T. Mauduit et al., RAFT TECTONICS - THE EFFECTS OF BASAL SLOPE ANGLE AND SEDIMENTATION-RATE ON PROGRESSIVE EXTENSION, Journal of structural geology, 19(9), 1997, pp. 1219-1230
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 stu
dy the effects of the two parameters mentioned above. Models are compo
sed of two-layer slabs, with Newtonian silicone putty at the base to r
epresent 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 degr
ees 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 d
omains also varies as a function of alpha. Overburden faulting always
starts with syn-kinematic grabens which evolve into symmetric or asymm
etric grabens or into tilted blocks bounded by listric normal faults.
While synthetic listric normal faults characterize the downslope defor
mation domains, both synthetic and antithetic listric normal faults ca
n occur in the upslope deformation domain. Ductile deformation within
the basal decollement layers results from variable combinations of pur
e shear and simple shear. Interaction of ductile deformation in the du
ctile decollement layer with faulting in the overburden locally produc
es complex and heterogeneous strain patterns, notably within salt roll
ers. Finally, increasing rates of syn-kinematic sedimentation are seen
to increase the rate of downslope displacement and to enhance and fav
our the development of listric normal faults. (C) 1997 Elsevier Scienc
e Ltd.