SLIDES ON THE FLANKS OF SUBMARINE CANYONS IN THE UPPER SLOPE OF THE ALGARVE

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1 Thalassas, 2008, 24 (1): An International Journal of Marine Sciences SLIDES ON THE FLANKS OF SUBMARINE CANYONS IN THE UPPER SLOPE OF THE ALGARVE SAYAGO-GIL, M. (1), PÉREZ-GARCÍA, C. (2), VÁZQUEZ, J. T. (3), HERNÁNDEZ-MOLINA, F. J. (4), FERNÁNDEZ-SALAS, L. M. (5), ALVEIRINHO- DIAS, J. M. (6), DÍAZ DEL RÍO, V. (7) & SOMOZA, L. (8) Key words: Gulf of Cadiz; Algarve; Upper slope; Sedimentary instabilities; Submarine canyons. ABSTRACT This work has the main aims of the sedimentary instabilities characterization on the flanks of submarine canyons in the upper slope of the Algarve continental margin (SW Iberian Peninsula). High resolution seismic (Sparker) has been used to carry out a morphometric study. Depth, slope, length, width and area on the canyons and sedimentary instabilities on their flanks have been measured. Six submarine canyons have been studied in the upper slope such as: Fado, Salema, Lagos Superior, Portimao, Meninas- C and Meninas-M. Morphometric features of the sedimentary instabilities are different depending on the sector where they are located (western sector and eastern sector, divided by the Portimao canyon). A recent tectonic activity has been inferred starting from the different morphology of the canyons and the sedimentary instabilities on their flanks in each sector (western and eastern). In addition, Mediterranean Outflow Water (MOW) dynamics could have had influenced on the asymmetry of the canyons flanks and the generation of sedimentary instabilities. (1) Instituto Español de Oceanografía, Centro Oceanográfico de Málaga. Puerto Pesquero, s/n, Apartado 285. Fuengirola 29640, Spain. cegema@ma.ieo.es (2) University of Tromso. Drammsveien Tromso, Norway. carolina.garcia@ig.uit.no (3) Departamento de Cristalografía y Mineralogía, Estratigrafía, Geodinámica y Petrología y Geoquímica. Facultad de Ciencias del Mar, CASEM - Campus de Puerto Real, Universidad de Cádiz. Puerto Real, 11510, Spain. juan.vazquez@uca.es (4) Facultad de Ciencias, Departamento de Geociencias Marinas, Universidad de Vigo. Vigo, 36200, Spain. fjhernan@uvigo.es (5) Instituto Español de Oceanografía, Centro Oceanográfico de Málaga. Puerto Pesquero, s/n, Apartado 285. Fuengirola 29640, Spain. luismi.fernandez@ma.ieo.es (6) Área Departamental de Ambiente e Ciencia da Terra. Facultade de Ciencias do Mar e do Ambiente- FCMA. Universidade do Algarve. Campus de Gambelas Faro, Portugal. jdias@ualg.pt (7) Instituto Español de Oceanografía, Centro Oceanográfico de Málaga. Puerto Pesquero, s/n, Apartado 285. Fuengirola 29640, Spain. diazdelrio@ma.ieo.es (8) Instituto Geológico y Minero de España (IGME). Servicio Geología Marina, Río Rosas 23, Madrid 28003, Spain. l.somoza@igme.es 65

2 M. Sayago-Gil, C. Pérez-García, J. T. Vázquez, F. J. Hernández-Molina, L. M. Fernández-Salas, J. M. Alveirinho- Dias, V. Díaz del Río & L. Somoza INTRODUCTION The Gulf of Cadiz is located in the south-western margin of the Iberian Peninsula, at the eastern end of the Azores-Gibraltar zone, is the location of the diffuse plate boundary between Eurasia and Africa. The present plate convergence between the African and Eurasia plates in the Gulf of Cadiz area, is at a rate of about 4 mm/yr with NW-SE trend, and is accommodated over that broad diffuse deformation zone (Argus et al., 1989; Olivet, 1996). The south-portuguese continental margin took shape as a consequence of the differential opening of the northern and central Atlantic Ocean during the Mesozoic. Subsequently, this area has been bend (Mougenot, 1988; Terrinha, 1998). Cabral, 1995 suggested that at present and at least from Upper Pliocene, the Portuguese margin is undergoing a general compression. This process is not known in great detail but it seems to be in relation to active compressive structures located in the western part of Iberia. From oceanographic point of view, the presentday circulation pattern is dominated by exchange of water masses through the Straits of Gibraltar. This