) and multichannel reflection seismic (MCS) experiment that took place in spring 2010. The experiment took place on two legs: The first leg with Spanish R/V Sarmiento de Gamboa and Italian R/V Urania collected five WAS profiles striking E-W across the entire basin recorded on ocean bottom seismic stations and land stations with a 4800 c.i. G-II gun array as source. The second leg with R/V Sarmiento de Gamboa collected 16 MCS profiles (about 1500 km) using a 3.75 km-long streamer and a 3100 c.i. G-II gun array as source. MCS profiles were shot coincident with WAS profiles. WAS – MCS transects were located in regions with different amount of extension the study the full structure including the two conjugate margins. Additional MCS lines were shot concentrated in the region where mantle exhumation has taken place. The seismic information is placed in a 3D context with the integration of the multibeam bathymetry that covers the entire basin. We present the interpretation of the tectonic structure from MCS images and bathymetry and the calibrated stratigraphy of the basin that gives information of timing, duration, and amount of the tectonic extension for the different transects. We compare those results with the final P-wave velocity models from the five WAS profiles that supply information on the nature of the crust. Each transect provides information of the relationships among extension rates, crustal thickness, nature of the crust, and style of deformation. This information allows to interpret mechanisms of deformation, to infer the importance of magmatism in the rifting process, and to interpret the changes leading of mantle exhumation. Furthermore, the data provide insight in the process of formation of the structure conjugated margins.

Description: We present 2-D seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter-plate boundary of the Nicaragua convergent margin along two wide-angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake. The trench-perpendicular profile runs over a seamount subducting under the margin slope, at the location where seismological observations predict large coseismic slip. Along this profile, the igneous basement shows increasing velocity both with depth and away from the trench, reflecting a progressive decrease in upper-plate rock degree of fracturing. Upper mantle-like velocities are obtained at approximate to 10 km depth beneath the fore-arc Sandino basin, indicating a shallow mantle wedge. A mismatch of the inter-plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by approximate to 15% velocity anisotropy, probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites. The presence of a shallow, partially serpentinized mantle wedge, and the fracture-related anisotropy are supported by gravity analysis of velocity-derived density models. The downdip limit of inter-plate seismicity occurs near the tip of the inferred mantle wedge, suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge. Near the trench, the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture, which could explain its tsunamigenic character.