L. M. Pinheiro

Seabed morphology and hydrocarbon seepage in the Gulf of Cádiz mud volcano area: Acoustic imagery, multibeam and ultra-high resolution seismic data

Abstract Extensive mud volcanism, mud diapirism and carbonate chimneys related to hydrocarbon-rich fluid venting are observed throughout the Spanish–Portuguese margin of the Gulf of Cadiz. All the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome/accretionary complex units which were emplaced in the Late Miocene in response to NW-directed convergence between the African and Eurasian plates. The study area was investigated by multibeam echo-sounder, high and ultra-high resolution seismic profiling, dredging and coring. The structures observed on multibeam bathymetry, at water depths between 500 and 1300 m, are dominated by elongate mud ridges, mud cones, mud volcanoes and crater-like collapse structures ranging in relief from 50 to 300 m and size from 0.8 to 2 km in diameter. The main morphotectonic features, named the Guadalquivir Diapiric Ridge (GDR) and the Cadiz Diapiric Ridge (CDR), are longitudinally shaped diapirs which trend NE–SW and consist of lower–middle Miocene plastic marly clays. The GDR field and the TASYO field, which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development, are described in this paper. The GDR field is characterised by numerous, single, sub-circular mud volcanoes and mud cones. The single mud volcanoes are cone-shaped features with relatively gentle slopes of 3°–6°, consisting of several generations of mud breccia deposition with indications of gas-saturation, degassing structures and the presence of H2S. The mud cones have asymmetric profiles with steep slopes of up to 25° and contain large surficial deposits of hydrocarbon-derived carbonate chimneys and slabs. The TASYO field is characterised by an extensive concentration of small, sub-circular depressions, oval and multi-cone mud volcanoes and large sediment slides. Mud volcanoes in this area are characterised by moderate slopes (8°–12°), have bathymetric relief ranging from 100 to 190 m and consist of sulphide-rich mud breccia, calcite chimneys, carbonate crusts and chemosynthetic fauna (Pogonophora tube worms). We propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic/accretionary wedge. This stress results in the uplifting and squeezing plastic marly clay deposits. Additionally, the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon-enriched fluids to migrate.

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.

Thin crust at the western Iberia Ocean‐Continent transition and ophiolites

Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean-continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

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.

Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup

This book summarizes our present understanding of the formation of passive continental margins and their ocean–continent transitions. It outlines the geological, geophysical and petrological observations that characterize extensional systems, and how such observations can guide and constrain dynamic and kinematic models of continental lithosphere extension, breakup and the inception of organized sea-floor spreading. The book focuses on imaging, mapping and modelling lithospheric extensional systems, at both the regional scale using dynamic models to the local scale of individual basins using kinematic models, with an emphasis on capturing the extensional history of the Iberia and Newfoundland margins. The results from a number of other extensional regimes are presented to provide comparisons with the North Atlantic studies; these range from the Tethyan realm and the northern Red Sea to the western and southern Australian margins, the Basin and Range Province, and the Woodlark basin of Papua New Guinea. All of these field studies, combined with lessons learnt from the modelling, are used to address fundamental questions about the extreme deformation of continental lithosphere.

Compressional deformation at the ocean–continent transition in the NE Atlantic

Local zones of compressional deformation, spatially coincident with the ocean-continent transition. occur in the NE Atlantic off western Iberia, on the northern margin of the Bay of Biscay and in the southern Rockall Trough. The deformation zones are typically broad oceanward-facing monoclines several tens of kilometres in width. The amount of shortening is small, although the structures may also accommodate some strike-slip motion. Deformation peaked during the Mid-Late Eocene in Biscay. off Galicia Bank and in Rockall Trough, and in the mid-Miocene off western Iberia. These deformation pulses were contemporary with the Pyrenean and Betic Orogenies in western Europe and may have resulted from structural reorganizations within the evolving orogens driven by changes in plate motion. Possible mechanisms which might concentrate deformation at the ocean-continent transition include slip within serpentinized peridotite underlying the transitional region and reactivation of a pre-existing detachment fault within the thinned continental crust. Our observations may imply that rifted continental margins, when reactivated, can develop into new sites of plate subduction.

Numerous methane gas‐related sea floor structures identified in Gulf of Cadiz

Until recently, the importance of intense sea floor emissions of hydrocarbon-enriched fluids related to the ongoing movement and development of the olistostrome-accretionary wedge complex located in the Gulf of Cadiz (Figure la), remained uncertain. The Gulf of Cadiz is located in the transitional zone between the Gloria transform fault zone, which is the African-Eurasian plate boundary in the Atlantic, and the western-most part of the Alpine-Mediterranean orogenic belt.

Tectonic expression of an active slab tear from high-resolution seismic and bathymetric data offshore Sicily (Ionian Sea)

Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high-resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two-branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike-slip kinematics as indicated by the morphology and steep-dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike-slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio-Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6–7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693.

The lateral extent of sequence boundaries

Abstract Sequence boundaries, as defined by the Exxon group, are regionally extensive surfaces (basin-wide scales) that are characterized in part by stratal discontinuity surfaces in the form of onlap, downlap and toplap (Mitchum et al. 1977). The idea that sequence boundaries are so areally extensive is closely connected to the ideology favoured by the Exxon Group that sequence boundaries are eustatic in origin. This concept of sequence boundaries was initially based on seismic stratigraphic observations of stratal reflection geometry, but has now been extended to rock stratigraphic relationships in general. One of the important issues in sequence stratigraphy is to decide whether concepts based on the low resolution seismic method are applicable at much higher orders of resolution, such as in outcrop. This paper examines the stratigraphic conditions necessary for the development of regionally extensive and discrete surfaces of onlap, downlap and toplap. The main conclusions are that in order for onlap, downlap and toplap surfaces to develop as discrete surfaces there must be no contemporaneous sediment accumulation beyond the lap-out position. If this condition is not met, then discrete lap-out surfaces do not develop. Instead, closely-related families of surfaces that individually are of limited areal extent are more likely to form. It is argued that in most depositional environments some form of sedimentation is likely to occur beyond the lap-out position. This suggests that sequence boundaries should only develop over extremely limited areas, and should not be expected to form regionally correlatable stratigraphic surfaces. The apparent regional extent of some sequence boundaries and their genetic linkage with eustatic changes is in conflict with predictions based on simple stratigraphic principles. The regional correlatability of sequence boundaries probably owes more to the limited resolution of seismic data and the map-driven need to correlate over large areas than to any physical continuity of a single surface.