J. H. Monteiro

Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise

A new deep water mud volcano field (between 2000 and 3500 m water depth) was discovered in the deep South Portuguese margin, as well as several new mud volcanoes in the South Spanish and Western Moroccan margins of the Gulf of Cadiz, during the TTR-10 (Training Through Research, UNESCO/IOC) cruise, in July/August 2000. This work followed the discovery of a large mud volcano field in the Gulf of Cadiz, first investigated during the TTR-9 cruise [Gardner (2001) Geophys. Res. Lett. 28, 339–342; Kenyon et al. (2000) IOC, Technical series no. 56]. The discoveries were made based on a SEAMAP side-scan sonar mosaic and multibeam bathymetry (SEABEAM) collected in the area by the Naval Research Laboratory (NRL), Washington, DC, USA, in 1992, kindly released for this purpose. Single-channel seismics, long-range side-scan sonar (OKEAN), TV-controlled grab, hull-mounted 3.5-kHz profiler and coring were used to investigate several seafloor features observed on the side-scan sonar imagery, in the South Iberia, Spanish and Moroccan margins of the Gulf of Cadiz, which were confirmed to be mud volcanoes. The typical structures related to fluid venting in the Gulf of Cadiz are essentially represented by conical mud volcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m. Some of these structures appear to be aligned along major conjugate NE–SW and NW–SE trending faults that can be identified on the side-scan sonar imagery. The new field discovered in the South Portuguese margin is the deepest in the Gulf of Cadiz area and includes three new mud volcanoes – Bonjardim, Olenin and Carlos Ribeiro – which seem to be quite active, with near-surface gas hydrate occurrence and a high saturation in H2S and hydrocarbon gases (mainly methane) in the mud breccia and overlying pelagic sediments. Gas hydrates were recovered from the Bonjardim mud volcano. An intensely gassified mud breccia, with one fragment of semi-consolidated claystone with a thin bituminous veneer at the surface, was recovered from the Carlos Ribeiro mud volcano. The fauna recovered consists mainly of pogonophoran worms belonging to several species and undetermined species of Foraminifera. Three new mud volcanoes were also discovered in the NW Moroccan margin: Rabat, Student and Jesus Baraza. These show a richer fauna that includes several species of molluscs, polychaetes, pogonophoran worms, crustaceans, echinoderms and some fragments of dead coral (Madrepora and Lophelia). Carbonate crusts were recovered from the Student mud volcano. The Ginsburg mud volcano, discovered during the previous TTR-9 cruise, was revisited and gas hydrates recovered once again. A new mud volcano was also discovered in the Spanish margin, Tasyo, where evidence was found of coral build-ups on the hard substratum of the mud volcanic edifice.

Tsunamigenic-seismogenic structures, neotectonics, sedimentary processes and slope instability on the southwest Portuguese Margin

Tectonically active structures prone to cause devastating earthquakes and tsunamis, e.g. the Lisbon 1755 earthquake, were investigated during the UNESCO/IOC Training Through Research-10 (TTR-10) cruise on the southwest Portuguese Continental Margin using single channel seismic profiles, a 3.5-kHz hull-mounted sea-bottom profiler, 10-kHz OKEAN long range side-scan sonar, 30-kHz ORETECH deep-towed side-scan sonar, and a high-resolution deep-towed sea-bottom profiler. These data allowed the definition of new active faults and the establishment of morphological criteria for the classification of active faults in the study area. Landslides associated with the activity of a major tectonic structure, the Marques de Pombal Fault, and other areas with clear signs of mass wasting phenomena were mapped. A slope-to-basin sedimentary system comprising 21 sedimentary ridges up to 20 km long was mapped and described. It was found that the sediments are mainly transported into the deep basins by mass transport processes across the steepest fault scarps forming a channel–levee system, while gravitational slides/slumps dominate the shallower slopes. The sedimentary ridges with an elevation of 40–50 m (50–60 ms TWT) above the seafloor are imaged on the high-resolution seismic profiles as an alternation of high and low amplitude reflectors. It is shown that the Pereira de Sousa Fault, its plateau and the Principes de Avis Plateau are experiencing uplift according to sedimentary and morphological criteria.

Mesozoic–Cenozoic evolution of North Atlantic continental-slope basins: The Peniche basin, western Iberian margin

New regional (two-dimensional) seismic reflection data, published Deep-Sea Drilling Project–Ocean Drilling Program reports, and unpublished shallow-offshore well information characterize the Mesozoic–Cenozoic evolution of the western Iberian continental slope north of 3845N. Two distinct sectors bounded by first-order transfer faults exist between the Galicia Bank and the Nazar fault. The northernmost sector 1 is filled by Triassic–Aptian (prebreakup) sequences, reaching more than 3.5 s two-way traveltime (TWTT) in thickness in distinct half grabens. Salt pillows, salt ridges, minibasins, and salt-detached overburden faults were generated during the Mesozoic and reactivated in the Cenozoic. Sector 2 shows Triassic–Jurassic units more than 2.0 s TWTT thick, underlying east-tilting half grabens of Early Cretaceous age. Salt structures in this sector evolved into mature salt diapirs. Postbreakup units are up to 2.0 s TWTT thick in both sectors. The evolution of the study area replicates evolutionary settings that have previously been proposed for nonvolcanic passive margins. However, some distinct features are noted: (1) widespread Triassic–Berriasian units deposited over rotated tilt blocks represent the early rifting stage; (2) Early Cretaceous subbasins showing rift-climax units, most likely formed during the advanced rifting stage, are spatially constrained to an approximately 100-km (62-mi)-wide region stretched along the continental slope; and (3) listric blocks and their associated low-angle (deep) detachment faults, formed on the distal margin during the advanced rifting and transition to sea-floor spreading stages, show no developed rift-climax units above them. From the early rifting stage onward, Mesozoic faults and halokinetic structures induced local differences in the thickness and character of seismic facies. Cenozoic (Alpine) tectonism promoted the reactivation of older Mesozoic structures.

