Probing into extensional sedimentary basins: comparison of recent data and derivation of tentative models

Abstract More and more deep Seismic profiling is revealing the complex evolution of the continental crust, which bears the imprints of former plate movements. Previous subduction zones and mountain belts affected the crust before the formation of intra-cratonic sedimentary basins or passive margins. Therefore the former geological setting must always be taken into account in deep reflection studies of such areas. Recent deep Seismic profiles in Europe and other parts of the world have demonstrated the largely heterogeneous nature of the continental crust, with dipping reflectors within the crust. While bedding heterogeneities or intrusive sill complexes may be invoked, these reflectors are most often associated with inherited thrust planes related to crustal shortening during previous orogenies. Some of these inherited faults have been imaged beneath sedimentary basins with contrasting styles and their reactivation depends on their strike in relation to the regional stress field. These pre-existing zones of weakness induce a heterogeneity in the upper crust, which may control the geometry of future extensional tectonics. When the strike of the inherited faults is perpendicular to the extensional stress field, they may be reactivated as a detachment surface controlling the sedimentary infill. More often, the obliquity of these structures induces local changes and segmentation of the rif: when they are crossed. In such a case, they correspond to transfer faults, acting as relays between major normal faults, or to “ twist zones” where changes occur in the throw of the normal faults, sometimes evolving to an inversion of the polarity in the tilting of the major blocks. During extension, the top of the lower crust may act as a mechanical decoupling boundary separating asymmetrical brittle failure within the upper crust and symmetrical ductile creep within the lower crust. Extensional modes may explain the great values of crustal thinning observed in both upper and lower crusts beneath rifts and passive margins related to the opening of an oceanic domain. It is more difficult to explain the comparatively small amount of horizontal extension generally observed.