T. Cunha

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.

Mesozoic–Cenozoic evolution of North Atlantic continental-slope basins: The Peniche basin, western Iberian margin

New regional (two-dimensional) seismic reflection data, published Deep-Sea Drilling Project–Ocean Drilling Program reports, and unpublished shallow-offshore well information characterize the Mesozoic–Cenozoic evolution of the western Iberian continental slope north of 3845N. Two distinct sectors bounded by first-order transfer faults exist between the Galicia Bank and the Nazar fault. The northernmost sector 1 is filled by Triassic–Aptian (prebreakup) sequences, reaching more than 3.5 s two-way traveltime (TWTT) in thickness in distinct half grabens. Salt pillows, salt ridges, minibasins, and salt-detached overburden faults were generated during the Mesozoic and reactivated in the Cenozoic. Sector 2 shows Triassic–Jurassic units more than 2.0 s TWTT thick, underlying east-tilting half grabens of Early Cretaceous age. Salt structures in this sector evolved into mature salt diapirs. Postbreakup units are up to 2.0 s TWTT thick in both sectors. The evolution of the study area replicates evolutionary settings that have previously been proposed for nonvolcanic passive margins. However, some distinct features are noted: (1) widespread Triassic–Berriasian units deposited over rotated tilt blocks represent the early rifting stage; (2) Early Cretaceous subbasins showing rift-climax units, most likely formed during the advanced rifting stage, are spatially constrained to an approximately 100-km (62-mi)-wide region stretched along the continental slope; and (3) listric blocks and their associated low-angle (deep) detachment faults, formed on the distal margin during the advanced rifting and transition to sea-floor spreading stages, show no developed rift-climax units above them. From the early rifting stage onward, Mesozoic faults and halokinetic structures induced local differences in the thickness and character of seismic facies. Cenozoic (Alpine) tectonism promoted the reactivation of older Mesozoic structures.

Structural evolution and timing of continental rifting in the Northeast Atlantic, West Iberian Margin

Regional (2D) seismic-reflection profiles, outcrop and borehole data are used to indicate that the bulk of Late Jurassic-earliest Cretaceous subsidence occurred in the present-day continental slope area. Five (5) principal regressive events (and their correlative basal unconformities) reflecting tectonic uplift and relative emersion in proximal basins are correlated with major rift-related tectonic events on the deeper margin. A significant portion of the west Iberian lower-plate margin was uplifted and eroded during the last stages of continental rifting. Such process was repeated at different times (and in different areas) as the locus of rifting and continental break-up migrated along the future passive margin. As a result, in west Iberia two distinct rift axes are recognised, a first axis extending from the Porto Basin to the Alentejo Basin and a second axis located on the outer proximal margin north of 380 30’N. We propose the late-rifting phases of tectonic quiescence, widespread erosion and sediment progradation on the inner proximal margin as marking the abandonment of extensional basins east of a major Slope Fault System (SFS), and the subsequent onset of syn-rift extension on the outer proximal margin. However, rift-related units of Triassic (and older?) to Middle Jurassic age are well represented on the outer proximal margin. This fact indicates that crustal extension on continental slope basins of west Iberia consisted of a prolonged process in which the last rifting episode is structurally imprinted over older – but not less important – rift episodes.