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Volcanic passive margins: another way to break up continents

Abstract : Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle. In general terms, if we consider frictional and non-linear ductile behavior of rocks and extension rates of the order of a few cm/yr, the extension of " normal " continental lithosphere requires high levels of differential stresses 1–3. However, passive margins record events that end with continental break-up, when the integrated strength of rifted lithosphere drops to zero 1,3. The role of magma intrusion in favoring and focusing extension may be important as the lithosphere can be both thermally weakened by hypothetical lithospheric-scale dykes 4 or compositionally strengthened in lower crustal section by cooled mafic intrusions 5. The distinction between between the volcanic (VPMs) and non-volcanic (in this case, sedimentary) passive margins (SPMs) is basically drawn on the basis of timing and degree of mantle melting in relation to lithosphere extension, break-up and plate separation 6–9
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Laurent Geoffroy, Evgueni Burov, P. Werner. Volcanic passive margins: another way to break up continents. Scientific Reports, Nature Publishing Group, 2015, 5, pp.14828 ⟨10.1038/srep14828⟩. ⟨insu-01235820⟩

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