We N110 15 Crustal Structure of the Southern Portuguese ... - Earthdoc

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Exploración) & R. Rocca (Repsol Exploración). SUMMARY. The southern Portuguese margin recorded significant extension and subsidence during the Jurassic ...
1-4 June 2015 | IFEMA Madrid

We N110 15 Crustal Structure of the Southern Portuguese Margin - Constraints from Potential Field Methods A. Ramos* (Universitat de Barcelona), L. Cascone (Repsol Exploración), A. Olaiz (Repsol Exploración), O. Fernández (Repsol Exploración), A. Sánchez de la Muela (Gessal), W. Hermoza (Repsol Exploración), A. Arnaiz (Repsol Exploración) & R. Rocca (Repsol Exploración)

SUMMARY The southern Portuguese margin recorded significant extension and subsidence during the Jurassic, resulting in the formation of the Algarve basin. Recent studies have revealed the existence of oceanic crust, of potentially Jurassic age, underlying significant portions of the Gulf of Cadiz to the south and west of the Algarve basin, but the exact relationship between this basin and the oceanic crust is still ambiguous. In this study we integrate data derived from field outcrop studies, interpretation of commercial seismic, and gravity and magnetic modelling to better understand the transition from the rifted continental crust underlying the Algarve basin to the oceanic crust of the Gulf of Cadiz. This integrated approach reveals that the Gulf of Cadiz oceanic crust is related to the WSW-ENE trending Jurassic passive margin of southern Portugal, and is separated from the Algarve Basin by a domain of highly thinned continental crust.

77th EAGE Conference & Exhibition 2015 IFEMA Madrid, Spain, 1-4 June 2015

1-4 June 2015 | IFEMA Madrid

Introduction The southern margin of Portugal lies in the complex zone of interaction between Tethyan and midAtlantic rifting. The structure of the southern Portuguese margin is dominated by roughly E-W trending extensional faults and NW-SE trending transform faults. The E-W trending Meso-Cenozoic Algarve Basin (Matias et al., 2011) is set within this framework. The southern margin of the Algarve basin is the Guadalquivir-Portimão Bank, an area of positive bathymetric expression and a pronounced gravimetric high (Figure 1), labelled the Gulf of Cadiz Gravity High (GCGH) by Gràcia et al. (2003). South of this interpreted NE-SW trending basement high, bathymetry increases rapidly and significantly, potential field modelling indicates a shallowing of the Moho, and seismic reflection profiles show a consistent drop in the depth to basement. The possible presence of oceanic crust of Jurassic age in this area has been discussed by multiple authors based on potential field data (e.g., Gutscher et al., 2009, Sallarès et al., 2011), but with no details on the nature of the transition between the interpreted oceanic crust south of the Guadalquivir Bank and the continental crust under the Algarve basin. Regional structure Four serial regional cross-sections have been modelled across the Algarve basin and extended beyond its southern limit, the Guadalquivir-Portimão Bank (Figure 1), although we show three models in this paper. These NNW-SSE trending cross-sections follow four regional 2D seismic lines from a survey acquired by TGS in 2000, and interpretation is locally supported by 3D seismic acquired by Repsol Exploración in 2012. Four main structural domains can be defined along the cross-sections: x Coastal area (in the north) in which basement rises rapidly from depths in excess of 3km to around 1km near the shore, x The Algarve basin sensu strictu whose basement structure is dominated by complex horst-andgraben structures while the Mesozoic and Tertiary cover is dominated by salt-related structures whose position is often controlled by underlying basement structures, x The Guadalquivir-Portimão Bank, a prominent seafloor feature which is related to a basement high and locally salt-related structures, x The deep offshore extension of the basin, in which the top of basement drops to depths well in excess of 5km. This portion also coincides broadly with the area covered by the Guadalquivir allochthonous unit or olistostrome, a tectonic/gravitational mélange interpreted to be the westernmost extension of the Betic-Rif orogen.

Gravity & magnetic modelling Whereas the shallow structure of the Mesozoic-Tertiary Algarve basin can be mostly inferred from 2D and 3D seismic images (e.g., Matias et al., 2011), the basement structure and in particular the crustal structure can only be accurately inferred or supported from the integration of potential field data. To this goal, both gravity and magnetic data acquired together with 2D, 3D seismic and satellite derived data were used to constrain the deep structure interpretation. Information from wells and public domain data has been used to constrain the densities of the modelled formations. Additional constraints from seismic interpretation have been incorporated to build the initial models. Moho geometry has been derived from the interpretation of a smooth deep high amplitude reflector in the 2D seismic. Potential field data modelling is a relatively fast process that allows to compare the synthetic signal response of our geological models with the real measured data. Models can be iteratively adjusted until a satisfactory fit between calculated and measured signals is achieved.

