Fault-related Folding in the Deep Waters of the Equatorial Margin of Brazil

Abstract

Two gravitational fold and thrust belts (GFTBs) from the deep waters of the Para-Maranhao and Barreirinhas basins were interpreted in two-dimensional seismic sections and analyzed using the concepts of fault-related folding and taper-wedge mechanics. These basins lie in the equatorial Atlantic Ocean margin of Brazil, a classic example of transform to oblique (transtensional) continental margin. There, deep-water anoxic shales of probably Turonian age served as decollement zones to Late Cretaceous and Paleogene predominantly siliciclastic wedges to slide down from upper slope (extension) to lower slope (contraction) realms. The structural style of both fold belts is typical thin-skinned tectonics with imbricate thrust faults branching upward from the detachment level with associated fault-related folding. The direction of tectonic transport is from the coast to offshore (southwest to northeast). The similarities and differences between the GFTBs were highlighted. The most striking difference regards the distribution of contractional strain throughout the fold belt. In the Para-Maranhao GFTB, a single major contraction event was achieved via eight regularly spaced imbricate thrust faults. Intervening slices present all types of classic fault-related folding (fault-bend, fault-propagation, and detachment folds). In the Barreirinhas GFTB, an earlier minor contraction was also spread over regularly spaced faults; however, a major late contraction was achieved by one main thrust fault with one major fault-related fold associated to it. This fold is predominantly a shear fault-bend fold whose geometry varies slightly along strike. Their main similarities regard the discrete nature of the detachment level, their structural coherence and narrowness, their low to moderate wedge taper, and their noncritical nature of the taper wedge. Both present syntectonic growth strata that record variations in the balance between the rates of sedimentation and structural uplift of the fold, and folding by limb rotation. The fault-related folding thus determined and the parameters established for wedge taper are slightly different from those presented by active submarine fold and thrust belts at convergent margins and passive margins throughout the world. The decollement dip of the GFTBs is significantly lower, and the bathymetric slope is somewhat higher than elsewhere around the globe.