Pedro Terrinha

Tsunamigenic-seismogenic structures, neotectonics, sedimentary processes and slope instability on the southwest Portuguese Margin

Tectonically active structures prone to cause devastating earthquakes and tsunamis, e.g. the Lisbon 1755 earthquake, were investigated during the UNESCO/IOC Training Through Research-10 (TTR-10) cruise on the southwest Portuguese Continental Margin using single channel seismic profiles, a 3.5-kHz hull-mounted sea-bottom profiler, 10-kHz OKEAN long range side-scan sonar, 30-kHz ORETECH deep-towed side-scan sonar, and a high-resolution deep-towed sea-bottom profiler. These data allowed the definition of new active faults and the establishment of morphological criteria for the classification of active faults in the study area. Landslides associated with the activity of a major tectonic structure, the Marques de Pombal Fault, and other areas with clear signs of mass wasting phenomena were mapped. A slope-to-basin sedimentary system comprising 21 sedimentary ridges up to 20 km long was mapped and described. It was found that the sediments are mainly transported into the deep basins by mass transport processes across the steepest fault scarps forming a channel–levee system, while gravitational slides/slumps dominate the shallower slopes. The sedimentary ridges with an elevation of 40–50 m (50–60 ms TWT) above the seafloor are imaged on the high-resolution seismic profiles as an alternation of high and low amplitude reflectors. It is shown that the Pereira de Sousa Fault, its plateau and the Principes de Avis Plateau are experiencing uplift according to sedimentary and morphological criteria.

Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin

Subduction initiation at passive margins plays a central role in the plate tectonics theory. However, the process by which a passive margin becomes active is not well understood. In this paper we use the southwest Iberia margin (SIM) in the Atlantic Ocean to study the process of passive margin reactivation. Currently there are two tectonic mechanisms operating in the SIM: migration of the Gibraltar Arc and Africa-Eurasia convergence. Based on a new tectonic map, we propose that a new subduction zone is forming at the SIM as a result of both propagation of compressive stresses from the Gibraltar Arc and stresses related to the large-scale Africa-Eurasia convergence. The Gibraltar Arc and the SIM appear to be connected and have the potential to develop into a new eastern Atlantic subduction system. Our work suggests that the formation of new subduction zones in Atlantic-type oceans may not require the spontaneous foundering of its passive margins. Instead, subduction can be seen as an invasive process that propagates from ocean to ocean.

Compressive Episodes and Faunal Isolation during Rifting, Southwest Iberia

Evidence for three short‐lived compressive episodes of late Carixian, late Callovian–early Oxfordian, and Tithonian‐Berriasian ages that lasted <5 m.yr. and occurred during the process of rifting of the Algarve Basin is presented. These tectonic‐inversion episodes are described at outcrop and cartographic scales and have been dated with the accuracy provided by the ammonoid scale. An uplift event of late Toarcian–Aalenian age of undetermined tectonic origin is also described. We show that these four tectonic episodes coincide in time with important ecological events, such as the onset of migration and/or the segregation of Boreal and Tethyan ammonite species and the confinement of the Algarve Basin. Stratigraphic and paleoecological data from the Algarve and Lusitanian Basins are compared and discussed together with eustatic and tectonic information. We propose that the tectonic‐inversion episodes that caused uplift are the origin of the Mesozoic sedimentary gaps and the intermittent opening and closure of the seaway located offshore the SW corner of Iberia between the Algarve and Lusitanian Basins (i.e., a seaway between the Boreal and Tethyan realms). Three tectonic mechanisms for the origin of these short‐lived compressive episodes are presented after comparing the tectonic setting of the Algarve Basin with other geological provinces of the world where similar phenomena also occurred.

Salt tectonics in the western Gulf of Cadiz, southwest Iberia

This study presents the results from the interpretation of an extensive and recent regional two-dimensional seismic survey focused on the understanding of the salt tectonics in the western Gulf of Cadiz (GoC). Two different salt units were identified: an autochthonous salt unit of the Late Triassic or the Early Jurassic (Hettangian) and an allochthonous unit that originated from the Hettangian salt. Interpretation of the pattern of distribution of the salt in the basin allowed subdivision of the area of study into three distinct salt domains: the eastern domain characterized by the presence of a conspicuous allochthonous salt nappe (Esperanca Salt), the central domain dominated by salt diapirs with mild deformation of Miocene strata and wide salt-withdrawal minibasins, and the southwestern domain where present-day tectonics induces impressive salt deformation affecting the sea floor. This complex pattern is mainly the result of the interaction of inherited basement structure, complex tectonic history, and stress regime of the basin. The intense halokinesis observed has created several salt-related trap geometries and fluid migration pathways. As the focus of worldwide exploration along passive margins is gradually shifting to deep-water regions, the western GoC has the potential to become a deep-water petroleum province in the near future.

