Contraction induced by block rotation above salt (Angolan margin)

Gravity spreading above salt at passive margins is the major mode of deformation of post-salt sediments. Whereas this process generally creates a structural zoning, extensional upslope and contractional downslope, discrepancies can however arise. For example, evidence of contractional deformation occurs in the extensional domain of the Angolan margin, to the south of the Congo delta fan. Slope-parallel seismic lines show grabens, rollover and extensional diapirs. Conversely, strike-parallel seismic lines present inversion of early grabens, apparently related to a regional-scale decrease in sedimentary thickness away from the Congo delta. As the spreading rate and the characteristic spacing of structures are direct functions of sedimentary loading, one can expect structural changes along strike due to sedimentary thickness variations. This hypothesis was tested using spreading-type experiments of brittle-ductile models lying on top of an inclined rigid substratum. The experiments simulate the progradation of a synkinematic sedimentary cover above salt, with a lateral variation of sedimentation rate. The models show that the spreading rate was higher in the thicker part. Early grabens initiated perpendicular to the slope direction. Where sedimentation rate was high, they kept their orientation during spreading and formed purely extensional synsedimentary structures: Grabens, rollovers and diapirs. Where sedimentation rate was low, blocks separated by grabens rotated in a domino-type fashion but this domain continued to extend in a slope-parallel direction. Strike slip between blocks was entirely localised within the early grabens, which inverted and formed anticlines. Structures obtained in experiments are directly comparable to those in seismic lines of the Angolan margin. In both the Angolan margin examples and the laboratory experiments, block rotation is interpreted as slope-parallel strike-slip shear zones due to lateral variations in spreading rate.