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.
Highlights
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 synmagmatic break-up of the lithosphere
In contrast with non-volcanic margins, continentwarddipping detachment faults accommodate crustal necking at both conjugate volcanic margins
Summary
This thermo-mechanical code handles free surface boundary conditions which allow the modelling of the topography and basement evolution, as well as the large strains and visco(ductile)-elastic-plastic(brittle) rheologies characteristic of different lithospheric and mantle units (see Supplementary Information) These conditions include Mohr-Coulomb failure for brittle deformation (faulting), pressure-temperature strain-rate dependent ductile flow for viscous deformation, thermo-dynamic phase transitions, internal heat sources and elastic compressibility. The numerical code Flamar is based on the FLAC37 and Parovoz algorithm[38], as described in many previous studies[30,39,40,41,42] For this reason, we limit the description of the code to the essential features: the ability to handle (1) large strains and multiple visco-elastic-plastic rheologies (EVP) including Mohr-Coulomb failure (faulting) and non-linear pressure-temperature and strain-rate dependent creep; (2) strain localization; (3) thermo-dynamic phase transitions; (4) internal heat sources; (5) free surface boundary conditions and surface processes. Using equations of heat transfer, with a heat advection term included in the Lagrangian derivative DT/Dt, as follows: ρC p
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