Leon R. (1), Somoza L.(1), Ivanov M.K. (2),  Diaz-del-Rio V.(3), Lobato A.(3),
Hernandez-Molina F.J.(4), Fernandez-Puga M.C.(1), Maestro A.(1), Medialdea T.(1), Alveirinho J.(5) and
Vazquez T (4).

(1) Marine Geology Dv., Geological Survey of Spain IGME , Madrid, Spain. Email: rleon@itge.es
(2) UNESCO Center on Marine Geology and Geophysics, Moscow State University, Geology faculty ,Moscow, Russia
(3) Instituto Español de Oceanografia IEO, Spain.
(4) Facultad de Ciencias del Mar UCA, Cadiz, Spain.
(5) Universidad do Algarve UALG, Faro, Portugal.

Cooperation between the UNESCO-IOC TTR programme (TTR9 and TTR10 cruises) and the Spanish marine science TASYO project (Tasyo-2000 and Anastasya-2000 cruises) have permitted to identify a large number of structures related to hydrocarbon rich fluid venting in the Gulf of Cadiz.
In 1999, during the TTR9 cruise aboard R/V Logachev, were identified the Yuma, Ginsburg, Kidd, Adamastor y TTR mud volcanoes and the San Petesburgh mud volcanoe along the border of the Spanish-Moroccan margins (Ivanov et al. 2000). According to the results of gas measurements, this area is characterized by relatively high background gas content (up to 292 ml/l) in comparison with other areas of extensive mud volcanism as the Eastern Mediterranean and the Black Sea (Stadniskaia et al. 2000). In this background, in May 2000, one of the main objectives of the Tasyo2000 cruise aboard R/V Hesperides were surveyed the Spanish-Portuguese continental margins in order to identify mud volcanism, geophysical gas-hydrates evidences and other gas-related sea-floor features along the slopes of the Gulf of Cadiz.
An extent new data of more than 1200 km has been obtained during the Tasyo2000 cruise. The  Simrad EM12S-120 system, a multibeam echo sounder system, was used for mapping sea-floor. It operates at a main frequency of 13 kHz, with 81 beams, which allow a maximum coverage angle of 120º ( about three times the depth).  This  system triggering with a range of pulse length of 2-10 ms, reaching a resolution of 0.6 m. A preliminary processing of data has been made aboard with the Neptune software. The swath mapping provided bathymetric map contoured at an interval of 10 m also a subsidiary map showing the strength of the sea-floor backscattering of the sonar signal.  The upper sediment column and sea floor were studied with high resolution methods. A Parasound echosounder TOPAS (Topographic Parametric Sound) , which is a sub-bottom profiles  was used. It works with CHIRP wavelet, operating at two simultaneous primary frequencies of 15kHz  and 18 kHz. An penetration of up to 100 m has obtained with a resolution of 1-0.5 m. A fishing-type echosound Simrad EK500 operating at a frequency 38 kHz (Splitbeam) was also used for observed water column targets plumes related to active gas venting.
The swath mapping survey of the Gulf of Cadiz resulted in three unexpected results: the probable presence of a number of mud volcanoes, the presence of numerous large pockmark craters, and of the remarkable features resembling scars of a large sediment slide migrating towards the Horseshoe abyssal plain. The main area of pockmarks craters and  mud volcanoes were identified as the "Tasyo field". The Tasyo field extend within 36º15´N-7º W and 35º 45´N-7º 30´, characterized by an irregular sea floor formed by the presence of numerous crater-like pockmarks and dome features. Depths range from 750 meter to 1050 meter. Deeper zones corresponds to large crater pockmarks whereas shallower ones are the top of dome structures. The Tasyo field is surrounded to the north by a main channel of the Mediterranean undecurrent (MOW Mediterranean Outflow Water) and is cut by distributary  channels flowing downslope.
In July 2000, during the TTR10 cruise aboard R/V Logachev, the Tasyo field was firstly surveyed with multifold seismic, and later, several gravity cores were collected. There were identified several large mud volcanoes (Rabat, Olenin, Baraza and Tasyo mud volcanoes),  that linkage the Morrocan and Spanish margins. In September 2000, during the Anastasya-2000 cruise aboard the R/V Cornide de Saavedra, gravity cores and dredge samples collected from dome structures of the Tasyo field, observed on the swath mapping, revealed at least four new gas venting structures within the Tasyo field: the Hesperides mud volcanoe complex and the Faro, Cibeles and Almazan single mud volcanoes. In all of them, mud breccia, carbonate slabs and dolomite chimneys were collected. The Hesperides mud volcanoe complex, with a seabed diameter of more than 5 km building over a plateau that reaches a height of 80 m, has a complex top with a conical structure and relatively steep slopes with a relief of 15 m over the plateau. The plateau is surrounded by steep slope depressions that are clearly reflected on the multibeam mosaic.  These depressions are interpreted as formed by gas venting and brines seepages (brine pools ?). Presently, these depressions are cutting and filled with sandy contourites from  overflowing channels of the main MOW channel. The thickness of this type of  mud volcanoe evidenced by transparent acoustic facies range from 15 m on the border to 45 m on the axis. Basis on the swath mapping , at least five single cones can be identified. Dredges collected from these cones revealed large amounts of dolomite slabs (ankerite) and some samples of chimneys and matrix breccia. Pogonophora sp. and pyrite were also collected, all characteristics of chemosyntectic communities. The Faro, Cibeles and Almazan mud volcanoes with smaller dimensions, has single cones with a diameter of about 1800 meter. The axis of this type of mud volcanoe show a conical transparent facies deforming the underlying sediments. They are arranged along a slide scar well observed on the swath mapping.  Dredge samples collected on these structures show the same characteristics than the Hesperides mud volcanoe: dolomite crusts with sulfide matrix breccias.
Observations on the very high-resolution seismic of the mud volcanoes seems to support a pattern of episodic venting. The main mechanism to explain the observed widespread and episodic venting gas on the sea-floor subsurface, of overpressured formations and of fluid circulation through the sediments and the sea floor are:
(a) Contractional faults regulated by salt/shale adjustment. (Somoza et al. 1999). Therefore, on the upper slope contractional faults provide avenues of vertical transport for overpressured compartments which provide the driving force for fluid and gas expulsion (Lowrie et al. 1999).
 (b) Destabilization of gas hydrates by warming  of the Mediterranean Outflow water (MOW) and/or  by mega-slides detached on gas-hydrate stability zone (GHZ) According to depth-temperature distribution of the sea water masses in this area and  with geothermal gradients of 2.89 ºC per 100m, the GHZ begin at 680 m on Atlantic water mass and at 850 m when MOW is present. Thus, the influence of MOW reduce drastically the existence of the gas hydrates. Both mechanism are  probably modulated by sea-level changes in response to lowstand sedimentary loading and changes in current strength and patterns of the Mediterranean outflow water.