Strike-slip faults mediate the rise of crustal-derived fluids and mud volcanism in the deep sea

We report on newly discovered mud volcanoes located at ~4500 m water depth ~90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a >400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87 Sr/ 86 Sr, d 18 O, dD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.

Extension and inversion structures in the Tethys–Atlantic linkage zone, Algarve Basin, Portugal

Abstract The Algarve Basin is a Meso-Cenozoic sedimentary basin overlying Carboniferous basement, located in the southwestern margin of the Iberian Peninsula. Its structure reveals a protracted tectonic history comprising various pulses of Mesozoic extension followed by Cenozoic compression. This work deals with the structure along the northern margin, where the Mesozoic extensional structures and Cenozoic inversion structures crop out. The strike of the extensional structures ranges from E–W to N–S, as controlled by a shift from Tethyan-dominated extension in the east to Atlantic-dominated extension in the west. Contractional structures are inverted extensional structures, following their same trends. It is argued that the thickness of the Hettangian evaporite layer exerts a strong control on the structural style throughout the basin during the extensional and inversion episodes. The basin is affected by thick-skinned deformation along the northern margin, where salt is thin or absent, basement involved fault systems and short-cut structures. Basinward, as the Hettangian salt thickens, the margin is affected by thin-skinned deformation, with listric and down-to-the-basin growth faults, diapirism and salt-cored detachment folds. The aim was to discuss the key tectonic features, the relevance of salt, and understand the nature, timing, and significance of all these structures in the regional tectonic evolution.

The 1755 Lisbon earthquake and the beginning of closure of the Atlantic

The 1755 Lisbon earthquake and tsunami had one of the highest magnitudes in the history of Europe. The source mechanism requires generation at a subduction zone. Intensity distribution and tsunami modelling excludes the Gorringe Bank as a source area and suggests generation by the incipient convergence of the Atlantic with the Southwest Iberia and Morocco margin rather than at the less active Gulf of Cadiz Accretionary Prism. The comparison with the 2004 Sumatra earthquake and tsunami supports this interpretation. A tsunami warning alert system is urgent for the Atlantic.

Comment on “Lisbon 1755: A Case of Triggered Onshore Rupture?” by Susana P. Vilanova, Catarina F. Nunes, and Joao F. B. D. Fonseca

The work published recently by Vilanova et al. (2003) in the Bulletin of the Seismological Society of America proposes that the accounts of destruction and other effects reported in the Lisbon area as a result of the 1 November 1755 earthquake are best explained by a local rupture on the Lower Tagus Valley fault (ltvf), triggered by the static stress change produced by the main offshore source located in the Gorringe area. Because of the potential impact of this hypothesis on the seismic hazard of the Lisbon area, we discuss and complement the evidence presented by Vilanova and co-workers, concluding that the “local rupture model” should remain, for the moment, as an unsubstantiated speculation. Vilanova et al. (2003) (referred to herein as “the authors” or “the paper”) compare the intensities reported for the 1755 earthquake in Lisbon and Algarve to conclude that Lisbon intensities are abnormally high for the same distance, in particular, when compared with what was observed for the 28 February 1969 earthquake. They interpreted this as the result of local rupture. In Figure 1 we re-plot the macroseismic intensity as a function of the distance to the Gorringe Bank source presumed by the authors. We can see that “Lower Tagus Valley intensities” do not show any abnormal behavior. Similar plots could be made for all source areas proposed by other authors (Baptista, 1998; Baptista et al. , 1998a, 1998b; Zitellini et al. , 1999, 2001; Terrinha et al. , 2003) with minor changes to the conclusion we reached previously. The conclusions reached by the authors were based on their figure 8A, from which intensity values for the Algarve and Portuguese west coast corresponding to distances greater than 350 km are missing. Figure 1. Modified Mercalli (MM) intensities for the 1755 earthquake, plotted as a …

Neogene to recent contraction and basin inversion along the Nubia‐Iberia boundary in SW Iberia

The SW of Iberia is currently undergoing compression related to the convergence between Nubia and Iberia. Multiple compressive structures, and their related seismic activity, have been documented along the diffuse Nubia-Iberia plate boundary, including the Gorringe bank west of the Gulf of Cadiz, and the Betic-Rif orogen to the east. Despite seismic activity indicating a dominant compressive stress along the Algarve margin in the Gulf of Cadiz, the structures at the origin of this seismicity remain elusive. This paper documents the contractional structures that provide linkage across the Gulf of Cadiz and play a major role in defining the present-day seismicity and bathymetry of this area. The structures described in this paper caused the Neogene inversion of the Jurassic oblique passive margin that formed between the Central Atlantic and the Ligurian Tethys. This example of a partially inverted margin provides insights into the factors that condition the inversion of passive margins.

Anatomy and tectonic significance of WNW-ESE and NE-SW lineaments at a transpressive plate boundary (Nubia-Iberia)

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).