J.T. Vázquez

Vast fields of hydrocarbon-derived carbonate chimneys related to the accretionary wedge/olistostrome of the Gulf of Cádiz

We report the first discovery and sampling of vast fields of hydrocarbon-derived carbonate chimneys along the Gulf of Cadiz continental slope, at depths between 500 and 1200 m. A large variety of carbonate chimneys (more than 200 samples) were recovered from four different areas, named the DIASOM, TASYO, GBF (Guadalquivir Basin) and ESF (East Moroccan) fields. Observations from an underwater camera revealed a spectacular high density of pipe-like chimneys, some of them longer than 2 m, lying over the sea floor, and some protruding from muddy sediment. Local fissures and alignment of isolated chimneys were also observed, suggesting that their distribution is controlled by fault planes. Chimneys collected show a wide range of morphological types (spiral, cylindrical, conical, mushroom-like and mounded) with numerous nodule protuberances and ramified fluid channelways. The chimneys are mainly composed of authigenic carbonates (ankerite, Fe-bearing dolomite and calcite) with abundance of iron oxides, forming agglomerates of pseudo-pyrite framboids. N 18 O isotopic values vary from 0.7 to 5.5x whereas N 13 C values indicate that chimney carbonates are moderately depleted in 13 C, ranging from 346x to 320x PDB, interpreted as formed from a mixture of deep thermogenic hydrocarbons and shallow biogenic methane. These vast fields of carbonate chimneys imply new considerations on the importance of hydrocarbon fluid venting in the Gulf of Cadiz and, moreover, on the active role of the olistostrome/accretionary wedge of the Gibraltar arc. B 2003 Elsevier Science B.V. All rights reserved.

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.

Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and the Weddell Gyre flows

Abstract New swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift, sediment wave morphologies, and seismic facies in the central Scotia Sea. The deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean: the eastward flowing Antarctic Circumpolar Current (ACC) and the northward outflow of the Weddell Sea Deep Water (WSDW). The contourite drifts are wedge-like deposits up to 1 km thick, that exhibit aggradational reflectors along axis thinning towards the margins. The contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles. The elongate-mounded drifts are best developed along the left-hand margins of channelized bottom current flows, due to the Coriolis force. A contourite fan has a main channel and two distributary channels that expand over a gentle seafloor. The proximal fan exhibits sediment waves with the distal fan incised by furrows. Sediment wave fields are well developed in areas of intensified bottom flows without channels, particularly at the confluence of the ACC and the WSDW. Small sediment waves occur where unidirectional bottom current flows predominate. Sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea-bottom relief. The stratigraphic sequence above the oceanic crust of Early to Middle Miocene age contains six seismic units separated by major reflectors. All the units were shaped under the influence of strong bottom current flows, although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time. The age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep-sea deposits in the region. The oldest, Units VI–IV, are of Early to Middle Miocene age and developed under the influence of the ACC. They are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5–8 cm/ky. Unit III, with an estimated Middle Miocene age, evidences the first incursion of WSDW into the central Scotia Sea, when plate movement caused openings in the South Scotia Ridge and allowed the connection with the northern Weddell Sea through Jane Basin and gaps in the ridge. Unit II, estimated to be of Late Miocene to Early Pliocene age, extends over the area and is characterized by internal unconformities. A major unconformity at the base of Unit II records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the Antarctic Peninsula shelf. Unit I, of Late Pliocene to Quaternary age, shows intensified bottom currents. The unconformity at the base of Unit I probably predates the onset of major Northern Hemisphere glaciations and the greater expansion of Antarctic ice sheets during the Late Pliocene. The extensive distribution of contourite deposits above the oceanic crust testifies to the long-term production of Antarctic Bottom Water. Cold, deep water was swept northward from the Weddell Gyre, interacting with the ACC, and possibly exerting profound influences on the global circulation system and the onset of major glaciations.