exchange is driven by the highly saline and warm MOW near the bottom and the turbulent, less saline, cool-water mass of AI (Atlantic Inflow) at the surface. The MOW forms a strong bottom current flowing towards the W and NW above NADW (North Atlantic Deep Water) (Madelain, 1970; Melières, 1974; Zenk, 1975; Thorpe, 1976; Ambar & Howe, 1979). After passing the Gibraltar seamount, MOW forms a turbulent flux only some 150 to 200 m and reaches velocities in excess of 250 cm/s. It then spreads westward into the Gulf of Cadiz, descending the slope and spreading out in two main cores: an upper core centred between m and a lower core centred between m water depths. This lower core then divides into three distinct branches eventually losing contact with the seafloor at around 1400 m water depth (Madelain, 1970; Zenk, 1975; Borenäs et al., 2002). Continuing within the North Atlantic Ocean, three MOW pathways can be identified: to Figure 1: Study area location. Bold lines represent tracks of FADO-9711 cruise and dotted lines are the tracks of Anastasya-99 cruise. 66

3 Lides on the Flanks of Submarine Canyons in the Upper Slope of the Algarve Figure 2: Cross-section of the seismic profiles placed on the canyons from each sector: western and eastern. 67

4 M. Sayago-Gil, C. Pérez-García, J. T. Vázquez, F. J. Hernández-Molina, L. M. Fernández-Salas, J. M. Alveirinho- Dias, V. Díaz del Río & L. Somoza Figure 3: Morphological sketch remarking main features of the Algarve submarine margin. Canyons location: 1-Fado Canyon; 2-Salema Canyon; 3-Lagos Superior Canyon; 4-Portimao Canyon; 5-Meninas-C Canyon; 6-Meninas-M Canyon. Modified of Pérez-García et al., the N along the Iberian slope; to the W from Cape St. Vincent, and to the SW as far as the Canary Islands and then westwards (Iorga & Lozier, 1999). Consequently, the present morphology of the continental margin of the Gulf of Cadiz has been influenced by both recent evolution of the tectonic and sedimentary processes (Maldonado et al., 1999; Medialdea et al., 2004; Hernández-Molina et al., 2006; Llave et al., 2006). Mougenot et al. (1979) mentioned two canyons crossing the continental slope: Portimao and Lagos. In addition, Pérez-García et al., 2004 establish two different sectors, western and eastern, separated by the Portimao submarine canyon. This article characterizes the sedimentary instabilities on the flanks of submarine canyons in the upper slope of the Algarve continental margin (SW Iberian Peninsula) (Fig. 1). In addition, control factors of these instabilities are argument. The study area is located at 36.50º-37º N and 7.50º-9.10º W between -140 m and -750 m depth. The study area is located in the Sector-5 (Submarine canyons sector) defined by Llave (2003) and Hernández-Molina et al., (2003, 2006) in the Gulf of Cadiz. It is characterised by the occurrence of erosive submarine canyons orientated approximately downslope (NE-SW) with steep margins and erosive floors a quite lineal axis (Vanney & Mougenot, 1981; Mulder et al., 2006). MATERIALS AND METHODS The methodology used to carry out this work, has been based on the high-medium resolution seismic profiles of Sparker (1000 J and 4500 J), up to 2 seconds of depth TWTT (Two Way Travel Time). In order to 68

5 Lides on the Flanks of Submarine Canyons in the Upper Slope of the Algarve obtain the position data the DGPS (Differential Global Positional System) was used. The total coverage of the seismic net is 3423 km. These seismic profiles have been collected during two oceanographic cruises: FADO9711 and ANASTASYA99, performed by Instituto Español de Oceanografía on board B/O Francisco de Paula Navarro and B/O Cornide de Saavedra, respectively. (Fig. 1). A regional morphological study of this area has been done from which the morphometric study is showed in this paper measuring depths, slopes, lengths, widths and areas on the submarine canyons and on the slides on their flanks. These results have allowed both identifying the main morphological features and determining the existence of a possible interaction between them. Seismic profiles