Rift and COT structure of the Brazilian Equatorial Margin

    The formation of the >1000 km long Brazilian Equatorial Margin (BEM) is not yet understood. Limited accessibility of data has caused its classification as a transform margin based on its geodynamic situation during the separation of Africa and South America. However, a newly available grid of seismic reflection lines imaging the entire crust along ~500 km of the BEM provides a comparatively high-resolution map of its structure that questions the classic interpretation of the system, but also does not agree with end-member models of Atlantic Margin rifting. The dataset consists of ~10k  km of 2D seismic reflection lines and several exploration wells provided by the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP). The area covered by the grid extends from the south of the Romanche Fracture Zone to Touros High. The imaged domains extend under the continental shelf, the continental slope, and the deep-water basin. The aim of this work is to discuss the crustal structure, the distribution and age of syn-rift sediment and how syn-rift deformation styles vary along the BEM

     We have interpreted and mapped the Moho reflection along most of the region, as well as the base of the sediment cover, defining the geometry of the possibly crystalline basement. The basement thickness thins from ~7-4 s Two-Way Time (TWT) under the continental shelf to ~4-2 s TWT under the continental slope and from ~2.0-1.5 s TWT to under the deep-water basin where the basement thickness ranges 4.9-2.2 s (TWT). We have mapped and age-calibrated syn-rift sediment deposits from under the continental shelf to the deep-water basin.

   The style of deformation and distribution of syn-rift strata changes from south to north along the study region. At the Touros High Plateau, the southernmost region of the Equatorial Margin, the basement and syn-rift strata across the continental slope and deep-water basin are cut by steep faults with a deformation pattern that may indicate a strike-slip transform-type kinematic opening. On the central to northern sectors of the study area, syn-rift strata fill the space created by normal faults. These faults, that define a complex pattern, can dip landward or seaward and cause blocks to be tilted. Apparently, most faults exhibit small offsets and only a few cut and offset (>0.3 s TWT) the top of the basement by a significant amount.

     The style of crustal thinning and the syn-tectonic strata and fault geometry indicate that only the southernmost sector of Touros High contains structures supporting transform tectonics. The central and north sectors display a gradual seaward crustal thinning and lack evidence of significant syn-rift magmatism. The often-well-imaged Moho suggests a deep-water margin floored by a fairly constant-thickness basement, which indicates the lack of mantle exhumation. The seismic structure supports a transition from faulted and gradually thinned crust overlaid by syn-rift strata to a constant-thickness basement that lacks significant faulting and syn-tectonic deposits, which may be interpreted as the first formed oceanic crust during the Cretaceous Magnetic Quiet Zone.