ALBERT

Description: The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M ~ 8.7 1755 Great Lisbon earthquake and the M = 7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11 months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.

Description: In the Gulf of Cadiz key segment of the Africa-Iberia plate boundary (North-East Atlantic ocean), three main different modes of tectonic interference between a recently identified wrench system (SWIM) and the Gulf of Cadiz Accretionary Wedge (GCAW) were tested through analog sand-box modeling: a) An active accretionary wedge on top of a pre-existent inactive basement fault; b) An active strike-slip fault cutting a previously formed, inactive, accretionary wedge; and c) Simultaneous activity of both the accretionary wedge and the strike-slip fault. The results we obtained and the comparison with the natural deformation pattern favor a tectonic evolution comprising two main steps: i) the formation of the Gulf of Cadiz Accretionary Wedge on top of inactive, Tethyan-related, basement faults (Middle Miocene to similar to 1.8 Ma); ii) subsequent reactivation of these basement faults with dextral strike-slip motion (similar to 1.8 Ma to present) simultaneously with continued tectonic accretion in the GCAW. These results exclude the possibility of ongoing active SWIM wrench system cross-cutting an inactive GCAW structure. Our results also support a new interpretation of the SWIM wrench system as fundamentally resulting from strike-slip reactivation of an old (Tethyan-related) plate boundary

Description: The Gulf of Cadiz and the passive continental margin of southern Iberia to the west of the Strait of Gibraltar locally accommodate the presently ongoing convergence between Africa and Eurasia by widespread, rather diffusive, seismic activity. Seismicity of the northern Gulf of Cadiz was derived from an amphibious seismological network, including 24 temporary marine offshore stations, besides the permanent networks in Portugal, Spain, and Morocco. During the 6 month of the offshore network operation, in total 86 local earthquakes were located at six or more offshore stations with the majority of earthquakes occurring to the southwest of Iberia and along the Algarve continental margin off southern Iberia. The distribution of events along the Algarve margin mimics features reported for the Atlantic passive continental margins of both South and North America. Focal mechanisms at the Portimão Bank support that seismically active areas are associated with compression. Similar stress patterns are reported for the east coast of South and North America. However, while earthquakes along the American east coast occur at crustal levels, earthquakes in the northern Gulf of Cadiz occur both in the lower crust and upper mantle, with the majority of events rupturing within the mantle, including a number of well-located earthquakes beneath crust forming the continent-ocean transition zone. The large number of earthquakes in the mantle might be caused by the unique geological setting, where deformation occurs in cool lithosphere of Mesozoic age. We suggest that seismicity along the Algarve margin is caused by re-activation of pre-existing margin-parallel faults rather than corresponding to newly formed structures related to a new developing plate boundary.

Description: The Alpine orogeny is well recorded onshore and offshore by tectonic inversion of the Mesozoic rift basins. Large scale linear seamounts (more than 250 km long and with up to 5 km of uplift) involving oceanic and continental lithosphere were carried on top of thrusts, such as the Gorringe seamount and the Estremadura Spur in the SouthWest and West Iberia Margin, respectively. The SouthWest Iberia Margin also recorded the westward migration of the Gibraltar Oceanic slab as the westwards propagation of the Neo-Tethys subduction. Rotation of the tectonic compression from NW-SE to WNW-ESE inPliocene times caused the development of large scale dextral wrench faults as the present day Africa-Iberia plate boundary. Neotectonics of this plate boundary caused large to mega-scale destructive earthquakes and tsunamis.

Description: The Cape Verde archipelago is a group of Ocean Islands in the Central Atlantic that forms two chains of islands trending Northwest and Southwest. Several of the islands are considered to be volcanically active, with frequent eruptions on Fogo. We examine the mineral chemistry and thermobarometry of the southern islands; Santiago, Fogo and Brava together with the Cadamosto Seamount. Our objective is to explore the magmatic storage system and implications for volcanic eruptions and associated hazards at Cape Verde. The volcanic rocks at Cape Verde are alkaline and dominantly mafic, whereas the island of Brava and the Cadamosto Seamount are unusually felsic. Clinopyroxene compositions range from 60 to 90 Mg# at Santiago and Fogo. In contrast, at Brava and the Cadamosto Seamount the clinopyroxene compositions are 5 to 75 Mg#. Mineral chemistry and zonation records fractional crystallization, recharge, aggregation of crystals, magma mixing and variations in thermal conditions of the magma at temperatures from 925 to 1250C. Magma storage depths at Santiago, Fogo, Brava and the Cadamosto Seamount are between 12 and 40 km, forming deep sub-Moho magma storage zones. Transient magma storage in the crust is suggested by fluid inclusion re-equilibration and pre-eruption seismicity. A global compilation of magma storage at Ocean Islands suggests deep magma storage is a common feature and volcanic eruptions are often associated with rapid magma ascent through the crust. Shallow magma storage is more variable and likely reflects local variations in crustal structure, sediment supply and tectonics. Petrological constraints on the magma plumbing system at Cape Verde and elsewhere are vital to integrate with deformation models and seismicity in order to improve understanding and mitigation of the volcanic hazards.