Northwestern Mediterranean Basin Research Articles

Tertiary sedimentary history and structure of the Valencia trough (western Mediterranean)

We present here main results of the Common Depth Point (CDP) data acquired during the Valsis 2 Cruise in 1988 in the Valencia trough. The profiles are tied in with industrial well data and this correlation allows the sedimentary and structural history of the region to be deduced. The Valsis Cruise seismic profiles have been supplemented by a very dense grid of industrial seismic lines and these data permit us to establish an accurate depth to basement map. The formation of the initial grabens, coeval with those of the Gulf of Lions, is related to the Early Miocene opening of the northwestern Mediterranean basin and the Barcelona graben is filled by the same sedimentary layers, including evaporites, as that of the Provençal region. Nevertheless, the Valencia-Catalan grabens have been reactivated by young extensional tectonics which could be a consequence of the convergence of Africa relative to Europe. The Valencia trough is segmented by transfer faults which trend NW-SE. These faults, which have a more accentuated structural expression than the Valencia and Catalonia grabens, may act as transform faults separating the individual Balearic Islands. The transfer faults are in strike with volcanic ridges which have been sampled during the DSDP Leg 13. The dense seismic grid allows us to delineate several widespread volcanic features in the Valencia trough which have been active from the Early Miocene to the Pleistocene. However, we note that the volcanic features are mainly Miocene in age whereas the recent volcanism is restricted to a narrow zone (Columbretes Islands). The compressional tectonics which deformed the Balearic Islands does not appear to extend far towards the North. We delineate the compressional front north of Ibiza, but we failed to determine any thrust or fold north of Mallorca, whereas an extensional tectonics is evident.

Fluxes and transport of anthropogenic and natural polycyclic aromatic hydrocarbons in the western Mediterranean Sea

Surface sediments were collected in the Gulf of Lions and in the Rhône delta and analyzed for composition of polycyclic aromatic hydrocarbons (PAH). After isolation of lipids, the hydrocarbons were separated into fractions by high-performance liquid chromatography (HPLC) and fractions were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). Annual PAH fluxes were calculated for the northwestern Mediterranean basin. The fluxes of open-sea sediments varied from 164 μg m −2 year −1, for a station located far from the coast and influenced by atmospheric deposition, up to 1420 μg m −2 year −1 for stations directly influenced by the Rhône river inputs, whereas stations located in the Rhône delta were characterized by higher fluxes, up to 12 250 μg m −2 year −1. PAH concentrations decreased significantly with distance from the river mouth. Detailed analysis of PAH revealed the presence of anthropogenic compounds, pyrolytic and petrogenic, and naturally derived compounds. The distribution and ratios of specific compounds of PAH are discussed as a function of morphological and sedimentological features to evaluate and characterize the respective importance of atmospheric and Rhône-derived river inputs. Pyrolytic inputs from anthropogenic combustion processes were predominant at all studied stations. Compounds of natural origin were quantified, such as retene and related abietic acid precursors, and pentacyclic triterpenes derived from α- and β-amyrins. Anthropogenic PAH fluxes, determined for an open-sea sediment corresponding to the end of the nineteenth century (∼ 100 years ago), were three to four times less than the current flux; however, the retene flux was equal to or higher than that of today.

Deep structure of the north-western Mediterranean Basin: Results of a two-ship seismic survey

Data from thirty-two Expanding-Spread Profiles carried out in the Gulf of Lions and Ligurian Sea in September 1981 were used to study the deep structure of Western Mediterranean Basin. P-refractions and precritical reflections were observed, down to the Mohorovicic discontinuity, despite the presence of a halokinetic Messinian salt layer, and they have been used in the x—t domain to derive the velocity structure of both sedimentary cover and crust. Three different geological domains are distinguished: a thick continental domain, with a velocity of 6.2 km s −1 and a crustal thickness of more than 20 km, that is continued seawards by a thinned continental domain with a crustal thickness of 5–7 km only, and then in the center of the basin, a double-layered crust with velocities of 5.8 and 6.9 km s −1 and total thickness of about 5 km, interpreted as an oceanic domain, with basement at about 10 km below sea level. The Gulf of Lions margin and the Ligurian Sea are two contrasting areas: the thinned continental domain is very wide in the Gulf of Lions (100 km) and considerably reduced in the Ligurian Sea. A paleo-oceanic ridge appears to be present in the Gulf of Lions. The two basins are separated by a major fracture zone. Ante-rift sediments are observed in the Gulf of Lions.

Les affleurements de series sedimentaires pre-pliocenes dans le bassin mediterraneen nord-occidental

The DSDP site 372 (leg 42) proved the outcropping of Miocene layers off Menorca Island. But a reexamination of several multichannel seismic profiles shows that these outcrops are common in many continental rises of the Northwestern Mediterranean basin. Three phenomena act together to explain these features. First, we have many evidences of deep contour currents which sweep the foot of the continental slope and removed the Plio-Pleistocene deposits. Second, extensive erosion occurring during the Messinian event caused outcropping of the Miocene beds. Third, the continental rise with a thinned continental crust has a lower subsidence rate than the oceanic crust and the Miocene beds are flexured seaward.

Northwestern Mediterranean Sedimentary Basin from Seismic Reflection Profile

A Flexotir seismic reflection profile made in 1970 from the Gulf of Valencia to the Ligurian Sea defines the sedimentary formations and the main structural provinces in the northwestern Mediterranean basin. This basin has a sedimentary section 6,000-7,000 m thick and contains a Pliocene-Quaternary group 1,000-1,500 m thick with a velocity of 1.7-2.8 km/sec; an evaporitic group consisting of a bedded upper unit of Messinian (late Miocene) age 600 m thick with a velocity of 3.5 km/sec and a homogeneous lower unit of salt, 0-1,200 m thick with a velocity of 4.3 km/sec; an infrasalt group 1,000-4,000 m thick; and an acoustic basement visible only near the basin margins. In the southern Gulf of Lion the salt group, although thick, is not disturbed by diapirs, whereas farther east (Ligurian Sea) diapirs are highly developed, probably in connection with Pliocene faults. The infrasalt group is horizontal; its composition is unknown; its age could be as old as Oligocene in some areas. Two types of basin margins were revealed. In the first type (Provence), all the formations in the basin pinch out against the abrupt continental slope. In the second type (north Balearic zone, Corsica), between the continental shelf and the abyssal plain, there is a broad intermediate marginal zone. This marginal zone is characterized by a shallow continental basement, a thick infrasalt group localized in restricted basins, the absence of salt, and the presence of a Pliocene-Quaternary group similar to the one in the abyssal plain. There are two different types of structural boundaries; the first, now at great depth, limits the continuous infrasalt group and the salt layer, and the second corresponds to the present margin related to Pliocene-Quaternary faulting. The structural history of the basin apparently is not governed solely by Pliocene-Quaternary tectonics; foundering began earlier, probably as early as the Oligocene or early Miocene.