Salt flow in the Angolan Margin

Abstract

Abstract Salt tectonics is characterized by two types of mechanical interactions, between brittle and ductile deformation and between deformation and sedimentation, respectively. Consequently, a broad spectrum of structures and, rather frequently, significant 3D complexities can result. In such settings, interpretations made from too local observations can easily be misleading. The purpose of this work is to point out the interest of basin-scale mechanical understanding of salt tectonics processes to provide reliable interpretations of structure development, even at local scales. These principles are illustrated by the Angolan margin. The 2D dynamics of the salt tectonics system can be defined by the conditions of salt flow imposed by the upslope and downslope basin boundaries, leading to a two sub-system organisation, extensional upslope and contractional downslope, respectively. However, the existence of contractional structures in the upslope domain is an obvious departure from the 2D basin-scale flow model. These " anomalies?? develop along lateral boundaries where a strong component of strike-slip shear is responsible for large block rotations. The observed contractional structures are a direct product of these block rotations. These 3D structural complexities likely result from a rejuvenation of the basal slope along a part of the margin. This example illustrates how a robust salt flow model at basin scale, with welldefined initial and boundary conditions, can contribute to the understanding of a non-conform sub-system. The results are sustained by maps of raft restoration and kinematic analysis. Introduction Salt tectonics is characterized by two types of mechanical interactions, between brittle and ductile deformation and between deformation and sedimentation, respectively. Consequently, a broad spectrum of structures and, rather frequently, significant 3D complexities can result. In such settings, interpretations made from too local observations can easily be misleading. The purpose of this work is to point out the interest of basin-scale mechanical understanding of salt tectonics processes to provide reliable interpretations of structure development, even at local scales. At regional scale, salt tectonics in the Angolan margin, Lower Congo and Kwanza basins, is driven by the combined effects of two major parameters that are the margin tilt and the sedimentary loading. Once created, as a result of lithosphere cooling after the continental break-up, the basal slope remains an efficient cause of gliding in a seaward direction. Synkinematic structures like, tilted blocks, rollovers, turtles, grabens and diapirs (Duval et al.,1992, Fort et al., 2004a) attest of the extensional regime upslope, whereas, downslope shortening lead to the formation of growth synclines/anticlines, pop-up structures, compressional diapirs, and salt nappes (Brun & Fort, 2004, Gottschalk et al., 2004). In response to the cylindricity of the margin, early deformation gave birth to structures trending almost parallel to the coastline. The deposition at regional scale of massive loading, so called Congo fan, also insure a salt mobilisation in a seaward direction from Oligocene to present day. However, the non-cylindricity of the sedimentary pile induces a lateral variation of the deformation in a NNW-SSE direction. This lateral segmentation of the margin is accommodated by transfer zones of ENE-WSW direction and raft rotation around vertical axes (Fort et al., 2004b). In response to gravity driven deformation, the basal salt layer is thinning upslope and thickening downslope. Thinning occurs by a combination of pure and simple shear below rollovers and rafts, whereas thickening results from shortening and salt flow in a downslope direction.