Similarities and Differences between Magma-Poor and Volcanic Passive Margins – Applications to the Brazilian Marginal Basins

Abstract

Passive margins are presently classified in a spectrum of types that have as end-members MagmaPoor (MPPMs) and Volcanic Passive Margins (VPMs). VPMs develop atop or in the close vicinity to the impingement of mantle plumes or of strong and concentrated mantle-derived thermal anomalies upon mega-continents. A series of thermal anomalies/plumes can encroach under a continental mega-plate causing intense volcanism and severe localized thermal weakening of the lithosphere. VPMs are invariably associated to Large Igneous Provinces (LIPs). Crustal deformation in VPMs during the rifting phase is dominated by volcanism under the form of Seaward-Dipping Reflectors (SDRs), severe crustal dyking and widespread hyper-extension. The propagation of fracturing/faulting/rifting linking the thermally weak zones leads to the breakage of the mega-continent into two or more continents and micro-continents. MPPMs develop along the linking rift zones in the areas situated at great distances from the surface expression of the thermal anomalies or plumes; thus, with very reduced influence of volcanism when compared to VPMs. Crustal deformation in MPPMs during the rifting phase is dominated by crustal stretching and thinning without the influence of thermal weakening due to intrusions or volcanism. Hyper-extension occurs in MPPMs in lesser amount than in VPMs. In both types of margins the final breakup of the lithosphere is always promoted by magma from the asthenosphere reaching the surface to create spreading ridges and oceanic crust. Consequently, even in MPPMs volcanism may play an important role in the latest stages of rifting, in the outermost realms of the continental margin. Transitional Passive Margins develop in intermediate positions where VPMs grade into MPPMs and vice-versa.