HR: 1330h
AN: T53A-06    [Abstracts]
TI: Basement Structure and Sediment Architecture in the Southern Iberia Abyssal Plain
AU: * P‚ron-Pinvidic, G
EM: Gwenn.Peron-Pinvidic@eost.u-strasbg.fr
AF: EOST, CNRS-ULP, 1 rue Blessig, Strasbourg, 67084 France
AU: Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: EOST, CNRS-ULP, 1 rue Blessig, Strasbourg, 67084 France
AB: Deep-sea drilling and geophysical surveys at the Southern Iberia Abyssal Plain (SIAP) have revealed the existence of an up to 130 km wide Ocean-Continent-Transition (OCT). This OCT presents geophysical characteristics that match neither those of oceanic crust nor those of a thinned continental crust. This zone is characterized by weak and non-linear magnetic anomalies, variable topography, particular seismic velocities structures with strong gradients and no clear Moho reflection. New models assume that the OCT is formed by mantle exhumation along downward concave faults, an idea that is also supported by numerical modelling and observations in ancient margins exposed in collisional orogens. This model has considerable implications for the sedimentary architecture and the internal structure and nature of basement rocks in the OCT which are not yet tested. In order to test this model we mapped seismic formations and reflection geometries in the basement and in the sedimentary cover, and distinguished five basement domains based on seismic characteristics throughout the abyssal plain using seismic reflection lines Lusigal 12, Sonne, and CAM. The results of this work enabled us to describe the 3D sedimentary architecture in the OCT and to define its relation to basement structures. Of particular interest is the distribution of the lowermost formation overlying the basement, which is dated, based on correlation with DSDP Site 398 further to the east, as Valanginian-Hauterivian to late Aptian. This formation shows tilted reflector sequences thickening onto the footwall in the northwest whereas comparable structures are not observed towards the south and in the adjacent basins further continentwards. This observation indicates the existence of growth structures in the OCT and suggests a migration of the tectonic activity in the OCT towards the northwest. A second aim of the study was to define the internal structure of the OCT which is so far unconstrained. Because inherited heterogeneities or compositional variations within the basement of the OCT are likely to control the distribution of the deformation within the overlying sediments during tectonic inversion, we mapped the strain distribution within the post-rift sediments and compared it with the distribution of the five basement domains that were defined based on the seismic characteristics. Preliminary results show that: (1) reactivation structures are localized in the SIAP within a zone overlying basement with well-defined seismic characteristics, and (2) deformation occurs within the OCT and not at its transition to continental or oceanic crust. These results suggest that the OCT can not be considered as a homogeneous domain and may give some insights on where and how subduction may form during reactivation of passive margins.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3025 Marine seismics (0935)
DE: 8105 Continental margins and sedimentary basins
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2005 Joint Assembly