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Lisbon, Portugal

Cunha T.A.,University of Oxford | Cunha T.A.,Instituto Don Luiz IDL LA | Watts A.B.,University of Oxford | Pinheiro L.M.,University of Aveiro | Myklebust R.,TGS NOPEC Geophysical Company ASA
Earth and Planetary Science Letters | Year: 2010

Multi-channel seismic and gravity anomaly data have been used to determine the extent of compressional deformation along the SW Portugal rifted continental margin and place constraints on the long-term (>1M.a.) strength of the lithosphere. The seismic sections suggest that the region of compressional deformation is broad (~100km) and has been active since the Miocene. Integration with recently compiled high-resolution bathymetric data shows that the main thrust front is located along the base of the continental slope, between north of the Gorringe Bank and the Setúbal Canyon. Gravity data show that the thrust front is associated with a narrow isostatic anomaly 'high' of up to 70mGal that is flanked on its NW edge by a broad 'low' of up to 20mGal. This high-low 'couple' can be explained by compressional loading of extended continental lithosphere that increased its flexural strength (or equivalent elastic thickness, Te) since rifting. Based on combined 2-D backstripping and gravity modelling techniques we estimate a Te of ~10km during the main stretching episode, in the Late Jurassic (maybe earliest Cretaceous?), and of 35-50km during the Miocene to Recent compression. The existence of a broad region of deformation off SW Portugal together with a strong lithosphere have implications for the rupture models of large earthquakes in the region, such as the 1755 Great Lisbon earthquake, particularly when accounting for a complex, multiple rupture in faults which cut through lithosphere of distinct nature and origin, as appears to be required by modellers to explain the historical observational data. © 2010 Elsevier B.V.

Cunha T.A.,Instituto Don Luiz IDL LA | Cunha T.A.,National Laboratory of Energy and Geology | Matias L.M.,Instituto Don Luiz IDL LA | Matias L.M.,University of Lisbon | And 8 more authors.
Geophysical Journal International | Year: 2012

We use a thin-shell approximation for the lithosphere to model the neotectonics of the Gulf of Cadiz, SW Iberia margin and the westernmost Mediterranean, in the eastern segment of the Azores-Gibraltar plate boundary. In relation to previous neotectonic models in the region, we utilize a better constrained structural map offshore, and the recent GPS measurements over NW Africa and Iberia have been taken into account, together with the seismic strain rate and stress data, to evaluate alternative geodynamic settings proposed for the region. We show that by assuming a relatively simple, two-plate tectonic framework, where Nubia and Eurasia converge NW-SE to WNW-ESE at a rate of 4.5-6 mm yr -1, the models correctly predict the amount of shortening and wrenching between northern Algeria-Morocco and southern Spain and between NW Morocco and SW Iberia, as estimated from both GPS data and geological constraints. The consistency between modelled and observed velocities in the vicinity of Gibraltar and NW Morocco indicates that forcing by slab sinking beneath Gibraltar is not required to reproduce current horizontal deformation in these areas. In the Gulf of Cadiz and SW Iberia, the modelling results support a diffuse Nubia-Eurasia Plate boundary, where the convergence is accommodated along NNE-SSW to NE-SW and ENE-WSW thrust faults and WNW-ESE right-lateral strike-slip faults, over an area >200 km wide, in good general agreement with the distribution of the seismic strain rate and associated faulting mechanisms. The modelling results are robust to regional uncertainties in the structure of the lithosphere and have important implications for the earthquake and tsunami hazard of Portugal, SW Spain and Morocco. We predict maximum, long-term average fault slip rates between 1-2 mm yr -1, that is, less than 50 per cent the average plate relative movement, suggesting very long return periods for high-magnitude (M w > 8) earthquakes on individual structures. © 2012 The Authors Geophysical Journal International © 2012 RAS.

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