Major Tectonic and Sedimentary Processes in Portuguese MarginsPrincipal research unit: CENTRO DE GEOFÍSICA DA UNIVERSIDADE DE LISBOA
Prime contractor: Instituto de Ciências da Terra e do Espaço
Other participating institutions:
Principal Investigator: Luis Alberto Mendes Victor
Abstract
Margins are the major loci of sedimentary accumulations that contain some of the best preserved records of global sea-level variations, climatic fluctuations, lithospheric deformation, ocean circulation geochemical cycles, organic productivity and sediment supply. Margins, therefore, record the variations in the solid earth-ocean-atmosphere system, essential to evaluating models for today’s global changes.
Portugal mainland faces two distinct margins, the western and southern margins with quite different Mesozoic and Cenozoic tectonic settings. The western margin started its formation during Triassic rifting and has been considered a passive margin from Cretaceous times (after oceanic break up between Newfoundland and Iberia) until Present. However, the moderate instrumental seismicity of this margin, the occurrence of highly destructive historical earthquakes and tsunami, the existence of large active thrusts (over 1000 m throw, Zitellini et al., 1999) and many other geological and geophysical features suggest that the Western Portuguese margin is in a state of incipient subduction (Ribeiro et al., 1996). One physiographic feature of this margin whose formation, structure and tectonic role remains unknown is the Madeira-Tore Rise. Recent studies (Monteiro, 1998) tend to corroborate the hypothesis firstly put forward by Laughton et al. (1975) that the Tore Seamount corresponds to a meteorite crater. Nevertheles, after data released by Roeser et al. (1992) the Madeira-Tore Rise displays a structure similar to that of the Gorringe Seamount, i.e. a shortening structure overthrust towards the continental margin. Unravelling of this structure is one of the main goals of this project because the Tore-Madeira Rise is one of the major puzzles in the Western Portuguese margin and also because it probably constitutes a first order seismogenic structure.
Work based on palinspatic restoration of plate movements of the North Atlantic Azores-Gibraltar region indicate consumption of approximately 150 km of crust between Portugal and Morocco from Late Cretaceous times to Present (Roest and Srivastava, 1991). However, the subduction zone responsible for this process has not been found yet nor any other type of plate boundary between Africa and Iberia. This plate boundary between Portugal and Morocco, still unfound, had a reduced activity during Miocene-Pliocene times. Mapping of active structures both in the offshore and onshore (Terrinha, 1998 and Dias et al, 1999, respectively) and earthquake epicentres and focal mechanism indicate that the Southern Portuguese Margin is presently under a constrictive state of stress.
An important subsidiary development of this tectonic process was the emplacement of an olistostrome in the Gulf of Cadiz, which extends south of the Guadalquivir Seamount until the Seine Abyssal Plain. This chaotic body of Middle Miocene-Upper Miocene age is covered by an important sedimentary infill deposited by the outflow current from the Mediterranean Sea. Sediments provenance and distribution are very complex due to the effect of the well-known exchange of water masses through the Gibraltar Strait.
The main goals of this project are the following. Firstly, the evaluation of the seismogenic and tsunamigenic potential of the western and southern margins of Portugal, a task that has been initiated with DETWS, GITEC I and GITECTWO EU projects, that is now pursued under BIGSETS project (end in June 2000). Secondly, the investigation of the Africa-Iberia plate boundary, its location, geometry and articulation with the low angle faults (both Mesozoic extensional and Neogene and Quaternary thrusts) of the western Portuguese margin. Thirdly, the establishment of a high level morphological and geophysical baseline data to study the coupling between regional tectonics and sediment dynamics, in the South Portuguese Margin and also to investigate the possibility of fluid escape from the Miocene olistostrome and younger sediments; large volumes of hydrocarbon fluids or organic gas are interesting both from an economic and risk point of view since their sporadic burst can cause landslides and trigger tsunamis.
The objectives of MATESPRO are in-line with major international initiatives such as MARGINS (USA), MARGES (France) and the ESF MARGINS programme.
http://www.cgul.ul.pt/projects/matespro.htm
