Papers Basin-forming processes and the deep structure of the Campos Basin, offshore Brazil W. U. Mohriak Petroleo Brasileiro S.A., Department of Exploration, Avenida Chile 65-S. 1301 20035, Rio de Janeiro-RJ, Brazil R. Hobbs BIRPS Group, Bullard Laboratories, University of Cambridge, UK J. F. Dewey Department of Earth Sciences, University of Oxford, Oxford, UK Received 26 June 1989; revised 19 November 1989; accepted 20 November 1989 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. Keywords: basin formation; lithospheric stretching; Campos Basin, Brazil; seismic reflection profiles Introduction Among the geophysical methods that have been used in constraining geological models of the deep structure of sedimentary basins, gravity is probably most intensively used (Karner and Watts, 1981; Rabinowitz, 1982). However, it does not determine a definitive geological model. There is an inherent non-uniqueness in modelling sedimentary basins because the distribution of densities and the geometry of the structures in the lower crust are not known, and different configurations might fit the observed anomalies (Hutchinson et al., 1983). Nonetheless, by assuming a realistic density distribution in the crust, it has been possible to constrain geological models and to infer the deep structure of several basins (Talwani, 1976; Keen et al., 1982; Klitgord and Berhrendt, 1979; Hutchinson, 1982; Keen, 1982). The method indicates that passive margin basins show variable crustal sections along the margin, 0264-8172/90/020094-29 $03.00 ~c~1990 Butterworth & Co. (Publishers) Ltd 94 Marine and Petroleum Geology, 1990, Vol 7, May that the relationship between sediment accumulation and thinning of the crust is complex (Beaumont et al., 1982; Dillon et al., 1982) and that some of the basins seem to have a high density layer in the lower crust, whereas others do not (Grow, 1981; Keen, 1982; White et al., 1987). With the advent of deep seismic reflection data processing in the last decade, geodynamic models can be constrained by imaging the lower crust underlying sedimentary basins. A global perspective on the deep structure of mobile belts and sedimentary basins is now possible, given the amount of data that has accumulated during the last ten years. However, some questions are still unresolved, particularly the geometry of major faults, the accommodation of strain in the lower crust and upper mantle, the effects of stretching the lithosphere heterogeneously, and the relationship between shear zones, sedimentary basins