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1. THE WESTERN IBERIA MARGIN: A GEOPHYSICAL AND GEOLOGICAL OVERVIEWL.M. Pinheiro, R.C.L. Wilson, R. Pena dos Reis, R.B. Whitmarsh, A. RibeiroABSTRACTINTRODUCTIONPLATE TECTONIC SETTING AND SEISMICITYPHYSIOGRAPHIC FEATURES OF THE CONTINENTAL MARGINGEOLOGICAL FRAMEWORK OF WESTERN IBERIAVariscan BasementMesozoic-Tertiary Basins and Structural HighsCrustal Structure Under Western IberiaGEOLOGICAL HISTORY OF THE WESTERN IBERIAN CONTINENTAL MARGINIntroductionMesozoic Rifting and SubsidenceKinematics and Timing of the DeformationCenozoic Compressional EpisodesEvidence OnshoreEvidence OffshoreMagmatic Activity in the MesozoicLate Triassic-Early Jurassic Alkaline CycleEarly Middle Jurassic Tholeiitic CycleEarly Cretaceous Transitional CycleLate Cretaceous Alkaline CycleTHE OCEAN/CONTINENT TRANSITION OFF WESTERN IBERIACrustal and Upper Mantle Structure Across the Ocean/continent TransitionTiming of the Onset of Seafloor SpreadingCONCLUSIONSACKNOWLEDGMENTSREFERENCESFIGURESFigure 1. The changing plate tectonic setting of Iberia. A-F: Plate tectonic setting between Chron M10 (Hauterivian) and Chron 6 (Burdigalian), after Srivastava et al., 1990a (with permission). AGFZ = Azores-Gibraltar Fracture Zone; KA = Kings Trough-AzFigure 2. A. Seismicity map for the area between the Azores triple junction and the western Mediterranean for the period 1962-1972, showing location of epicenters of events with body-wave magnitude greater than or equal to 4. From Udias et al. (1976). B. Earthquake focal mechanisms in the area of the eastern end of the Azores-Gibraltar plate boundary (after Grimison and Chen, 1988).Figure 2 (continued).Figure 3. Physiographic features of the west Iberia Margin. Bathymetry is shown in meters, simplified after Lallemand et al. (1985). Submarine canyons (in italic): P = Porto; A = Aveiro; DCV = Dom Carlos Valley; C = Cascais (note that the Lisboa canyon Figure 4. Zones and terranes of the Variscan orogen in the Hesperic Massif of Iberia. A. Detailed map of the main terranes in the Portuguese part of the Hesperic Massif adjacent to the study area (modified from Ribeiro and Silva, in press). 1 = Iberian Figure 5. Sketch map showing the distribution of marginal sedimentary basins along the west Iberia Margin (from Murillas et al., 1990). S A = Serra da Arrábida; P-C-T = Porto-Coimbra-Tomar Fault; VDG = Vasco da Gama Seamount. The lines labeled A-F show Figure 6. Sketch of structural sections along the west Iberia Margin (for locations, see Figure 5. A-C from Murillas et al. (1990); D-F from Wilson et al. (1989). The seismic units referred to in the key for D-F are those of Groupe Galice (1979). Note thaFigure 7. Simplified summaries of stratigraphic successions along the west Iberia Margin.Figure 8. Neogene tectonic structures in Portugal. A. Map of structures active in Portugal during the Neogene. The inset map of Iberia shows the location of B (from Ribeiro et al., 1990). LSF = Lousã-Seia fault; LTFZ = Low Tagus Fault Zone; MF = MessejFigure 9. A. Areas of occurrence of Mesozoic igneous rocks (in black) in western Iberia. SAML = Serra d'Aire-Montejunto Lineament; Naz. F. = Nazaré Fault. B. Inset showing the location of A in the Iberian Peninsula.Figure 10. A. Bathymetric chart of the west Iberia Margin (after Lallemand et al., 1985; contours in meters) showing the main features in the region of the ocean-continent transition. Stippled area shows distribution of probable serpentinized peridotite aFigure 10 (continued).TABLETable 1. Summary of data sets available from sectors of the Western Iberian Margin.Whitmarsh, R.B., Sawyer, D.S., Klaus, A., and Masson, D.G. (Eds.), 1996Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 1491. THE WESTERN IBERIA MARGIN: A GEOPHYSICAL AND GEOLOGICAL OVERVIEW1L.M. Pinheiro,2 R.C.L. Wilson,3 R. Pena dos Reis,4 R.B. Whitmarsh,5 A. Ribeiro6ABSTRACTThis paper presents a general overview of the geology and geophysics of western Iberia, and in particular of the westernPortuguese Margin. The links between the onshore and offshore geology and geophysics are especially emphasized. The westIberia Margin is an example of a nonvolcanic rifted margin. The Variscan basement exposed on land in Iberia exhibits strike-slip faults and other structural trends, which had an important effect on the development, in time and space, of subsequent rift-ing of the continental margin and even perhaps influences the present-day offshore seismicity. The margin has had a long tec-tonic and magmatic history from the Late Triassic until the present day. Rifting first began in the Late Triassic; after about 70Ma, continental separation began in the Tagus Abyssal Plain. Continental breakup then appears to have progressively migratednorthwards, eventually reaching the Galicia Bank segment of the margin about 112 Ma. Although there is onshore evidence ofmagmatism throughout the period from the Late Triassic until 130 Ma and even later, this was sporadic and of insignificant vol-ume. Important onshore rift basins were formed during this period. Offshore, the record is complex and fragmentary. An ocean/continent transition, over 150 km wide, lies beyond the shelf edge and is marked on its western side by a peridotite ridge andthin oceanic crust characterized by seafloor spreading anomalies. Rifted fault blocks are recognized within the ocean/continenttransition along the whole margin. Mostly, they merge westwards into a transitional zone where the basement often has lowrelief of unknown origin, and linear magnetic anomalies parallel the seafloor spreading anomalies. There is indirect geophysi-cal evidence that this zone is underlain by intrusions in the lower crust. The most plausible, but not the only, explanation seemsto be that this part of the ocean/continent transition consists of fragments of magmatically disrupted and intruded thinned conti-nental crust. The margin also underwent important postrift compression in Eocene and Miocene time, as demonstrated by fold-ing and nondeposition or erosion of abyssal plain sediments. The Eocene deformation is clearly visible off Galicia but theMiocene deformation is dominant in the rest of the margin, where it may have overprinted the former compressional episode.The margin is still seismically active in its southern part at the present day, mainly because of its proximity to the Azores-Gibraltar plate boundary.INTRODUCTIONThe purpose of this paper is to provide a general overview of thegeology and geophysics of


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