Basin-forming processes and the deep structure of the Campos Basin, offshore Brazil

Abstract

This paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins. The events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors: shear zones, thin layers of intrusive rocks, underplated material, gneissic banding, fluids and heterogeneities in the crust and mantle. Several types of sedimentary basins have been recognized with the aid of deep seismic reflection data: basins formed along active master faults; basins formed along inactive faults; basins with no fault control; basins formed by low angle detachment faults; and basins formed by pervasive pure shear, or an approximately pure shear, where the lower crust has been locally extended by a different amount than the upper crust. The mechanisms of basin forming processes recognized in deep seismic profiles are: (a) simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle; (b) thinning of the lower crust and mantle by ductile processes, convective mechanisms or flow in the mantle and the upper crust by faulting, resulting in an overall pure shear; and (c) thinning of the lower crust associated with phase changes in the mantle or non-conservation of mass. Two end-member models of basin formation involving lithospheric stretching may be recognized in the South Atlantic: simple shear stretching, where only the upper crust is involved in the process and mantle uplift is offset relative to the basin; and pure shear stretching, where uplift of the mantle vertically balances the basin fill. The first mechanism has been suggested for the onshore Tucano Basin. The data obtained for the offshore Campos Basin suggests regional lithospheric stretching and crustal thinning with Moho uplift compensating for sediment accumulation in the basin depocentre. Complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the Campos basin.