Cenozoic tectono-sedimentary evolution of the western Iberian margin

Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN) and seismic reflection data are used to characterise the Cenozoic tectono-sedimentary evolution of the Porto, Lisbon and Alentejo continental margins, offshore Portugal, where the presence of submarine fans, slope gullies, submarine canyons and seamounts was confirmed. Sediment drifts, some of probable contouritic origin, were recognised over the upper continental slope (500–1000 m) and surrounding the Vigo seamount. Seven echo types offshore Porto (types IA-1, IB, IB-2, IC-2, IIA, IIB, IIIA) and five echo types offshore Lisbon (types IB, IIA, IIB, IB-2, IIIA) were identified on 3.5-kHz profiler data. In addition, 11 Meso–Cenozoic seismic units off Lisbon and Alentejo, plus three post-Turonian seismic packages offshore Porto were interpreted and dated by well, dredge and DSDP/ODP stratigraphic data. During the Cenozoic, the tectono-sedimentary evolution of the studied areas depended on their position in relation to the locus of compression in Iberia. Accommodation space and sediment pathways varied in relation to distinct pulses of uplift or subsidence occurring at different times in the three studied regions. The Porto and Lisbon margins record extensional collapse respectively after the early Eocene and early Chattian, interrupted by short episodes of uplift related to distinct tectonic phases. As a result, gravity flows dictated deposition on these margins in most part of the Cenozoic. Seamounts and halokinetic structures controlled deposition on the Porto margin by inducing topographic barriers to the westward progradation of slope-derived sediment. The relative proximity of the Alentejo margin to the Azores–Gibraltar Fracture Zone resulted in folding and exposure during the middle Oligocene, but subsidence after the early Chattian generated a palaeoslope buried under Neogene units. Oligocene and Burdigalian canyon incision offshore Alentejo preceded the emplacement of modern channels during the Pliocene. These presently transport sediment derived from the shelf and major onshore drainage catchments into abyssal areas.

The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia

New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobaca sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobaca sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in fault-bounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded sub-basins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems.

Structural evolution and timing of continental rifting in the Northeast Atlantic, West Iberian Margin

Regional (2D) seismic-reflection profiles, outcrop and borehole data are used to indicate that the bulk of Late Jurassic-earliest Cretaceous subsidence occurred in the present-day continental slope area. Five (5) principal regressive events (and their correlative basal unconformities) reflecting tectonic uplift and relative emersion in proximal basins are correlated with major rift-related tectonic events on the deeper margin. A significant portion of the west Iberian lower-plate margin was uplifted and eroded during the last stages of continental rifting. Such process was repeated at different times (and in different areas) as the locus of rifting and continental break-up migrated along the future passive margin. As a result, in west Iberia two distinct rift axes are recognised, a first axis extending from the Porto Basin to the Alentejo Basin and a second axis located on the outer proximal margin north of 380 30’N. We propose the late-rifting phases of tectonic quiescence, widespread erosion and sediment progradation on the inner proximal margin as marking the abandonment of extensional basins east of a major Slope Fault System (SFS), and the subsequent onset of syn-rift extension on the outer proximal margin. However, rift-related units of Triassic (and older?) to Middle Jurassic age are well represented on the outer proximal margin. This fact indicates that crustal extension on continental slope basins of west Iberia consisted of a prolonged process in which the last rifting episode is structurally imprinted over older – but not less important – rift episodes.

Reassessment of sedimentary evidence for deep contour currents west of Iberia

The 6.5 kHz GLORIA long range sidescan sonar images from west of Portugal were previously interpreted (Fig. 1) as showing evidence for mud waves and for drifts extending northwards from basement highs (Roberts and Kidd 1984; Gardner and Kidd 1987).A deep, northwards flowing contour current, existing over a long period at depths of between about 2,000 m and 4,000 m, was invoked to explain these observations. No profiles across these supposed contourite mud waves were published, although a profile across supposed mud waves at about 3,500 m in the Gulf of Cadiz (further south) had been shown (Gardner and Kidd 1983). Even this example could be due to downslope processes, as shown by (2000) for waves on other slopes. Open image in new window Fig. 1. Trends of features mapped from early GLORIA sidescan sonar data (Roberts and Kidd 1984) compared with the area of the OKEAN survey. Later analysis of the same GLORIA data by Gardner and Kidd (1987) differentiated channel-associated features and edges of seamounts from the majority of lineations, which were attributed to contourite mud waves. The Dom Carlos valley trend and the edges of seamounts were also seen on the OKEANdata (Fig. 2) but none of the other lineat ions were present on either OKEAN or high resolution seismic profiles Open image in new window Fig. 2. Mosaic of OKEAN long range sidescan sonar data. High backscatter shown in lighter tones