Model of distributed deformation, block rotations and crustal thickening for the formation of the Spanish Central System

Abstract The Spanish Central System constitutes an important morphostructural intraplate feature of Iberia that resulted from a concentration of crustal strain derived from stresses applied on the boundaries of the Betics and Pyrenees. We propose a tectonic model in which the deformation of this zone was produced by a mid-late Mesozoic shear zone, a type of secondary plate boundary, that absorbed part of the Iberia-Eurasia relative motion. The deformation within this E-W shear zone was compensated by means of block rotations that resulted from the reorganization of multiple and previous faults and fractures. The subsequent N-S Africa-Eurasia convergence was partly accommodated within this intraplate zone of weakness. This caused a concentration of strain that was responsible for the homogeneous thickening of the lower crust, and for the blocking of block rotations and forced uplifting of the upper crust.

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.

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

Abstract The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.

Fluid Venting Through the Seabed in the Gulf of Cadiz (SE Atlantic Ocean, Western Iberian Peninsula): Geomorphic Features, Habitats, and Associated Fauna

Publisher Summary The Gulf of Cadiz is located in the SW Iberian Peninsula and can be considered as the structural front of the Betic-Rifian Arc, located at the westernmost tectonic belt of the Alpine–Mediterranean convergence zone between the African and Eurasian plates. This front is an olistostrome, comprising a large volume of shale and salt diapirism. A variety of seep-related geomorphic features has been described in this region, such as mud volcanoes, carbonate mounds, pockmarks, brine pools, and gas-related sediments. These structures contain precipitates in the form of slabs, pavements, and chimneys, generated by aggregation of carbonate cement resulting from microbial oxidation of gas emissions. These increase the complexity of the soft bottoms where they are formed, promoting a mixture of hard- and soft-bottom fauna. On stable hard substrates, corals, gorgonians, and sponges increase this complexity even more, resulting in a higher number of species than in adjacent soft bottoms. In adjacent soft bottoms, fisheries for different demersal and benthic species occur, although no detailed information is available on their impact upon mud volcano communities. Mud volcanoes, pockmarks, and other fluid escape structures are common along the Iberian margin of the Gulf of Cadiz. They are interpreted as indicators of gas-rich, overpressured sediments occurring at different depths. Geomorphic features related to fluid flow and availability of hard bottoms (e.g., chimneys, slabs) is influenced by the erosion patterns that affect each volcano. Therefore, different habitat types occur in association with different volcanoes.

Seismic Stratigraphy of Miocene to Recent Sedimentary Deposits in the Central Scotia Sea and Northern Weddell Sea: Influence of Bottom Flows (Antarctica)

Multichannel and high resolution seismic profiles from the central Scotia Sea and northern Weddell Sea show a sequence of seismic units interpreted to be the result of high-energy bottom currents. The seismic character of the units is indicative of active bottom flows, which developed extensive drifts under the influence of the Weddell Sea Bottom Water (WSBW) and the Antarctic Circumpolar Current (ACC). The opening of the connection between Jane Basin and the Scotia Sea is marked by a major regional unconformity that recorded a reorganization of bottom flows. The uppermost deposits are characterized by intensified bottom currents, which may reflect increased production of WSBW.

From chemosynthesis-based communities to cold-water corals: Vulnerable deep-sea habitats of the Gulf of Cádiz

The Gulf of Cádiz (GoC) represents an area of ecological importance within the northeastern Atlantic Ocean due to the presence of Mediterranean and Atlantic water masses, a heterogeneous seafloor and a biological confluence. Nevertheless, information on the presence of vulnerable deep-sea habitats is still very scarce and it is of importance for further habitat monitoring within the context of the Habitats and Marine Strategy Framework Directives and for improving conservation and resource extraction management. From 2010 to 2012, fluid migration and emission related edifices (e.g., mud volcanoes, diapirs) from the Spanish continental margin of the GoC have been explored using a remotely operated vehicle (ROV; Liropus 2000) and an underwater camera sled (UCS; APHIA 2012) as well as several devices for collecting sediment and fauna. Different vulnerable deep-sea habitats have been observed, including anoxic bottoms with bacterial mats, sea-pen communities, sponge aggregations, antipatharian and gorgonian communities and also cold-water coral banks. Some of these habitats are included in conservation lists of the habitat directive and in international conventions (OSPAR, RAC/SPA), however some of them are located in areas of the GoC that are exposed to intense trawling. The diversity of habitats detected in the Spanish continental margin of the GoC highlights the importance of seepage related edifices as inducers of seabed and habitat heterogeneity in deep-sea areas.