Raft tectonics: the effects of basal slope angle and sedimentation rate on progressive extension

In the Gulf of Guinea, the Upper Cretaceous-Tertiary sedimentary cover has undergone gravity gliding above an Aptian salt layer for various basal slope angles from 0 degrees to 4 degrees. Gliding started during Albian time and evolved up to the present with variable sedimentation rates. Faulting patterns are seen to vary in particular as a function of the basal slope angle and the syn-kinematic sedimentation rates. A series of laboratory experiments on small-scale models is used to study the effects of the two parameters mentioned above. Models are composed of two-layer slabs, with Newtonian silicone putty at the base to represent a basal salt decollement and dry sand on top to represent the sedimentary overburden. Models are allowed to deform under their own weight for various basal slope angles ranging from 0 degrees to 5 degrees with two different syn-kinematic sedimentation rates. It is shown that the basal slope angle alpha controls the location of faulting in the overburden, within a single downslope deformation domain for alpha values smaller than 2 degrees and within two domains lying downslope and upslope for higher values of alpha. The width of the deformation domains also varies as a function of alpha. Overburden faulting always starts with syn-kinematic grabens which evolve into symmetric or asymmetric grabens or into tilted blocks bounded by listric normal faults. While synthetic listric normal faults characterize the downslope deformation domains, both synthetic and antithetic listric normal faults can occur in the upslope deformation domain. Ductile deformation within the basal decollement layers results from variable combinations of pure shear and simple shear. Interaction of ductile deformation in the ductile decollement layer with faulting in the overburden locally produces complex and heterogeneous strain patterns, notably within salt rollers. Finally, increasing rates of syn-kinematic sedimentation are seen to increase the rate of downslope displacement and to enhance and favour the development of listric normal faults. (C) 1997 Elsevier Science Ltd.