Compressional salt tectonics (Angolan margin)

We present an analysis of compressional deformation at the front of a gravity spreading system above salt using seismic data from the Angolan margin and laboratory experiments. The geological setting and Structural zonation is briefly reviewed and illustrated with two cross sections parallel to the margin slope in the Kwanza Basin. Experiments are carried out using sand and silicone putty to represent sediments and salt, respectively. The silicone layer was double-wedge-shaped to simulate more precisely the initial geometry of the Aptian salt basin of the Angolan margin. Models based on the Angolan margin example display the same structural zonation consisting of an upslope domain of extension, a downslope domain of compression and an abyssal undeformed domain. We present three different models, with different input parameters, showing the lateral variability of compressional structures in the downslope compressional domain. Models show two main stages of compression, which first appears in a domain located at some distance from the toe of the ductile wedge and then propagates both downslope to the distal salt pinchout and upslope in the formerly extensional domain. The initial zone of compression evolves into a domain of strong shortening characterised by folds, thrusts and squeezed diapirs. Synclines undergo strong pinching and can become detached as pod-like structures encapsulated within the underlying ductile layer. Anticlines are also pinched, thus isolating blobs of ductile material forming compressional diapirs that can extrude up to the surface. Unfolded layers develop into pop-Lip-type anticlines flanked by growth synclines. The propagation of compression, both downslope and upslope, creates domains of moderate shortening on each sides. Close to the domain of strong shortening, double-wavelength folds form a transition to a domain where compression is Superposed onto the lower part of the upslope extensional domain, leading to extensional diapir squeezing. In the Angolan margin, propagation of compression downslope is characterised by recent folding affecting a sedimentary sequence of constant thickness and even the seafloor. Characteristic structures identified in the models are compared with seismic examples. We tentatively apply the mechanisms of sediment incorporation within the underlying ductile layer, as demonstrated in models, to the zone of apparently thick massive salt of the Angolan margin. (C) 2004 Elsevier B.V. All rights reserved.