with a transversal position respect to the canyons located in the upper slope have been used (Fig. 2). It is worthy to clarify the meaning of slide and sedimentary instabilities in this work: both terms are used without distinction to refer to a mass of sediment sliding down, moving along in continuous contact with the slip plane. RESULTS In order to been able to explain the sedimentary instabilities development, a morphometric study of the canyons, in which are located the instabilities on their flanks, is presented. A total number of six submarine canyons have been studied. From west to east they are the following: Fado, Salema, Lagos Superior, Portimao, Meninas-C y Meninas-M (Fig. 3). The main morphological features of the submarine canyons are listed below (Fig. 2 and Fig. 3) (note that the following data belong to the upper slope of the Algarve): - Fado. This canyon is located in front of St. Vicente Cape. The canyon head is at 500 m deep in the upper slope and it comes out onto Lagos Moat. The cross-section is V shaped (Fig. 2a). Fado is an asymmetric canyon with a greater presence of slides on the E-flank. The W-flank slope is 6.3º and 7.5º to the E-flank. The width ranges between 1.45 km and 8.19 km. - Salema. The head of the canyon is at 350 m deep in the upper slope. Salema is an asymmetric canyon in which the main presence of slides is on its W-flank. The canyon shows a V shape. The slope of the W-flank and E-flank are 3.8º and 9º respectively. The breadth varies from 0.86 km to 2.72 km (Fig. 2b). - Lagos Superior. The head of the canyon is located in the shelf break at 200 m deep. A cross-section shows an asymmetrical V. Otherwise, the surface is covered by slides on the W-flank rather than on the E-flank. The canyon shows a W-flank and E- flank slopes of 9.25º and 11.75º respectively. The width varies from 4.98 km (in the canyon head) to 2.68 km (towards offshore) (Fig. 2c). - Portimao. The canyon is placed in the outer continental shelf at 100 m deep. The limit in between upper-middle slope is linked to the Alvarez-Cabral Moat. In the upper slope, Portimao Canyon presents an asymmetrical V cross-section. Slides presence on the E-flank of the canyon is less than on the W-flank. The slopes are 6.2º and 14.3º in the W-flank and the E-flank respectively. The width of the canyon varies from 1.84 km to 4.72 km (Fig. 2d). - Meninas-C. The head of the canyon is located at 350 m deep in the upper slope and it comes out onto Alvarez-Cabral Moat. The unique slide observed is in the axis towards the offshore. The W-flank slope is 5.3º and 4.5º the E-flank. The average width is 0.96 km. Nowadays, this canyon is in a silting stage (Fig. 2e). - Meninas-M. The canyon is placed in the shelf break at 200 m deep and it comes out onto Alvarez- 69

6 M. Sayago-Gil, C. Pérez-García, J. T. Vázquez, F. J. Hernández-Molina, L. M. Fernández-Salas, J. M. Alveirinho- Dias, V. Díaz del Río & L. Somoza Figure 4: Slopes, lengths, widths and areas of the landslides: a) Landslides features on W-flank and E-flank of the canyons in the Western Sector; b) Landslide features measure in the canyon of the Eastern Sector. Cabral Moat. Slides are not observed. The width range varies between 1.68 km in the canyon head and 2.07 km towards offshore (Fig. 2f). Consequently, average values used to the morphometric study carried out to the canyons shows that the canyon heads are located in depths between m, being shallower from the western area to Portimao submarine canyon. The slope gradient varies between 3.8º-9.3º on the W-flanks of submarine canyons and from 4.5º-14.3º on the E-flanks. The widths of the canyon heads reach values in between 0.86 and 1.8 km, except the Lagos Superior canyon head with 5 km. Nevertheless, the canyon widths to the offshore area vary from 1.4 to 8.2 km. Morphometric features of slides are different depending on the location of the canyon in the western 70

