--> Magma-Poor and Volcanic Passive Margins — Characterization of the End-Members of Rifted Margins

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Magma-Poor and Volcanic Passive Margins — Characterization of the End-Members of Rifted Margins

Abstract

Passive margins are classified in a spectrum of types that have as end-members Magma-Poor (MPPMs) and Volcanic Passive Margins (VPMs). VPMs develop atop or in the vicinity of mantle plumes or of strong and concentrated mantle-derived thermal anomalies that encroach under mega-continents, causing severe localized thermal weakening of the lithosphere. Crustal deformation in VPMs during rifting is dominated by intense volcanism (Large Igneous Provinces), severe crustal dyking and hyper-extension. The propagation of faulting linking the thermally weak zones leads to the breakage of the mega-continent into two or more continents. MPPMs develop in the areas situated at great distances from the thermal anomalies/plumes. Crustal deformation in MPPMs is dominated by crustal stretching and thinning without the influence of thermal weakening. Hyper-extension occurs in MPPMs in lesser amount than in VPMs. In both margins the final breakup of the lithosphere is promoted by magma from the asthenosphere reaching the surface to create spreading ridges and oceanic crust. Transitional Passive Margins develop in intermediate positions where VPMs grade into MPPMs. The rifts in VPMs are filled mostly by volcanic/volcaniclastic rocks. Huge growth wedges consistently accrete laterally from the proximal to the outer margin, being rotated and dipping invariably seaward (SDRs). They fill the grabens showing growth towards listric faults that dip consistently continent ward. The crustal structure changes dramatically from the original thickness to a hyper-extended domain via a distinct narrow necking zone. SDRs tend to appear ocean ward of this necking zone and extend all the way to the COB. The Moho is usually a strong and well defined seismic reflector. The rifts in MPPMs are filled predominantly by sedimentary rocks. Volcanic flows may be present as minor constituents. Rift faults dip mostly ocean ward. The crustal structure displays a consistent set of domains, from the original thickness under shallow waters, to the stretched, thinned, necked and hyper-extended domains in the ultra-deep waters. Mantle exhumation marks the COB. The Moho is rarely a distinct seismic reflector. Interpretations of ultra-deep seismic sections from several passive margins and spectacular outcrops are used to build reference models for each end-member. MPPMs present high petroleum potential in both Rift and Post-Rift sections, while the petroleum potential in VPMs seems to be restricted to the last.