References

Ivanov, M.K., Kenyon, N., Nielsen, T.,Wheeler, A., Monteiro, J.,Gardner, J., Comas, M., Akhmanov, G., Akhmetzhanov, A., and Scientific Party of the TTR-9 cruise. (2000): Goals and principle results of the TTR-9 cruise. Geological processes on European Continental Margin (TTR-9) Post-Cruise Conference. Abstract Volume p24-25.
Lowrie, A., Hamiter, R., Moffett, S., Somoza, L., Maestro, A, Lerche, I. (1999): Potential Pressure Compartments Sub-Salt in the Gulf of Mexico and Beneath Massive Debris Flows in the Gulf of Cadiz. GCSSEPM Foundation 19th Annual Research Conference Advanced Reservoir Characterization .p271-280.
Stadniskaia, A., Ivanov, M., and Gardner, J. (2000): Hydrocarbon gas distribution in mud volcanic deposits of the Gulf of Cadiz. Preliminary results. Geological processes on European Continental Margin (TTR-9) Post-Cruise Conference. Abstract Volume p45-46.
Somoza, L., Maestro, A. and Lowrie, A. (1999): Allochtonous Blocks as Hydrocarbon Traps in the Gulf of Cadiz. Offshore Technology Conference OTC 10889: p571-577.