7 Lides on the Flanks of Submarine Canyons in the Upper Slope of the Algarve sector (Fado, Salema, Lagos Superior and Portimao) or in the eastern sector (Meninas-C and Meninas-M): - Western sector. The slides on the flanks of the canyons show lengths which range from 2.7 to 16.9 km on the W-flanks and from 5.8 to 13.5 km on the E-flanks. The average width of these gravitational deposits on the W-flanks of the canyons is 0.9 km and 2 km on the E-flanks. The slides show areas which ranged from 1.4 to 28.8 km 2 on W-flanks and from km 2 on the E-flanks. The thicknesses of these deposits vary from milliseconds (Fig. 4a). - Eastern sector. A guessed unique slide is located on the Meninas-C canyon. It is located in the canyon axis towards offshore. This slide is 1.43 km length and 0.83 km width, and the total area covers 1.3 km 2 approximately. Slides have not been observed in Meninas-M submarine canyon (Fig. 4b). DISCUSSION The results from the study about submarine slides confirm the existence of two physiographic sectors in the upper slope of the Algarve continental margin identified by Pérez-García et al., 2004: western and eastern. The positions of the canyon heads are shallower eastward in the western sector. In addition, the western sector reveals a lower gradient of the slopes of the W-flanks of the canyons than the slopes of the E-flanks. The western sector shows more canyons and longer than in the eastern sector. This is a sign of the greater gravitational processes activity in the western sector. The slides which are located on the W-flanks of the submarine canyons are longer, narrower and take up a bigger area than the slides located on the E-flanks. This feature matches up with minor inclinations of the W-flanks of the canyons. All these features could be due to a regional tilting of this area toward the west as consequence of recent compressive processes in this part of the margin. In the eastern sector, canyons come out onto Alvarez-Cabral Moat which could to cause a greater influence of the MOW on their flanks. Slides occurrences on the flanks of the canyons is minor than in the western sector and it is located on the axis. The different position and features of the slides in both sectors are probably arranged because a differential origin of the morphology of the recent canyons in each sector occurs. In this way, the canyons located in the western sector could have a structural control rather than those canyons located in the eastern sector. The Mediterranean Upper Water is flowing northwestward as a tabular water mass along the upper slope at a water depth of m. Moreover, the along-slope processes are clearly dominant with regard to interaction between MOW and the seafloor. (Hernández-Molina et al., 2003). Consequently, is possible that the MOW is responsible that the sedimentary instabilities on the canyon flanks. CONCLUSIONS Several evidences confirm that a regional tilting of the margin toward the west occurs in the area affecting to the morphology of the canyons and instabilities generation. Two sectors can be distinguished in the upper slope of Algarve: western and eastern and the morphology canyons genesis is different on each one: structural control in the western sector and depositional influence in the eastern sector. Abundance of slides located on the flanks of these canyons is greater in the western sector than in the eastern sector. Narrower, longer and bigger slides are present on the W-flank of the submarine canyons in the western sector. The differential control suggests that the processes associated to the MOW dynamics, could have had influence on the asymmetry of the canyons flanks and the generation of sedimentary instabilities. 71

