In foldbelt faults, layers with ductile behaviour can form levels of décollement [Byerlee, 1978]. When these levels are prekinematic, they play a significant role in the genesis, evolution and final geometry of the foldbelt faults, as, for example in the Appalachian Mountains [Davis and Engelder, 1985], the Jura [Sommaruga, 1999], or the Pyrenees [Vergés et al., 1992]. Previous studies based on analogue modelling have shown how a prekinematic décollement level can influence the geometry of foldbelt faults and structures [Ballard, 1989; Colletta et al., 1991; Letouzey et al., 1995; Merle et Abidi, 1995]. However, no study has yet described the influence of synkinematic sedimentation of incompetent levels on the genesis and evolution of compressive structures. The laboratory experiments presented here are designed to explore some of the mechanisms of formation of synsedimentary thrust faults, in relation with the occurrence of a décollement layer during syntectonic sedimentation.
Analogue modelling – Experimental procedure
The models presented here were designed to simulate geological situations comparable to those observed on the border of an overthrust belt. The modelling techniques are similar to those usually applied in experiments on brittle-ductile systems at the Laboratory of Experimental Tectonics of the Geosciences department (Rennes University), and have been fully described in previous studies [e.g. Faugère and Brun, 1984; Vendeville et al., 1987; Davy and Cobbold, 1991].
The prekinematic and synkinematic brittle levels are represented by sand, while the prekinematic and synkinematic ductile levels are represented by silicone.
The experimental apparatus is composed of a fixed and rigid basal plate over which a thin mobile plate is pushed at a constant rate. During shortening (of 5 cm), brittle sedimentation is simulated by sprinkling fresh sand onto the model, and ductile sedimentation is simulated by the deposition of a thin silicone plate onto the model. Photographs of the model surface are taken at regular time intervals to study the development of the structures. The internal structure is recorded from serial cross-sections cut after the experiments.
The parameters tested are the sedimentation rate [see also Tondji Biyo, 1995; Nalpas et al., 1999; Barrier et al., 2002], and the presence and location of a synkinematic décollement layer. The sedimentation is homogeneously distributed on both sides of the relief developed above the thrust front, with a variable ratio R between the rate of sedimentation (vsed) and the rate of uplift (vup), with R taking the values (1) R = vsed/vup = 1/2, (2) R = 1 and (3) R = 2 [Barrier et al., 2002]. The décollement level is deposited at the beginning of sedimentation, either over the whole model or in front of the thrust throughout sedimentation.
In all models, the progressive shortening is accommodated by two conjugate reverse faults. The major fault is antithetic to the displacement of the mobile wall. The synthetic fault is transitory [Ballard, 1989; Tondji Biyo, 1995]. In experiments without ductile sedimentation, the main thrust zone shows an increasing dip with each depositional increment [Barrier et al., 2002]. When the ductile level is deposited, (1) the dip of the main thrust decreases as it reaches the silicone, (2) a wedge of sand then penetrates the silicone forming a detachment, and (3) this wedge is abandoned and the main thrust fault cuts through the wedge, allowing the fault to propagate upward.
At low sedimentation rate, the final geometry shows a major reverse fault made up of a ramp in the prekinematic sand and a flat in the synkinematic silicone. At high sedimentation rate, the major reverse fault is made up of a ramp in the prekinematic sand and a flat in the synkinematic silicone forming a distinctive wedge of sand and a prolongation of the ramp rear the sand wedge.
The presence of a synkinematic ductile level in the model at the beginning of shortening favours decoupling between the prekinematic and the synkinematic sand: the faults in the prekinematic sand are not directly connected to the faults in the synkinematic sand. In addition, the deformation of the sand is different according to whether it is underneath or above the synkinematic ductile level. The prekinematic or synkinematic sand under the synkinematic ductile level is undeformed, whereas the synkinematic sand overlying the synkinematic ductile level is folded.
In the presence of a ductile level, the reverse fault forms a flat in the silicone. The silicone leads to different behaviours of the fault and the synkinematic sand. This raises the question of how to identify synkinematic deposits in compressive basins. In most cases, only the geometry of the strata is used: if progressive unconformity is observed, the strata are synkinematic (growth strata), if not, the strata are deposited before or after the deformation. However, the evolution of growth-strata geometry is also related to the rheology of the rocks. Since geometrical criteria are insufficient, it is also necessary to take account of facies variations.
The presence of a synkinematic ductile level results in the development of a low angle thrust.
The presence of synkinematic ductile levels facilitates deformation and the development of progressive unconformity in growth strata.
Synkinematic sediments with brittle behaviour, deposited in front of a thrust fault, cannot develop a progressive unconformity.
The absence of a progressive unconformity does not necessarily rule out a formation being synkinematic.