Volcanic Versus Non-Volcanic Passive Margins: Two Different Ways to Break-up Continents
Abstract:Volcanic passive margins (VPMs) are distinctive features of Larges Igneous Provinces. They characterize continental breakup associated with the extrusion and intrusion of large volumes of magma, predominantly mafic. In Large Igneous Provinces, regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere, suggesting that mantle melting is a cause of continental break-up, not a consequence. Early melt covers as volcanic traps large cratonic or/and cratonic-edge continental areas. Crustal dilatation through dyking in the upper crust and magma underplating at Moho level is thought to occur massively during this early stage. Lithosphere extension leading to break-up and VPMs development is coeval with a 3D focusing of mantle melting, giving rise to VPMs.
From a combination of deep seismic reflection profiles and onshore observations, we show that the mechanism of continental breakup at volcanic passive margins is very different from the one generally proposed for non-magmatic systems. Crustal extension and coeval extrusion of thick wedges of seaward-dipping basalts are accommodated by continentward-dipping detachment-faults at both conjugate margins. Those faults root on a deformed ductile crust whose composition seems partly magmatic.
Our numerical modeling show that hardening of deep continental crust during the early magmatic stages provokes a divergent flow of the ductile lithosphere (mantle and lower crust) away from a central continental block which thins through advection with time. Magma-assisted crustal-scale faults dipping continentward root over this flowing material, isolating micro-continents which may be lost in the future oceanic domain.
The structure and tectonic evolution of volcanic passive margins cannot therefore be compared to non-volcanic ones, where major detachment faults dip oceanward during the necking-stage and where mantle is finally exhumed during the mechanical breakup. Confusions may exist where ancient hyper-extended rift systems parallel VPMs. In such cases, VPMs develop within a continental lithosphere made harder by previous crust thinning and lithosphere thermal relaxation.