8 M. Sayago-Gil, C. Pérez-García, J. T. Vázquez, F. J. Hernández-Molina, L. M. Fernández-Salas, J. M. Alveirinho- Dias, V. Díaz del Río & L. Somoza ACKNOWLEDGEMENTS This work is based on the data obtained by the Instituto Español de Oceanografía, in the PB C03-03 project (FADO) and a bilateral agreement with the Universidade do Algarve, Portugal. REFERENCES Ambar, I. & Howe, M.R Observations of the Mediterranean Outflow I. Mixing in the Mediterranean Outflow. Deep-Sea Research, 26A: Argus, D.F., Gordon, R.G., Demets, C. & Stein, S Closure of the Africa-Eurasia-North America plate motion circuit and tectonics of the Gloria fault. Journal of Geophysical Research, 94: Borenäs, K. M., Wahlin, A. K., Ambar, I. & Serra, N The Mediterranean outflow splitting- a comparison between theoretical models and CANIGO data. Deep-sea Research II, 49: Cabral, J Neotectónica em Portugal Continental. Memoria Instituto Geológico e Mineiro, 31: 265 pp. Hernández-Molina, F.J., Llave, E., Somoza, L., Fernández-Puga, M.C., Maestro, A., León, R., Medialdea, T., Barnolas, A., García, M., Díaz del Río, V., Fernández-Salas, L.M., Vázquez, J.T. Lobo, F.J., Alveirinho-Dias, J.A., Rodero, J. & Gardner, J Looking for clues to paleoceanographic imprints: A diagnosis of the Gulf of Cadiz contourite depositional systems. Geology, 31 (1): Hernández-Molina, F. J., Llave, E., Stow, D. A. V., García, M., Somoza, L., Vázquez, J. T., Lobo, F., Maestro, A., Díaz del Río, V., Leon, R., Medialdea, T. & Gardner, J The contourite depositional system of the Gula of Cadiz: a sedimentary model related to the bottom current activity of the Mediterranean Outflow Water and the continental margin characteristics. Deep-Sea Research II, 53: Iorga, M. & Lozier, M. S Signatures of the Mediterranean outflow from a North Atlantic climatology. 1. Salinity and density fields. Journal of Geophysical Research, 194: Llave, E Análisis Morfosidementario y Estratigráfico de los Depósitos Contorníticos del Golfo de Cádiz. Implicaciones Paleoceanográficas. Ph. D. Thesis, Universidad de Cádiz, 303 pp. Llave, E., Hernández-Molina, F. J., Somoza, L., Stow, D., Díaz del Río, V., Quaternary evolution of the Contourite Depositional System in the Gulf of Cadiz. In: Economic and Paleoceanographic Importance of Contourites, Viana, A. and Rebesco, M. (Eds.). London Geological Society. Special Publication (In press). Madelain, F Influence de la topographie du fond sur l ecoulement méditerranéen entre le Detroit de Gibraltar et le Cap Saint-Vincent. Cahiers Océanographiques, 22: Maldonado, A., Somoza, L. & Pallarés, L., The Betic orogen and the Iberian-African boundary in the Gulf of Cádiz: geological evolution (central North Atlantic). Marine Geology, 155: Medialdea, T., Vegas, R., Somoza, L., Vázquez, J.T., Maldonado, A., Díaz-del-Río, V., Maestro, A., Córdoba, D. & Fernández- Puga, M.C Structure and evolution of the Olistostrome complex of the Gibraltar Arc in the Gulf of Cádiz (eastern Central Atlantic): evidence from two long seismic crosssections. Marine Geology, 209 (1-4): Melières, F Recherches sur la dynamique sédimentuire du Golfe de Cadiz (Espagne). Thesis, Univ. Paris A: 235 pp. Mougenot, D., Géologie de la marge portugaise. Ph. D. Thesis, University Pierre Curie, 155 pp. Mougenot, D.; Monteiro, J.H.; Dupeuble, P.A. and Malod, J.A La marge continentale sud-portugaise : évolution structurale et sédimentaire. Ciencias da Terra (UNL), 5: Mulder, T., Lecroart, P., Hanquiez, V., Marches, E., Gonthier, E., Guedes, J.C., Thiébot, E., Jaaid, E.B., Kenyon, N.H., Voisset, M., Pérez, C., Sayago, M., Fuchey, Y. & Bujan, S., The western part of the Gulf of Cadiz: contour currents and turbidity currents interactions. Geo-Marine Letters, 26: Olivet, J. L La cinématique de la Plaque Ibérique. Bulletin de Centres Recherche. Exploration et Production Elf Aquitaine, 20: Pérez-García, C., Sayago-Gil, M., Vázquez, J.T., Hernández-Molina, F.J., Llave, E., Díaz del Río, V. & Vicente, J Morfología del talud superior del margen ibérico surroccidental: implicaciones tectónicas y sedimentarias. Geo-Temas, 6(5): Terrinha, P Structural Geology and Tectonic Evolution of the Algarve Basin, South Portugal. Ph. D. Thesis, University of London, 430 pp. Thorpe, S. A Variability of the Mediterranean in the Gulf of Cadiz. Deep-Sea Research, 23: Vanney, J.R. & Mougenot, D La plataforme continentale du Portugal et des provinces adjacents: analyse géomorphologique. Memoria del Servicio Geológico de Portugal, 28: 1-86, Lisboa.0 Zenk, W On the Mediterranean outflow west of Gibraltar. Meteor Forscir. Ergebuisse A (16): (Received: November, 9, 2008; Accepted: March, 28, 2008) 72