77th EAGE Conference & Exhibition 2015 IFEMA Madrid, Spain, 1-4 June 2015

1-4 June 2015 | IFEMA Madrid

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Figure 1 First vertical derivative Bouguer anomaly map of SW Iberia. Location of the grav-mag models across the Algarve Basin in white. Grav-mag modelling of the four regional sections has revealed the following: x Salt thickness is highly variable along the basin, but in most cases remnant salt is thin excepting in diapirs and salt walls. x The positive gravity anomaly along the Guadalquivir-Portimão Bank is in part related to a basement high in this area, as has been traditionally interpreted. However, locally, very short wavelength gravity anomalies require the presence of accumulation of low-density material (salt) in narrow structures (diapirs). x The wavelength of the positive gravity anomaly increases to the southwest. This can only be accounted for by a progressive westward shallowing of the Moho and deepening of the top of basement (thinning of the crust) on the southern side of the Guadalquivir-Portimão Bank. x Magnetic signal over the Algarve basin indicates laterally variable composition of the basement, and to the thinning of the upper crust towards the west. These results are all compatible with the key observations on seismic both in terms of the shallow structure north of the Guadalquivir-Portimão Bank, and the deepening of basement to its south. Discussion Strong indications of the presence of oceanic crust under the Gulf of Cadiz have only been provided for the westernmost and central portion of this area (Sallarès et al., 2011, Ranero, pers. comm.). The extension of this oceanic crust eastward towards the Strait of Gibraltar is not supported by hard geophysical data other than the presence of a subducting slab under the Betic-Rif Arc (Gutscher et al., 2009). Given the prominence of the Guadalquivir-Portimão Bank, this feature has been consistently used as the potential continent-ocean boundary between both locations (Gutscher et al., 2009). However, the modelling presented in this study demonstrates that whereas highly thinned, potentially oceanic crust does underlie the Gulf of Cadiz south of the westernmost Guadalquivir-Portimão Bank. There is no evidence for such dramatic crustal contrast in the central part of the Gulf of Cadiz.

77th EAGE Conference & Exhibition 2015 IFEMA Madrid, Spain, 1-4 June 2015

1-4 June 2015 | IFEMA Madrid

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Figure 2 Gravity-magnetic models along the Algarve Basin and the Guadalquivir-Portimão Bank

77th EAGE Conference & Exhibition 2015 IFEMA Madrid, Spain, 1-4 June 2015

1-4 June 2015 | IFEMA Madrid

Figure 3 Schematic map of crustal structure in the Gulf of Cadiz Seismic interpretation and field work in the onshore Algarve basin indicate that the main extensional faults in this area strike W-E to WSW-ENE. This extensional system is compartmentalized by NWSE trending transfer faults. It is therefore inferred that the contact between the Portimão Bank and oceanic crust is across a transform fault of the Jurassic extensional system, whereas the possible oceanic crust domain further to the east would lie southeast of a domain of transitional continental crust outboard of the Guadalquivir Bank (Figure 3). This Jurassic-age margin is now juxtaposed against northern Africa along the Iberia-Africa plate boundary. Conclusions x The Guadalquivir-Portimão Bank is the boundary between continental and oceanic or highly thinned crust only to the west of the 8ºW meridian. x East of the 8ºW meridian the Guadalquivir-Portimão Bank is only a basement high separating two domains of the Mesozoic-Tertiary Gulf of Cadiz basins resting on continental crust. x Grav-mag modelling has proven crucial in understanding the crustal structure of the Gulf of Cadiz. x Further work is needed to accurately delineate the continent-ocean boundary extending east into the Gulf of Cadiz. References Dañobeitia, J. J., Bartolomé, R., Checa, A., Maldonado, A., Slootweg, A. P. [1999] An interpretation of a prominent magnetic anomaly near the boundary between the Eurasian and African plates (Gulf of Cadiz, SW margin of Iberia). Marine Geology, 155, 45 – 62. Gràcia, E., Danobeitia, J., Vergés, J., Bartolomé, R., Córdoba, D. [2003]a. Crustal architecture and tectonic evolution of the Gulf of Cadiz (southwest Iberian margin) at the convergence of the Eurasian and African plates. Tectonics, 22, 1033. Gutscher, M.-A., Dominguez, S., Westbrook, G.K., Le Roy, P., [2009]a. Deep structure, recent deformation and analog modeling of the Gulf of Cadiz accretionary wedge: implications for the 1755 Lisbon earthquake. Tectonophysics, 475, 85–97. Matias, H., Kress, P., Terrinha, P., Mohriak, W., Menezes, P.T. L., Matias, L., Santos, F., Sandnes, F. [2011] Salt tectonics in the western Gulf of Cadiz, southwest Iberia. AAPG Bulletin, 95, 1667-1698. Pérez-López, A., Sanz de Galdeano, A. C. [1994] Tectónica de los materiales triásicos en el sector central de la Zona Subbética (Cordillera Bética). Revista Sociedad Geológica España, 7 (1-2), 141153. Sallarès, V., Gailler, A., Gutscher, M.-A., Graindorge, D., Bartolome, R., Gracia, E., Diaz, J., Zitellini, N. [2011] Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberia). Earth and Planetary Science Letters, 311, 112–123. 77th EAGE Conference & Exhibition 2015 IFEMA Madrid, Spain, 1-4 June 2015